Field studies investigating anthelmintic resistance in young cattle on five farms in New Zealand

Simultaneous resistance to multiple anthelmintic classes in nematode parasites of cattle in New Zealand

Resistance to the benzimidazole and macrocyclic lactone anthelmintics is widespread in Cooperia spp. on cattle farms in New Zealand. Since this was first documented in 2006 little has changed in cattle farming systems except for the widespread use of levamisole to control Cooperia spp. in young cattle (i.e., parasite control has maintained an almost total reliance on use of anthelmintics). Here we report the emergence of simultaneous resistance to the benzimidazole, macrocyclic lactone and levamisole anthelmintics in Cooperia spp. and in Ostertagia spp. Anthelmintic efficacy against nematode parasites of cattle was investigated on four commercial farms following reports of poor animal growth rates and welfare, and positive faecal egg counts, despite routine treatment with combination anthelmintics, which included levamisole. Faecal egg count reduction tests involved 15 animals per treatment group, individual egg counts (paired samples) conducted pre- and post-treatment, with eggs counted to ≤ 15 eggs per g faeces and larval cultures for morphological identification. Actives tested varied between farms but always included levamisole alone and several combination products containing levamisole. Of the 20 tests conducted (i.e., 5 products on each of 4 farms) only 3 exceeded 90% efficacy against Cooperia spp. even though 8 of the products tested were combinations containing levamisole and at least one other broad-spectrum anthelmintic. Levamisole used alone achieved efficacies between 44% and 71% against Cooperia spp. across the four trials. The only product to exceed 95% efficacy against Cooperia spp. was a combination of monepantel + abamectin which was 100% effective against all parasites. Resistance to oxfendazole in Ostertagia spp. was indicated on 3 farms, while on one farm efficacy of all the tested products was ≤75% against this parasite. All the farms involved in this study were farming intensive cattle operations with an almost total reliance on anthelmintics to control parasitism. The results clearly demonstrate the emergence of simultaneous resistance to oxfendazole, levamisole and the macrocyclic lactone anthelmintics. Despite years of advice and recommendations to change farming practices away from intensive monocultures, many farmers have continued with the practice, and some are now faced with the very real possibility of being unable to control cattle parasites on their farms.

Efficacy of a fixed-dose combination injectable (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) in New Zealand cattle against naturally acquired gastrointestinal nematode populations with demonstrated resistance to doramectin

Intensive farming practices and heavy reliance on anthelmintics have contributed significantly to the problem of macrocyclic lactone (ML) resistance in New Zealand. Farmers now have few options for effectively controlling cattle gastrointestinal nematodes (GINs) and regularly experience sub-optimal efficacy against economically important species. We present a novel fixed-dose combination injectable (FDCI) that simultaneously delivers 0.2 mg/kg doramectin and 6 mg/kg levamisole hydrochloride (HCl) to target a broad spectrum of cattle GINs in a single dose, providing an additional solution to endoparasite control in an environment of anthelmintic resistance. A dose confirmation study was conducted using naturally acquired infections of GINs in beef cattle in New Zealand. Cattle with GIN infections confirmed by fecal egg count (FEC) were randomly allocated (n = 12 per group) to the control (saline-treated), FDCI-treated or doramectin-treated group. On Day 0, cattle were weighed and administered a single subcutaneous injection of saline or endectocide. Rectal fecal samples were collected from each animal on Day 7 for individual duplicate fecal egg count (FEC) analysis, and coprocultures were conducted on pooled fecal samples within each treatment group. All animals were euthanized and necropsied for worm recovery on Days 14 through 16. Treatment efficacy was calculated based on reduction in FECs and worm burdens. All enrolled cattle were positive for GINs based on Day -5 FECs, with geometric mean (GM) FECs ranging from 337 to 521 eggs per gram (EPG). All saline-treated cattle remained positive for GIN infections for the study duration (Day 7 GM FEC = 427 EPG). Necropsy and worm recoveries revealed the presence of doramectin-resistant Cooperia oncophora, C. surnabada and Trichostrongylus longispicularis, as evidenced by ≤ 72.3 % efficacy of doramectin against these species. The new FDCI was ≥ 99.8 % effective against all GIN species, including ML-resistant C. oncophora, C. surnabada and T. longispicularis, providing broad-spectrum efficacy and eliminating economically important cattle GINs, including ML-resistant populations.

Comparative growth performance of backgrounded beef heifers treated with an injectable fixed-dose combination (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) or single-active (0.2 mg/kg ivermectin) endectocide

Gastrointestinal nematodes (GINs) can negatively impact all production classes of cattle, particularly growing cattle. A global decline in efficacy of broad-spectrum single-active anthelmintics requires alternative GIN control methods without the aid of novel drug classes. Here, we present a new fixed-dose combination injectable (FDCI) endectocide for cattle that combines doramectin (5 mg/ml) and levamisole hydrochloride (150 mg/ml). A 56-day comparative performance confinement backgrounding trial was conducted in stocker beef heifers (n = 1548) with confirmed GIN infections to (1) compare the Day 14 post-treatment effectiveness of the new FDCI endectocide to pen mates treated with the injectable single-active endectocide ivermectin, as evidenced by fecal egg counts (FECs) conducted for a randomly selected subset (10%) of both treatment groups, and (2) determine if the greater GIN control by the FDCI evidenced in the subsample improved growth performance in all FDCI-treated heifers. Heifers were procured in four cohorts, with a 10-week timeframe between enrollment of the first and last cohort. Treatment groups were comingled within dirt-floor pens (n = 31; 7-8 per cohort) and offered a standard backgrounding diet ad libitum for the study duration. Heifers with enrollment FEC ≥ 30 eggs per gram (EPG) were randomly allocated to receive the FDCI (n = 773) or ivermectin (n = 775) on Day 0. Day 0 FECs conducted on 10% of enrolled heifers (FDCI, n = 78; ivermectin, n = 79) were not different between treatment groups (p = 0.491). Day 14 FECs for the same heifers were reduced compared to Day 0 within each treatment group. Heifers given the FDCI had lower Day 14 AM FECs and higher FEC reduction test (FECRT) result (0.07 EPG; 0.999) than ivermectin-treated heifers (21.58 EPG; FECRT = 0.850). Mean body weight (BW) was not different between treatment groups on Day 0 (p = 0.2762) and Day 14 (p = 0.2010) but was significantly greater (p = 0.0007) for FDCI-treated heifers compared to ivermectin-treated heifers on Day 56. Compared to ivermectin-treated heifers, overall average daily gain from all evaluation periods (Day 0-14, Day 14-56, and Day 0-56) was greater (p ≤ 0.0052) in FDCI-treated heifers, and FDCI-treated heifers had 4.223 kg greater total weight gain over the 56-day study. The FDCI (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) was highly effective in reducing GIN infections and thus promoted improved growth performance in beef heifers over a 56-day backgrounding period.

Reproductive and margin of safety of a fixed-dose combination injectable endectocide (0.2 mg/kg doramectin; 6.0 mg/kg levamisole hydrochloride) in cattle

We present a fixed-dose combination injectable (FDCI) solution for cattle formulated for a single subcutaneous administration at a dose rate of 1 ml/25 kg of body weight to deliver a dose of 0.2 mg/kg of doramectin and 6.0 mg/kg of levamisole hydrochloride (5.1 mg/kg base equivalent). This drug product is marketed in the United States under the tradename Valcor® and in Australia and New Zealand under the tradename Dectomax V®. Both levamisole and doramectin have histories of safe and effective use in ruminants, with safety margins of 3X and 25X, respectively. Three studies were conducted to demonstrate the safety of the new FDCI: margin of safety (Study 1), and reproductive safety in sexually nulliparous beef heifers (Studies 2 and 3). In Study 1, 3-month-old sexually intact male and female calves were given either saline (control) or 1X, 2X, or 3X FDCI on Days 0, 14, and 28. General health, clinical, and neurological observations were made throughout the study, and clinical and pathology evaluations were made at study end. Studies 2 and 3 demonstrated the reproductive safety of the FDCI on sexually nulliparous beef heifers using estrus synchronization and timed artificial insemination. Treatments of either saline (control) or 3X FDCI were administered to coincide with either folliculogenesis, implantation, organogenesis, early gestation, or late gestation. Reproductive safety was demonstrated by evaluating rates of conception, calving, abortion, and stillbirth, dystocia scores, and calf health. In all studies, the FDCI at 1X, 2X, or 3X dosages was well tolerated. In the margin of safety study, 3X calves showed increased incidence of salivation for up to 8 h post-dosing compared to other groups. Injection sites were palpable post-dosing in all three FDCI groups but resolved by Day 28 in all but one animal each in 2X and 3X. In the reproductive safety studies, the FDCI had no effect on conception, pregnancy, fetal development, or postnatal viability. Injection site swelling was increased in frequency and duration compared to controls. The studies demonstrate the safety of the new FDCI in cattle from 3 months of age and in reproducing heifers during all reproductive stages from folliculogenesis through gestation and up to a month post-partum.

Efficacy of a new fixed-dose combination injectable (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) in Australian cattle against artificial infections of gastrointestinal nematodes

We describe a new fixed-dose combination injectable (FDCI) formulated with doramectin and levamisole hydrochloride (HCl) to target broad and overlapping spectra of gastrointestinal nematodes (GINs) through two distinct modes of action. Here, we demonstrate the superior efficacy of the FDCI against mixed populations of cattle GINs in two dose confirmation studies conducted in Australia using artificially induced adult (Study 1) and immature (Study 2) GIN infections. Artificial infections consisted of Cooperia spp., Haemonchus placei, Ostertagia ostertagi, and Trichostrongylus axei. In both studies, cattle were inoculated with third-stage larvae and infections were confirmed by fecal egg count (FEC). Treatment groups in both studies were as follows: (1) negative control (saline, 0.9% sodium chloride), (2) positive control injectable endectocide (Study 1-0.2 mg/kg ivermectin; Study 2-0.2 mg/kg doramectin), (3) positive control injectable anthelmintic (7.5 mg/kg levamisole HCl), and (4) FDCI (0.2 mg/kg doramectin + 6.0 mg/kg levamisole HCl). Cattle were treated either 28 days (Study 1) or 6 days (Study 2) post-infection. On Days 14-16 (Study 1) or Days 20-21 (Study 2) post-treatment, cattle were euthanized and necropsied for the recovery, identification, and enumeration of worms. Treatment efficacy was calculated as reduction in worm burdens of treated cattle compared to saline-treated cattle, and treatments were considered effective if the geometric mean worm burden in the treatment group was reduced by ≥ 95% compared to the negative control group. In both studies, saline-treated cattle remained positive for GIN infections for the study duration. Ivermectin was less than 95% effective against Cooperia spp. (80.2%) and H. placei (24.8%) in Study 1, and levamisole HCl was less than 95% effective against Ostertagia spp. (47.1%) in Study 2. In contrast, the novel FDCI was 100% effective in treating adult and immature life stages of all cattle GINs included in the artificial infections, with no worms recovered at necropsy from doramectin + levamisole HCl-treated cattle. These data show a single administration of the FDCI provides broad-spectrum treatment of economically important cattle GINs.

Dose confirmation of a novel fixed dose combination injectable (0.2 mg/kg doramectin + 6.0 mg/kg levamisole hydrochloride) against naturally acquired gastrointestinal nematodes in US cattle

Macrocyclic lactone (ML) resistance in cattle gastrointestinal nematodes (GINs) is an increasing problem. Concurrent combination anthelmintic therapy incorporating an existing ML with a second drug class has been proposed to control cattle GINs while slowing the development of ML resistance. Two dose confirmation studies were conducted to investigate the efficacy of a new fixed-dose combination injectable (FDCI) anthelmintic against common cattle GINs known to negatively impact production. The FDCI is formulated with 5 mg/ml doramectin and 150 mg/ml levamisole hydrochloride (HCl). Cattle enrolled in the two studies were sourced from either the Southern (Study 1, n = 30) or Midwest (Study 2, n = 36) United States. Animals with GIN infections confirmed by fecal egg count (FEC) were randomly allocated to one of three treatment groups. On Day 0, cattle with positive FECs on Day -5( ± 2) were weighed and administered a single subcutaneous injection of either saline (0.9% sodium chloride) at 0.04 ml/kg, 10 mg/ml doramectin at 0.02 ml/kg (to provide 0.2 mg/kg doramectin) or the FDCI at 0.04 ml/kg (to provide 0.2 mg/kg doramectin and 6.0 mg/kg levamisole HCl). On Day 14, fecal samples were collected, animals were euthanized, and worms were collected from the intestinal tract of each animal. Treatment efficacy was calculated using worm burdens and the fecal egg count reduction test (FECRT). Pre-treatment (Day -5, Study 1; Day -3, Study 2) mean FECs were 999.4-1136.2 eggs per gram (EPG) in Study 1 and 137.1-226.6 EPG in Study 2. The FDCI was active against cattle GIN populations in both studies, with FECRT ≥ 99.98% in both studies. Compared to saline-treated cattle, FDCI-treated cattle had significantly fewer adult and immature worms of all identified species on Day 14. In Study 1, Day 14 efficacy of the FDCI was 96.9% for Cooperia spp. (C. oncophora (99.7%) and C. punctata (95.9%)), 99.1% for Nematodirus helvetianus, and 99.8% for Ostertagia spp. In Study 2, the FDCI provided 100% efficacy against all adult GIN species identified, including all GINs identified in Study 1 and Trichostrongylus axei. The FDCI also provided 95.5% efficacy against immature Ostertagia spp. and 100% efficacy against immature Cooperia spp. (Study 2). Doramectin was effective against all adult cattle GINs (except N. helvetianus) in Study 2 but was only effective against adult Ostertagia spp. in Study 1. Additionally, doramectin was only effective against immature Cooperia spp. (and not immature Ostertagia spp.) in Study 2. A single administration of the doramectin + levamisole HCl FDCI provides a new and effective approach to the treatment and control of common cattle GINs, including those exhibiting decreased susceptibility to doramectin alone.

The Economic Impact of Parasitism from Nematodes, Trematodes and Ticks on Beef Cattle Production

Global human population growth requires the consumption of more meat such as beef to meet human needs for protein intake. Cattle parasites are a constant and serious threat to the development of the beef cattle industry. Studies have shown that parasites not only reduce the performance of beef cattle, but also negatively affect the profitability of beef agriculture and have many other impacts, including contributing to the production of greenhouse gases. In addition, some zoonotic parasitic diseases may also threaten human health. Therefore, ongoing cattle parasite research is crucial for continual parasite control and the development of the beef cattle industry. Parasitism challenges profitable beef production by reducing feed efficiency, immune function, reproductive efficiency, liveweight, milk yield, calf yield and carcass weight, and leads to liver condemnations and disease transmission. Globally, beef cattle producers incur billions (US$) in losses due to parasitism annually, with gastrointestinal nematodes (GIN) and cattle ticks causing the greatest economic impact. The enormity of losses justifies parasitic control measures to protect profits and improve animal welfare. Geographical differences in production environment, management practices, climate, cattle age and genotype, parasite epidemiology and susceptibility to chemotherapies necessitate control methods customized for each farm. Appropriate use of anthelmintics, endectocides and acaricides have widely been shown to result in net positive return on investment. Implementing strategic parasite control measures, with thorough knowledge of parasite risk, prevalence, parasiticide resistance profiles and prices can result in positive economic returns for beef cattle farmers in all sectors.

Establishment of Cooperia oncophora in calves

The establishment rate of Cooperia oncophora related to host age and previous infection was investigated in young calves. Calves of similar age were kept on a feed pad and allocated into multiple groups, based on their age and weight. Two groups (each n = 16) received trickle infections with an ivermectin-susceptible C. oncophora isolate of 2000 or 10,000 infective stage larvae per week while another group (n = 16) was kept as an uninfected control. At intervals over a period of 11 months, two animals from each group were challenged with 15,000 infective stage larvae of an ivermectin-resistant isolate, 25 days later orally treated with ivermectin and 5 days after that slaughtered for worm counts. On three occasions additional calves (n = 2), subjected to the high trickle infection rate, received an ivermectin treatment to remove the existing worm burden, prior to challenge as above. Further calves (n = 4) of similar age were introduced at the beginning and the end of the experiment to determine the effect of larval age on establishment rate. The establishment in the two trickle infection groups declined to <10% within the first three months, which was significantly different from the control group. In the animals receiving the high trickle infection, but an anthelmintic treatment before challenge the establishment rate was not significantly different from the controls. Over the duration of the experiment establishment in the control group declined from 53% to <20%, which was similar to the decrease recorded at the beginning and the end of the experiment in the animals to determine the effect of larval age. The findings indicate that an existing C. oncophora burden had a strong effect on the establishment of incoming larvae in the trickle infected groups, but this was not observed if the existing burden was removed before the final challenge. The decline in establishment rate in the control group was attributed to the age of the larvae and not the age of the calves per se.

The effect on liveweight gain of using anthelmintics with incomplete efficacy against resistant Cooperia oncophora in cattle

A replicated field trial was conducted to measure the effect on liveweight gain of failing to adequately control anthelmintic resistant populations of Cooperia oncophora and to determine whether populations, and hence production losses, increased with time. Eight mobs of 10 Friesian-Hereford calves were run on independent farmlets from January to December, over each of two years. All mobs were routinely treated with a pour-on formulation of eprinomectin every six weeks, which controlled parasites other than Cooperia. Four mobs also received six weekly treatments with an oral levamisole plus albendazole combination anthelmintic to control Cooperia. Liveweights, condition scores, faecal egg counts and larval numbers on pasture were measured throughout. In the first year animals treated with eprinomectin alone were 12.9 kg lighter in November than those treated with eprinomectin plus albendazole and levamisole, however, in the second year there was no difference between the treatment groups. The data, therefore, support the view that while C. oncophora is less pathogenic than other cattle parasite species it can still cause production losses when present in sufficient numbers. In the first year of the study, parasite load, as measured by faecal nematode egg count and larval numbers on herbage, tended to be higher and calf growth rates lower than in the second year. In both years, counts of infective larvae on herbage declined over winter-spring to be at low levels before mid-summer. This suggests that the carry-over of infection from one crop of calves to the next was relatively small and hence that the level of challenge to the young calves at the start of each year was largely due to the effectiveness of the quarantine treatments administered when the animals arrived on the trial site. Low survival of larvae on pasture between grazing seasons, resulting in small larval populations on pasture when drenching programmes start each summer, might help to explain the widespread development of anthelmintic resistance in this parasite under New Zealand grazing systems.

The effect of mid-lactation treatment with topically applied eprinomectin on milk production in nine New Zealand dairy farms

The New Zealand dairy industry has shown little interest in exploring the impact of gastrointestinal parasitism on productivity in adult dairy cattle and as a consequence there has been little research completed. A randomised clinical trial was conducted to evaluate the effect of mid-lactation anthelmintic treatment on milk production under the extensive New Zealand pastoral dairying system. A random sample of cows from nine commercial dairy herds in the Manawatu region of New Zealand was treated once with pour-on eprinomectin at the dose rate of 0.5mg/kg during the afternoon milking in the period 6 Jan 2013 to 24 Jan 2013. The treatment cows (n=540) were matched to control cows (n=540) within their respective herds and the change in milk production, recorded at the first two herd tests after treatment, was analysed using a linear mixed effects model. The study found a small but significant increase in energy corrected milk production of 0.35kg/day (95% CI 0.05-0.64) at the first and second herd tests after treatment. There were no significant interactions found between treatment and age or between treatment and the bulk tank Ostertagia antibody ELISA ODR indicating that the beneficial response to eprinomectin treatment appeared independent of age of animal or ODR result. A concurrent North Island drought may have affected the response to treatment. Even so, the small effect of eprinomectin treatment on milk production found in this trial is, we believe, not sufficient to advocate whole herd treatment of dairy cows in New Zealand.

Immunogenomics of gastrointestinal nematode infection in ruminants - breeding for resistance to produce food sustainably and safely

Gastrointestinal nematode (GIN) infection of ruminants represents a major health and welfare challenge for livestock producers worldwide. The emergence of anthelmintic resistance in important GIN species and the associated animal welfare concerns have stimulated interest in the development of alternative and more sustainable strategies aimed at the effective management of the impact of GINs. These integrative strategies include selective breeding using genetic/genomic tools, grazing management, biological control, nutritional supplementation, vaccination and targeted selective treatment. In this review, the logic of selecting for "resistance" to GIN infection as opposed to "resilience" or "tolerance" is discussed. This is followed by a review of the potential application of immunogenomics to genetic selection for animals that have the capacity to withstand the impact of GIN infection. Advances in relevant genomic technologies are highlighted together with how these tools can be advanced to support the integration of immunogenomic information into ruminant breeding programmes.

Confirmation of ivermectin resistance in Ostertagia ostertagi in cattle in New Zealand

Six suspected cases of ivermectin resistance in Ostertagia spp. in cattle were investigated after routine anthelmintic efficacy testing on commercial farms. On four farms a comprehensive faecal egg count reduction test (FECRT) was undertaken using oral formulations of ivermectin (0.2mg/kg), albendazole (10mg/kg) and levamisole (7.5mg/kg) while on two farms only ivermectin was tested. The proportions of Ostertagia spp. in the untreated control and post-treatment larval cultures were used to apportion egg counts to genera and determine efficacy against this genus. Isolates of Ostertagia spp. recovered from three of the farms were each used to infect 18 six month old calves. The efficacy of oral formulations of ivermectin and moxidectin, both at 0.2mg/kg, was determined against each isolate by slaughter and worm count. The efficacy of ivermectin against Ostertagia spp., based on differentiated FECRT for each of the farms varied from 0% to 88%. The efficacy of ivermectin based on worm counts in the slaughter trial varied from 13% to 75% but moxidectin was >99% effective against all isolates. In addition, in the FECRT albendazole, at a dose rate of 10mg/kg, failed to achieve 95% efficacy against Ostertagia spp. on two farms (82% and 85%). Levamisole consistently failed to achieve 95% efficacy against Ostertagia spp. which is consistent with its known lesser efficacy against this parasite. These results confirm the presence of macrocyclic lactone resistant O. ostertagi in cattle in New Zealand and the likely presence of dual resistance, to macrocyclic lactones and albendazole, in some isolates. Resistant populations of this highly pathogenic parasite are probably not uncommon in New Zealand and pose a significant threat to animal production and welfare in the future.

Reduced efficacy of ivermectin treatments in gastrointestinal nematode infections of grazing cattle in Japan

Fecal egg count reduction tests (FECRT) and larval migration inhibition tests (LMIT) were conducted to assess the efficacy of ivermectin (IVM) against gastrointestinal nematodes on 2 cattle farms in northern Japan in 2009 and 2010. Twelve to 20 calves on each farm were treated topically with 0.5 mg IVM/kg 2 (Farm 2) or 4 times (Farm 1) during the grazing season (May–October). On Farm 1, fecal egg count (FEC) reduction at 14 days post-treatment ranged from 16 to 87% in 2009 and from 24 to 96% in 2010, with relatively low reductions in August and October (16–53%). Conversely, IVM treatment on Farm 2 reduced FEC by 97% in September 2009. Larvae obtained from fecal cultures and identified by PCR-RFLP analysis revealed that the dominant species on both farms prior to IVM administration was Cooperia oncophora. In 2009, the FEC reduction of C. oncophora on Farm 1 decreased from 85% in May to 56% in August. In 2010, the reduction in C. oncophora in August was 28%. In the LMIT using larvae collected from the fecal cultures on Farm 1 in May and August 2009, the EC₅₀ value of IVM in C. oncophora in August (0.892 µg/ml) was 3 times higher than that in May (0.296 µg/ml). The results of the LMIT corroborated the FECRT data, indicating the presence of IVM-resistant C. oncophora on Farm 1, at least in August. This is the first report of IVM-resistant nematodes in Japanese cattle.

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

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-levamisole 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 impression 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.

Anthelmintic resistance in New Zealand: A perspective on recent findings and options for the future

A recent national survey on anthelmintic resistance in cattle and sheep in New Zealand indicated that the magnitude of the problem has increased from very low levels only a few years ago to disturbingly high levels now. There is a particular problem with multiple resistance to all three action families of anthelmintic currently available in Ostertagia (= Teladorsagia) spp in sheep, and to both macrocyclic lactones (ML) and benzimidazoles in Cooperia spp in cattle. The prevalence and extent of resistance indicate that all cattle farmers and most sheep farmers should now be using a combination anthelmintic on most occasions just to achieve effective control of all parasites. Despite this, the presence of resistant parasites has generally not been appreciated by the majority of affected farmers, possibly because most have not formally tested to determine the resistance status of nematodes on their farms. Anthelmintics will remain the cornerstone of gastrointestinal nematode control in sheep and cattle for the foreseeable future but to ensure their continued effectiveness farmers need to be constantly aware of the need to maintain adequate reservoirs of unselected nematodes, i.e. worms in refugia, to minimise the expansion of the resistant population. High-risk practices in relation to selection of resistance need to be identified and avoided or at least their use limited. These include: treating adult animals where there is no identified need, moving newly treated animals onto 'clean' pasture, and failing to effectively quarantine-drench bought-in animals. None of these are new concepts but many have not been adopted or practised. In particular, sheep farmers should endeavour to avoid treating ewes pre-lambing with long-acting anthelmintics. Farmers needs to negotiate a balance between achieving good parasite control and the sustainability of their control options.

Anthelmintic resistance in nematode parasites of cattle: a global issue?

Acceptable performance of grazing cattle frequently depends on the availability of effective broad-spectrum anthelmintics to remove, or prevent infection with, gastrointestinal nematodes. This control is increasingly threatened by populations of nematodes resistant to the most commonly used anthelmintics. Although this appears to have developed more slowly than in nematodes infecting small ruminants, the number of reports in the literature over the past five years suggests a rapidly escalating problem. This review discusses this literature, several issues unique to cattle parasitism and anthelmintics, and how previous research in small ruminants can improve the management of anthelmintic resistance in cattle.