Physiological and parasitological responses to nematode infections of fattening cattle in the Western Pampas of Argentina

The epidemiology of nematode infection was studied in fattening grazing cattle from weaning (April 1994) to market at the end of their second autumn (July 1995). Sixty Aberdeen Angus calves of seven months of age were randomly allocated by weight to two groups: GT, treated every three weeks with doramectin (200 mcg/kg); and GI, an infected group, only treated with fenbendazole (7.5 mg/kg) at weaning and on the 1st of October. The two groups were grazed together on contaminated lucerne pastures until July, on 'clean' oat pastures until October and again on contaminated lucerne until the end of the trial. Fecal egg counts (epg), herbage larvae (L3), serum pepsinogen (Pep) and blood eosinophils (Eo) were evaluated monthly. Eight steers were slaughtered for worm recovery, three in July 1994, three in December 1994 and two in July 1995. Grazing feed intake was estimated by fecal output (chromic oxide method)/l-diet digestibility and to measure non-specific response, Brucella antibodies were detected at 11 and 40 days post-vaccination in early winter. Fecal egg counts, Pep and Eo of GI increased (P<0.01) from April to July when there was a moderate-to-high level of infection. Ostertagia, Trichostrongylus and Cooperia were the predominant genera. By late winter, all parameters decreased on oat 'clean' pastures and increased again when cattle returned to moderately infected lucerne. During summer, the parameters measured reflected the negligible numbers of L3 on pastures until early autumn. At this time, increased numbers of L3 were followed by a moderate rise (P<0.01) of epg, Pep and Eo values. During winter, GT showed higher (P<0.04) Brucella antibody IgG titers while feed intake of GI was 24.9% depressed (P<0.02). There were total cumulative weight-gain (WG) differences (P<0.001) between groups (GT=263.1 kg; GI=214.3 kg). During the second autumn, the mean WG of GT steers was 16.6 kg greater (P<0.04) than that of GI. Vaccination titres against Brucella suggested non-specific depression of immunity, while higher Pep and Eo levels in second-year steers may have reflected hypersensitivity reactions.

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.

Fitness of benzimidazole-resistant and -susceptible worms of Teladorsagia circumcincta, a nematode parasite of small ruminants

We compared, some fitness-related traits of benzimidazole resistant (rr) and susceptible (rS, SS) worms of Teladorsagia circumcincta, a gastrointestinal parasite of the small ruminants, under laboratory conditions. PCR was used to determine the genotypes (rr, SS, rS) and the fitness of each was compared within the same strain. There was no significant difference in egg production, development rate from egg to infective larvae stage, establishment of these larvae in the host or the survival of adult worms and infective larvae for the 3 genotypes. The same results were obtained for the establishment rate of larvae in the host and the production of infective larvae under conditions of strong competition between resistant and susceptible worms. The fact that there were no differences in fitness suggests that the installation of benzimidazole resistance in a worm population is irreversible. This agrees with field observations.

Field efficacy of doramectin pour-on against naturally-acquired, gastrointestinal nematodes of cattle in North America

Seven studies were conducted under field conditions in North America to evaluate the therapeutic efficacy of doramectin in a pour-on formulation at a dosage of 500 microg/kg (1 ml/10 kg) for cattle harboring naturally-acquired infections of gastrointestinal nematodes, including species of Haemonchus, Ostertagia, Trichostrongylus, Bunostomum, Cooperia, and Oesophagostomum. In each study, 40 to 100 cattle were randomly allocated to a saline- or doramectin-treated group in a tiered manner based on Day -7 bodyweight. On Day 0, the cattle received either saline or doramectin topically, according to their treatment group. Weather and safety observations were made following treatment. No adverse reaction to treatment was observed at any time during these studies. Fecal egg count (FEC) determinations were carried out on each animal on Days -7, 0, 7, 14, and 21. Reductions in FEC for the doramectin-treated animals compared to saline-treated cattle were > or = 96.0% by Day -7 and > or = 99.0% on Days 14 and 21 for each study. Across all studies regardless of weather conditions, the reduction by Day 21 for the doramectin-treated animals compared to saline controls was 99.7% (p < or = 0.0001) and compared to pretreatment levels in doramectin-treated cattle was 99.9% (p < or = 0.0001). Doramectin pour-on should provide a useful new treatment for controlling nematode parasites of cattle.

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.

The efficacy of Michel's dose and move system on gastrointestinal nematode infections in diary calves

The efficacy of a move to aftermath in July combined with moxidectin or fenbendazole treatment for the control of parasitic gastroenteritis (PGE) in calves was evaluated in three field experiments in the Netherlands. In all five treated groups high gastrointestinal nematode infections and PGE were prevented by a dose and move in July. Cooperia infections increased to moderate levels in two groups treated with moxidectin and one group treated with fenbendazole. In both other groups and also for Ostertagia in these three groups, low to extremely low infections were acquired. In the first experiment high primary infections, resulting in high faecal egg counts and a moderate increase of blood pepsinogen values occurred before the dose and move. Nevertheless, these primary infections were not high enough to result in PGE. In both other experiments primary infection levels were low and faecal egg counts increased to 100-650 eggs/g faeces at the end of the grazing season. The blood pepsinogen values of non-treated control groups demonstrated that it took more than a month after their move to aftermath before substantial reinfection occurred on the new pasture. In the first and the last experiment only, high Ostertagia and Cooperia infections developed in the control group at the end of the grazing season, though it did not result in clinical PGE. The experiments demonstrate all theoretical risks of the dose and move system: (1) PGE early in the grazing season as a result of high overwintered pasture infectivity. (2) PGE just before the move as a result of an early midsummer increase in pasture infectivity. (3) PGE around housing as a result of insufficient suppression of pasture infectivity late in the grazing season. (4) Underexposure to nematode infections due to a high suppression of nematode infections. Nevertheless, it can be concluded that under normal conditions the dose and move system remains to be a valuable and easily applicable system for the control of PGE.

The effect of different infection levels on acquired resistance to gastrointestinal nematodes in artificially infected cattle

The objective of this study was to evaluate the effect of different infection levels of Ostertagia ostertagi and Cooperia oncophora in a simulated 'first grazing season' on the resistance of calves to an artificial challenge infection. The infection levels were determined by the infection schedules and the chemoprophylaxis used. Thirty six 7-11-month old Holstein-Friesian bull calves were randomly divided into four groups. The animals of group B received an ivermectin sustained release bolus (ISRB) on day 0. The calves of group D were treated on days 0 and 56 with a subcutaneous injection of doramectin (0.2 mg kg(-1) BW). Group C was the untreated control group. The calves of group N were used as helminth-naive controls, while the animals of groups B, C and D were trickle infected for 24 weeks. The infection schedules were designed to simulate the expected infection pattern for each treatment group under set-stocked conditions in temperate climate areas. After the last infection, all animals were treated with oxfendazole. One week later, all animals received a challenge infection of 50,000 O. ostertagi L3 and 100,000 C. oncophora L3, spread over 10 consecutive days. During the primary infection period the faecal egg output and the serum pepsinogen and antibody levels reflected the different levels of host-parasite contact between the groups (group C > group D > group B > group N). After the challenge infection, faecal egg counts, total Ostertagia burden, size of the adult worms and abomasal globule leucocyte counts all indicated a positive relationship between the level of Ostertagia infection during the primary infection period and the level of acquired resistance. A reduction of host-parasite contact during the primary infection period, as a consequence of the infection schedule and the chemoprophylaxis used, resulted in a diminished level of resistance to the artificial challenge infection with O. ostertagi. Faecal cultures and small intestine worm counts indicated that all previously infected groups had acquired a high degree of resistance to the Cooperia challenge infection.

Multiple anthelmintic resistance on a goat farm in Kenya

The anthelmintic efficacy of benzimidazoles, levamisole, rafoxanide and ivermectin was evaluated on an institutional farm in Kenya using faecal egg count reduction test, larval cultures and a controlled slaughter trial. The results of this study indicated simultaneous resistance of Haemonchus contortus against benzimidazoles, levamisole and rafoxanide and Trichostrongylus columbriformis and Oesophagostomum sp. against levamisole on the same farm. Injectable and orally administered ivermectin was effective against the benzimidazole and levamisole resistant H. contortus.

Moxidectin as an endectocide in reindeer

During the winter 1991-92, 42 reindeer hinds of the Kaamanen Experimental Reindeer Herd in Finnish Lapland, naturally infected with various parasites, were allocated to 3 groups. One group was an untreated control group and the other 2 groups received either moxidectin or ivermectin at a dose of 200 micrograms kg-1 subcutaneously. The efficacy of treatment was followed with monthly faecal examinations for nematode eggs and counting of warbles, Hypoderma tarandi larvae, and throat bots, Cephenemyia trompe larvae, from live animals in spring. The efficacy of moxidectin against warbles (92.8%) and throat bots (70.8%) did not match that of ivermectin, which was 100% against both species. Both moxidectin and ivermectin were effective against gastrointestinal trichostrongylid egg production over the December to May trial period indicating good efficacy against adult and inhibited trichostrongylids. Only non-significant differences were seen in weight development and calf birth weights between the groups. Because of its only moderate insecticidal efficacy, moxidectin cannot be recommended as an endectocide in reindeer.

The results of anthelmintic-abbreviated infections of Trichostrongylus colubriformis and Teladorsagia circumcincta on fecal egg counts in goats on pasture

Twenty, 2-yr-old angora goats kept on nematode larvae-contaminated pasture since birth were divided into 2 equal groups. Goats from group 1 were immunized by drug-abbreviated infection, a procedure that gave high protection against field challenge in 12-moold sheep. Group 1 was orally dosed 3 times with increasing numbers of Trichostrongylus colubriformis and Teladorsagia circumcincta infective larvae. Each time, the infection was abbreviated with Oxfendazole (OXF) 15 days after dosing. Group 2 received only OXF. After the third dose of OXF, the goats were grazed together on the same pasture and fecal egg counts determined. No protection in immunized goats was achieved. In fact, immunized goats produced significantly more nematode eggs than the nonimmunized group.

Use of free living stages to study the effects of thiabendazole, levamisole, pyrantel and ivermectin on the fine structure of Haemonchus contortus and Heligmosomoides polygyrus

Ultrastructural changes induced in vitro by thiabendazole, levamisole, pyrantel and ivermectin in the free living larval stages of two trichostrongyles (Heligmosomoides polygyrus and Haemonchus contortus) were analysed. The observed damage for each anthelmintic is related to the known mode of action and compared to the damage commonly described in adults. The advantage of using larvae to study the effects of anthelmintics on the fine structure of nematodes rather than adults is described. Thiabendazole induced alteration of the cellular organization especially epithelial cells of the digestive tract. Changes in mitochondria were also seen. Levamisole caused contraction of muscle fibres whereas no specific lesions were observed with pyrantel. Ivermectin caused an hypertrophy of muscular groups. The interest of such a technique in research on the modes of action of anthelmintics is emphasized.

Strategic effects of early season treatments with Moxidectin on trichostrongylosis in young calves

This study was conducted to evaluate the effects of strategic early season treatments with Moxidectin on trichostrongyles in first-season grazing heifer calves on a permanent pasture. Three groups of Black-Pied Friesian heifer calves were turned out in early May on a permanent pasture naturally infected with trichostrongyle larvae. Two of these groups were treated with Moxidectin at turnout or at turnout and again 8 weeks later, while one group served as untreated controls. For the first three weeks of the experiment all animals grazed together in one flock on one pasture. Subsequently, the pasture was divided into three comparable plots, which from then and until housing in mid October were grazed by each calf group. The results showed that the treatments significantly suppressed faecal egg excretion in the early part of the season, leading to reduced herbage infectivity and parasitism over the rest of the season. Results from faecal egg counts determined at genus level revealed that the persistent activity of Moxidectin appeared to be longer against Ostertagia spp. than against Cooperia spp. Two Moxidectin treatments, given at turnout and on week 8, protected the calves to a higher degree than a single Moxidectin treatment given at turnout. However, there was also a significant effect of only one Moxidectin treatment which possibly could be explained by a low overwintered larval population and a drought period in the early summer.

Cryopreservation of the first-stage larvae of trichostrongylid nematode parasites

First stage (L1) larvae of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta can be cryopreserved in the presence of DMSO using a two-step freezing protocol involving an initial period at -80 degrees C prior to transfer to liquid nitrogen. Thawed L1 larvae continue development in vitro producing third stage (L3) larvae that are infective to sheep when dosed per os. Establishment rates for L3 larvae grown from thawed L1 larvae were 40 and 80% for H. contortus and T. colubriformis, respectively. There was no difference in survival or infectivity between benzimidazole (BZ)-susceptible and BZ-resistant H. contortus isolates and cryopreservation caused no shift in their BZ-resistance status as indicated in an in vitro larval development assay. Cryopreservation also had no effect on the sensitivity of these isolates to the avermectins or levamisole in vitro. High survival rates (60-70%), good levels of establishment and the stability of anthelmintic resistance status of isolates indicate that little if any selection occurs during the cryopreservation process. L1 larvae of all 3 species have been successfully recovered after 16 months storage in liquid nitrogen, cultured to the L3 stage and established in sheep.

Host movement and the genetic structure of populations of parasitic nematodes

Mitochondrial DNA (mtDNA) sequence data were used to compare the population genetic structures of five species of parasitic nematodes from three different hosts: Ostertagia ostertagi and Haemonchus placei from cattle, H. contortus and Teladorsagia circumcincta from sheep, and Mazamastrongylus odocoilei from white-tailed deer. The parasites of sheep and cattle showed a pattern consistent with high gene flow among populations. The parasite of deer showed a pattern of substantial population subdivision and isolation by distance. It appears that host movement is an important determinant of population genetic structure in these nematodes. High gene flow in the parasites of livestock also indicates great opportunity for the spread of rare alleles that confer resistance to anthelmintic drugs. All species, including the parasite of deer, had unusually high within-population diversities (averages of 0.019-0.027 substitutions per site between pairs of individuals from the same population). Large effective population sizes (Ne), perhaps in combination with rapid mtDNA evolution, appear to be the most likely explanation for these high within-population diversities.

Production losses in Merino ewes and financial penalties caused by trichostrongylid infections during winter and spring

The productivity of Merino ewes treated with ivermectin and a controlled-release capsule containing albendazole was compared with untreated ewes grazing the same pastures on each of 3 farms. Treated breeding ewes had significantly increased greasy fleece weights (GFW, 6.5% and 7.1%) compared with untreated breeding ewes, on 2 of 3 farms but treatment caused no significant increase in the GFW of non-breeding ewes. Other benefits of treatment on all farms were a significant increase in body weight gain (from 1.7 to 3.7 kg) and a significant decrease in the weight of dag removed at crutching (from 42 to 622 g). These benefits occurred despite the presence on each farm of worms resistant to benzimidazoles. One disadvantage of treatment was an increase in mean fibre diameter of wool from treated ewes of from 0.12 to 0.41 microns. This increase reduces the value of the wool. Partial budgets indicated a net loss of from 8 to 62 cents per ewe for treatment. However, potential benefits such as increased growth rate and wool production from lambs of treated ewes were not included in the analysis. On one farm ewes were classified as 'susceptible' to severe dag if they had a dag score > or = 4 at the time of capsule treatment. Ewes classified as 'susceptible' were about 7 times more likely to develop severe dag than were other ewes. There was no significant difference between the GFW of untreated 'susceptible' ewes and untreated ewes not classified as 'susceptible'.(ABSTRACT TRUNCATED AT 250 WORDS)

[Levamisole- and tetramisole-resistant gastrointestinal nematodes in sheep]

Two experiments were carried out with sheep naturally infected with gastrointestinal nematodes to evaluate efficacy of anthelmintics using in vivo faecal egg count reduction (FECR) test. In experiment 1 with 28 ewes, the following efficacy of anthelmintics given at recommended dose rates was observed: albendazole 99.4%, ivermectin 99.3% and levamisole 81.8%. In experiment 2 with 18 ewes, tetramisole exhibited 71.3% efficacy. Suspected resistance to imidothiazole anthelmintics was confirmed by in vitro larval development test (LDT)--minimal inhibition concentration (MIC) values were estimated at 2.0 micrograms/ml. Infective larvae L3 cultivated from eggs produced by the population of resistant helminths were identified as Ostertagia and Trichostrongylus spp.

Survey for drug-resistant trichostrongyle nematodes in ten commercial goat farms in west Malaysia

The prevalence of drug-resistant parasites in goats in West Malaysia has rarely been reported. Ten commercial goat farms were surveyed for resistance to anthelminthics by measuring the reduction in faecal egg counts (FECs) after treatment with levamisole, pyrantel pamoate and fenbendazole. Resistance to fenbendazole was seen in most farms; there was no evidence of resistance to levamisole but some resistance to pyrantel pamoate was detected on one farm. The significance of the findings are discussed.

Moxidectin: pharmacokinetics and activity against horn flies (Diptera: Muscidae) and trichostrongyle nematode egg production

The concentration of moxidectin, a macrocyclic lactone endectocide, in the blood serum of cattle resulting from single and daily subcutaneous injections and oral dosing was determined as a function of time. When given as a single subcutaneous (SC) injection, the drug peaked between 4 and 6 h post-treatment. As a single oral dose, the peak serum level occurred at 1 day post-treatment. Daily SC injections and oral doses resulted in a gradual increase in blood serum level over the 21 days of treatment but did not reach a plateau during this time. Horn flies, Haematobia irritans (Linnaeus), feeding on the blood of treated cattle drawn on Day 21 of daily treatment showed a decline in survival and egg production, but a negligible effect on egg hatching. Dose-mortality data on adult horn flies showed an LC-50 and LC-90 value of 10 ppb and 19 ppb in the blood, respectively. Moxidectin was also found to have larvicidal activity against the immature stages of the horn fly in the manure of treated cattle. Moxidectin administered at 100, 50 and 25 micrograms kg-1 as a daily oral medication was 100% effective in eliminating trichostrongyle egg counts by Day 3 of the treatment. Counts remained negative to the end of the trial.

A survey of anthelmintic resistance in Slovakia

A survey of prevalence of anthelmintic resistant nematode strains was carried out in 77 sheep farms in the Slovak Republic. An egg hatch assay was used for the detection of resistance to the benzimidazole group of anthelmintics. Results could not be obtained from five farms because of low egg counts in faecal samples. On 64 farms, the effective dose (ED50) values were under 0.1 microgram ml-1 while on six farms, trichostrongylid populations with resistance to benzimidazoles were found. Their ED50 values ranged from 0.10 to 0.12 microgram ml-1.

Resistance to benzimidazole anthelmintics by trichostrongyles in sheep and goats in north-west Cameroon

Following the routine use of tiabendazole at monthly intervals for several years, the trichostrongyle parasites of sheep and goats on a government research station in North-West Cameroon had become resistant to benzimidazole anthelmintics.

Moxidectin: systemic activity against common cattle grubs (Hypoderma lineatum) (Diptera: Oestridae) and trichostrongyle nematodes in cattle

Moxidectin, a systemic insecticide, was evaluated for its efficacy against the migrating first instars of the common cattle grub, Hypoderma lineatum, and against nematode egg production in beef cattle. It was observed that all three levels (0.1, 0.2 and 0.4 mg moxidectin kg-1) were 100% effective against cattle grubs when administered as a s.c. injection. The same levels of treatment were very effective (90-100%) in reducing trichostrongyle nematode egg production. However, there was a slight indication that at least one species, Cooperia oncophora, was not completely eliminated, as it was observed that small numbers of eggs began to appear after 2 weeks post-treatment when there had been no opportunity for reinfection.

The effect of parasite control on fertility in beef heifers

The effect of anthelmintic treatment on the growth and conception rate in beef heifers was studied in southwestern Virginia, USA. Forty mixed-breed heifers were divided into two groups at weaning and placed on experimental pastures. Control heifers did not receive anthelmintic treatment, while animals in the treatment group were given ivermectin (200 micrograms kg-1) at the time of allocation to groups in October and again in April the following year. Supplemental feed was provided throughout the winter to ensure adequate weight gains. In May, the heifers were exposed to bulls for natural breeding for a period of 6 weeks. Weight gains, fecal egg counts, serum pepsinogen levels and pasture larvae counts were monitored throughout the experiment. No difference in conception rate was observed between the two groups, although the cumulative weight gain was significantly greater in treated than non-treated animals. Serum pepsinogen levels and fecal egg counts were also generally lower in treated than non-treated heifers.

Critical controlled test to evaluate resistance of field strains of Haemonchus contortus to thiophanate

Following reports of suspected gastrointestinal strongylid nematode resistance to thiophanate in sheep and goats at Sokoine University of Agriculture (SUA), Tanzania, field strains of Haemonchus contortus, one from SUA and another from Kimamba, Tanzania, were selected for a critical controlled test to evaluate the efficacy of thiophanate. The SUA H. contortus strain in goats showed a marked resistance to thiophanate at an oral dose of 50 mg kg-1 liveweight (efficacy = 24.1%), while the Kimamba H. contortus strain showed very marked susceptibility (efficacy = 99.7%) to thiophanate at the same oral dose. This is the first report from East Africa of a field strain of H. contortus resistant to thiophanate. The incidence of resistant strains of strongyloid parasites to benzimidazole anthelmintics in sheep and goats in East Africa is on the increase. The magnitude of this problem should be evaluated and remedial solutions sought.

Effect of selection pressure for thiabendazole resistance on fitness of Haemonchus contortus in sheep

A strain of Haemonchus contortus showing moderate levels of resistance to thiabendazole (TBZ) (egg hatch assay LC50 2.1 microM) was subjected, in alternating sequence, to 5 in vitro and 4 in vivo selections with TBZ. This resulted in a significant increase in resistance (egg hatch assay LC50 2.8 microM). The isolates obtained after each combined in vitro and in vivo selection were compared in terms of egg hatch assay, establishment, net egg output, per capita egg output and degree of pathology caused in infected hosts. The increase in resistance was accompanied by a significant increase in larval establishment in sheep (22.8-39.2%) and was associated with more severe pathology, higher egg count and worm recovery 3 months post-infection. The resistant strains were also compared with a TBZ-susceptible strain (egg hatch assay LC50 1.0 microM). The TBZ-susceptible strain had the highest net egg output, it caused the most severe pathology and it resulted in higher worm recovery than any of the TBZ-resistant strains, although the most resistant strain was very similar to the TBZ-susceptible strain. Neither the per capita egg output nor biological features of the free-living stages were significantly affected by the degree of resistance. The implications of these observations for anthelmintic use and for management of nematode anthelmintic resistance in the field are discussed.

A novel approach to the control of anthelmintic-resistant Haemonchus contortus in sheep

This paper reports attempts to control a resistant strain of Haemonchus contortus on pasture by replacing it with a susceptible strain. By making use of artificially infected donor sheep, six camps (paddocks) were seeded with a resistant field strain of H. contortus until it was confirmed by means of worm-free tracer lambs that the grazing had become infective. Thereafter, using donor sheep infected with a susceptible laboratory strain of H. contortus for seeding the pasture, attempts were made at various times of the year to replace the resistant strain on the pasture with the susceptible strain in five of the camps. The sixth remained as a control camp, in which no attempt was made to replace the resistant strain. In two of the five test camps, the susceptible strain was introduced in the autumn after 8-10 weeks of nil grazing; in the remaining three camps the introduction was made in spring (two camps) or summer without having a period of nil grazing. The susceptibility of the worm strains introduced initially, as well as of those that developed in the various camps, was gauged both by controlled non-parametric anthelmintic slaughter trials at the beginning and at the conclusion of the trial, and by an in vitro egg hatch test. A reversion to susceptibility occurred in three of the five camps. These included both of the camps infested with the susceptible strain in the spring and one of the two infested in the autumn. Possible reasons for the failure to replace the resistant strain in the remaining two test camps are discussed.

A larval paralysis assay for the detection of thiabendazole resistance in trichostrongyles

Infective-stage larvae of trichostrongyle nematodes, either resistant or susceptible to thiabendazole (TBZ), were incubated in eserine, an acetylcholinesterase inhibitor. Paralysis occurred in larvae treated with eserine but significant differences were observed in the percentage of larva immobilized between TBZ-resistant and TBZ-susceptible strains of the nematodes. These differences are probably related to the presence of higher levels of acetylcholinesterase in the TBZ-resistant strains than in the susceptible strains. This could be used as a rapid and inexpensive method of detecting resistance to TBZ in trichostrongyles.

Specific interaction of benzimidazole anthelmintics with tubulin: high-affinity binding and benzimidazole resistance in Haemonchus contortus

The specific (high-affinity) binding of tritiated benzimidazole [(3H]BZ) anthelmintics-mebendazole [(3H]MBZ) and oxibendazole [(3H]OBZ) to, and the specific displacement (inhibition) of these radioligands by unlabelled BZs (oxibendazole, mebendazole, oxfendazole, albendazole, fenbendazole and thiabendazole) from crude tubulin extracts prepared from thiabendazole (TBZ)-susceptible (S) or TBZ-resistant (R) strains of adult Haemonchus contortus, have been examined. The most striking difference between R and S was that the drug specifically bound at infinite ligand concentration (Bmax), was markedly reduced for the R strain, with no apparent change in association constant (Ka). Thus, resistance was associated with a loss of high-affinity receptors. TBZ-resistance was not associated with a change in low-affinity binding. There was a greater loss of high affinity receptors for [3H]OBZ than for [3H]MBZ. Using the displacement data. BZs were ranked according to their Ka and IC50 (concentration of BZ inhibiting 50% of radioligand binding) values. The Ka and IC50 values and the rank order of the BZs were approximately independent of the radioligand displaced or source (S or R) of the tubulin extracts used. The results are consistent with tubulin binding being the primary mechanism of action for all of these BZs.

South African field strains of Haemonchus contortus resistant to the levamisole/morantel group of anthelmintics

A strain of Haemonchus contortus from the Pietermaritzburg district of Natal was found to be resistant to levamisole (geometric mean efficacy 76.5%), morantel (41.9%), the benzimidazoles (oxfendazole: 33.7%) and rafoxanide (82.0%), but apparently fully susceptible to closantel and disophenol. In the case of ivermectin, a mean of 5.2% of the H. contortus was not removed at a dosage of 200 micrograms kg-1 live mass. A second strain of H. contortus, from Amsterdam in the south-eastern Transvaal, showed reduced susceptibility to levamisole (80.8%) and morantel (46.2%), the only 2 drugs tested. This is apparently the first report of resistance to the levamisole/morantel group of anthelmintics in sheep in South Africa.

Comparison of three in vitro techniques to estimate benzimidazole resistance in Haemonchus contortus of sheep

Three in vitro assays to detect benzimidazole resistance, namely, the egg-hatch assay, tubulin-binding assay, and a larval-development assay, were evaluated by estimating the level of benzimidazole resistance in three field isolates of Haemonchus contortus compared with a susceptible reference strain. Comparisons were also made with estimates of benzimidazole resistance of the three field strains obtained from an in vivo controlled anthelmintic efficacy test. All three in vitro tests showed similar, consistent results which also suggested greater sensitivity than the in vivo assay. These results indicate that selection of an in vitro technique to determine benzimidazole resistance should therefore be based on considerations other than precision, such as technical expertise, availability of equipment, cost and speed in which diagnosis is required.

Effects of the chitin synthesis inhibitor diflubenzuron on development of Ascaris suum and Haemonchus contortus

The potential of the chitin synthesis inhibitor diflubenzuron (DFB) to alter the development of the parasitic nematodes (Ascaris suum and Haemonchus contortus was investigated. DFB given orally (10 mg kg-1 per day for 30 days) to sheep inoculated with H. contortus infective larvae did not prevent the establishment of adults or affect fecal egg output. However, there was a significant (greater than 90%) decrease in the number of infective larvae recovered from fecal cultures derived from lambs harboring H. contortus adults that were treated with DFB. DFB did not affect egg hatching. Oral administration (10 mg kg-1 per day for 20 days) of DFB to swine harboring adult A. suum adults had no effect on the adult worm burden or on egg morphology, but eggs removed from worms obtained from DFB-treated swine contained less chitin than eggs removed from untreated control swine. DFB also inhibited chitin synthesis in vitro in the isolated reproductive tract of A. suum adults. These results indicate that DFB at high doses can inhibit the subsequent development of H. contortus larvae in the feces. Since H. contortus larvae lack chitin, DFB may act on these larvae by a mechanism independent of a direct effect on chitin synthesis.

Motility response of levamisole/benzimidazole-resistant Haemonchus contortus larvae

Electronic measurements of the motility of third stage larvae of a susceptible and a levamisole/benzimidazole-resistant strain of Haemonchus contortus were made after incubation in solutions of anthelmintics for 24 h. Results confirmed the resistance to benzimidazoles, but failed to show differences in response to levamisole. Visual observations of paralysis of first and third stage larvae in solutions of levamisole also failed to show significant differences between the two strains. The tests as conducted failed to demonstrate levamisole resistance, suggesting that nematode strains may vary in their mechanisms of resistance to levamisole.

Colorimetric assay for the detection of benzimidazole resistance in trichostrongyles

A modified version of the aphid tile-test, which is used to detect insecticide resistance in single adult aphids that are resistant or susceptible to organophosphate or carbamate insecticides, was used to compare the levels of non-specific esterases in strains of the trichostrongyle nematodes Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis which were known to be resistant or susceptible to benzimidazole (BZ) anthelmintics. This colorimetric assay has shown that there is significantly more non-specific esterase in the infective-stage larvae of BZ-resistant strains than in susceptible strains and this may prove to be of use in the detection of resistance to benzimidazole anthelmintics.

Evaluation of the morantel sustained release trilaminate in the control of parasitic gastroenteritis in first season grazing cattle

A novel intraruminal bolus developed for the sustained delivery of the anthelmintic morantel tartrate was evaluated in the seasonal control of parasitic gastroenteritis in first season grazing calves. The morantel sustained release trilaminate is a trilaminate sheet consisting of a central lamina of a morantel tartrate/ethylene vinyl acetate matrix coated on both sides with a thin impermeable layer of ethylene vinyl acetate. A symmetrical pattern of circular perforations punched through the device controls the release of morantel. Administration of the trilaminate to calves significantly reduced their faecal egg output compared with untreated controls and thus reduced pasture larval contamination. Clinical parasitic gastroenteritis was prevented in the treated calves and there were significant reductions in their worm burdens compared with the untreated control calves both during and at the end of the grazing season. The control of parasitic gastroenteritis resulted in a significantly greater (P less than 0.0001) weight gain, of 45 kg, by the treated calves.

[Primary anthelminthic screening on the L4 and L5 developmental stages of the lungworm, Dictyocaulus viviparus, in guinea pigs]

The study was aimed at expanding the primary anthelmintic screening to cover a model of the group of pulmonary nematodes; in this particular case to introduce the lungworm, D. viviparus, in laboratory animals. A method of primary screening for the fourth and fifth larval states of D. viviparus in guinea-pigs was worked out after the selection of a suitable laboratory host. In the primary screening, three well-known anthelmintics of the benzimidazol series, including fenbendazole, mebendazole and levamisol, were tested by the method of controlled test. The anthelmintics were administered at the recommended doses of 7.5 and 10.0 mg per kg of live weight for two days in succession. The effectiveness of the control of the 4th and 5th larval states of D. viviparus was 93.4% in fenbendazole, 89.0% in mebendazole, and 89.9% in levamisol. It is confirmed by the results of the trials that guinea-pigs can be used for the testing model of the lungworm, D. viviparus, in anthelmintic screening.

Changes in anthelmintic resistance status of Haemonchus contortus and Trichostrongylus colubriformis exposed to different anthelmintic selection pressures in grazing sheep

This experiment was designed to study, over a 5-year-period, the effect of different frequencies of treatment with three different anthelmintic groups, namely, benzimidazoles, levamisole and ivermectin, and different frequencies of alternation between them, on existing levels of anthelmintic resistance in the nematode parasites Haemonchus contortus and Trichostrongylus colubriformis of grazing sheep. No evidence of ivermectin resistance emerged, even in suppressively treated groups. Likewise, H. contortus failed to develop resistance to levamisole under a similar selection regimen. Thiabendazole was shown to select positively against levamisole resistance in T. colubriformis resulting in significantly greater susceptibility to this drug than for the natural reversion which occurred in the untreated control. There was no evidence that an anthelmintic treatment combined with a movement of sheep to pastures of low infectivity selected more rapidly for resistance than where the same number of treatments were given to set-stocked sheep. Rotation between anthelmintic groups at yearly intervals appeared to be more beneficial in delaying resistance than rotation of drugs with each treatment.

Resistance of Libyostrongylus douglassi in ostriches to levamisole

While fenbendazole at 15 mg kg-1 either alone or in combination with resorantel reduced the total burden of Libyostrongylus douglassi by more than 98%, levamisole alone at 30 mg kg-1 was only 28% effective, and in combination with resorantel its efficacy was 67%. These results with levamisole do not correspond with the registered rate of efficacy which is given as at least 80% in South Africa. This is apparently the first reported case of anthelmintic resistance in ostriches.

In vitro response of Haemonchus contortus larvae harvested during different times of the patent period to anthelmintics

Three commercially available ruminant anthelmintics and an investigational drug were evaluated for effects on the motor function/motility of third stage (ensheathed) Haemonchus contortus. Helminth ova were collected at one, five, nine and 13 weeks during the patent period, cultured to the third larval stage and assayed for sensitivity to four different drugs. All four drugs (100 and 10 micrograms ml-1) significantly affected the motility of third stage H contortus larvae cultured from eggs passed at each of the times examined. However, the investigational drug (p-toluoyl chloride phenylhydrazone, 10 micrograms ml-1) had a significantly greater effect on the motility of larvae harvested at 13 weeks than those cultured at nine weeks (32 per cent difference). No other significant differences in the motility response during the patent period were observed.