Ellagitannins Inhibit the Exsheathment of Haemonchus contortus and Trichostrongylus colubriformis Larvae: The Efficiency Increases Together with the Molecular Size

Worldwide, parasitic gastrointestinal nematodes continue to threaten animal health, welfare, and production in outdoor breeding systems of small ruminants. For more than 50 years, the control of these parasitic worms has relied on the use of commercial synthetic anthelmintics. However, anthelmintic resistance in worm populations is nowadays widespread and requires novel solutions. The use of tannin-rich plants has been suggested as an alternative to synthetic anthelmintics to control gastrointestinal nematodes. The majority of previous studies have focused on the activity of proanthocyanidins (syn condensed tannins), and less is known about ellagitannins. In this study, the effects of 30 structurally unique ellagitannins on the exsheathment of third-stage infective larvae were examined on Haemonchus contortus and Trichostrongylus colubriformis by the in vitro larval exsheathment inhibition assay. Ellagitannins were found to be promising natural anthelmintics as they showed direct inhibition on larval exsheathment for both nematode species. In general, ellagitannins were more efficient at inhibiting the exsheathment of H. contortus larvae than those of T. colubriformis. The efficiency of inhibition increased as the degree of oligomerization or the molecular weight of the ellagitannin increased. Otherwise, we found no other structural features of ellagitannins that significantly affected the anthelmintic activity on the third-stage infective larvae. The effective concentrations were physiologically relevant and should be achievable in the gastrointestinal tract also in in vivo conditions.

Genotypic characterisation of monepantel resistance in historical and newly derived field strains of Teladorsagia circumcincta

Recent reports of monepantel (MPTL) resistance in UK field isolates of Teladorsagia circumcincta has highlighted the need for a better understanding of the mechanism of MPTL-resistance in order to preserve its anthelmintic efficacy in this economically important species. Nine discrete populations of T. circumcincta were genotypically characterised; three MPTL-susceptible isolates, three experimentally selected MPTL-resistant strains and three field derived populations. Full-length Tci-mptl-1 gene sequences were generated and comparisons between the MPTL-susceptible isolates, MPTL-resistant strains and one field isolate, showed that different putative MPTL-resistance conferring mutations were present in different resistant isolates. Truncated forms of the Tci-mptl-1 gene were also observed. The genetic variability of individual larvae, within and between populations, was examined using microsatellite analyses at 10 'neutral' loci (presumed to be unaffected by MPTL). Results confirmed that there was little background genetic variation between the populations, global FST <0.038. Polymorphisms present in exons 7 and 8 of Tci-mptl-1 enabled genotyping of individual larvae. A reduction in the number of genotypes was observed in all MPTL-resistant strains compared to the MPTL-susceptible strains that they were derived from, suggesting there was purifying selection at Tci-mptl-1 as a result of MPTL-treatment. The potential link between benzimidazole (BZ)-resistance and MPTL-resistance was examined by screening individual larvae for the presence of three SNPs associated with BZ-resistance in the β-tubulin isotype-1 gene. The majority of larvae were BZ-susceptible homozygotes at positions 167 and 198. Increased heterozygosity at position 200 was observed in the MPTL-resistant strains compared to their respective MPTL-susceptible population. There was no decrease in the occurrence of BZ-resistant genotypes in larvae from each population. These differences, in light of the purifying selection at this locus in all MPTL-resistant isolates, suggests that Tci-mptl-1 confers MPTL-resistance in T. circumcincta, as in Haemonchus contortus, but that different mutations in Tci-mptl-1 can confer resistance in different populations.

Selection for anthelmintic resistant Teladorsagia circumcincta in pre-weaned lambs by treating their dams with long-acting moxidectin injection

Administration of long-acting anthelmintics to pregnant ewes prior to lambing is a common practice in New Zealand. Today, most of these products contain macrocyclic lactone (ML) actives, which because of their lipophilic nature, are detectable in the milk of treated animals and in the plasma of their suckling offspring. This study was conducted to confirm the transfer of ML actives to lambs in the ewe's milk, and to assess whether this could result in selection for ML resistant nematodes in the lamb. Ninety, twin bearing Romney ewes were treated before lambing with a long-acting injectable formulation of moxidectin, a 100-day controlled release capsule (CRC) containing abamectin and albendazole, or remained untreated. After lambing, seven ewes from each treatment group were selected for uniformity of lambing date and, along with their twin lambs, relocated indoors. At intervals, all ewes and lambs were bled, and samples of ewe's milk were collected, for determination of drug concentrations. Commencing 4 weeks after birth all lambs were dosed weekly with 250 infective larvae (L3) of either an ML-susceptible or –resistant isolate of Teladorsagia circumcinta. At 12 weeks of age all lambs were slaughtered and their abomasa recovered for worm counts. Moxidectin was detected in the plasma of moxidectin-treated ewes until about 50 days after treatment and in their lambs until about day 60. Abamectin was detected in the plasma of CRC-treated ewes until the last sample on day 80 and in the plasma of their lambs until about day 60. Both actives were detectable in milk of treated ewes until day 80 after treatment. Establishment of resistant L3 was not different between the treatment groups but treatment of ewes with moxidectin reduced establishment of susceptible L3 by 70%, confirming the potential of drug transfer in milk to screen for ML-resistance in the suckling lamb.

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Long-acting anthelmintics were administered to pregnant ewes.

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Moxidectin and abamectin were detected in ewe's milk for >60 days.

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Moxidectin and abamectin were detected in plasma of lambs for >60 days.

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Abamectin treatment had not effect on establishment of larvae in the lambs.

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Moxidectin treatment reduced establishment of susceptible, but not resistant larvae.

Dexamethasone treatment interferes with the pharmacokinetics of ivermectin in young cattle

An experiment was carried out to study the possible interaction between dexamethasone (DXM) treatment and the efficacy of ivermectin (IVM) treatment in young cattle. Two groups, each of seven calves, were experimentally inoculated with an equal mixture containing 15,000 third stage larvae of Cooperia oncophora and Ostertagia ostertagi each, and with no history of being resistant to any anthelmintics. However, in this study C. oncophora was unexpectedly classified as IVM-resistant according to the outcome from the faecal egg count reduction test (FECRT). Blood parameters and faecal egg counts (FEC) were monitored from 0 to 35 days post infection (d.p.i.). The calves in one group received intramuscular injections of short and long-term acting DXM at 22 and 24 d.p.i., respectively. The other group remained as a control. Three days post patency (24 d.p.i.) both groups were injected subcutaneously with IVM (Merial) at the recommended dose (0.2mg/kg). A significant difference (p<0.001) in FEC patterns was observed between groups. Although both groups still excreted eggs (100-200 eggs per gram faeces) 11 days post anthelmintic treatment, the control group had a significantly higher reduction between 23 and 35 d.p.i. (p=0.025). After 35 days, four animals per group were euthanized, and worms in the gastrointestinal tract were counted. No O. ostertagi were found in the abomasums, but low to high numbers (800-6200) of C. oncophora remained in the small intestines in both groups. Overall, these findings indicated that there was an interaction between the efficacy of IVM and DXM treatment. As significantly lower plasma levels of IVM were observed in the DXM group, we conclude that the impaired efficacy of ivermectin was most likely due to the altered pharmacokinetics.

The use of a simplified faecal egg count reduction test for assessing anthelmintic efficacy on Belgian and German cattle farms

Anthelmintic resistant cattle nematodes have been reported in different regions around the world. However, in Western Europe the assessment of the problem relies largely on case reports and no prevalence data based on wide-scale surveys are available. Therefore, we performed a survey to (1) screen for reduced anthelmintic efficacy in Belgian and German cattle farms; (2) evaluate the usefulness of a simplified faecal egg count reduction test (FECRT), where efficacies are based on the mean FECs of 10 at random collected faecal samples pre- and post-treatment per farm and (3) identify possible risk factors for reduced anthelmintic efficacy. Of 88 farms included in this study, 84 farms used macrocyclic lactones (MLs). A FECR <95% was observed on 39% of these 84 farms. However, using a Markov chain Monte Carlo simulation analysis, to correct for the used McMaster FEC technique with a detection limit of 50 epg, reduced efficacy could only be confirmed in 25% of the farms (21/84). Only Cooperia spp. were found in significant numbers in the coprocultures post-treatment. Reduced efficacy was significantly associated with farm type and with a lower efficacy in beef herds compared to dairy herds. Four farms were revisited and a standardized FECRT was performed to confirm anthelmintic resistance (AR). Surprisingly, macrocyclic lactone resistance against Cooperia oncophora was only confirmed in one of four farms. In conclusion, our results show that a reduced efficacy observed in a FECRT are not only caused by AR but that the detection limit of the FEC technique used and the (in)correct administration of the anthelmintic drugs are confounding factors of major importance.

Honest sexual signalling mediated by parasite and testosterone effects on oxidative balance

Extravagant ornaments evolved to advertise their bearers' quality, the honesty of the signal being ensured by the cost paid to produce or maintain it. The oxidation handicap hypothesis (OHH) proposes that a main cost of testosterone-dependent ornamentation is oxidative stress, a condition whereby the production of reactive oxygen and nitrogen species (ROS/RNS) overwhelms the capacity of antioxidant defences. ROS/RNS are unstable, very reactive by-products of normal metabolic processes that can cause extensive damage to key biomolecules (cellular proteins, lipids and DNA). Oxidative stress has been implicated in the aetiology of many diseases and could link ornamentation and genetic variation in fitness-related traits. We tested the OHH in a free-living bird, the red grouse. We show that elevated testosterone enhanced ornamentation and increased circulating antioxidant levels, but caused oxidative damage. Males with smaller ornaments suffered more oxidative damage than those with larger ornaments when forced to increase testosterone levels, consistent with a handicap mechanism. Parasites depleted antioxidant defences, caused oxidative damage and reduced ornament expression. Oxidative damage extent and the ability of males to increase antioxidant defences also explained the impacts of testosterone and parasites on ornamentation within treatment groups. Because oxidative stress is intimately linked to immune function, parasite resistance and fitness, it provides a reliable currency in the trade-off between individual health and ornamentation. The costs induced by oxidative stress can apply to a wide range of signals, which are testosterone-dependent or coloured by pigments with antioxidant properties.

Resistance of trichostrongyles to benzimidazoles in Italy: a first report in a goat farm with multiple and repeated introductions

Anthelmintic resistance is widely distributed in small ruminants throughout the world. The extension of resistance seems lower in southern European countries and has not been reported previously in Italy. In the present study, resistance to benzimidazoles, levamisole and ivermectin was evaluated in a multi-breed goat farm of southern Italy. The farm had a history of repeated goat introductions from other flocks and a moderate regimen of anthelmintic treatments using alternatively the three above-mentioned drugs. Resistance of gastrointestinal strongyles was studied on the basis of faecal egg counts, egg hatch assay and necropsies. Resistance to anthelmintics was evidenced for benzimidazoles only, and Trichostrongylus colubriformis was the only resistant strongyle species. Single drug and single species resistance suggest that resistance is on its beginning and that measures for reducing the spread of resistance are of interest and should be promoted.

Evaluation of broad-spectrum anthelmintic activity in a novel assay against Haemonchus contortus, Trichostrongylus colubriformis and T. sigmodontis in the gerbil Meriones unguiculatus

The gerbil Meriones unguiculatus, infected with three species of nematodes, each located in a separate part of the gastrointestinal tract, provided a reliable laboratory assay for the evaluation of broad-spectrum anthelmintic activity. Gerbils harbouring 6-day-old infections of Haemonchus contortus, Trichostrongylus colubriformis and T. sigmodontis were given selected broad-spectrum anthelmintics by gavage. Three benzimidazoles, thiabendazole, oxfendazole and albendazole, a tetrahydropyrimidine, morantel, an imidazothiazole, levamisole hydrochloride, a macrocyclic lactone, ivermectin and an experimental natural product, paraherquamide, were active against all three nematodes at various dosages. Trichostrongylus colubriformis was most sensitive to levamisole hydrochloride, morantel, thiabendazole and paraherquamide whereas ivermectin, oxfendazole and albendazole were more effective against H. contortus. All compounds were active against the caecal nematode T. sigmodontis although it was less sensitive than T. colubriformis. Haemonchus contortus was more sensitive than T. sigmodontis to all anthelmintics tested except thiabendazole.

Mathematical models to assess strategies for the control of gastrointestinal roundworms in cattle 2. Validation

A computer model of the population biology of Ostertagia ostertagi in young cattle and the effect of the parasite on animal growth has been constructed. It was validated against results from field trials where worm populations and rate of growth of cattle treated with anthelmintics were compared to similar groups of untreated animals. A close correlation between observed and predicted values for faecal egg and pasture larval counts was seen in the cattle which had been treated with an oxfendazole pulse release bolus at turnout. Timings and peak values were less accurately predicted in the untreated cattle. Although predictions for live weight gain during the grazing season indicated that the model may be overestimating the potential growth rate of cattle, it is considered that the model provides a suitable tool for comparing the effectiveness of different worming programmes under farm conditions. The computer simulation also allowed examination of the underlying influences and interactions between parasitological factors, such as numbers of immature and adult worms, and animal and pasture factors, such as sward height, grass consumption and feed conversion, in the parasite's influence on animal performance.

The effect of chicory ( Cichorium intybus ) and sulla ( Hedysarum coronarium ) on larval development and mucosal cell responses of growing lambs challenged with Teladorsagia circumcincta

The aim of this study was to investigate the effects of grazing different bioactive forages on acquired immunity against Teladorsagia circumcinta infection. The development of immunity was assessed by following the response of trickle-infected lambs grazing chicory (Cichorium intybus; IC), sulla (Hedysarum coronarium; IS) or grass/clover (Lolium perenne/Trifolium repens; IGC), to a single challenge infection. Parasite-naive lambs, grazing grass/clover, were also challenged with the single infection dose providing the uninfected control (UGC) group. Trickle infection significantly reduced worm establishment, inhibited larval development and increased mucosal mast cell (MMC) and globule leucocyte (GL) cells. Grazing treatment (chicory, sulla or grass/clover) significantly affected adult worm (P<0.05), late-L4 (P<0.01) and mid-L4 (P<0.01) larval-stage recoveries of the trickle-infected lambs, with IGC group always carrying higher worm burdens than either IC or IS lambs. MMC and GL cells of trickle-infected lambs were positively correlated with the proportion of early-L4 worms recovered and negatively correlated with both the proportion of adult worms recovered and the total worm establishment, suggesting that the observed effects were due to an enhanced immune response. The results suggest elevated immune responses against T. circumcincta infections in growing lambs grazing on either sulla or chicory compared to those grazing on grass/clover, probably due to differences in forage nutritional values.

In vivo anthelmintic activity of Butea monosperma against Trichostrongylid nematodes in sheep

Seeds of Butea monosperma administered as crude powder (CP) at doses of 1, 2 and 3 g/kg to sheep naturally infected with mixed species of gastrointestinal nematodes exhibited a dose and a time-dependent anthelmintic effect. The maximum reduction of 78.4% in eggs per gram of feces (EPG) was recorded on day 10 after treatment with 3 g/kg. Levamisole (7.5 mg/kg), a standard anthelmintic agent, exhibited 99.1% reduction in EPG.

Further characterisation of a triple resistant field isolate of Teladorsagia from a Scottish lowland sheep farm

Anthelmintic efficacies against juvenile developing populations of Teladorsagia species that were known to be resistant to anthelmintics from all three broad spectrum families were examined using a controlled efficacy test. Fenbendazole (FBZ), levamisole (LEV), ivermectin (IVM), combinations of these anthelmintics and moxidectin (MOX) were assessed in parasite naïve lambs artificially infected with 8,000 third stage larvae (Tci5) and treated orally 8-day post-infection with the compounds at the manufacturers recommended dose rates, FBZ, 5 mg/kg body weight (BW); LEV, 7.5 mg/kg BW; IVM, 0.2 mg/kg BW; MOX (0.2 mg/kg BW). The lambs were slaughtered 14-day post-treatment. The arithmetic mean worm burden reductions resulting from oral treatments with FBZ; IVM; LEV; FBZ+IVM; FBZ+LEV; FBZ, LEV+IVM or MOX were 36%, 82%, 38%, 86%, 60%, 88% and 97%, respectively. The results illustrate that combination treatments showed improved efficacies against the juvenile population compared to individually administered treatments but that these improvements were not wholly effective. Moxidectin was the only treatment that was over 95% effective, though caution should be noted when advising the use of MOX prophylactically since 3% of the infection still survived this treatment. Treatments directed at juvenile stages of Tci5 were less effective, with the exception of IVM, compared to a similar trial using Tci5 where the same treatments were directed against a predominantly adult population. No interaction was detected comparing the timings of treatments and its effectiveness with the exception of IVM (two-way ANOVA, p < 0.05). These findings suggest that, on the whole, the selection processes for anthelmintic resistance (AR) may occur at an early stage of development within the parasites, having severe implications for the early detection of AR.

Comparing different formulae to test for gastrointestinal nematode resistance to benzimidazoles in smallholder goat farms in Mexico

The objective was to examine the coincidence in the prevalence of benzimidazole (Bz) resistance in smallholder goat herds, as determined by three average-based and two individually-based faecal egg count reduction (FECR) tests. Nineteen smallholder goat herds with more than 30 animals were selected from 84 herds in Yucatan. Animals shedding 150 eggs/g of faeces (EPG) on day zero were randomly divided into two groups. The control group did not receive treatment and the treated group received fenbendazole (10mg/kg body weight per os). Feed was withdrawn for 16 h before treatment. Ten days after treatment, both groups were sampled to determine their FEC. Faecal cultures and identification of infective larvae were performed for estimating the proportions of genera of gastrointestinal nematodes (GIN) present. Presence of resistant GIN was determined with three different average-based FECR (FECR(1), FECR(2) and FECR(3)) and two individually-based FECR (iFECR(1) and iFECR(2)). The prevalence of herds with Bz resistant nematodes (and 95% confidence interval (95% CI)) was calculated using each formula. Coincidence among formulae was estimated with Kappa values. The prevalence (+/- 95% CI) of Bz resistance calculated with FECR(1) (57.89 +/- 22.20) had a high coincidence with iFECR(1) and iFECR(2) (Kappa values of 0.86 and 0.79, respectively). The prevalence with FECR(2) (31.58 +/- 20.90) and FECR(3) (21.05 +/- 18.33) had a low coincidence with FECR(1) (Kappa < 0.50). Trichostrongylids found on Bz resistant farms were mainly Haemonchus spp., however, some Trichostrongylus spp. and Oesophagostomum spp. were found too. The high coincidence between the standard average-based FECR(1) and the individually based formulae is encouraging and may suggest that either formula could be applied to smallholder farmers. Further laboratory studies are needed to confirm the resistance status in the herds.

Confirmation of resistance to ivermectin by Cooperia curticei in sheep

AIM

To investigate the occurrence of resistance to a full dose of oral ivermectin by Cooperia curticei in sheep.

METHODS

Twelve lambs on a sheep and cattle property in the North Island of New Zealand were randomly allocated to one of two equal-sized groups. One group was treated orally with a single dose of ivermectin at the manufacturer's recommended dose rate of 0.2 mg/kg, while the other remained as an untreated control. Worm counts were carried out post mortem on the abomasa and small intestines of all animals in both groups 7 days after treatment.

RESULTS

While treatment with ivermectin reduced the numbers of all other worm genera to almost zero, those of Ostertagia(= Telodorsagia) circumcincta and C. curticei were reduced by only 37% and 19%, respectively.

CONCLUSIONS

These results provide clear evidence of resistance to ivermectin by O. circumcincta and C. curticei. They also appear to represent the first record of macrocylic lactone (ML) resistance in C. curticei in sheep in New Zealand or elsewhere.

The effect of grazing forage containing condensed tannins on gastro-intestinal parasite infection and milk composition in Angora does

The objective of this study was to evaluate effects of the condensed tannin (CT)-containing forage sericea lespedeza (sericea lespedeza (SL); Lespedeza cuneata; 15.2% CT), on fecal egg count (FEC), larval development (larvae/10 g of feces), worm burden and immune response compared with a crabgrass (Digitaria ischaemum)/Kentucky 31 tall fescue (Festuca arundinacea; control forage (CTF)) forage low in CT (0.32% CT) in grazing Angora does and their kids. Fifty worm-free mixed-age does were randomly allocated to three treatments. One treatment (10 does; initial liveweight (LW) = 45+/-1.5 kg) entailed grazing of SL forage from April 25 to July 15, 2002 with a second treatment of CTF (20 does; initial LW = 43+/-1.4 kg) grazing during the same period. Does of the third treatment (20 does; initial LW = 44+/-1.4 kg) grazed a sward of SL for 2 weeks and then one of CTF for 2 weeks followed by alternating between the two pastures every 2-week rotational grazing (ROT). To gauge levels of infective larvae on pasture, three worm-free Angora kids (initial LW = 3.6+/-0.2 kg) were randomly selected as tracers. Tracers grazed for final 60 days and were euthanized for determination of worm burden. The immune response of does was measured by skin thickness reaction after the intradermal injection of 250 microg phytohemagglutinin (PHA). Mean FEC for SL and ROT were substantially lower (P < 0.01) than for CTF does (145, 329 and 894 eggs/g, respectively). The FEC for kids was lower (P < 0.05) for SL than for ROT and CTF (550, 2757 and 3600 eggs/g, respectively). Total fecal egg output (3.3, 6.0 and 26.9 x 10(5) eggs/day, respectively) and larval development (242, 263 and 792 larvae/10 g, respectively) were lower (P < 0.05) for SL and ROT than for CTF. Tracers grazing on SL had lower total worm burdens than ROT and CTF (P < 0.01). The immune response was higher (P < 0.01) for SL (4.9 mm) and ROT (6.0 mm) than for CTF (3.0 mm) at 12 h after injection of PHA. The packed cell volume (PCV) in does was higher (P < 0.01) for SL and ROT than for CTF (27, 26 and 23%, respectively). Does that grazed CT-containing forage had considerably lower milk somatic cell counts (SCC) than does grazing non-CT-containing forage. In summary, grazing CT forages reduced FEC, larval development and worm burden, and also appeared to enhance immune response. The CT-containing forage SL reduced gastro-intestinal parasite infections of Angora does and kids.

The larval feeding inhibition assay for the diagnosis of nematode anthelmintic resistance

A larval feeding assay for detection of nematode anthelmintic resistance to macrocyclic lactones and imidazothiazoles is described. The estimated concentration of anthelmintic required to inhibit larval feeding in 50% of L1's (IC50) that were resistant to either macrocyclic lactones or imidazothiazoles were significantly higher (P < or = 0.05) than those of susceptible isolates. Some variations in IC50 values were observed during the patent period of infection in all strains, although the pattern of the IC50 followed the same course. IC50 values varied in larvae developing from eggs shed throughout the patent period, with low values in the earliest larvae followed by higher values as the infection progressed, before decreasing at 70-90 days post-infection, although the low values of the first part of the patent period were not reached. However, the IC50 differences between all resistant and susceptible strains were significant throughout the whole patent period for ivermectin and levamisole. These results suggest that this technique may provide an alternative in vitro to detect anthelmintic resistance.

Real time PCR for the diagnosis of benzimidazole resistance in trichostrongylids of sheep

This report describes a new molecular method for the diagnosis of benzimidazole susceptibility or resistance in three main species of trichostrongylids of sheep (Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus vitrinus). This assay is based on the use of real time polymerase chain reaction (PCR) to detect mutations of residue 200 on isotype 1 of beta-tubulin. The technique allows calculation of the proportion of each allelic variant as it combines kinetic (real time quantitative) PCR with allele-specific amplification and requires no post-PCR processing. The level of resistance in the population is determined by the proportion of the beta-tubulin codon 200 TAC allele. Hence, it was observed that the proportion of the resistant allele in susceptible strains ranged between 24% and 32.3%; in resistant strains this value increased to between 71.3% and 86.3%. Furthermore, there was a good correlation between real time PCR, faecal egg count reduction test, egg hatch assay and conventional allele-specific PCR, in both resistant and susceptible strains. A sensitive, rapid, highly reproducible and inexpensive technique for detecting resistance to benzimidazoles in a worm population has been developed.

Failure of moxidectin to control benzimidazole–, levamisole– and ivermectinresistant Teladorsagia circumcincta in a sheep flock

Control of a benzimidazole-, levamisole- and ivermectin-resistant population of Teladorsagia circumcincta was attempted using moxidectin administered orally at the manufacturer's recommended dose rate of 200 microg/kg bodyweight. Ewes were dosed after lambing with the aim of controlling their periparturient rise in faecal egg output and lambs were dosed at six-week intervals throughout the summer. This regimen failed to suppress the establishment of significant numbers of infective helminth larvae on the pasture, resulting in unsatisfactory lamb production. Oral dosing with moxidectin was effective in removing adult female burdens of ivermectin-resistant T. circumcincta, but the effect of the drug did not persist against the resistant helminth population.

Duration of activity of topical eprinomectin against experimental infections with Teladorsagia circumcincta and Trichostrongylus colubriformis in goats

The immediate as well as the persistent anthelmintic efficacies of topically applied eprinomectin were evaluated in goats against induced infections with Teladorsagia circumcincta (2800 L3) and Trichostrongylus colubriformis (6000 L3). Twenty-three culled dairy goats were allocated to the following groups: control animals (group 1), animals treated 21 days prior to nematode infection (group 2), animals treated 7 days prior to nematode infection (group 3) and animals treated 21 days after nematode infection (group 4). Eprinomectin was applied at twice the cattle dose rate (1.0 mg/kg BW). According to the groups, necropsies were undertaken 28 days after nematode infection (groups 1-3) or 14 days after the anthelmintic treatment (group 4). Worm counts were determined for abomasum and small intestine. The curative anthelmintic efficacy of eprinomectin at 1.0 mg/kg BW on existing worm burdens was 100% against T. circumcincta and T. colubriformis. Quite similar worm burdens reductions were observed when eprinomectin was administered 7 days before infection whereas they were only 52.4 and 17.8% for T. circumcincta and T. colubriformis, respectively, for an administration of the drug 21 days prior to the nematode infection.

Characterisation of two triple resistant field isolates of Teladorsagia from Scottish lowland sheep farms

The anthelmintic resistance status of two field isolates derived from farms (farm A and B) located near Edinburgh were examined using both controlled efficacy tests (CET) and faecal egg count reduction tests (FECRT). Efficacies against fenbendazole (FBZ), levamisole (LEV) and ivermectin (IVM) and, for one isolate, against combinations of these anthelmintics and moxidectin were determined in naïve lambs, artificially infected with the isolates and treated with the compounds at the manufacturers recommended dose rates. (FBZ, 5mg/kg bodyweight (BW); LEV, 7.5mg/kg BW; IVM, 0.2mg/kg BW; Moxidectin (MOX) 0.2mg/kg BW). In both field isolates, the predominant species found pre-treatment and the only species found post-treatment was Teladorsagia circumcincta. Resistance to FBZ, LEV and IVM was confirmed in CET and FECRT on farm A and to the latter two compounds on farm B, which had a history of benzimidazole resistance and where TBZ resistance was also demonstrated using an egg hatch assay (EHA). For the farm A isolate CET efficacies against FBZ; IVM; LEV; FBZ + IVM; FBZ + LEV; FBZ, LEV + IVM and MOX were 59, 60, 88, 94,93, 92 and 98%, respectively. The CET efficacies for the farm B isolate were 51% and 72% for LEV and IVM, respectively.

Resistance to benzimidazole and macrocyclic lactone anthelmintics in cattle nematodes in Argentina

In April 2003, persistent scouring and ill-thrift that was reported in calves form an intensive beef rearing operation in central Argentina despite treatments with benzimidazole and ivermectin. In order to conduct a controlled faecal egg count reduction test on this herd, 40 calves 5-8-months-old were selected on the basis that they had a nematode eggs per gram (epg) of faeces count greater than 150. Animals were divided into four groups (1-4) of 10 calves. Calves of groups 1-3 were treated, respectively, with subcutaneous injection of ivermectin (200 mcg/kg), ricobendazole (4 mg/kg) and levamisole (7.5 mg/kg), while calves of group 4 remained as untreated controls. The egg count reductions carried out 10 days later were lower than 15% in calves treated with ivermectin and ricobendazole, but 100% in animals receiving levamisole. Pooled post-treatment faecal cultures showed larval percentages of 92 and 95 for Haemonchus and 8 and 5 for Cooperia in the faeces of calves treated with ivermectin and ricobendazole, respectively. This is the first reported case of Haemonchus parasiting cattle showing simultaneous resistance to avermectins and benzimidazole type anthelmintics.

Efficacy of ivermectin in calves against a resistant Cooperia oncophora field isolate

Since 1999, two Cooperia oncophora isolates, originally obtained from the United Kingdom, have been maintained by regular passage through calves at the Macdonald Campus, McGill University farm. One isolate, IVS, was originally susceptible to ivermectin, while the IVR isolate was originally resistant to ivermectin. These two isolates have been used to study the mechanisms of ivermectin resistance. To confirm the susceptible/resistant status after 4 years of passaging through calves, a controlled study was performed in which two worm-free calves were experimentally infected with IVS and another two worm-free calves with the IVR infective larvae. The calves were treated with ivermectin (0.2 mg/kg) subcutaneously (Ivomec Injection) 21 days after infection. Ivermectin at the recommended dose was 100% effective at eliminating the IVS isolate, since no eggs were found in feces, and no adult worms were found in the small intestine of the treated IVS-infected calf. In contrast, the IVR-infected calf continued to pass eggs in feces even after treatment with ivermectin, and adult worms (250) were found in the small intestine at necropsy. The untreated calves had 1,330 and 848 adult worms, respectively, for the IVS and IVR infected animals.

In vitroeffects of three woody plant and sainfoin extracts on 3rd-stage larvae and adult worms of three gastrointestinal nematodes

Most studies on the effects of tanniferous plants on nematodes have examined forages but have neglected the woody plants. Therefore,in vitroeffects of extracts from 3 woody plants (Rubus fructicosus,Quercus robur,Corylus avellana) have been tested on trichostrongyles and compared to sainfoin, a legume forage. Because somein vivoresults indicated that the effects of tannins differed depending on the parasitic species and/or stages, the effects were measured on 3rd-stage larvae (L3) and adult worms ofTeladorsagia circumcincta,Haemonchus contortusandTrichostrongylus colubriformis. The effects of plant extracts varied according to the plant sources, the parasite species and stages. For the woody plants, significant inhibitory effects were obtained on both stages of abomasal species. Results forT. colubriformiswere more variable. Effects of sainfoin extracts were significant onT. colubriformisandH. contortusL3, and on abomasal adult worms. In order to assess the implications of tannins, polyethylene glycol (PEG), an inhibitor of tannins, was added to hazel tree, oak and sainfoin extracts. Without PEG, significant inhibitory effects on L3 and adult worms were confirmed. After addition of PEG, the larval migration and motility of adult worms were restored in most cases. These results confirm variations in effects depending on factors related to plants or parasites and suggest that tannins are partly responsible for the effects.

Treatment of small lungworm infestation in sheep by using moxidectin

The use of moxidectin (MXD) in the treatment of small lungworm infestation (Cystocaulus ocreatus, Muellerius capillaris, Neostrongylus linearis and Protostronglylus rufescens) in sheep, was evaluated. Twenty-one sheep naturally infested with small lungworms, were divided into three groups (n = 7) and treated as follows: group A with moxidectin 1% injectable solution at a dose rate of 0.2mgkg(-1) bodyweight, group B with moxidectin 0.1% oral drench at a dose rate of 0.2 mgkg(-1) bodyweight and group C being controls. Before treatment, mean faecal larval counts were 30.7, 21.1 and 26.7 lpg in group A, B and C, respectively; 14 days after treatment respective counts were 0.4, 2.3 and 63.0 lpg, (percentage reduction after moxidectin administration >96.0%); 60 days after treatment respective counts were 0.0, 0.0 and 26.4 lpg, (percentage reduction after moxidectin administration 100%). It is concluded that treatment of small lungworm infestation of sheep can be effected by using moxidectin.

Benzimidazole-resistant beta-tubulin alleles in a population of parasitic nematodes (Cooperia oncophora) of cattle

Three anthelmintic classes with distinct mechanisms of action are commercially available. Selection of nematode populations resistant to all these drugs has occurred, particularly in trichostrongyloid parasites of sheep. Anthelmintic resistance in cattle parasites has only recently been recognized and appears to be less pronounced, even though very similar species infect both hosts. To understand the bases for differences in the rate of resistance development in sheep versus cattle parasites, it is important to first demonstrate that the same kinds of resistance alleles exist in both. The benzimidazoles (BZ), which have been used for more than 40 years, were chosen as an example. BZ-sensitive (BZ(S)) and BZ-resistant (BZ(R)) nematodes that parasitize sheep have been distinguished at the molecular level by a single nucleotide change in the codon for amino acid 200 of a beta-tubulin gene, a switch from TTC (phenylalanine) to TAC (tyrosine). PCR primers were designed to completely conserved regions of trichostrongyloid beta-tubulin genes and were used to amplify DNA fragments from Haemonchus contortus (cDNA from a BZ(S) and a BZ(R) library) as positive controls. The technique was then extended to the cattle parasites, Cooperia oncophora and Ostertagia ostertagi (from genomic DNA). Sequence analysis proved the presence of amplified BZ(S) alleles in all three species and BZ(R) alleles in the BZ(R) population of H. contortus. Based on these data, nested PCR primers using the diagnostic T or A as the most 3' nucleotide were designed for each species. Conditions for selective PCR were determined. To demonstrate feasibility, genomic DNA was recovered from individual H. contortus L(3) larvae from both BZ(S) and BZ(R) populations. Genomic DNA was also isolated from >70 individual adult male C. oncophora collected from a cattle farm in New Zealand with reported BZ resistance. Allele-specific PCR discriminated among heterozygotes and homozygotes in both species. This method could find utility in studying the molecular epidemiology of BZ resistance in cattle parasites and for defining the variables that limit the development and spread of anthelmintic resistance in this host.

Dose confirmation of moxidectin 0.5% pour-on against adults and fourth-stage larvae of various Cooperia spp. and Trichostrongylus colubriformis in Louisiana

Thirty-five castrate or intact Holstein male calves, ranging in weight from 122 to 210kg, were used in the study. On study Days -15 and -14, all study calves were treated with fenbendazole 10% paste at 10mg/kg to remove existing nematode infections. All cattle had zero egg counts on Day -1. Experimental infections of a mixed species inoculum were administered to all calves on Day 0. The inoculum consisted of Cooperia spp. (primarily C. punctata, but also C. pectinata, C. spatulata, C. oncophora and C. surnabada-total 40,961); Ostertagia ostertagi-1550; Trichostrongylus colubriformis-4996; and Oesophagostomum radiatum-38. Necropsy results from two of three monitor calves slaughtered on Day 6 after infection indicated that Day 6 was an appropriate time to evaluate efficacy of moxidectin against fourth larval stages (L(4)). The remaining 32 calves were randomly allotted to four groups of eight based on body weights. Eight calves (Group 1) were treated with moxidectin 0.5% pour-on at 0.5mg/kg on Day 6 to evaluate efficacy against nematode larval stages; eight control calves (Group 2) were matched with these principals. Eight calves (Group 3) were also treated with moxidectin pour-on at the same dosage on Day 23 in order to determine efficacy against adult nematodes; eight control calves (Group 4) were matched with these principals. In both cases, principals and controls of the Days 6 and 23 treatments were necropsied at 14-15 days after treatment. C. punctata was the only species found in a sufficient number of controls to evaluate moxidectin efficacy against the L(4). Moxidectin pour-on was not effective (P<0.05) against C. punctata L(4) by arithmetic means, but was highly effective based on geometric means. Regardless of whether arithmetic or geometric means were used for percent efficacy calculations, moxidectin pour-on was demonstrated to be highly effective (>99%) against the adult stages of C. oncophora, C. punctata, C. spatulata, C. surnabada, Cooperia spp. adult females and T. colubriformis adults.

The effect of treatment with a 1% injectable formulation of moxidectin during the rainy season and at the beginning of the dry season on gastrointestinal nematodes in cattle from communal areas in Zimbabwe

The effect of 1% moxidectin/cydectin at 0.2 mg/kg live weight on gastrointestinal nematodes and on the growth of calves, weaners and cows was investigated in five communal areas on the highveld of Zimbabwe. Three field experiments were carried out between March 1996 and June 1997. In experiment 1, treatment was administered in all five areas at the end of the rainy season in March 1996, followed by a further treatment at the beginning of the dry season in May/June 1996. In experiment 2, the treatment was administered in three areas at the end of the rainy season in March 1997. In experiment 3, treatment was administcred in one area at the beginning of the dry season in April 1997. Large numbers of eggs were present in the faeces of calves and weaners at the start of experiments 1 and 2. Epg values were lower in cows and in all age categories in experiment 3. There was a statistically significant reduction in epg values in calves, weaners and cows following treatment with a reduction of 90-99% in all cases except in cows in experiment 3, where no meaningful assessment was possible owing to the low egg counts in both the treated and control cows. The dominating larval types in faecal cultures were Cooperia and Haemonchus. Trichostrongylus, Oesophagostomum and Bunostomum were also found. Following treatment, Haemonchus was suppressed far more than Cooperia. This may be related to a longer residual effect against abomasal parasites like Haemonchus in comparison to small intestinal worms like Cooperia. Anthelmintic treatment conferred significant weight gain advantages (p < 0.05) on treated calves. weaners and cows. The weight gains are discussed in relation to disease and nutrition.

Are Teladorsagia circumcincta (Nematoda) morphs equally able to survive under anthelmintic treatment in sheep on pastures?

The nematode Teladorsagia circumcincta is a polymorphic species, morphs T. c. trifurcata and T. c. circumcincta being the most frequent. Among other traits, T. c. trifurcata has lower infectivity than T. c. circumcincta. Anthelmintic treatments that regularly remove populations should favour the more infective T. c. circumcincta. The present investigation was intended to test the respective capabilities of the two morphs to withstand anthelmintic pressure. The nematode strain was susceptible to imidazothiazole and partly resistant to benzimidazole at the onset of experiment. A 2-year grazing season study was undertaken under four levels of anthelmintic pressure [from untreated controls to frequently treated with a poorly efficient benzimidazole (fenbendazole) or an efficient imidazothiazole (levamisole)]. The strain previously maintained in laboratory conditions regained its natural, original proportion of T. c. trifurcata during the first grazing season. The two morphs were equally resistant to fenbendazole; and the proportion of resistant nematodes did not vary between the two morphs. During the second grazing season, the treatments indirectly depleted the proportions of T. c. trifurcata, probably due to the poor infectivity of this morph.

Effect of benzimidazole under-dosing on the resistant allele frequency in Teladorsagia circumcincta (Nematoda)

This experiment was designed to determine the effects of under-dosing on the frequency of benzimidazole resistant allele in the nematode Teladorsagia circumcincta. Fenbendazole (FBZ) was tested at 1/32, 1/16, 1/8 and 1/4 of the recommended dose for sheep (5 mg/kg body weight). The fraction of the susceptible homozygote (SS), susceptible heterozygote (RS) and resistant homozygote (RR) genotypes were compared among FBZ dose groups to evaluate differences between SS and RS genotype selective advantage. Almost all SS genotype worms were eliminated by 1/4 of the FBZ recommended dose, whereas a significant fraction of the RS genotype worms survived treatment. The selective advantage was 4.5 times higher for the RS genotype. This selective advantage was determined at 1/4 of the manufacturer's recommended dose of FBZ. This value should be taken as an indictor of the selective advantage of RS over the SS genotype when lambs are under-dosed. A computer simulation was used to study the putative spread of anthelmintic resistance over a range of RS selective advantages (2, 4.5 and 10-fold), with two average sizes of individual host worm population (20 or 2000 worms/host) and two initial R allele frequencies (0.1%, or 1%). In all situations, the lowest selective advantage of the RS genotype over the SS genotype was sufficient to promote the spread of resistance in susceptible populations. When the RS genotype had no selective advantage over the SS genotype, genetic drift almost always led to the loss of the R allele, except in the largest populations (average size = 2000 worms).

Drought and flock isolation may enhance the development of anthelmintic resistance in nematodes

A survey of anthelmintic resistant nematodes was conducted in sheep and goat flocks in Greece using in vivo and in vitro tests. Faecal egg count reduction tests in Macedonia were all greater than 99% indicating very high sensitivity of the nematodes to anthelmintics. In vitro tests showed benzimidazole resistant Teladorsagia circumcincta in 17 out of 106 flocks on small islands. On the mainland there were only three cases of benzimidazole resistance out of 310 flocks and animals had recently been introduced to the flocks. Flocks on the islands are isolated and there are higher temperatures than on the more mountainous mainland, where flocks tend to intermingle. It is concluded that drought and isolation are likely to be the major factors accounting for the development of anthelmintic resistance in nematodes in the island flocks.

Control of anthelmintic resistant endoparasites in a commercial sheep flock through parasite community replacement

An effort was undertaken to replace a community of sheep endoparasites that had been classified as resistant to levamisole and albendazole with a community of more susceptible parasites using a dilution approach that could be integrated into the management of a commercial flock. For this study, pastures on this sheep farm were divided into two areas: north and south. Strategically timed anthelmintic treatments combined with pasture management reduced to nondetectable levels the endemic community of anthelmintic resistant parasites in this flock and on these pastures by early summer. A group of 102 ewes, lambs, and rams were experimentally infected with third stage larvae from the more susceptible community of parasites. These sheep then seeded the south pastures with the new parasite community, while sheep on the north pastures maintained the endemic resistant community. Despite its insensitivity as a technique for detecting anthelmintic resistance, fecal egg count reduction tests at the end of the grazing season indicated that the more susceptible parasites were present on the south pastures while resistant parasites were present on the north. The following grazing season, similar protocols were used to introduce the more susceptible parasites onto the north pastures. At the end of the grazing season, fecal egg count reduction tests indicated that the new community of parasites had become established on both groups of pastures of the farm.

Resistance of Cooperia to ivermectin treatments in grazing cattle of the Humid Pampa, Argentina

A field experiment and a controlled efficacy trial were conducted to demonstrate resistance of cattle trichostrongylid nematodes to endectocidal compounds in grazing cattle on the Humid Pampa, Argentina. Male Polled Hereford calves, aged 9-11 months old, with a history of frequent treatments with compounds of the avermectin/milbemycin class, were used for the trial. The field experiment involved six groups of 10 animals each, which were subcutaneously treated with either ivermectin (IVM), long-acting ivermectin (LA-IVM), moxidectin (MXD) or doramectin (DRM) at a dosage of 0.2mg/kg BW. A fifth group was treated orally with fenbendazole at a dosage of 5mg/kg BW; the sixth group of calves served as non-treated control. Faecal samples were collected from each animal on the day of treatment and at 14 days after treatment. Nematode egg counts were performed to estimate the faecal egg count reduction test (FECRT). The FECRT showed that reductions were 65% (IVM), -20% (LA-IVM) and 85% (DRM). Egg counts from calves treated with MXD or fenbendazole were reduced by 95 and 100%, respectively. For the controlled efficacy trial, six animals with the highest egg counts from the IVM, LA-IVM and non-treated control groups, were necropsied on day 18 after treatment to determine numbers and nematode species in the abomasum and small intestine. The results indicated that efficacy against Cooperia oncophora was 62.7 and 48% for IVM and LA-IVM, respectively. Neither of the IVM formulations showed efficacy against Trichostrongylus spp. in the small intestine. However, the absence of Trichostrongylus spp. in some animals of both treated and control groups precluded a valid assessment of efficacy or resistance. It was concluded that resistance of trichostrongylid nematodes to the avermectin/milbemycin class of compounds in grazing cattle of the Humid Pampa, Argentina, may be strongly established on farms where cattle are treated at frequent intervals throughout the year.

Multispecific resistance of trichostrongyles to benzimidazoles in a goat herd in Germany

Anthelmintic resistance of nematodes to benzimidazoles and ivermectin was suspected from field observations in an Angora goat herd in northern Hesse. Larvae of the caprine trichostrongyle isolate were used for artificial infections of helminth-naive sheep. Groups of the lambs were treated with albendazole (3.8 mg/kg p.o.) or ivermectin (0.2 mg/kg s.c.) four weeks after infection or remained untreated as controls and were necropsied one week later. Albendazole reduced the mean faecal egg counts by 82% and the mean worm counts of Haemonchus contortus by 22%, Ostertagia circumcincta by 10% and Trichostrongylus colubriformis by 41% as compared with the controls. Ivermectin was 100% effective. This is the first report from Germany of multispecific resistance to benzimidazoles in a trichostrongyle population of goats.

Gastric Ollulanus tricuspis infection identified in captive cheetahs (Acinonyx jubatus) with chronic vomiting

Gastritis, vomition and weight loss are common in captive cheetahs (Acinonyx jubatus). Gastric spiral bacteria (Helicobacter spp.) and the very small, viviparous nematode Ollulanus tricuspis, a stomach worm of cats, are believed to be important causes. Three sibling cheetahs at Wellington Zoo, New Zealand, developed chronic vomiting, diarrhoea and debility. Their parents were both South African-born. Response to antibacterial treatment was poor. Endoscopic examinations revealed chronic lymphoplasmacytic gastritis and Ollulanus infection. Treatment with oxfendazole and pyrantel embonate resulted in clinical improvement; however, 1 cheetah, which died 7 months later as a result of a ruptured liver due to hepatic amyloidosis, still had Ollulanus worms present in her stomach. Ollulanus tricuspis is a significant cause of gastritis and vomiting in captive cheetahs, lions and tigers, as well as wild cougars and tigers. The parasite has not yet been found in sub-Saharan Africa. Because of the unusual characteristics of this parasite, the literature on its life history and techniques for diagnosis is reviewed.

Localization and migration of benzimidazole resistant and susceptible adult Cooperia curticei in the small intestine of sheep after treatment with thiabendazole

Four groups of three lambs per group were experimentally infected with Cooperia curticei susceptible (two groups) or resistant (two groups) to benzimidazoles, and distributions of adult worms in the small intestine were studied. For each Cooperia isolate, one group was treated with thiabendazole (TBZ) (5 or 50mg/kg bodyweight) 28 days after infection. In the two untreated groups, the population of C. curticei were present from the second to the tenth meter of intestine from the pylorus with a maximum in the sixth meter for both isolates. After treatment with TBZ, the size of the resistant worm population did not significantly decrease but a large number of worms were found towards the proximal sections of the intestine. In contrast, the susceptible population was reduced by about 40% but the surviving worms remained at this same site of predilection after treatment. Measurements of the concentration of TBZ and 5OH-thiabendazole (5OH-TBZ) in the intestinal segments do not indicate a clear relationship between the localization of worms and TBZ or 5OH-TBZ concentrations at least 12h after the anthelmintic treatment. The hypothesis of an enhanced expression of the mechanisms of resistance in the first few meters of small intestine is suggested.

Efficacy of copper oxide needles for the control of nematode parasites in dairy goats

The spread of benzimidazole-resistant nematodes in dairy goat farms is of a great concern as probably more than 70% of the flocks are involved. As there are very few other anthelmintic options during the lactating period, we have evaluated the efficacy of copper oxide needles (CON, Copinox, Bayer, UK) in both experimental and natural infections in goats. The curative effect of CON (2-4 g) on existing worm burdens was assessed in goats experimentally infected with Teladorsagia circumcincta, Haemonchus contortus and Trichostrongyus colubriformis, compared to controls. The preventive effect of CON (4 g) on worm establishment was monitored for 2 months in animals experimentally infected with H. contortus and for 3 months in naturally infected animals on a farm exhibiting predominant infections with T. circumcincta and Oesophagostomum venulosum. In both experimental and natural conditions, the efficacy of CON was nil against Teladorsagia, Trichostrongylus and Oesophagostomum infections. In contrast, the efficacy of CON against Haemonchus was clearly established in reducing the worm burden (75%) as well as in lowering the egg output (37-95%) in relation to the establishment of new infections over several weeks. Copper oxide needles may represent an alternative to conventional anthelmintics in the control of Haemonchus infection in some goat farms.

Evaluation of the persistent efficacy of doramectin and ivermectin injectable against Ostertagia ostertagi and Cooperia oncophora in cattle

The persistent efficacy of doramectin and ivermectin injectable against moderate and high infection levels of Ostertagia ostertagi and Cooperia oncophora were evaluated in cattle. Calves were allocated to six groups of six animals. On Day 0 animals of Groups I1/I2 and D1/D2 were treated with 0.2mg/kg ivermectin and doramectin injectable, respectively. Animals of the C1, I1 and D1 groups received a daily (moderate) infection of 1000 L3 of O. ostertagi and 1000 L3 of C. oncophora, and animals of the C2, I2 and D2 groups received a daily (high) infection of 10,000 L3 of each species. The animals were infected for 21 days with both species, the infections with C. oncophora and O. ostertagi started from Days 8 and 15 post treatment, respectively. Animals were necropsied on Day 40. The calculation of the persistent activity of ivermectin and doramectin was based on the efficacy against the different developmental and adult stages of both parasites. The present study confirmed that infection levels may influence the duration of persistent efficacy of an anthelmintic. Doramectin had at the moderate infection level a persistent efficacy of at least 35 days against O. ostertagi and at least 28 days against C. oncophora; at the high infection dose persistent efficacy was somewhat shorter i.e. up to 33 days and approximately 28 days, respectively. The duration of persistent efficacy of ivermectin against O. ostertagi at the moderate infection level was between 14 and 25 days, at the high dose level up to 25 days. Persistent efficacy of ivermectin against C. oncophora could, at both infection doses, not be measured, with the present experimental design.

Comparative cost-effectiveness of ivermectin versus topical organophosphate in feedlot yearlings

A replicated-pen field trial was performed under commercial feedlot conditions in western Canada to determine the cost-effectiveness of administering ivermectin to yearling beef cattle upon entry to the feedlot after the grazing season, and to establish the level of trichostrongylid gastrointestinal parasite infection in this population, as estimated by fecal egg counts. Six thousand eight hundred and eighty-three, mixed breed, yearling steers were randomly allocated upon arrival at the feedlot to one of 2 experimental groups as follows: Ivermectin, which received topical ivermectin (0.5%) at the rate of 1.0 mL/10 kg body weight; or Fenthion, which received topical fenthion (20%) at the rate of 12 mL/295 kg body weight. There were 15 pens in each experimental group. Final weight, weight gain, average daily gain, and dry matter intake to gain ratio were significantly (P < 0.05) improved in the Ivermectin group as compared with the Fenthion group. There were no significant (P > or = 0.05) differences in initial weight, days on feed, or daily dry matter intake between the experimental groups. The geometric mean fecal egg counts at the time of allocation were 14.7 eggs/5 g and 16.6 eggs/5 g for the Ivermectin and Fenthion groups, respectively (P > or = 0.05). There were no significant (P > or = 0.05) differences in morbidity or mortality between the experimental groups. In the economic analysis, the significant improvements in feedlot performance in the Ivermectin group resulted in a net economic advantage of $4.20 CDN per animal.

Evaluation of the effect of simulated rainfall on the efficacy of Ivomec Pour-On against Cooperia spp. infection in cattle

Four controlled studies, one each in Australia, Germany, the United Kingdom, and the United States, involving 30 calves each were conducted to evaluate the effect of simulated rainfall on the efficacy of Ivomec Pour-On against infections of Cooperia spp. At 3 weeks before treatment the calves were infected orally with third-stage larvae of Cooperia spp. In each study a recent, locally derived field isolate was used. The calves were allocated by restricted randomization based on body weight within sex to one of the following treatments: unmedicated control with no rain, Ivomec Pour-On with no rain, Ivomec Pour-On with rain starting at 40 min before treatment, Ivomec Pour-On with rain starting at 10 min after treatment, and Ivomec Pour-On with rain starting at 60 min after treatment. Ivomec Pour-On was applied topically at a dose rate of 1 ml/10 kg body weight (500 microg ivermectin/kg body weight). The simulated rainfall was equivalent to a heavy shower of approximately 12.5 mm of water during a 30-min period. The calves were necropsied for worm counting at 14 or 15 days after treatment. An evaluation of the pooled data showed that as compared with the untreated controls, the Ivomec Pour-On-treated calves with no rain had significantly (P < 0.01) fewer C. oncophora (> 99%), C. punctata (> 99%), C. surnabada (> 98%), and combined Cooperia spp. (> 99%). The reduction in Cooperia numbers noted for calves exposed to simulated rainfall was > 96% for all Cooperia species, regardless of when the rainfall started relative to the application of Ivomec Pour-On. There was no significant (P > 0.1) difference between the Ivomec Pour-On-treated calves with no rain and the pooled groups with simulated rainfall or between the group with rain before treatment and the pooled groups with rain after treatment. Ivomec Pour-On was highly effective against established infections of Cooperia spp. when applied to wet animals or to animals becoming wet shortly after treatment.

Persistent efficacy of doramectin injectable against artificially induced infections with Cooperia punctata and Dictyocaulus viviparus in cattle

Three studies were conducted to evaluate the persistent efficacy of doramectin injectable solution against experimental challenges with infective larvae of Cooperia punctata and Dictyocaulus viviparus. In each study, four groups of ten randomly-assigned calves, negative for trichostrongyle-type eggs on fecal examination, were treated subcutaneously in the midline of the neck with saline (1 ml/50 kg) on Day 0 or doramectin (200 microg/kg = 1 ml/50 kg) on Day 0, 7, or 14. Two additional calves from the same pool of animals were randomly assigned as larval-viability monitors and received no treatment. On Days 14-28, approximately 1000 and 50 infective larvae of Cooperia spp. and D. viviparus, respectively, were administered daily by gavage to each animal in Groups T1-T4. On Day 28, the two larval-viability monitor calves were inoculated in a similar manner with a single dose of approximately 30000 and 2000 larvae of Cooperia spp. and D. viviparus, respectively. Equal numbers of calves from each treatment group were killed on Days 42-45, as well as the two viability monitor animals to enumerate worm numbers. A 2% or 5% aliquot of small intestinal contents and washings were examined for worm quantification and identification, while 100% of the lung recoveries were quantified and identified. For each study and across the three studies, geometric mean worm recoveries for each treatment group were calculated from the natural log transformed data (worm count + 1) and were used to estimate percentage reduction. In the three studies, doramectin injectable solution was 97.5% efficacious against lungworms for up to 28 days and was 99.8% efficacious in reducing infection resulting from challenge with infective larvae of C. punctata for at least 28 days post-treatment.

Pharmacology of anthelmintic resistance

Anthelmintic resistance has grown from a curiosity to an important economic problem in several animal industries and is now set to threaten the control of human parasites. The pharmacology of anthelmintics and anthelmintic resistance has been studied most extensively in the nematode parasites of sheep. Here, Nick Sangster and Jenny Gill discuss this veterinary experience, summarizing the progress made in understanding anthelmintic resistance and highlighting the tools available for research.

Comparison of arithmetic and geometric means as measures of a central tendency in cattle nematode populations

Efficacy calculations in anthelmintic studies require estimates of the central tendency for the nematode populations. Confusion exists among practitioners regarding which measures of central tendency are most appropriate; although the arithmetic mean is frequently used, there are theoretical reasons for preferring the geometric mean. To investigate this controversy, arithmetic and geometric means were compared for their suitability for use in measuring efficacy. Arithmetic and geometric means were compared as measures of central tendency for skewed distributions. The following criteria were developed to facilitate the comparison: (1) probability around the parameter, (2) influence of extreme values, and (3) proximity to the median. Under log-normality, theoretical results demonstrated the superiority of the geometric mean. Modified-bootstrap simulations using empirical data from cattle were used to confirm theoretical expectations. Simulations on log-normal data supported the geometric mean as the better indicator of the central tendency. Additionally, for data not confirmed as log-normal, the superiority of geometric means was demonstrated. In a comparison of precision, it was shown that mean squared error was always smaller for sample geometric means than for arithmetic means when n> or =2. Simulation results added support to that conclusion.

Influence of ruminal bypass on the pharmacokinetics and efficacy of benzimidazole anthelmintics in sheep

Oxfendazole, fenbendazole and albendazole were each administered at 5mgkg(-1) to sheep fitted with abomasal cannulae as a single bolus intra-ruminally or infused intra-abomasally at a declining exponential rate, with half-life equivalent to the rate of rumen fluid outflow. The pharmacokinetic disposition of parent compound and metabolites in plasma and abomasal fluid was determined by high performance liquid chromatography. Compared with intra-ruminal administration, intra-abomasal infusion of fenbendazole lowered the area under the concentration-time curve of drug in both plasma and abomasal fluid; intra-abomasal infusion of albendazole substantially increased maximum drug concentration and the concentration-time curve in abomasal fluid and lowered the plasma concentration time curve of the sulphoxide metabolite; intra-abomasal infusion of oxfendazole increased maximum concentration and the concentration-time curve of drug in plasma and abomasal fluid. The greater availability in abomasal fluid of oxfendazole and albendazole when given at commercial dose rates of 5 mg kg(-1) and 3.9 mg kg(-1), respectively, by intra-abomasal infusion correlated with increased efficacy of both drugs against benzimidazole-resistant Trichostrongylus colubriformis and of albendazole against benzimidazole-resistant Haemonchus contortus over that achieved by intra-ruminal administration as a single bolus.

Anthelmintic resistance revisited: under-dosing, chemoprophylactic strategies, and mating probabilities

Deterministic and stochastic models are used to examine the evolution of anthelmintic resistance among trichostrongylid parasites of domestic ruminants. We find that the relative selection pressures exerted by chemoprophylactic (preventive) control strategies, chemotherapeutic (salvage) control strategies, and regimens involving "under-dosing" are critically dependent on a variety of host and parasite parameters (particularly host immunity and grazing behaviour, parasite fecundity, and the survival of the free-living stages on the pasture). Chemoprophylactic strategies are not necessarily more likely to exert a stronger selection pressure than chemotherapeutic strategies. Similarly, as one reduces dosage levels, there is a range of dose levels where under-dosing promotes resistance and a range of dose levels where under-dosing impedes resistance. The most dangerous dose is either that necessary to kill all the susceptible homozygotes, or that necessary to kill all the susceptible homozygotes and all the heterozygotes. Which one prevails depends upon model parameters. The stochastic formulation indicates that spatial heterogeneity in transmission may be a significant force in promoting the spread of resistant genotypes--at least when infection is at low levels.

Persistence of the efficacy of doramectin against Ostertagia ostertagi and Cooperia oncophora in cattle

The persistence of the efficacy of doramectin injectable against Ostertagia ostertagi and Cooperia oncophora was evaluated in two studies in calves. In both, the calves were allocated to six groups of six. Calves in the first control group (C1) and first treated group (T1) received a daily infection of 200 L3 of O ostertagi and 200 L3 of C oncophora; the calves in groups C2 and T2 received a daily infection of 1000 L3 of each species, and groups C3 and T3 received 10,000 L3 of each species per day. The calves in the three treated groups each received 0.2 mg/kg doramectin injectable on day 0. In the first study, the calves were infected for 21 days with Cooperia and for 28 days with Ostertagia, and they were slaughtered on day 33. In the second study, the calves were infected for 21 days with both species, the infections with Cooperia and Ostertagia starting from eight and 15 days, respectively, after the treatment, and the animals were slaughtered on day 40. The calculation of the persistence of the activity of doramectin was based on its efficacy against the different developmental and adult stages of the two parasites. The data from both studies indicated that the efficacy of doramectin against Ostertagia persisted for at least five weeks, but no conclusions could be drawn about the effect of the size of the infective doses on the persistence of the activity. In contrast, the Cooperia worm counts from the second study suggested that the efficacy of doramectin against Cooperia persisted for at least four weeks when the calves were exposed to a low or moderate infection level, whereas at the highest infection level it persisted for between three and four weeks.

Avermectin/milbemycin resistance in trichostrongyloid nematodes

Resistance to levamisole and the benzimidazoles appears to be achieved by one or, at most, two mechanisms in the common trichostrongyloid parasites of sheep. For the avermectin/milbemycin anthelmintic class the picture is more complex. In-vitro assays employing the free-living stages of trichostrongyloid nematodes were used to investigate structure-activity relationships for the avermectins/milbemycins. While avermectin/milbemycin-susceptible isolates of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta were found to differ in their intrinsic sensitivities to avermectin/milbemycin inhibition of larval development and L3 motility, structure-activity profiles against all three species were similar. In-vivo avermectin/milbemycin resistance was associated with a reduced sensitivity to avermectin/milbemycin inhibition of larval motility and/or development in some, but not all, isolates. Where a reduced sensitivity to avermectin/milbemycin inhibition of larval development was observed, different groups of resistant isolates displayed different structure-activity profiles. Many avermectin/milbemycin-resistant isolates showed an increased sensitivity to paraherquamide. These in-vitro data have allowed the classification of avermectin/milbemycin-resistant isolates into a number of distinct types. Study of the inheritance of avermectin/milbemycin resistance in two resistance types suggests that the in-vitro differences between resistant isolates reflect important differences in the mechanism of resistance present. The kinetics of expulsion of H. contortus, T. colubriformis and O. circumcincta from sheep following treatment with ivermectin indicate that, in vivo, the critical action of avermectins/milbemycin against O. circumcincta may be different to that which results in H. contortus and T. colubriformis elimination. This observation provides some explanation for the differences between resistant isolates. If, for different species, the critical event(s) leading to expulsion are different, then it follows that the mechanisms of resistance observed may also differ.