Cross infection of Haemonchus contortus between calves and lambs.

Haemonchus contortus can frequently be found infecting pre-weaned beef calves on sheep and beef farms around the North Island of New Zealand. The purpose of this study was to determine whether parasites cycling in young cattle constitute a potentially important source of infection for sheep. A field isolate of H. contortus was cycled through either calves or lambs for 3 generations. The larvae resulting from the third cycle of infection were then used to infect both lambs and calves and the resulting faecal nematode egg count (FEC), worm burden, adult worm length and in utero egg count were measured. Larvae derived from lambs inoculated into calves exhibited lower establishment rates, the adult worms were shorter, had lower in utero egg counts, and the resulting faecal egg counts were also lower than when inoculated into lambs (p < 0.01). H. contortus' lack of ability to passage freely between lambs and calves indicates that large populations are unlikely to occur under mixed grazing, resulting in limited potential as a source of infection in sheep. However, indications of an ability to adapt to the alternative host suggest that some investigation of infection in cattle dominant farming operations in the north of the country might be warranted.

The production costs of Haemonchus contortus and other nematode parasites in pre-weaned beef calves in New Zealand.

Haemonchus contortus can frequently be found infecting pre-weaned beef calves on sheep and beef farms around the North Island of New Zealand. The purpose of this study was to consider whether the presence of this parasite alone, or as part of a mixed infection, could be impacting growth rates of young animals, on three commercial farms in the North Island of New Zealand. Trials were conducted on commercial sheep and beef farms in each of the Northland, King Country and Gisborne regions, in late summer/autumn (February to April) of 2016 to measure the effect of treatment with narrow and broad spectrum anthelmintics on liveweight gain of spring-born calves pre-weaning. Each farm was chosen based on the presence of Haemonchus and that it was a beef cow/calf system with the cows and calves grazing the same pastures as sheep at some stage. Three sampling visits were made to each farm with the animals being weighed, faecal sampled and treated with one of two anthelmintics (Closantel alone to remove only Haemonchus or a triple combination containing moxidectin, levamisole and oxfendazole to remove all nematodes) or left untreated, on each of the first two visits. There was no significant difference in liveweight gain between any of the treatment groups, hence there was no evidence for an impact of Haemonchus alone, or a mixed nematode infection, on pre-weaned calf growth rates on these farms. It remains unclear whether there may be a justification to consider treatment of calves should they constitute a significant source of pasture larval infestation with H. contortus, in an integrated cattle-sheep system.

Route of administration influences the concentration of ivermectin reaching nematode parasites in the gastrointestinal tract of cattle.

An animal trial was conducted to measure the concentrations of ivermectin occurring in abomasal and small intestinal contents and mucosa, and in the target parasites (Ostertagia ostertagi and Cooperia oncophora) following administration by subcutaneous, oral and pour-on routes. Twenty-five steers were infected with ivermectin-resistant isolates of O. ostertagi and C. oncophora and following patency randomly allocated to 3 treatment groups of 7 and 1 untreated control group of four. On day 0, animals in the treatment groups were administered ivermectin via the oral, injectable or pour-on routes. On days 1, 2, 3, 4, 5, 6 and 8, blood samples were collected from all live animals, one animal from each treatment group was euthanised and the abomasum and small intestine recovered. Control animals were euthanised on each of days 4, 5, 6 and 8. Samples of gastrointestinal tract organs, their contents, mucosa and parasites were collected and assayed for ivermectin concentration using HPLC. The highest plasma concentrations occurred following subcutaneous administration. In the gastrointestinal contents the highest levels occurred following oral administration, although one high value occurred following pour-on administration, which was attributed to self-licking by the treated animal. The lowest GI content levels followed subcutaneous injection. Ivermectin concentrations in the gastrointestinal mucosa were highest following subcutaneous injection. Drug levels in the abomasal parasite O. ostertagi were most closely correlated with levels in the abomasal mucosa whereas levels in the intestinal C. oncophora were most closely correlated with those in the intestinal contents. Thus, the maximun levels of drug reached C. oncophora in the small intestine following oral administration. In contrast, the highest levels of ivermectin in O. ostertagi followed subcutaneous injection. Therefore, route of administration is likely to influence the exposure to ivermectin for different parasite species.

Dealing with double trouble: Combination deworming against double-drug resistant cyathostomins.

An alternative control regimen for drug-resistant parasites is combination deworming, where two drugs with different modes of action are administered simultaneously to target the same parasite. Few studies have investigated this in equine cyathostomins. We previously reported that an oxibendazole (OBZ) and pyrantel pamoate (PYR) combination was not sustainable against a cyathostomin population with high levels of OBZ and PYR resistance. This study consisted of a field study and two computer simulations to evaluate the efficacy of a moxidectin-oxibendazole (MOX-OBZ) combination against the same cyathostomin population. In the field study, anthelmintic treatments occurred when ten horses exceeded 100 eggs per gram. Fecal egg counts and efficacy evaluations were performed every two weeks. The two simulations utilized weather data as well as equine and parasite population parameters from the field study. The first simulation repeated the treatment schedule used in the field study over a 40 year period. The second evaluated efficacies of combination treatments using selective therapy over 40 years. In the field study, efficacies of MOX and both combination treatments were 100%. The egg reappearance period for MOX was 16 weeks, and the two combination treatments were 12 and 18 weeks. The first (46.7%) and last (40.1%) OBZ efficacies were not significantly different from each other. In the simulation study, the combination treatment delayed MOX resistance development compared to when MOX was used as a single active. This occurred despite the low efficacy of OBZ. The second set of simulations identified combination treatments used with selective therapy to be the most effective at delaying MOX resistance. Overall, this study supports the use of combination treatment against drug-resistant cyathostomins, when one of the actives exhibits high efficacy, and demonstrates benefits of this approach despite substantially lowered efficacy of the other active ingredient.

Anthelmintic efficacy of single active and combination products against commonly occurring parasites in foals.

Parasite control in foals is complicated by the concurrent presence of biologically diverse parasites with differing levels of anthelmintic resistance. Several combination anthelmintic products are available for use in horses, but information on their efficacies against important equine parasites is scarce. Two trials were performed in New Zealand during 2008 and 2011 on four different farms with substantially different anthelmintic treatment histories. The first trial evaluated the efficacy of an ivermectin/praziquantel/oxibendazole combination, a single active oxibendazole, and a single-active macrocyclic lactone (ML) in 49 foals located on three farms. The second trial evaluated two combination anthelmintic products and three single-active ML products and enrolled a total of 110 foals on three farms. Foals in the second trial were allocated to one of six anthelmintic treatment groups; oxfendazole/pyrantel embonate, pyrantel embonate/ivermectin/praziquantel, ivermectin/praziquantel, abamectin/praziquantel, moxidectin/praziquantel, and a placebo-treated control. In both trials, foals were monitored monthly prior to treatment, and fecal egg counts (FECs) of Parascaris spp., strongylid, and Strongyloides westeri were determined. A "rolling enrolment" process was implemented whereby foals were systematically allocated to a treatment group and treated with the corresponding anthelmintic following the first appearance of Parascaris spp. eggs in the faeces. A generalised linear model was used to evaluate the effect of farm and treatment on Day14 FEC (ln) for each parasite. Three different FECR calculation methods were employed as follows; i) FECR(T) pre and post treatment ii) FECR (C) in the treated group compared with control, and iii) FECR (P) pre- and post- treatment in the treated and control groups. Across both trials, treatment with ML single active products failed to achieve >95% reduction in Parascaris spp. FEC on two of three farms. The pyrantel embonate/oxfendazole and ivermectin/ praziquantel/oxibendazole combinations demonstrated full efficacy against Parascaris spp. This is in contrast to the anti-strongylid efficacies determined, where the pyrantel embonate/oxfendazole combination and single active oxibendazole had reduced efficacy on one farm, while the macrocyclic lactones generally had good efficacy. Strongyloides egg counts were sporadic in both trials, and allowed limited insight into anthelmintic efficacy. The study illustrated the importance of keeping an untreated or placebo-treated control group in studies evaluating anti-Parascaris efficacy and it demonstrated the utility of a rolling enrolment procedure, where foals are enrolled over the course of a defined period of time. Furthermore, the study demonstrated the value of a farm specific FECR monitoring programme and the complexity of parasite control in foals, where combination anthelmintic products can be employed to target multiple species of parasites.

Nematode parasites in young cattle: what role for unexpected species?

AIMS To investigate the timing of infection of beef calves with sheep nematode species on three sheep and beef farms, and to determine the prevalence of cross-infection in calves before weaning across a larger number of farms. METHODS Farms in the Far North, Gisborne and Tararua districts, in the North Island of New Zealand, were enrolled in 2014. Fresh faecal samples were collected from approximately 10 calves on each farm between birth and up to 5 months after weaning. In 2016, faecal samples were collected from calves before weaning from 22 farms across the upper North Island. For both trials faecal samples were assessed for faecal nematode egg counts and cultured to determine parasite genus. For samples from the three farms, larvae were identified to species using a multiplex PCR assay. RESULTS On the three farms, the median percentage of sheep nematode species detected in faecal cultures was 25 (min 3, max 77)%. The main sheep species detected were Cooperia curticei and Haemonchus spp. (putatively contortus). In faecal samples collected before weaning from 22 farms, Haemonchus spp. were present in 19/22 samples, and the median prevalence was 15 (min 0, max 73)% of the total larvae cultured. CONCLUSIONS AND CLINICAL RELEVANCE The implications of sheep nematode species being present in calves should be considered by farmers and veterinarians when undertaking anthelmintic efficacy testing, as they may contribute to false conclusions regarding anthelmintic efficacy. Pre-weaning calves may also be a possible source of contamination and/or refugia for Haemonchus spp. on farms and should be considered when developing parasite control plans for sheep.

Production responses and cost-benefit of long-acting pre-lambing anthelmintic treatment of yearling ewes in two commercial flocks in New Zealand.

AIMS: To investigate the production responses and cost-benefit of administering a controlled-release anthelmintic capsule (CRC) to pregnant yearling ewes prior to lambing. METHODS: Yearling ewes from two commercial sheep flocks (A, n=489; B, n=248) in the North Island of New Zealand were enrolled in the study. Prior to lambing, CRC containing albendazole and abamectin were administered to half the ewes while the other half remained untreated. Ewe liveweights and body condition scores were measured prior to lambing, at weaning and, for Flock B, prior to subsequent mating. Lambs were matched to dams shortly after birth and the weight and number of lamb weaned per ewe were determined. A cost-benefit analysis was undertaken for Flock B considering the increased weight of lamb weaned per ewe, and the weight of ewes at the next mating and the benefit in terms of lambs born. RESULTS: The mean weight at weaning of treated ewes was greater for treated than untreated ewes by 2.76 (95% CI 0.64-4.88) kg in Flock A (p<0.001) and 2.35 (95% CI -0.41-5.12) kg in Flock B (p=0.003); the weight of lamb weaned per ewe was greater for treated than untreated ewes by 1.43 (95% CI -0.71 to -3.49) kg in Flock A (p=0.041) and 3.97 (95% CI 1.59-6.37) kg in Flock B (p<0.001), and ewe liveweight prior to subsequent mating was greater for treated than untreated ewes in Flock B by 4.60 (95% CI 3.6-5.6) kg (p<0.001). There was no difference in the percentage of lambs reared to weaning between treated and untreated ewes in either flock (p>0.8). The overall cost-benefit of treatment for Flock B was NZ$9.44 per treated ewe. CONCLUSIONS AND CLINICAL RELEVANCE: Pre-lambing CRC administration to yearling ewes resulted in increased ewe weaning weights and weight of lamb weaned in both the flocks studied. There was an economic benefit in the one flock where this was assessed.

The efficacy and plasma profiles of abamectin plus levamisole combination anthelmintics administered as oral and pour-on formulations to cattle.

In phase I, faecal egg count reduction tests (FECRT) were conducted on six commercial cattle farms to compare the performance of two pour-on and one oral combination anthelmintic. Groups of 12-15 calves were sampled for faecal nematode egg count (FEC) before treatment with either abamectin oral, levamisole oral, an abamectin+levamisole oral combination or one of two abamectin+levamisole combination pour-ons. Samples were collected again 14days after treatment to calculate the percentage reduction in FEC. The proportions of infective stage larvae (L3) in faecal cultures were used to apportion egg counts to, and calculate efficacy against, the main parasite genera. Abamectin oral was effective against Ostertagia except on one farm where resistance was indicated, but had reduced efficacy against Cooperia on four farms. Levamisole oral was effective against Cooperia on all farms, but had variable efficacy against Ostertagia. The abamectin+levamisole oral was effective against both species on all farms. The abamectin+levamisole pour-ons were effective on some farms but not on others. In particular, pour-on 2 failed to achieve 95% efficacy in 45% of evaluations, 4/6 against Cooperia and 1/5 against Ostertagia. On some farms the combination pour-ons were less effective than their constituent actives administered alone as orals. In phase II, 8 groups of 6 calves, grazing parasite-free pasture, were infected with putatively ML-resistant isolates of Cooperia oncophora and Ostertagia ostertagi. Once infections were patent groups were treated with oral or pour-on formulations of abamectin alone, levamisole alone, abamectin+levamisole (two pour-ons) or remained untreated. Blood samples were collected for analysis and after 8days all calves were euthanized and abomasa and intestines recovered for worm counts. All treatments were effective against O. ostertagi and all treatments containing levamisole were effective against C. oncophora. Animals treated with the oral combination had higher Cmax and AUC values for abamectin in plasma than animals treated orally with abamectin alone. In contrast, animals treated with the combination pour-ons tended to have lower plasma levels for abamectin than those treated with abamectin alone as a pour-on, with differences in the Cmax and AUC values approaching statistical significance (p-values ≤0.07). There were no differences detected in plasma concentrations of levamisole. The inconsistent and sometimes poor efficacy of the combination pour-ons on-farm is likely due to reduced levels of abamectin in the plasma and hence less active reaching the target worms in the gut.

Chilling requirements for hatching of a New Zealand isolate of Nematodirus filicollis.

The eggs of some species of the parasitic nematode Nematodirus require a period of chilling before they can hatch; N. filicollis is one such species. This study investigated this requirement for chilling in a New Zealand strain of this species. Eggs of N. filicollis were extracted from lamb's faeces and incubated at 20°C to allow development to the third stage larvae within the egg. These eggs were then placed into tissue culture plates and incubated at: 2.7°C (±0.99), 3.6°C (±0.90), 4.7°C (±0.35), 6.4°C (±0.37), 8.0°C (±1.54) or 9.9°C (±0.14) for up to 224 days. At 14day intervals until day 84, then every 28 days, one plate was removed from each temperature and placed at 13.1°C (±0.44) for 14 days. Eggs were then assessed for hatching. From this data, chill units were calculated by subtracting the culture temperature from a constant threshold of 11°C and multiplying by the number of days for which the sample was cultured; then the Gompertz model fitted. Even though hatching overall was low, a greater proportion of eggs hatched with chill accumulation. Maximum hatching of eggs required 800-1000 chill units. Consequently in the field, more than one season of chilling would be required before hatching. As such a generation time could take more than one year to complete. This is different to the hatching dynamics of N. spathiger, the other main species found in New Zealand sheep, which does not display this requirement for chilling and hatches immediately once the third stage larvae are developed.

Benzimidazole resistance in Nematodirus spathiger and N. filicollis in New Zealand.

AIM: To determine the prevalence of benzimidazole resistance in Nematodirus spathiger and N. filicollis from a sample of New Zealand farms. METHODS: The efficacy of albendazole (ABZ) against Nematodirus spp. was assessed by faecal nematode egg count reduction (FECR) tests undertaken in lambs aged 3-8 months old on 27 sheep farms throughout New Zealand. On each farm, groups of 10-16 lambs were either treated with ABZ (4.75 mg/kg) or remained as untreated controls. Faecal samples were collected from all animals at the time of treatment and 7-10 days later. Faecal nematode egg counts (FEC) were performed using a modified McMaster technique. Larvae were cultured from pooled faecal samples, collected 7-10 days after treatment from each group, by incubation at 20°C for 6 weeks, 4°C for 26 weeks then 13°C for 2 weeks. The resulting third stage larvae were identified to species using a multiplex PCR assay, that identified species-specific sequences in the second internal transcribed spacer region of ribosomal DNA. The efficacy of ABZ for N. spathiger and N. filicollis was calculated from the proportion of the two species in culture and the group mean FEC before and after treatment. Only farms with a mean of 10 epg for each species in untreated samples were included for analysis. Resistance was defined as an efficacy <95%. RESULTS: On farms that met the threshold of 10 epg in faecal samples, benzimidazole resistance was found on 20/21 (95%) farms for N. spathiger compared with 4/10 (40%) farms for N. filicollis (p<0.05). In samples collected following treatment, a mean of 83 (min 46, max 100)% of Nematodirus spp. larvae recovered from the untreated groups were N. spathiger, compared with 94 (min 45, max 100)% in the ABZ treated groups (p=0.03). This change in percentage was not influenced by the overall efficacy of treatment based on the FECR test (p=0.324). CONCLUSION: The results confirm the high level of resistance in N. spathiger in New Zealand and that benzimidazole resistance was more common in N. spathiger than N. filicollis. While resistance to benzimidazole anthelmintics has been reported previously in New Zealand, this is the first report of N. filicollis being resistant to benzimidazole anthelmintics.

A cost-benefit analysis of pre- and post-lambing anthelmintic treatments to twin-bearing ewes on commercial farms in the southern North Island of New Zealand.

AIM: To conduct a cost-benefit analysis of the administration of anthelmintics to adult ewes around lambing. METHODS: Production data from comparisons of different anthelmintic treatments with no treatment were used in a cost-benefit analysis. The data were from 14 trials (part of an experiment carried out on one farm in 1 year) conducted on sheep and beef farms (eight in 2011 and six in 2012) in the Wairarapa region of New Zealand. The cost structure involved the purchase price of products and the labour cost of administration. The four key benefits identified for the calculation of economic returns, relative to untreated ewes, were: increased value of ewes sold (culled) at weaning, additional lambs weaned related to ewe liveweight at mating, increased total weight of lamb weaned per ewe, and reduced number of ewes requiring removal of soiled wool at weaning due to a lower dag score. Commercial values for these variables as at December 2013 were used, with the measured production data, to calculate a net (NZ$) benefit for every treatment-trial combination. RESULTS: The economic return on treating ewes around lambing with anthelmintics was highly variable and across all trials treatment resulted in a financial loss in 18/38 (47%) groups of ewes. The mean net benefit from pre-lambing administration of a controlled release capsule (CRC) containing albendazole and abamectin was 5.36 (95% CI=-2.64 to 13.35) $/ewe, but overall was not different from zero (p=0.171). A breakdown of the overall gross benefit into its various components showed that weight of lamb weaned per ewe had the largest influence (a mean benefit of $5.68/ewe), followed by ewe liveweight pre-mating ($2.45/ewe), ewe liveweight at weaning ($0.66/ewe) and reduced dag score ($0.15/ewe). Other anthelmintic treatments all showed highly variable responses amongst trials, with some negative cost-benefits. There was no significant difference between any of the treatments except a short-acting oral treatment at tail-docking had a lower net benefit than a CRC containing albendazole administered pre-lambing (p<0.05). CONCLUSIONS: A positive financial return resulting from the anthelmintic treatment of adult ewes around lambing is neither consistent nor predictable, and is often not achieved. Given that the additional costs of accelerating the development of anthelmintic resistance were not included in these calculations, farmers need to consider carefully the merits of administering anthelmintics to ewes around lambing.

Production benefits from pre- and post-lambing anthelmintic treatment of ewes on commercial farms in the southern North Island of New Zealand.

AIMS: To measure the magnitude and variability in production responses to anthelmintic treatments administered to adult ewes around lambing. METHODS: Ewes carrying twin lambs, from sheep and beef farms (eight in Year 1 and six in Year 2) in the Wairarapa region of New Zealand, were enrolled in 14 trials (part of an experiment carried out on one farm in one year). Experiment 1 compared ewes treated 2-4 weeks pre-lambing with a controlled release capsule (CRC) containing abamectin, albendazole, Se and Co, to ewes injected pre-lambing with a long-acting Se plus vitamin B12 product, and to untreated ewes. Experiment 2 included these treatments, plus a CRC administered at pregnancy scanning. Experiment 3 included the same treatments as Experiment 1, plus administration of a CRC containing albendazole, Se and Co, injectable moxidectin or oral derquantel plus abamectin, all administered pre-lambing, or oral derquantel plus abamectin administered 4-6 weeks after lambing. Variables compared were ewe liveweight at weaning and pre-mating, lamb liveweight at weaning, total weight of lamb weaned per ewe and ewe dag score at weaning. RESULTS: Ewes treated with a CRC pre-lambing were heavier than untreated ewes (mean 3.2 kg) at weaning in 12/14 trials, and pre-mating (mean 2.8 kg) in 9/14 trials (p<0.001). Compared with mineral-treated ewes the mean difference was 2.8 kg pre-lambing (9/14 trials) and 1.7 kg pre-weaning (6/14 trials). Lambs reared by treated ewes were heavier (mean 1.55 kg) at weaning in 6/14 trials (p<0.001), but there was no effect of CRC treatment on total weight of lambs weaned per ewe (p=0.507). Variation in weight of lamb weaned per ewe was largely explained by differences in lamb survival from birth to weaning (p<0.001), with no effect of CRC treatment (p>0.65). Treatment of ewes with a CRC at pregnancy scanning was neither better nor worse than a pre-lambing treatment (p=0.065). There was no difference in the response from treatment with either of the two CRC or moxidectin. Treatment with short-acting oral anthelmintics resulted in no consistent benefit. CONCLUSIONS: Anthelmintic treatments administered to ewes around lambing resulted in variable responses between farms and years, which in some trials were negative for some variables, and some of the variability was due to the mineral component of the CRC. The widespread perception amongst farmers and veterinarians that anthelmintic treatment of ewes around lambing will always result in positive benefits is not supported.

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

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

A survey of the prevalence of Nematodirus spathiger and N. filicollis on farms in the North and South Islands of New Zealand.

AIM: To compare the prevalence of Nematodirus spathiger and N. filicollis on a sample of farms in the lower South Island with the lower and central North Island of New Zealand by way of a presence/absence survey. METHODS: A presence/absence non-random survey was conducted in which pooled faeces from lambs from 50 farms in the lower and central North Island (n=22) and lower South Island (n=28) were cultured, with and without a post-culture period of chilling, to produce third-stage parasitic nematode larvae (L3). After recovery using the Baermann technique, individual L3 were identified to Nematodirus spp. using a PCR assay. This identified the species that were present on each farm that were then used to compare the regions. RESULTS: Nematodirus filicollis was present in 38/50 (76%) samples from which Nematodirus spp. larvae were cultured, compared with 50/50 (100%) in the case of N. spathiger. No regional differences were observed in the prevalence of either species (p=0.74). Of the Nematodirus spp. L3 recovered from the unchilled samples, 415/428 (97%) were N. spathiger and 13/428 (3%) N. filicollis. After chilling 594/695 (85%) of the Nematodirus spp. L3 were N. spathiger and 101/695 (15%) were N. filicollis. CONCLUSION: Despite the non-random nature of the study, the evidence suggests that both these species are likely to occur sympatrically on most sheep farms throughout New Zealand. In general N. filicollis eggs did not hatch without a period of chilling and this has implications for the identification of these larvae using conventional culture methods.

Anthelmintic resistance in equine parasites--current evidence and knowledge gaps.

Anthelmintic resistance is becoming increasingly prevalent among equine nematode parasites. The first reports documenting resistance were published in the 1960s, just a short time after introduction of the first modern anthelmintics phenothiazine and thiabendazole. Several factors are known to influence development of resistance, but evidence specific to equine parasites is limited. Most current knowledge and applications have been extrapolated from research with trichostrongylid parasites of sheep. The number of cyathostomin species co-infecting horses adds to the complexity of investigating drug resistance but, given their apparent limited biological diversity, viewing these in a unispecific context remains a pragmatic approach. Factors affecting resistance development in cyathostomins include parasite seasonality, life span and fecundity, host immunity, and the existence of encysted stages. Further, parasite refugia have been shown to play a vital role in resistance development in other parasites, and likely is also important in equine parasites. Specific genetic factors for drug resistance and possible modes of inheritance have been identified for trichostrongylid nematodes, but it is widely accepted that several more remain undiscovered. Current evidence with equine and ruminant parasites suggests that fitness is not significantly compromised in drug resistant strains. Attempts to develop in vitro and molecular assays for diagnosing anthelmintic resistance in equine nematodes have had only limited success, standardized guidelines are sorely needed for performing the fecal egg count reduction test in horse populations. Taken together, this review illustrates the complexity of understanding anthelmintic resistance in equine nematodes, and emphasizes the need for further research.

The management of anthelmintic resistance in grazing ruminants in Australasia--strategies and experiences.

In many countries the presence of anthelmintic resistance in nematodes of small ruminants, and in some cases also in those infecting cattle and horses, has become the status quo rather than the exception. It is clear that consideration of anthelmintic resistance, and its management, should be an integral component of anthelmintic use regardless of country or host species. Many years of research into understanding the development and management of anthelmintic resistance in nematodes of small ruminants has resulted in an array of strategies for minimising selection for resistance and for dealing with it once it has developed. Importantly, many of these strategies are now supported by empirical science and some have been assessed and evaluated on commercial farms. In sheep the cost of resistance has been measured at about 10% of the value of the lamb at sale which means that losses due to undetected resistance far outweigh the cost of testing anthelmintic efficacy. Despite this many farmers still do not test for anthelmintic resistance on their farm. Many resistance management strategies have been developed and some of these have been tailored for specific environments and/or nematode species. However, in general, most strategies can be categorised as either; identify and mitigate high risk management practices, maintain an anthelmintic-susceptible population in refugia, choose the optimal anthelmintic (combinations and formulations), or prevent the introduction of resistant nematodes. Experiences with sheep farmers in both New Zealand and Australia indicate that acceptance and implementation of resistance management practices is relatively easy as long as the need to do so is clear and the recommended practices meet the farmer's criteria for practicality. A major difference between Australasia and many other countries is the availability and widespread acceptance of combination anthelmintics as a resistance management tool. The current situation in cattle and horses in many countries indicates a failure to learn the lessons from resistance development in small ruminants. The cattle and equine industries have, until quite recently, remained generally oblivious to the issue of anthelmintic resistance and the need to take pre-emptive action. In Australasia, as in other countries, a perception was held that resistance in cattle parasites would develop very slowly, if it developed at all. Such preconceptions are clearly incorrect and the challenge ahead for the cattle and equine industries will be to maximise the advantages for resistance management from the extensive body of research and experience gained in small ruminants.

The influence of temperature on the development and survival of the pre-infective free-living stages of nematode parasites of sheep.

AIM: To develop a model for the development dynamics of the pre-infective stages of nematode parasites on pasture and to use it to investigate the importance of temperature fluctuations on the proportion of eggs developing to infective larvae. METHODS: Data from published literature was used to derive relationships for development and survival rates in response to temperature for Teladorsagia (=Ostertagia) circumcincta, Trichostrongylus colubriformis and Haemonchus contortus. A model describing development from the egg to infective third larval stage (L3) for each species under the influence of temperature was constructed. It was assumed that moisture is never limiting. The model was initially used to estimate the proportion of eggs developing to L3 under 12-hourly temperature oscillations of different magnitude around constant temperatures of 10, 20 and 30°C. Subsequently the model was run on hourly temperature data collected from the field, and on daily and monthly means calculated from the hourly temperatures. As each set of comparisons had the same mean temperature, differences in proportional development were the result of the cumulative effects of temperature fluctuations on the development and survival of the developing stages. RESULTS: Oscillating temperatures always resulted in a lower estimated proportion of eggs developing to L3 than the constant mean, and the larger the oscillations the greater the reduction in development. Similarly, development estimated on hourly temperatures measured in the field was always lower than those estimated on daily mean temperatures calculated from the hourly values, sometimes by a large amount. Monthly mean temperatures resulted in estimates for development which were sometimes higher and sometimes lower than those based on hourly or daily means. CONCLUSIONS: Consistent accurate estimation of the proportion of eggs developing to L3 in the field will require the integration of temperatures at relatively short time intervals. This will be necessary to capture the effect of temperatures fluctuating, even for short periods, to temperature values which are less than optimal for development. The use of daily mean temperatures, or means taken over longer periods, is likely to yield inaccurate estimates of development at least some of the time.

Efficacy of oral, injectable and pour-on formulations of moxidectin against gastrointestinal nematodes in cattle in New Zealand.

The efficacy of moxidectin administered by different routes, against naturally acquired infections of gastrointestinal nematode parasites of cattle, was compared using faecal egg count reduction tests on 14 commercial farms throughout New Zealand. On each farm, groups of 15 calves were sampled for faecal nematode egg count and then treated with ivermectin administered orally, or with moxidectin administered either by the oral, subcutaneous injection or topical (pour-on) route. Samples were again collected 14 days after treatment and efficacy was calculated as the percentage reduction in-group mean egg count between the pre- and post-treatment samples. In addition, efficacy was calculated for individual animals, in order to compare the variability of the different treatments. On four farms untreated control groups were run and five animals from each of the control and all of the moxidectin-treated groups were bled over time to estimate plasma-moxidectin concentrations. Averaged across all tests, the reduction in faecal egg count was significantly greater after treatment with moxidectin oral (91.1%) than following treatment with moxidectin injection (55.5%) or with moxidectin pour-on (51.3%). Low efficacies were invariably against Cooperia oncophora. The oral treatments were significantly less variable in efficacy than the injection and pour-on treatments. Moxidectin concentrations in plasma were highest following subcutaneous injection and lowest following pour-on administration. Plasma levels following oral administration were intermediate, being significantly lower than post-injection and significantly higher than post-pour-on. There was no evidence of transfer of moxidectin to untreated animals through licking. Based on these results, along with those of other studies, it is proposed that oral administration of macrocyclic lactone anthelmintics results in higher concentrations of active reaching the target worms in the gastrointestinal tract than following either administration by injection or by pour-on.

Managing anthelmintic resistance--use of a combination anthelmintic and leaving some lambs untreated to slow the development of resistance to ivermectin.

A field study was conducted to test the hypotheses that use of a combination anthelmintic and/or increasing the pool of unselected worms 'in refugia' by leaving a proportion of lambs untreated would slow the development of resistance to ivermectin. Twelve suites of four paddocks (farmlets) were seeded with a mixture of resistant and susceptible isolates of both Trichostrongylus colubriformis and Teladorsagia (Ostertagia) circumcincta calculated to yield a 95% reduction in faecal nematode egg count (FEC) after treatment with ivermectin. Each year for three years the farmlets were stocked in the spring with mobs of lambs which were treated five times at 28-day intervals with either ivermectin or an ivermectin+levamisole combination. In addition, in half the mobs the heaviest 10% of lambs remained untreated at each treatment occasion, resulting in a 2 × 2 factorial treatment structure (i.e. two drench types × two percentage treated) with three complete replicates. The development of resistance to ivermectin, and at the end to levamisole, was measured by larval development assays (LDA) and worm counts from treated and untreated tracer lambs. For T. colubriformis the development of resistance to ivermectin, as measured by tracer lamb worm burdens, was delayed by treatment with the combination and by leaving 10% of lambs untreated. In addition, the interaction between these factors approached significance (p=0.052). Similarly, results of the LDAs indicated a slower development of resistance when lambs were treated with the combination and when 10% of lambs were left untreated. For T. circumcincta, results were compromised by the rapid development of resistance, which appears to be the result of low viability in the field of the susceptible isolate used to contaminate the pastures. Although a small delay in the development of resistance to ivermectin was indicated, this was off-set by an increase in the level of resistance to levamisole. A post-study modelling experiment simulating the conditions of the field study and the starting efficacies for the two nematode species produced equivalent outputs to those measured in the field. Overall, results support the conclusions that use of combination anthelmintics and deliberately increasing 'refugia' of unselected genotypes will slow the development of anthelmintic resistance. However, as indicated in modelling studies, once resistance to all the constituent actives is well developed (efficacy<70%) the value of combinations for slowing the development of resistance is largely lost.

The production cost of anthelmintic resistance in lambs.

The economic impact of anthelmintic resistance was investigated in lambs by comparing productivity parameters in groups of animals treated either with a highly effective anthelmintic, or an anthelmintic to which three species of resistant worms were known to be present. Ten farmlets, each stocked with 30 lambs, were rotationally grazed for 5 months, with monthly treatments of either albendazole, to which resistance existed, or a new combination product containing derquantel and abamectin (DQL-ABA) to which there was no resistance. Stock on five farmlets were treated with each anthelmintic and productivity measures, including liveweights, body condition and faecal soiling were assessed throughout. In addition, fleece weights and information on carcass weight and quality was collected at the end of the trial. Anthelmintic efficacy was measured at the last two treatment dates by faecal egg count reduction test with larval cultures. Albendazole demonstrated efficacies of 48.4% and 40.9% for Trichostrongylus spp. and Teladorsagia circumcincta respectively. By contrast, the DQL-ABA treatments were >99% effective against all genera. The difference in live-weight gain was 9 kg in favour of the DQL-ABA treatments. This translated into a 4.7 kg increase in carcass weight with a 10.4% increase in carcass value. Significant differences in body condition scores, faecal breech soiling and fleece weights were also recorded, all in favour of the DQL-ABA treatments. The time required for 50% of the animals to reach a target live-weight of 38 kg was significantly shorter (by 17 days) in those animals treated with DQL-ABA. The results show that the production cost of subclinical parasitism as a result of using an anthelmintic product which is less than fully effective due to resistance can greatly exceed the cost of routine testing of anthelmintic efficacy and the adoption of new anthelmintic classes. There is a strong case for many farmers to re-evaluate their position on some of these issues in order to optimise financial performance.