Dm Leathwick

Prevalence of anthelmintic resistance on 62 beef cattle farms in the North Island of New Zealand

Abstract AIM: To establish the prevalence of anthelmintic resistance in parasitic nematodes on a random sample of beef cattle herds in the North Island of New Zealand. METHODS: A cross-sectional prevalence study was conducted using a standardised faecal nematode egg count (FEC) reduction (FECR) test (FECRT) for ivermectin, levamisole and albendazole on 60 calves on each of 62 farms in the North Island chosen at random from farms that conformed with the selection criteria. Resistance to an anthelmintic was inferred when there was <95% reduction in FEC 7-10 days after treatment. Larval cultures were performed for all control groups and for treated groups for which resistance was evident. RESULTS: Of the farms that completed the FECRT, 4/61 (7%) showed ≥95% reduction in FEC for all anthelmintics tested. Resistance to ivermectin was evident on 56/61 (92%) farms, to albendazole on 47/62 (76%) farms, and to both ivermectin and albendazole on 45/61 (74%) farms. Resistance to levamisole was evident on only 4/62 (6%) farms. The parasites most prevalent in resistant populations cultured were Cooperia spp. On 45/61 (74%) farms where Cooperia spp were present in suffi cient numbers, resistance to both ivermectin and albendazole was evident. No cases of levamisole-resistant Cooperia spp were detected. Resistance of Ostertagia spp to ivermectin was evident on 4/45 (9%) farms, to albendazole on 15/46 (35%) farms, and to levamisole on 4/46 (9%) farms. CONCLUSION: Anthelmintic resistance in parasitic nematodes of cattle is common in the North Island of New Zealand. Beef farmers need to be aware of the risks posed by anthelmintic resistance, and routine FECR testing is recommended to ensure optimal productivity and to guide decision-making when purchasing anthelmintics to be used on-farm.

Prevalence of anthelmintic resistance on sheep farms in New Zealand

Abstract AIM: To establish the prevalence of anthelmintic resistance in parasitic nematodes on sheep farms in New Zealand. METHODS: A cross-sectional prevalence study was conducted, using a standardised faecal nematode egg count (FEC) reduction (FECR) test (FECRT) for ivermectin, at a full (0.2 mg/kg) and half (0.1 mg/kg) dose rate, and albendazole, levamisole and albendazole-levamisole in combination, on 60 lambs (n=10 per group) on farms selected from throughout New Zealand. Farms that conformed with selection criteria were chosen at random (n=80) or with a history of suspected resistance to macrocy- clic lactone (ML) anthelmintics (n=32). Resistance to an an- thelmintic was inferred when there was <95% reduction in FEC 7-10 days after treatment. Larval cultures were performed for all control groups and for treated groups for which resistance was evident. RESULTS: Of the farms randomly selected, 36% showed ≥95% FECR for all anthelmintics tested; resistance to ivermectin at 0.1 and 0.2 mg/kg liveweight was evident on 36% and 25% of these farms, respectively. Resistance to both ivermectin (0.2 mg/kg) and levamisole was evident on 8/80 (10%) farms, to ivermectin and albendazole on 10/80 (13%) farms, and to iver- mectin, levamisole and albendazole on 6/80 (8%) farms. The prevalence of resistance to a half dose of ivermectin tended to be more prevalent on farms with a history of suspected ML resistance (p=0.06). Resistance to albendazole was seen across all the main parasite genera, and to levamisole in Nematodirus, Ostertagia (= Teladorsagia) and Trichostrongylus species. Resistance to ivermectin was dominated by Ostertagia spp, although Cooperia, Nematodirus and Trichostrongylus species were also implicated. CONCLUSION: Anthelmintic resistance in parasitic nema-todes of sheep is common in New Zealand. Not only was resistance to albendazole and levamisole common, but resistance to the ML, ivermectin, was at a higher prevalence than expected. Sheep farmers and advisors in New Zealand need to re-evaluate the way they manage parasites, and more research is urgently needed if the steady decline in anthelmintic susceptibility is to be halted.

Anthelmintic resistance in New Zealand

Abstract Anthelmintic resistance was first confirmed in New Zealand in 1979 and since then has become common-place; more than 50 % of sheep farms now have detectable levels of resistance to one or more chemical classes of anthelmintic. Farmer drenching practices have changed little over the last 15–20 years and are clearly exerting a significant level of selection for resistance. In the absence of new chemical classes of anthelmintics, current parasite control practices will be unsustainable in the long-term. Once substantial resistance has developed, significant reversion to susceptibility is unlikely and re-introduction of failed drugs is likely to result in the rapid re-emergence of control problems. The number of anthelmintic treatments applied is not necessarily a reliable indicator of selection pressure and should not be the only factor considered in strategies for minimising the development of resistance. The relative potential of the different anthelmintics now available, particularly the long- acting products, to select for resistance varies with the way they are used and with other epidemiological and management factors; generalisations about their respective roles in the development of resistance are often unreliable. In many cases, literal extrapolation of recommendations for the management of resistance from Australia to New Zealand is unsupportable, given the differences in climate, parasite ecology and farming practices between the 2 countries. In the absence of a refuge for susceptible genotypes, as occurs when anthelmintic treatments are used as a means of generating low-contamination ‘safe’ pasture for young stock, the rapid development of resistance is likely. Anthelmintic treatments applied to animals with a high level of immunity, or which become immune while the anthelmintic is active, are likely to select for resistance faster than treatments applied to non-immune stock.

Controlling internal parasites in grazing ruminants without recourse to anthelmintics: Approaches, experiences and prospects

Interest in ways of raising stock without using anthelmintics has been stimulated by the desire for intensive grazing systems to adopt more sustainable methods of internal parasite control and by potential premiums for organic produce. This requires grazing management systems which are practicable and achieve levels of parasite control sufficient to meet realistic production objectives. This paper will summarise 3 years experience of lamb and cattle production without a recourse to anthelmintics on 2 production systems: a mixed cropping, lamb finishing, and cattle rearing unit and an all grass, mixed-stock, hill country unit in which all non-replacement lambs are sold at weaning. On both units parasite control was almost entirely dependent on integrated grazing management of sheep and cattle and strict systems of grazing management had to be rigidly applied. Acceptable productivity could be more readily achieved in sheep than cattle. Rams selected for resistance to nematodes were also used in both sheep flocks. Their influence on lamb production was equivocal. There is a need for more information on factors influencing parasite epidemiology and for consideration of strategies other than alternate grazing. The impact of pasture species on parasite epidemiology needs to be clarified. Current research indicates substantial differences between grasses in terms of parasite burdens acquired and production losses suffered by lambs grazing them. Grazing management may need to vary with pasture species. Also, specialty forage crops, particularly those containing condensed tannins (i.e. Hedysarum coronarium, Lotus pedunculatus and Lotus corniculatus) hold special promise as a means of countering parasite-induced production losses and dagginess. Using biological control of free-living larval stages and vaccination, may in the long term, also prove useful. Developing effective and acceptable systems for raising stock without using anthelmintics presents a considerable challenge, to parasitologists, as well as to plant breeders, agronomists, and farming systems researchers.

A model for nematodiasis in New Zealand Lambs : the effect of drenching regime and grazing management on the development of anthelmintic resistance

A model for nematodiasis in lambs was expanded to incorporate both the contribution of ewes to nematode epidemiology and the genetic parameters required to simulate the development of anthelmintic resistance in the nematode population. The expanded model was used to assess the impact of various drench and grazing management strategies for ewes and lambs on the rate of development of anthelmintic resistance. Three grazing management options, under a range of drenching schedules, were compared: one in which lambs and ewes were rotationally grazed as separate flocks over the same area after weaning (common grazing); a second in which lambs were grazed, after weaning, on areas from which ewes were excluded (separate grazing); and a third in which lambs were moved to safe pasture at weaning and again in early autumn (integrated control). Drenching strategies examined under the first 2 grazing options included a 5 lamb-drench preventive programme with 0, 1, 2, 3 or 4 additional lamb drenches, and 0 or 1 ewe drench treatment at either tall-docking or mating. Under the third grazing option, lambs were given either 1 or 2 drench treatments at or following each move to safe pasture and ewes 0 or 1 drench treatment at either tail-docking (i.e., 3-4 weeks after lambing) or mating. Model output suggests that drenching ewes prior to any lamb drenching programme is likely to significantly increase selection for drench resistance by pre-selecting the larval challenge to the lambs and, under some grazing systems, by reducing the diluting effect of eggs of susceptible genotypes passed by undrenched ewes. The results highlight the potential importance of undrenched ewes as a refuge for susceptible worm genotypes and indicate that on its own, drenching frequency is likely to be a poor indicator of selection pressure for resistance and thus of limited value in selecting strategies for the management of anthelmintic resistance.

A model for nematodiasis in New Zealand lambs

A strategic model is described for the epidemiology of mixed nematode infections in New Zealand lambs. The model successfully reproduces known patterns of parasite epidemiology and production loss in lambs under currently implemented control strategies. The variation in model output during sensitivity analysis was within acceptable limits defined by field data. Model output was most sensitive to variation in parameters affecting survival and migration of the free-living stages and host resistance to infection, suggesting that these factors are most influential in regulating parasite populations. It is intended to use the model to focus research on key aspects of nematode epidemiology and control and, following the incorporation of appropriate genetic mechanisms, anthelmintic resistance.

Management of gastrointestinal nematode parasites on sheep farms in New Zealand

Abstract AIM: To report current farmer opinions and farming practices relating to control of gastrointestinal nematodes and anthelmintic resistance on sheep farms in New Zealand. METHODS: An interview-based cross-sectional study of grazing management and anthelmintic useage was conducted by veterinarians on 80 randomly selected sheep farms in New Zealand. RESULTS: Useable data were returned by 74/80 (92%) farmers who participated in the study. However, despite contacting 400 farmers the target sample size of 100 farms was not reached. The results indicated that only 31% of farms had previously tested for drench resistance, that effective quarantine-drenching of imported stock was not always carried out, and that farmers were more likely to integrate cattle than ewes into their grazing management of lambs. Furthermore, the number of drenches given to lambs had changed little in 25 years. The use of faecal egg counting by farmers has increased. CONCLUSIONS: Dependence on anthelmintics continues to be high on sheep farms in New Zealand. Whilst the number of drench treatments has changed little, there is more widespread use of persistent or long-acting treatments. Farmers need to be encouraged to monitor the resistance status of nematode populations on their farms and use this information to develop strategies aimed at maintaining susceptible alleles within the parasite populations and conserving the efficacy of existing drug families.

Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand

Abstract AIM: To identify farm practices associated with the presence of resistance to a macrocyclic lactone (ML) anthelmintic on sheep farms in New Zealand. METHODS: A cross-sectional study was conducted to test for associations between the presence of resistance to an ML anthelmintic (ivermectin) and management practices on sheep farms in New Zealand. Selection of farms was both random (n=80) and purposive (n=32; being farms with a history of suspected ML resistance). Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <95% reduction in FEC 7–10 days after treatment with a half dose of ivermectin (0.1 mg/kg). A logistic regression model was built to identify farm-level factors that were associated with the presence or absence of ML resistance. RESULTS: Of the 112 flock managers that were approached for interview, 103 (92%) returned useable questionnaires. The odds of ML resistance were increased: on farms that had used long-acting ML products in ewes as a pre-lambing treatment for ≥3 of the previous 5 years (odds ratio (OR) = 7.2; 95% confidence interval (CI) = 1.7–30.3); on farms where <70% of the total stock units mid-winter were from sheep (OR=6.5; 95% CI=1.6–25.6); on farms which over the year purchased ≥10% of the number of sheep present mid-winter (OR=7.1; 95% CI=1.5–34.7); and on farms where the average wool diameter of the main flock was <37 (OR=4.1; 95% CI=1.1–14.7) microns. The model provided a good fit to the data (pseudo R2=0.64; Hosmer-Lemeshow statistic = 0.38). CONCLUSIONS: Explanatory factors identified as associated with the presence of ML (ivermectin) resistance on farms included the use of long-acting anthelmintic formulations in ewes pre-lambing, sources of refugia of unselected parasites on the farm, breed of sheep and their requirements for anthelmintic treatments, and the importing of resistant parasites with purchased stock. The study provides support for controls that aim to provide refugia of susceptible worms and that minimise the risk of introduction of resistance through effective quarantine-drenching.

Anthelmintic resistance and management of nematode parasites on beef cattle-rearing farms in the North Island of New Zealand

Abstract AIM: To provide information on current farmers’ opinions and farming practices thought to be related to anthelmintic resistance, and to test for associations between the presence of anthelmintic resistance and management practices on beef cattle- rearing farms in the North Island of New Zealand. METHODS: A study using an interview-based questionnaire about management of internal parasites was conducted on 62 beef cattle-rearing farms in the North Island of New Zealand, using case-control analyses to test for associations between management practices and the presence or absence of resistance to ivermectin or albendazole. Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <90% reduction in FEC 7-10 days after treatment of calves <12 months of age. RESULTS: Of the 59 farmers who completed the questionnaire, most (n=40) ranked parasites highly, and at about the same level as quality and quantity of feed, as important production-limiting factors for their enterprises. In contrast, anthelmintic resistance was not perceived to be a problem on 13 farms, and its importance was rated low on 24, moderate on 15, and high on only six farms. Despite all farms having planned parasite control programmes, there was heavy reliance on clinical signs of parasitism to determine frequency of treatments. About one in three farmers with beef breeder cows routinely treated their calves at marking, one in five treated mixed-age cows, and almost half treated rising 2-year-old cows before calving. One in four farmers used anthelmintics on calves on 8–12 occasions in their first year of life. Co-grazing with other species was rare, but follow-on grazing within 3 months after older cattle or sheep was common. On most farms, grazing cattle was restricted to part of the farm, a finding with implications for parasite control and persistence of larvae in refugia. Macrocyclic lactone (ML) anthelmintics or their combinations with other action families were currently, and for the past 5 years, used more frequently than benzimidazoles and levamisole, and benzimidazole-levami- sole combinations. The prevalence of resistance to ivermectin was high (82%) and no plausible model of associations could be constructed from the data. The prevalence of resistance to albendazole was 60%, and the risk of resistance increased as the number of rising 1-year-old cattle present mid-winter increased, and decreased as the number of breeding cows >2 years old present mid-winter increased. CONCLUSION: It is clear that in practice anthelmintic resistance is a secondary consideration to obtaining productivity advantages from the use of anthelmintics in beef cattle. Farmers’ opinions were divided on many issues and the overall impres- sion was of confused and diverse thinking regarding the principles of the use of anthelmintics. The overall outlook regarding anthelmintic resistance in cattle is bleak unless the need for integrated and long-term research activities is acted upon soon.

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.