Gastrointestinal nematode infections of first-grazing season calves in Western Europe: associations between parasitological, physiological and physical factors

Analysis of 85 studies on gastrointestinal nematode infections in first-grazing season (FGS) calves is presented. The studies cover a 26-year period and were carried out in 13 countries in Western Europe. Both control and chemoprophylactic-treated (early in the season) FGS calf groups were included. In 53 of the 85 studies, clinical outbreaks of parasitic gastroenteritis (PGE) were observed during the FGS in the control groups. The mean initial age (and weight) of the control calf group was significantly associated with PGE outbreaks: 82% of control calf groups < or = 6 months of age had outbreaks of PGE, compared to only 33% of control calf groups > 6 months of age. In 92% of trials where the geometric mean faecal egg count (MFEC) was > or = 200 EPG on Day 56, PGE outbreaks were observed, but where it was < 200 EPG, only 29% had PGE. The use of these two factors in assessing the likelihood of PGE outbreaks in untreated calf groups in the future FGS is therefore, proposed. No chemoprophylactic-treated groups had PGE, but there was a highly significant negative relationship between maximum faecal egg counts in the chemoprophylactic-treated calves and the proportion of the trial covered by the different chemoprophylactic systems. Higher stocking densities were significantly associated with higher pasture contamination in both control and chemoprophylactic-treated calves. A highly significant positive relationship between the weight gained in the chemoprophylactic-treated groups and the estimated duration of the various chemoprophylactic systems was found, but there were large variations in weight gains (60-160 kg) between groups even with the same chemoprophylactic. Despite this and other highly significant associations, it was not possible to indicate what weight gains were obtained by the end of the FGS, from factors measured early in the FGS.

Interpreting comparative experiments on parasite burdens in animals using randomization and canonical variates

Comparative experiments on parasite burdens present difficulties for modelling and interpretation: data tend to have highly skewed distributions, some standard methods of testing for effects have low power, and there is a need to make allowance for dependencies among the various measurements. Randomization testing and canonical variates analysis are advocated as a means of minimising these problems.

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.

Comparative efficacy of ivermectin pour-on, albendazole, oxfendazole and fenbendazole against Ostertagia ostertagi inhibited larvae, other gastrointestinal nematodes and lungworm of cattle

An experiment was conducted to evaluate the current efficacy of albendazole (ABZ), oxfendazole (OXF) and fenbendazole (FBZ) compared with ivermectin pour-on (IVM-PO) against inhibited early fourth-stage larvae (IEL4) of Ostertagia ostertagi, other gastrointestinal nematodes and lungworm of cattle during spring in Louisiana. Twenty-five crossbred beef heifer calves of 235 kg average weight and 10-12 months of age were acquired in late winter and grazed for 9 weeks on pasture contaminated with O. ostertagi and other nematodes until May 15. The cattle were weighed and randomly allotted into 5 groups of 5 calves on May 16 (day 0) and treatments were as follows: group 1, nontreated controls (CONT); group 2, IVM-PO on mid-backline at 500 micrograms/kg; group 3, ABZ suspension (oral) at 10 mg/kg; group 4, OXF suspension (oral) at 4.5 mg/kg; group 5, FBZ suspension (oral) at 5 mg/kg. After treatment and confinement in separate pens for each group, approximately equal numbers of cattle from each group were necropsied daily between days 29-31. Mean numbers of O. ostertagi developmental stages present in CONT were: adult, 5234; developing (DL4), 3130; IEL4, 44,077. The mean percentage of IEL4 was 84.1. Cooperia spp. were the second most prevalent in CONT (20,307) and smaller numbers of abomasal and intestinal species and Dictyocaulus viviparus were present in nearly all CONT. Percent reductions for the four compounds against O. ostertagi adult, DL4 and IEL4, respectively, were IVM-PO: 99.7, 98.3, 98.1; ABZ: 74.1, 76.5, 75.3; OXF: 78.5, 42.1, 32.0; FBZ: 63.6, 17.7, 39.7. Efficacy of IVM-PO was greater (P < 0.05) against all O. ostertagi stages than the benzimidazole (BZ) drugs, except for ABZ (DL4). There were no significant differences in group means (except for C. punctata adult males, P < 0.05 lower for IVM-PO) or wide variation in reduction percentages for other abomasal and intestinal species and D. viviparus between IVM-PO and BZ drugs. The low efficacy of all three BZ drugs for O. ostertagi, but especially for OXF and FBZ, under conditions of this experiment, were suggestive of drug tolerance to dosages employed or resistance. However, problems of drug availability/concentration in the abomasum, and the factor of IEL4 and DL4 replacing adult worms after treatment cannot be excluded. Efficacy of the BZ drugs against all other species were essentially similar to that of IVM-PO.

A survey of anthelmintic resistance in nematodes of sheep in The Netherlands

The prevalence of anthelmintic resistance in nematodes of sheep was surveyed in 1994 on 70 farms in the Netherlands. An in vitro egg hatch assay, faecal egg count reduction (FECR) 14 days after treatment, and larval cultures were used as methods of investigation. Oxfendazole was tested on 69, ivermectin on 51, and levamisole on 36 farms. The median effective dose (ED50) of thiabendazole could be determined on 64 farms. On 60 farms (94%) the ED50 value was > or = 0.12 microgram ml-1, which is indicative of the presence of benzimidazole (BZ) resistance. On two farms egg output was too low to do a FECR test. Based on the results of the FECR test, BZ resistance was present on 56 farms (84%), on 2 farms there was a suspicion of resistance and on 9 farms no resistance could be found. No clear indications were found for the presence of resistance against ivermectin or levamisole. BZ resistance was demonstrated in Haemonchus contortus, Cooperia curticei, Ostertagia spp. and/or Trichostrongylus spp. No resistance was observed in species from the genus Nematodirus, Chabertia ovina and/or Oesophagostomum spp.

Prophylactic efficacy of persistent anthelmintics against challenge with drug-resistant and susceptible Ostertagia circumcincta

Three groups of newly-weaned Romney lambs were given either a standard oral dose of albendazole, a controlled-release capsule containing albendazole, or a standard oral dose of moxidectin. At 10, 20, 30 and 40 days after treatment, sub-groups of lambs were given 10,000 infective-stage larvae of either a drug-resistant or a drug-susceptible strain of Ostertagia circumcincta. The recommended oral dose of albendazole removed 32 per cent of the resistant strain and over 99.9 per cent of the susceptible O. circumcincta. The recommended oral dose of moxidectin removed 91 per cent of the resistant strain and over 99.9 per cent of the susceptible parasites. None of the lambs treated with controlled-release capsules was challenged at 20 or 30 days after treatment. Twenty-one days after challenge, samples of faeces were taken to determine the presence of nematode eggs and cultured to establish the proportion of eggs developing to infective-stage larvae (L3). Abomasa were recovered after slaughter and worm burdens determined. In the lambs given controlled-release capsules only the resistant parasites were able to establish, and there were significantly fewer than in the lambs treated orally with albendazole. The proportion of the eggs from resistant parasites which developed to L3 was not reduced by the presence of the capsules. Oral moxidectin provided no protection against the establishment of the resistant strain and viable L3 were recovered after challenge with resistant parasites 10 days after treatment; however, the establishment of susceptible O. circumcincta was reduced by more than 99 per cent. The establishment of the susceptible parasites in the lambs treated with moxidectin increased with time and was not significantly lower than in the other groups by 30 days after treatment.

Efficacy of a pour-on formulation of eprinomectin (MK-397) against nematode parasites of cattle, with emphasis on inhibited early fourth-stage larvae of Ostertagia spp

OBJECTIVE

To evaluate efficacy of topically applied eprinomectin against inhibited early fourth-stage larvae (IL4) of Ostertagia spp in calves.

ANIMALS

4 groups (n = 6 [replicates]) for dose titration; 2 groups (n = 8 calves [replicates]) for dose confirmation.

PROCEDURE

2 dose titration studies-0, 125, 250, and 500 micrograms of eprinomectin/kg of body weight-Louisiana and Georgia- and 2 dose confirmation studies of selected therapeutic dosage (500 micrograms/kg) in Scotland and France. Monitor calves were used to determine inhibition percentage of Ostertagia IL4. Test calves were ranked by weight in replicates of 4 (titration trials) or 2 (confirmation trials) animals each, and within replicates, were randomly allocated to treatment groups. Drug treatments were done on day 0, and animals were euthanatized by replicate, with holding time between treatment and euthanasia varying among trials from 14 to 27 days.

RESULTS

Observations indicated high efficacy (> 99%) of 500 micrograms of eprinomectin/kg in removal of Ostertagia IL4. Ostertagia and Cooperia were only genera common across sites, with efficacy of aforementioned dosage against adult and larval stages of both genera consistently high (> 99%). Results of 1 or both titration studies (500 micrograms/kg) indicated > 99 to 100% efficacy against adult Haemonchus placei, Trichostrongylus axei, T colubriformis, Bunostomum phlebotomum, Dictyocaulus viviparus, and Oesophagostomum radiatum. Lower efficacy values were observed at minimal (125 micrograms/kg) dosage. In France, 500 micrograms/kg was 85% effective against Trichostrongylus spp adults; however, numbers of control calves infected with Trichostrongylus spp and degree of infection were low. Adverse reactions were not evident.

CONCLUSION

Eprinomectin given topically (500 micrograms) was highly effective against Ostertagia IL4 and other common nematodes of cattle.

The death rate of Ostertagia circumcincta and Trichostrongylus colubriformis in lactating ewes: implications for anthelmintic resistance

Lactating adult Romney ewes were infected, 4 weeks post-lambing, with benzimidazole (bz) resistant strains of Ostertagia circumcincta and Trichostrongylus colubriformis. Commencing 4 weeks after the initial infection the ewes were subjected to challenge 3 times weekly with 5000 L3 of bz-susceptible strains of both parasite species. At weekly intervals over the following 6 weeks, groups of ewes were drenched with a bz anthelmintic (oxfendazole) to remove bz-susceptible parasites and slaughtered to determine adult worm burdens of the bz-resistant parasites. The O. circumcincta infection declined exponentially with a mean daily death rate of 10.6% day-1 and no worms were recovered after 4 weeks or more of challenge. The T. colubriformis infection did not decline significantly over the 6 weeks of continuous challenge, indicating that the death rate could not be distinguished from zero. The upper 95% confidence limit for the death rate of T. colubriformis was 4.9%. The implications of these death rates on selection for drug resistance following ewe drenching during the post-partum period are discussed with selection pressure likely to be greater for T. colubriformis than for O. circumcincta.

The presence of an early L4 population in relation to the acquired resistance of calves naturally infected with Ostertagia ostertagi

The aim of the present study was to determine if the presence of inhibited L4 Ostertagia ostertagi populations during the housing period influenced the level of acquired immunity in calves at the start of the second grazing season. Four groups (A-D) of first season grazing calves were turned out onto a heavily infected pasture for five months, while the fifth group (E) was kept as uninfected controls. At housing all group A calves were necropsied for worm counts. A mean total of 78915 Ostertagia worms were found, of which 85% were in the early L4 stage. The calves of group C were dewormed with ivermectin at housing, while groups B and D remained untreated. After the calves were housed, egg counts, pepsinogen and antibody levels all declined in groups B, C and D, but during the second half of the housing period pepsinogen and antibody levels increased again in groups B and D. The effect of housing on the resident worm population was evaluated by slaughtering all group B calves for worm counts at the end of the housing period. A mean burden of 57,950 Ostertagia was present, with 78% as early L4 larvae. The remaining three groups (C, D and E) were treated with oxfendazole and turned out on the same pasture as the previous year for a challenge period of four weeks. In the control calves (group E) the egg output rose steeply, while egg counts remained low in groups C and D. In addition the worm burdens of the previously infected groups C and D were significantly reduced (75-90%), and the percentage of inhibited L4 larvae was significantly increased compared to the control group E. No significant difference was observed between the calves that were treated at housing (group C) and the untreated animals of group D. It was concluded that the continued presence of an inhibited L4 population during housing was not essential for maintenance of immunity.

Evaluation of a larval development assay for the detection of anthelmintic resistance in Ostertagia circumcincta

Two experiments were carried out to evaluate a larval development assay for the detection of anthelmintic resistance in O. circumcincta. In Experiment I, the dose responses to levamisole (LEV), thiabendazole (TBZ) and ivermectin (IVM) of 8 isolates of O. circumcincta were measured 34 days after infection (DAI). Four of these isolates were shown to be resistant to 1 or more anthelmintics. With 2 exceptions, all isolates considered to be resistant had higher LD50 values than the susceptible isolates for that anthelmintic. One exception was isolate RM8, which was considered to be resistant to all 3 anthelmintics based on faecal egg count reduction tests in goats, but the LD50 value for LEV did not differ from that for the susceptible isolates. The other exception was an isolate considered to be susceptible to TBZ which had a relatively high LD50 value. In an unrelated trial that was prompted by this finding, this isolate was confirmed to be benzimidazole-resistant. Isolate RM8 and an isolate susceptible to all 3 anthelmintics (SK2) were used in the second experiment, which was conducted to monitor changes in the LD50 values of LEV, TBZ and IVM over time following a single infection of 35,000 infective larvae in young sheep. Faecal samples were collected weekly from 24 to 115 DAI. With all 3 anthelmintics, the LD50 values increased with time to a peak around 50-60 DAI, and then declined to levels similar to those observed soon after patency. This trend was consistent for both isolates. The highest mean LD50 values for isolates SK2 for IVM and TBZ and RM8 for IVM and RM8, respectively, were 1.7 and 1.8 times, and 2.2 and 2.9 times higher than the initial mean LD50 values. There was a clear distinction in LD50 values between isolates at each sampling day for both IVM and TBZ. However, as a consequence of the changes in LD50 values with time, the peak LD50 values of IVM for isolate SK2 were higher than the minimum LD50 values of isolate RM8. As there was no apparent difference in LEV efficacy between these 2 isolates, the data were pooled. The highest mean LD50 value was 2.3 times higher than the initial LD50 value.

Hypobiosis in the ostertagids of red deer and the efficacy of ivermectin and fenbendazole against them

An abattoir survey was carried out during the 1991 to 1992 slaughter season to examine the structure of the ostertagid populations in red deer. Most of the animals were 15- to 22-month-old stags, but there were also hinds on two occasions and calves on one. Ostertagids of mixed species were present in all of the animals, and 43 of 120 (35.8 per cent) stags and 10 of 17 (58.8 per cent) hinds, but no calves, also carried small numbers of Trichostrongylus axei. Most animals carried adult and developing ostertagids and after September they all contained hypobiotic larvae (EL4). EL4 were present in three of nine stags in September and they increased in number through the autumn. In stags killed between November and February the group mean numbers of EL4 represented 62 to 94 per cent of the total burdens present, compared with 37 and 55 per cent on April 1 and 7, respectively. In the hinds, the number and the structure of the ostertagid populations was similar to that in the stags. Of 14 calves killed on October 9, only two contained a few EL4. Subcutaneous ivermectin at 400 micrograms/kg removed 100 per cent of the adult and developing worms, but although it also removed 95 per cent of EL4, some larvae remained in every treated animal. The efficacy of fenbendazole at 15 mg/kg given over five days was 75 per cent against EL4 and although the long interval after treatment obscured the result, it was probably much higher against later stages.

Persistence of the efficacy of pour-on and injectable moxidectin against Ostertagia ostertagi and Dictyocaulus viviparus in experimentally infected cattle

The persistence of the efficacy of moxidectin 0.5 per cent pour-on and moxidectin 1 per cent injectable against Ostertagia ostertagi and Dictyocaulus viviparus in calves was studied in two experimental trials. In the first trial two groups of seven calves were treated with either the pour-on or the injectable formulation, while a third group remained untreated. All the animals were infected daily from Monday to Friday with infective stages of O ostertagi and D viviparus between the day of treatment (day 0) and day 33, and were necropsied for worm counts three days later. The experimental design of the second trial was similar to that of the first but the period of infection was from 28 to 45 days after the treatment, and the necropsy was five days after the last infection. In both trials both moxidectin formulations had very high efficacies (99.6 per cent) against adult and developing stages of O ostertagi and D viviparus. The higher efficacy of the moxidectin pour-on preparation against early fourth stage larvae in both trials suggested that its effect was more persistent. It was calculated that the efficacy of moxidectin against O ostertagi persisted for at least five weeks for the injectable formulation and six weeks for the pour-on. The efficacy of moxidectin against D viviparus lasted for at least six weeks for both formulations.

The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: Paraguay

This survey to detect anthelmintic resistance in nematode parasites of sheep was conducted on 11 farms in the Occidental and 26 farms in the Oriental Regions of Paraguay using the faecal egg count reduction test (FECRT). The anthelmintic groups tested were the benzimidazoles, levamisole and avermectins (both oral and injectable). Overall the levels of resistance were 73%, 68%, 73% and 47%, respectively. Levels of resistance were similar for all three important nematode genera, viz. Haemonchus contortus, Ostertagia and Trichostrongylus. This survey clearly indicates that a large, and ever increasing, proportion of sheep farmers are rapidly approaching the time when they will have exhausted all chemotherapeutic options to control parasites. Unless they face having to abandon their sheep farming operations, radical changes will need to be implemented with urgency.

The prevalence of anthelmintic resistance in nematode parasites of sheep in southern Latin America: Uruguay

This survey was conducted on 252 farms randomly distributed over all the sheep raising areas in Uruguay. The study involved farms with more than 600 sheep, which represented 80% of the total sheep population of the country. Three anthelmintic groups were assessed, namely, benzimidazoles, levamisole and avermectins. Overall, the results showed 80% of sheep flocks had benzimidazole resistance, 71% had resistance to levamisole, and 1.2% of flocks showed resistance to avermectins. Approximately 28% of farms had resistance to one anthelmintic group, 64% to two anthelmintic groups, and 1% resistance to all three groups. Only 7.5% of farms had no detectable levels of anthelmintic resistance. More than 80% of farms had Trichostrongylus populations resistant to both benzimidazoles and levamisole. Resistance was recorded in all three anthelmintic groups for Haemonchus and resistance also occurred to benzimidazoles and levamisole in Ostertagia.

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.

Anthelmintic profile of the cyclodepsipeptide PF1022A in in vitro and in vivo models

A novel cyclodepsipeptide of fungal origin, PF1022A, recently was reported to have anthelmintic activity. To supplement published reports and determine potential utility of PF1022A as a ruminant anthelmintic, the compound was examined in in vitro and in vivo models. Assays used measured motility of Haemonchus contortus (intrinsic drug potency), ATP levels (parasite death), and activity against H. contortus, Ostertagia ostertagi, and Trichostrongylus colubriformis in the jird (spectrum, potency, and efficacy by various routes). The potency of PF1022A in reducing motility is greater than commercial anthelmintics. Examination of ATP levels in PF1022A-paralyzed H. contortus indicates that worms are not killed, suggesting the compound acts as a neurotoxin in nematodes. In the jird, PF1022A has activity orally against each of the parasites studied and at doses comparable to all commercial anthelmintics, except the macrocyclic lactones which are more potent. Unfortunately, for some nematode species, parenteral delivery is ineffective at realistic doses.

The influence of a heavy infection with sensitive and resistant strains of Ostertagia circumcincta and with Trichostrongylus colubriformis on the pharmacokinetics of febantel in lambs

Plasma concentrations of febantel and its major metabolites, fenbendazole, oxfendazole and fenbendazole sulphone, were determined after oral administration of 7.5 mg/kg febantel in lambs before and 28 days after infection with 100,000 L3 larvae of a benzimidazole (BZ)-sensitive or BZ-resistant strain of Ostertagia circumcincta or with 75,000 L3 larvae of a BZ-sensitive Trichostrongylus colubriformis strain. The febantel concentrations were always low, and in only a few samples were higher than the limit of detection. A mean decrease in the area under the curve (AUC) for the three metabolites of 10.2%, 16.4% and 4.9% in lambs infected, respectively, with BZ-sensitive O. circumcincta, BZ-resistant O. circumcincta and T. colubriformis was observed. The Cmax for all the metabolites was higher in the BZ-sensitive O. circumcincta group than in the naive sheep, while the Tmax occurred earlier. The Cmax and the Tmax values for all the metabolites were lower in the BZ-resistant O. circumcincta group than in their own naive controls. In the T. colubriformis group the Cmax values of the metabolites were lower and the Tmax occurred much later.

Atanine (3-dimethylallyl-4-methoxy-2-quinolone), an alkaloid with anthelmintic activity from the Chinese medicinal plant, Evodia rutaecarpa

Phytochemical studies using a range of chromatographic and spectroscopic techniques coupled with in vitro bioassays against larval Schistosoma mansoni, L4 larvae of Ostertagia circumcincta, and adults and larvae of Caenorhabditis elegans have led to the isolation of an active anthelmintic compound in the Chinese medicinal plant Evodia rutaecarpa (Rutaceae) and its identification as atanine (3-dimethylallyl-4-methoxy-2-quinolone). Atanine has not previously been found to possess antiparasitic activity.

Detection of anthelmintic resistant nematodes in sheep in southern England by a faecal egg count reduction test

On five farms identified by in vitro tests to have benzimidazole resistance, and on one farm with apparent anthelmintic failure, faecal egg count reduction tests were performed using benzimidazoles, levamisole and ivermectin. With benzimidazoles efficacy was less than 70 per cent on all farms, confirming the validity of the in vitro tests to detect benzimidazole resistance. On two farms levamisole had an efficacy of less than 90 per cent but laboratory tests failed to demonstrate levamisole resistance. Ivermectin reduced egg counts by 100 per cent on all farms.

Treatment of multiple resistant field strain of Ostertagia spp. in Cashmere and Angora goats

A multiple resistant field strain of gastro-intestinal nematodes was detected in Cashmere and Angora goats imported from New Zealand. Different treatments with three types of broad spectrum anthelmintics (albendazole, levamisole/tetramisole and ivermectin) at various dose rates are described. Satisfactory effect in Angora goats was not achieved until all three anthelmintics were administered simultaneously at twice the normal sheep dose. Post mortem worm counts and identification revealed that the resistant population consisted of 89% Ostertagia circumcincta, 6% Ostertagia ostertagi and 5% Ostertagia trifurcata. However, egg hatch assay carried out seven months later detected the presence of resistant nematodes again.

Acetylcholinesterase in infective-stage larvae of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis resistant and susceptible to benzimidazole anthelmintics

The amounts of acetylcholinesterase (AChE) in homogenates of infective-stage larvae of strains, from different countries, of Haemonchus contortus, Ostertagia circumcincta and Trichostrongylus colubriformis resistant or susceptible to benzimidazole (BZ) anthelmintics, were compared using a rapid colorimetric assay. The levels of AChE were much greater in those strains which were BZ-resistant than in BZ-susceptible populations. The larvae of susceptible and resistant H. contortus contained less AChE than did the larvae of the other two species. One-way analysis of variance showed that the observed differences between susceptible and resistant strains were significant in each of the species studied. It is suggested that the extra enzyme may act as an anthelmintic-detoxifying enzyme, may inactivate the active site of the anthelmintic or may prevent binding of the anthelmintic to its receptor.

Mutual resistance to avermectins and milbemycins: oral activity of ivermectin and moxidectin against ivermectin-resistant and susceptible nematodes

To determine whether there is mutual resistance to avermectin and milbemycin anthelmintics, ivermectin and moxidectin sheep drenches were tested against ivermectin-resistant and susceptible isolates of Ostertagia circumcincta and Trichostrongylus colubriformis in sheep. None of the isolates had been exposed to moxidectin previously. The dosage of ivermectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 23 times and six times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. The dosages of moxidectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 31 times and nine times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. It is concluded that the worms resistant to ivermectin were also resistant to moxidectin.

In vitro activity of paraherquamide against the free-living stages of Haemonchus contortus, Trichostrongylus colubriformis and Ostertagia circumcincta

Paraherquamide, an oxindole alkaloid recently reported to have potent nematocidal activity, was shown to have a marked inhibitory effect on the motility of the free-living larval stages of H. contortus, T. colubriformis and O. circumcincta. The effect of paraherquamide on larval motility could be distinguished from that caused by levamisole and the avermectins. After 72 h exposure, the concentration of paraherquamide required to inhibit the motility of 50% of L3 larvae present was 0.033, 0.058 and 2.7 micrograms ml-1 for O. circumcincta, T. colubriformis and H. contortus, respectively. Ivermectin (IVM)-resistant isolates of H. contortus were significantly more sensitive to the paralytic effects of paraherquamide than IVM-susceptible isolates of this species. Paraherquamide had no effect on the time for development from the egg to the L3 larval stage of H. contortus, T. colubriformis and O. circumcincta.

Control strategies to prevent resistance

Anthelmintics will continue to play a central role in nematode control programs of livestock for the foreseeable future. However, to prevent the emergence of resistance, anthelmintics need to be either so effective that there are no survivors following treatment or treatment should be so infrequent that those that survive make little or no contribution to subsequent generations of the parasite populations. In this regard, the record for the sheep, horse and particularly the goat industry is not good, whereas the second factor is almost certainly responsible for the lack of a significant resistance problem in the cattle industry. In Australia, where high levels of resistance prevail, control programs designed to limit the increase and spread of resistance have been promoted for some time. However, their development, and certainly their implementation, has tended to be reactive rather than proactive--attempting to deal with the problem when it threatens future productivity, rather than preventing it in the first instance. Regions and/or livestock industries with relatively less of a resistance problem should heed these developments and endeavour to establish a responsible ethos amongst users of anthelmintics. In concert with the general philosophy of Integrated Pest Management (IPM), parasite control schemes should focus on the planned integration of a range of techniques, rather than relying almost entirely on anthelmintics. By these means, sustainable control strategies to prevent resistance could be achieved.

Anthelmintics

Anthelmintics remain the principal means for the prevention and control of subclinical and clinical ostertagiasis. The selection of an appropriate anthelmintic depends on whether one is controlling or preventing Type I ostertagiasis (caused by the establishment of adult worms derived from recently acquired infective larvae), preventing Type II (treating pre-Type II or inhibited larvae) or controlling Type II ostertagiasis (caused by the development of inhibited larvae to adults), or using the anthelmintic as part of an epidemiologically based plan to reduce pasture contamination with infective Ostertagia ostertagi larvae. In the latter case, the choice of an anthelmintic may depend on whether the targets for treatment are only adult worms and developing larvae or whether the targets include hypobiotic larvae. Thus for Ostertagia control, anthelmintics must be divided into those that normally control all stages, such as the avermectin group (ivermectin, abamectin and moxidectin) and some of the benzimidazoles (albendazole, oxfendazole and fenbendazole at appropriate dose rates), and those that only control adult worms and developing larvae (levamisole, morantel, coumaphos, phenothiazine and thiabendazole).

Evidence of multiple anthelmintic resistance in a strain of Teladorsagia circumcincta (Ostertagia circumcincta) isolated from goats in Scotland

The efficacy of ivermectin, fenbendazole and levamisole was determined in lambs individually infected with 5000 third stage larvae of Teladorsagia (Ostertagia) species isolated from a herd of goats with a history of benzimidazole resistance and where the efficacy of ivermectin had also suffered an apparent decline. Efficacies, calculated using geometric means, for groups where treatments were directed against adult worms were 90.3 per cent (ivermectin), 43.4 per cent (fenbendazole) and 100 per cent (levamisole). The efficacy of ivermectin against immature stages was 83.2 per cent. Ivermectin was highly effective (99.9 per cent) against a known susceptible ovine strain of Teladorsagia.

Efficacy of ivermectin delivered from a sustained-release bolus against inhibited early fourth-stage larvae of Ostertagia ostertagi and other nematodes in cattle

The anthelmintic efficacy of ivermectin (IVM) delivered from a sustained-release (SR) bolus was evaluated against natural infections with gastrointestinal tract nematodes in 12 crossbred beef heifers in spring. The 12 calves were randomly allotted to 2 groups of 6 calves each. Group-1 calves were treated with an SR bolus designed to deliver 8 mg of ivermectin/d. Group-2 calves were nontreated controls. Cattle groups were kept in separate concrete-floored pens (grass hay nutrition) and slaughter was performed at 35 days after treatment. Fecal egg counts for group-1 calves remained zero after treatment, except for detection of less than 1 egg/g of feces in 1 calf at the time of slaughter; counts in nontreated calves increased. Mean and range of Ostertagia ostertagi inhibited larvae in nontreated calves were 27,093 and 10,622 to 56,368, respectively. Efficacy of the IVM SR bolus was 100% against O ostertagi developing fourth-stage larvae (L4) and inhibited early L4, Haemonchus placei adults, Cooperia punctata and C spatulata adult males, Cooperia spp adult females, Cooperia spp L4, Trichostrongylus colubriformis adults, Bunostomum phlebotomum adults, and Oesophagostomum radiatum adults. Efficacy for O ostertagi and T axei adults was 99.9%. Numbers of nontreated calves infected with C pectinata adult males and Oes radiatum L4 were too low to evaluate efficacy. Calves treated with the IVM bolus gained 10.2 kg, whereas nontreated calves lost 1.8 kg. Abomasal lesions were clearly greater in nontreated calves on the basis of index comparisons of abomasal weight and total live weight and gross pathologic features.

Efficacy of abamectin against natural infections of gastrointestinal nematodes and lungworm of cattle with special emphasis on inhibited, early fourth stage larvae of Ostertagia ostertagi

The anthelmintic efficacy of abamectin (avermectin B1) was evaluated against gastrointestinal nematodes, including Ostertagia ostertagi inhibited larvae and lungworm, in yearling crossbred beef heifers during late spring. The calves were grazed on contaminated pasture for 10 weeks and then held under conditions free of nematode infection for 3 weeks prior to allotment and treatment on 5 June. Thirteen calves were randomly assigned to two groups of six by restricted randomization on body weights; the extra lightest calf was assigned to the non-treated control group. Group 1 calves were treated with abamectin at 200 micrograms kg-1 body weight by s.c. injection and Group 2 calves were not treated; all were killed at 14 days after treatment. Ostertagia ostertagi was present in all controls; arithmetic mean numbers of adults, developing fourth stage larvae (L4) and inhibited EL4 were 7683, 605 and 36,102, respectively. Other nematode genera present in controls in sufficient numbers for the experiment were Haemonchus placei adults, Trichostrongylus axei adults, Cooperia spp. adults, Oesophagostomum radiatum adults, Bunostomum phlebotomum adults, Dictyocaulus viviparus adults and E5 (immature adults). Abamectin was highly effective (consistently greater than 99% efficacy and P less than 0.05) in removing all nematodes present in treated calves as represented in non-treated controls, including the primary target of Ostertagia ostertagi inhibited EL4. The lowest efficacy was 93.8%, against D. viviparus E5.

Studies on an Ostertagia ostertagi strain suspected to be resistant to benzimidazoles

Four calves were infected with a susceptible (laboratory) strain of Ostertagia ostertagi and four with a field strain suspected to be resistant to benzimidazoles. After 25 days two calves from each group were treated with 3.5 mg kg-1 fenbendazole. Egg output fell to zero in all treated calves. Treated calves did not harbour worms at slaughter 35 days after infection. Significant differences between the strains were shown for ED50 values for thiabendazole in the egg hatch assay, but not in the tubulin binding assay. It is concluded that benzimidazole resistance in the suspected strain cannot be confirmed.

Further studies with a strain of Ostertagia ostertagi resistant to morantel tartrate

Two experiments with a morantel resistant strain of Ostertagia ostertagi were carried out. In the first experiment five groups of five calves were infected with 60,000 larvae of this resistant strain. Calves of one group remained untreated, calves of the other groups were treated with morantel tartrate, oxfendazole, levamisole or ivermectin in the recommended doses. It was demonstrated that there was side resistance to levamisole, but not to oxfendazole or ivermectin. Compared with the untreated controls the reduction percentages of the worm burdens were 45.3 (morantel tartrate), 99.7 (oxfendazole), 83.5 (levamisole) and 100 (ivermectin). In the second experiment a comparison was made between the effect of levamisole against a morantel susceptible strain of O. ostertagi and the resistant strain. Two groups of five calves were infected with the susceptible strain and two groups with the resistant one. One group of each pair remained untreated, the other was treated with levamisole. The reduction percentages of the worm burdens were 99.6 (susceptible strain) and 63.6 (resistant strain). This result confirms the efficacy of levamisole to susceptible O. ostertagi and the side resistance of the morantel resistant strain.

Efficacy of albendazole, levamisole and fenbendazole against gastrointestinal nematodes of cattle, with emphasis on inhibited early fourth stage Ostertagia ostertagi larvae

An experiment was conducted to compare the anthelmintic efficacy of albendazole (ABZ), levamisole (LEV) and fenbendazole (FBZ) against inhibited early fourth stage larvae (EL4) of Ostertagia ostertagi during April in Louisiana. Forty cross-bred beef heifers (average weight 165 kg) were acquired during winter and grazed on pastures contaminated with O. ostertagi and other nematodes until early April. The cattle were weighed and randomly allotted into four groups of ten animals on 6 April and treatments were administered on 7 April. Experimental groupings were: Group 1, non-treated controls; Group 2, ABZ by oral drench at 10 mg kg-1; Group 3, LEV by topical, pour-on administration to back midline at 10 mg kg-1; Group 4, FBZ by oral drench at 10 mg kg-1. Equal numbers of cattle from each group were slaughtered daily between 10 and 13 days after treatment. Mean numbers of O. ostertagi developmental stages present in untreated controls were: adults, 13,714; developing L4 (DL4), 6487; inhibited EL4, 21,719. The mean percentage of inhibited EL4 was 51.8. Smaller numbers of Haemonchus placei, Trichostrongylus axei and Cooperia spp. were recovered uniformly in control cattle. Percentage reduction values for the three compounds against O. ostertagi adults, DL4 and EL4, respectively, were: ABZ, 99.0, 95.3, 84.9; LEV, 1.0, 21.8, 32.1; FBZ, 99.2, 97.2, 97.5. Differences between ABZ and LEV EL4 counts were not significant, but in all other cases worm counts in ABZ and FBZ-treated cattle were significantly lower (P less than or equal to 0.05) than in LEV-treated and control cattle. Both ABZ and FBZ were 98-100% effective against Haemonchus adults and L4, T. axei adults, and Cooperia spp. adults and L4. LEV was 100% effective against Haemonchus adults and L4, 85.6% against T. axei, and 94.6% and 89.59% effective against Cooperia spp. adults and L4, respectively.