Shortened strongylid egg reappearance periods in horses following macrocyclic lactone administration - The impact on parasite dynamics.

Over the past three decades, equine strongylid egg reappearance periods (ERPs) have shortened substantially for macrocyclic lactone anthelmintics. The ERPs of ivermectin and moxidectin were originally reported in the 8-10 and 12-16 week ranges, respectively, but several recent studies have found them to be around 4-5 weeks for both actives. This loss of several weeks of suppressed strongylid egg output could have substantial implications for parasite control. This study made use of a computer simulation model to evaluate the impact of shortened ERPs on the anthelmintic performance of ivermectin and moxidectin against equine cyathostomins. The original ERPs were set to 7.1 and 15.4 weeks for ivermectin and moxidectin, respectively, while the reduced ERP was set to 4.6 weeks for both actives. Simulations were set to compare model outputs between original and reduced ERP scenarios and results expressed as percent increase in strongylid egg output, infective third stage larvae on herbage (L3h), and encysted early third stage larvae (EL3). For each drug, simulations were evaluated for two different treatment scenarios (2 and 4 treatments annually), two different age groups (yearlings and adults), and for four different climates (cold humid continental, temperate oceanic, humid subtropical, and hot/cold semi-arid). Across all simulations, there was a substantial increase of the three evaluated parameters. With the ivermectin simulations, all three parameters increased in the 100-300% range across climates, age groups and treatment intensities. The moxidectin simulations displayed a wider range of results with parameters increasing from a few hundred to several thousand percent. The increases were most pronounced for L3h in the two cooler climates, reaching as high as 6727%. Overall, the loss of anthelmintic performance was at a magnitude of 10 times larger for moxidectin compared to ivermectin. This performance loss was climate dependent, and was also affected by treatment intensity, but not by horse age. This is the first study to evaluate consequences of shortened ERPs in horses and demonstrated a substantial loss in anthelmintic performance resulting from this development. The results illustrate that anthelmintic efficacy is more than the percent reduction of fecal egg counts at 14 days post treatment, and that substantial anthelmintic performance can be lost despite FECRTs remaining at 100%.

Carbohydrate larval antigen (CarLA IgA) responses to mixed species nematode infection in pasture grazed Angora goats.

The genetics of indicator traits for resistance of Angora goats to gastrointestinal nematode parasite infections, and their relationships with productivity traits, were investigated on a commercial mixed-enterprise farm in the eastern North Island of New Zealand. Faecal egg counts (FEC), specific Immunoglobulin A (IgA) and Immunoglobulin G (IgG) antibody titres against carbohydrate larval antigen (CarLA) in saliva, live weight and fleece weights were recorded from 278 goats of 19-20 months of age, run as four separate mobs (breeding bucks, castrated males (wethers), or 2 groups of breeding does). Summary statistics showed the mobs differed significantly in liveweight, loge (FEC+50), loge (IgA) and loge (IgG). Genetic parameters were estimated using an animal model with repeated records where appropriate, after adjusting for the different contemporary animal groups, using the restricted maximum likelihood (REML) package. Heritability estimates from repeated measures were 0.19 ± 0.16 for FEC, 0.28 ± 0.16 for CarLA specific IgA and 0.23 ± 0.15 for CarLA specific IgG. The CarLA specific IgA response was negatively genetically correlated with FEC (-0.99 ± 0.31) suggesting that it could be used as a selection tool for breeding resistant animals. Although the genetic and phenotypic correlations between CarLA IgA and IgG were high and significant, the analysis between loge (FEC+50) and loge CarLA IgG did not converge. Further, both FEC and CarLA IgA showed significant and favourable genetic correlations with live weight. In contrast, CarLA IgG showed an unfavourable phenotypic correlation with liveweight. While this is only a preliminary study, the results do suggest that the immunoassay measuring salivary CarLA IgA response may have utility as a selection tool for parasite resistance in some breeds of goats.

In vitro evaluation of fitness parameters for isolates of Teladorsagia circumcincta resistant and susceptible to multiple anthelmintic classes.

Anthelmintic resistance (AR) is an ever increasing problem for the sheep industry. Several studies worldwide have investigated reversing the trend of increasing AR and documented evidence for reversion toward susceptibility has been found. The hypothesis that resistance mutations compromise parasite fitness was drawn from this evidence. The aim of this study was to assess whether there were measurable differences in the fitness of Teladorsagia circumcincta isolates depending on their AR status. Four isolates were selected for the trial based on their known resistance status; D and M were multi-drug resistant, and T and W were susceptible to the benzimidazole, levamisole, and macrocyclic lactone anthelmintic classes. A secondary aim was to develop a series of in vitro bioassays for assessing fitness characteristics of parasites. The in vitro assays included; the cold stress test measured the number of third stage larvae (L3) developing from eggs stored at 4 °C for different lengths of time. Larval aging measured the locomotory activity of L3 after storage at 30 °C for different lengths of time. The exsheathment assay measured the exsheathment percentage of L3. Larval Length used length as a proxy for fecundity. The egg hatch assay evaluated egg hatch rate in water at room temperature. All isolates exhibited a decrease in the number of L3 recovered after storage of eggs at 4 °C (p < 0.001). Storage of L3 at 30 °C significantly influenced the ability of L3 to migrate through a 20 µm sieve (p < 0.001), however, there were no differences between isolates (p > 0.05). Exsheathment rate was higher for isolate D in comparison to isolates M and W, and for isolate T compared to isolate W. Isolate W was significantly longer than all other isolates (p < 0.05), whilst isolate M was significantly longer than isolate D (p < 0.05). No significant differences were found between isolates in egg hatch (p > 0.05). Overall, the results do not support differences in fitness associated with anthelmintic resistance status, even though differences were seen between the isolates for some assays. This suggests there is considerable variation in fitness parameters between isolates, making it difficult to determine whether resistance genotypes come with lower fitness.

Three-year study to evaluate an anthelmintic treatment regimen with reduced treatment frequency in horses on two study sites in Belgium.

In the present study, an anthelmintic treatment regimen with reduced treatment frequency was evaluated in horses on two study sites in Belgium during three consecutive summer pasture seasons. Historically, the horses on both study sites were treated up to 6 times a year with ivermectin (IVM) or up to 4 times a year with moxidectin (MOX), and previous efficacy evaluations indicated a reduced egg reappearance period in some of the treated horses for both IVM (28 days) and MOX (42 days). In the present study, all horses were treated with IVM or MOX in the spring and in autumn. Faecal egg counts (FEC) were conducted every two weeks during the summer pasture season and whenever the individual FEC exceeded 250 eggs per gram of faeces, the specific horse was treated with pyrantel embonate. No increase in parasitic disease over the three-year period of the study was observed. The FEC data collected in the study as well as the age of the animals and local weather data were then imported into a cyathostomin life-cycle model, to evaluate long term effects of the newly applied treatment regimen on the selection pressure for anthelmintic resistance, and compare to the previous high frequency treatment regimen. The model simulations indicated that the whole-herd treatment regimen with at least 4 macrocyclic lactone treatments annually led 2-3 times faster resistance development than any of the alternative treatment regimens evaluated under the specific conditions of these two study sites. Further lowering the treatment frequency or applying even more selective treatments enhanced the delay in resistance development, but to a lesser extent.

Route of administration affects the efficacy of moxidectin against Ostertagiinae nematodes in farmed red deer (Cervus elaphus).

The influence of route of administration on the pharmacokinetics and efficacy of macrocyclic lactone anthelmintics has been a subject of interest due to its potential to influence the development of anthelmintic resistance. For most parasite species studied so far, oral administration results in the highest concentrations of drug in the parasites and the highest efficacy against resistant genotypes. However, a recent study in cattle measured the highest levels of ivermectin in the abomasal Ostertagia ostertagi following subcutaneous injection, but it was not possible to correlate these elevated levels with efficacy. Therefore, the current study was initiated to determine whether injectable delivery might be optimal for attaining high efficacy against this important group of parasites. Three on-farm trials were conducted to measure the efficacy of moxidectin administered by the oral, injectable, and pour-on routes against Ostertagiinae parasites in farmed red deer. Groups of rising 1-year old stags (red or red-wapiti crossbreds) in the 84-104 kg weight range were randomised on liveweight into treatment groups of 6 (1 farm) or 8 (2 farms). Animals were treated to individual liveweight with moxidectin oral (0.2 mg/kg), injectable (0.2 mg/kg), pour-on (0.5 mg/kg) or remained untreated. Twelve days later all animals were euthanised and abomasa recovered for worm count. Adult worms were counted in a 2% aliquot of abomasal washings, and adult and fourth stage larvae in a 10 % aliquot following mucosal incubation in physiological saline. In addition, blood was collected from the same 5 animals in each of the treatment groups on days 0, 1, 2, 3, 5, 7 and 12 after treatment and moxidectin levels in plasma were determined using a mass spectrometer. The number of Ostertagiinae surviving treatment was significantly different for each of the treatment groups with injectable administration being most effective, oral administration being the next most effective and pour-on administration the least effective. This applied to both adult worms and fourth stage larvae. A similar pattern was seen in the levels of moxidectin in plasma with both the peak value and area under the concentration curve being highest following injectable administration and lowest following pour-on treatment. Although undertaken in a different host species, the results support the proposition that injectable administration of macrocyclic lactone anthelmintics is likely to be optimal for efficacy against Ostertagiinae parasites and potentially useful in slowing the emergence of resistance in these parasites.

Host effects on the free-living stages of Haemonchus contortus.

A group of 5 lambs (Host 1-5) was infected with the same batch of Haemonchus contortus and after patency individual faecal samples were collected, separately incubated at 23 °C for 14 days and third stage larvae collected through Baermannisation. Life-history traits were compared between larvae from different hosts: the length of the larvae was measured by microscope image analysis, larval survival in water at 35 °C, larval susceptibility to ivermectin (EC50) in a migration assay, the proportion of larvae exsheathing in vitro and the proportion establishing to the adult stage in young lambs. For all traits there were significant differences between the host animals, with larvae from specific hosts following a consistent pattern of displaying the highest or lowest trait results. Compared with larvae from Host 1 the larvae from Host 5 were () shorter (741-692 µm, p < 0.05), had a longer median survival at 35 °C (3.6-6.4 days, p < 0.05), were less susceptible to ivermectin (EC50 of 1.2 v 4.5 µM, p < 0.05), exsheathed to a lesser degree (83.6-58 %, p < 0.05), but showed a higher establishment rate in the consecutive host (15.2-31.4 %, p < 0.05). Regarding the survival time, anthelmintic susceptibility (under most commercial farming practices) and establishment rate as indicators for fitness, the parasites populating Host 5 produced progeny of higher fitness. The findings indicate that the host animal of the parental parasite generation has a significant effect on the parasite progeny.

Effects of long-acting, broad spectra anthelmintic treatments on the rumen microbial community compositions of grazing sheep.

Anthelmintic treatment of adult ewes is widely practiced to remove parasite burdens in the expectation of increased ruminant productivity. However, the broad activity spectra of many anthelmintic compounds raises the possibility of impacts on the rumen microbiota. To investigate this, 300 grazing ewes were allocated to treatment groups that included a 100-day controlled release capsule (CRC) containing albendazole and abamectin, a long-acting moxidectin injection (LAI), and a non-treated control group (CON). Rumen bacterial, archaeal and protozoal communities at day 0 were analysed to identify 36 sheep per treatment with similar starting compositions. Microbiota profiles, including those for the rumen fungi, were then generated for the selected sheep at days 0, 35 and 77. The CRC treatment significantly impacted the archaeal community, and was associated with increased relative abundances of Methanobrevibacter ruminantium, Methanosphaera sp. ISO3-F5, and Methanomassiliicoccaceae Group 12 sp. ISO4-H5 compared to the control group. In contrast, the LAI treatment increased the relative abundances of members of the Veillonellaceae and resulted in minor changes to the bacterial and fungal communities by day 77. Overall, the anthelmintic treatments resulted in few, but highly significant, changes to the rumen microbiota composition.

Monitoring equine ascarid and cyathostomin parasites: Evaluating health parameters under different treatment regimens.

BACKGROUND: Strongylid and ascarid parasites are omnipresent in equine stud farms, and ever-increasing levels of anthelmintic resistance are challenging the industry with finding more sustainable and yet effective parasite control programs. OBJECTIVES: To evaluate egg count levels, bodyweight and equine health under defined parasite control protocols in foals and mares at two Standardbred and two Thoroughbred stud farms. STUDY DESIGN: Longitudinal randomised field trial. METHODS: A total of 93 foals were enrolled and split into two treatment groups, and 99 mares were enrolled and assigned to three treatment groups. All horses underwent a health examination, and episodes of colic or diarrhoea were recorded at each faecal collection date. Bodyweights were assessed using a weight tape, and mares were body condition scored. Group A foals (FA) were dewormed at 2 and 5 months of age with a fenbendazole/ivermectin/praziquantel product, while group B foals (FB) were dewormed on a monthly basis, alternating between the above-mentioned product and an oxfendazole/pyrantel embonate product. Group A mares (MA) were dewormed twice with fenbendazole/ivermectin/praziquantel, group B mares (MB) were dewormed with the same product, when egg counts exceeded 300 strongylid eggs per gram, and group C mares (MC) were dewormed every 2 months, alternating between the two products. Health data were collected monthly for 6 months (foals) and bimonthly for 13 months (mares). Data were analysed with mixed linear models and interpreted at the α = 0.05 significance level. RESULTS: There were no significant bodyweight differences between foal groups, but MA mares were significantly lighter than the other two groups. Very few health incidents were recorded. Foals in group FA had significantly higher ascarid and strongylid egg counts, whereas no significant differences were observed between mare groups. MAIN LIMITATIONS: Study duration limited to one season. CONCLUSIONS: Anthelmintic treatment intensity was lowered from the traditional intensive regimes without measurable negative health consequences for mares and foals.

Climate change is likely to increase the development rate of anthelmintic resistance in equine cyathostomins in New Zealand.

Climate change is likely to influence livestock production by increasing the prevalence of diseases, including parasites. The traditional practice of controlling nematodes in livestock by the application of anthelmintics is, however, increasingly compromised by the development of resistance to these drugs in parasite populations. This study used a previously developed simulation model of the entire equine cyathostomin lifecycle to investigate the effect a changing climate would have on the development of anthelmintic resistance. Climate data from six General Circulation Models based on four different Representative Concentration Pathways was available for three New Zealand locations. These projections were used to estimate the time resistance will take to develop in the middle (2040-49) and by the end (2090-99) of the century in relation to current (2006-15) conditions under two treatment scenarios of either two or six yearly whole-herd anthelmintic treatments. To facilitate comparison, a scenario without any treatments was included as a baseline. In addition, the size of the infective and parasitic stage nematode population during the third simulation year were estimated. The development of resistance varied between locations, time periods and anthelmintic treatment strategies. In general, the simulations indicated a more rapid development of resistance under future climates coinciding with an increase in the numbers of infective larvae on pasture and encysted parasitic stages. This was especially obvious when climate changes resulted in a longer period suitable for development of free-living parasite stages. A longer period suitable for larval development resulted in an increase in the average size of the parasite population with a larger contribution from eggs passed by resistant worms surviving the anthelmintic treatments. It is projected that climate change will decrease the ability to control livestock parasites by means of anthelmintic treatments and non-drug related strategies will become increasingly important for sustainable parasite control.

Managing anthelmintic resistance in cyathostomin parasites: Investigating the benefits of refugia-based strategies.

Selective anthelmintic therapy has been recommended as a sustainable strategy for cyathostomin control in horse populations for several decades. The traditional approach has been to determine strongyle fecal egg counts (FEC) for all horses, with treatment only recommended for those exceeding a predetermined threshold. The aims are to achieve a reduction of overall egg shedding, while leaving a proportion of the herd untreated, which lowers anthelmintic treatment intensity and reduces selection pressure for development of anthelmintic resistance. This study made use of the cyathostomin model to evaluate the influence of treatment strategies with between 1 and 8 yearly treatment occasions, where either 1) all horses were treated, 2) a predetermined proportion of the herd remained untreated, or 3) horses were treated if their FEC exceeded thresholds between 100 and 600 strongyle eggs per gram. Weather data representing four different climatic zones was used and three different herd age structures were compared; 1) all yearlings, 2) all mature horses 10-20 years old, and 3) a mixed age structure of 1-20 years of age. Results indicated a consistent effect of age structure, with anthelmintic resistance developing quickest in the yearling group and slowest among the mature horses. Development of anthelmintic resistance was affected by treatment intensity and selective therapy generally delayed resistance. Importantly, the results suggest that the effects of selective therapy on resistance development are likely to vary between climatic zones and herd age structures. Overall, a substantial delaying of resistance development requires that the average number of treatments administered annually across a herd of horses needs to be about two or less. However, results also indicate that an age-structured prioritisation of treatment to younger horses should still be effective. It appears that a 'one-size-fits-all' approach to the management of anthelmintic resistance in cyathostomins is unlikely to be optimal.

Modelling the development of anthelmintic resistance in cyathostomin parasites: The importance of genetic and fitness parameters.

Previously described models for the free-living and parasitic phases of the cyathostomin life-cycle were combined into a single model for the complete life-cycle. The model simulates a single free-living population on pasture utilising parasite egg output from the horses and localised temperature and rainfall data to estimate infective larval density on herbage. Multiple horses of different ages are possible, each with an individualised anthelmintic treatment programme. Genotypes for anthelmintic resistance are included allowing for up to three resistance genes with 2 alleles each. Because little is known of the genetics of resistance to anthelmintics in cyathostomins, the first use of this model was to compare the effect of different assumptions regarding the inheritance of resistance on model outputs. Comparisons were made between single and two-gene inheritance, where the heterozygote survival was dominant, intermediate or recessive under treatment, and with or without a fitness disadvantage associated with the resistance mechanism. Resistance developed fastest when the heterozygotes survived anthelmintic treatment (i.e., were dominant) and slowest when they did not (i.e., were recessive). Resistance was slower to develop when inheritance was poly-genic compared to a single gene, and when there was a fitness cost associated with the resistance mechanism, although the latter variable was the least influential. Importantly, while these genetic factors sometimes had a large influence on the rate at which resistant genotypes built up in the model populations, their order of ranking was always the same, when different anthelmintic use strategies were compared. Therefore, the described model is a useful tool for evaluating different treatment and management strategies on their potential to select for resistance.

The effect of climate, season, and treatment intensity on anthelmintic resistance in cyathostomins: A modelling exercise.

Anthelmintic resistance is widespread in equine cyathostomin populations across the world, and with no new anthelmintic drug classes in the pharmaceutical pipeline, the equine industry is forced to abandon traditional parasite control regimens. Current recommendations aim at reducing treatment intensity and identifying high strongylid egg shedders in a targeted treatment approach. But, virtually nothing is known about the effectiveness of these recommendations, nor their applicability to different climatic regions, making it challenging to tailor sustainable recommendations for equine parasite control. This study made use of a computer model of the entire cyathostomin life-cycle to evaluate the influence of climate and seasonality on the development of anthelmintic resistance in cyathostomin parasites. Furthermore, the study evaluated the impact of recommended programs involving selective anthelmintic therapy on delaying anthelmintic resistance development. All simulations evaluated the use of a single anthelmintic (i.e., ivermectin) over the course of 40 model years. The study made use of weather station data representing four different climatic zones: a cold humid continental climate, a temperate oceanic climate, a cold semi-arid climate, and a humid subtropical climate. Initially, the impact of time of the year was evaluated when a single anthelmintic treatment was administered once a year in any of the twelve months. The next simulations evaluated the impact of treatment intensities varying between 2 and 6 treatments per year. And finally, we evaluated treatment schedules consisting of a combination of strategic treatments administered to all horses and additional treatments administered to horses exceeding a predetermined fecal egg count threshold. Month of treatment had a large effect on resistance development in colder climates, but little or no impact in subtropical and tropical climates. Resistance development was affected by treatment intensity, but was also strongly affected by climate. Selective therapy delayed resistance development in all modelled scenarios, but, again, this effect was climate dependent with the largest delays observed in the colder climates. This study is the first to demonstrate the value of cyathostomin parasite refugia in managing anthelmintic resistance, and also that climate and seasonality are important. This modelling exercise has allowed an illustration of concepts believed to play important roles in anthelmintic resistance in equine cyathostomins, but has also identified knowledge gaps and new questions to address in future studies.

A model for the dynamics of the parasitic stages of equine cyathostomins.

A model was developed to reproduce the dynamics of the parasitic stages of equine cyathostomins. Based on a detailed review of published literature, a deterministic simulation model was constructed using the escalator boxcar-train approach, which allows for fully-overlapping cohorts of worms and approximately normally distributed variations in age/size classes. Key biological features include a declining establishment of ingested infective stage larvae as horses age. Development rates are constant for all the parasitic stages except the encysted early third stage larvae, for which development rates are variable to reflect the sometimes extended arrestment of this stage. For these, development is slowed in the presence of adult worms in the intestinal lumen, and when ingestion of infective larvae on herbage is high or extended. In the absence of anthelmintic treatments, the life span of adult worms is approximately 12 months, and the presence of an established adult worm burden largely blocks the transition of luminal fourth stage larvae to the adult stage, resulting in mortality of the larvae. This inhibition is removed by effective anthelmintic treatment allowing the rapid replacement of adult worms from the pool of mucosal stages. Within the model, the rate and seasonality at which infective stage larvae are ingested strongly influences the dynamics of the pre-adult stages. While the adult worm burden remains relatively stable within a year, due to the negative feedback they have on developing stages, the numbers and proportions of larval stages relative to the total worm burden increase with the numbers of infective larvae ingested. Further, within the model, the seasonal rise and fall of encysted stages is largely driven by the seasonal pattern of infective larvae on pasture. Because of this, the model reproduces the contrasting seasonal patterns of mucosal larvae, typical of temperate and tropical environments, using only the appropriate seasonality of larvae on pasture. Thus, the model reproduces output typical of different climatic regions and suggests that observed patterns of arrested development may simply reflect the numbers and seasonality of free-living stages on pasture as determined by different management practices and weather patterns.

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.

A climate-driven model for the dynamics of the free-living stages of Cooperia oncophora.

Experimental results and published literature data regarding the development, survival and herbage translocation of Cooperia oncophora larvae were used to develop a climate-driven model to simulate the dynamics of the free-living stages. From daily maximum and minimum temperature the model estimated hourly development and survival rates of the pre-infective stages and daily survival of infective third stage larvae (L3) inside the faecal pat and in the herbage. In addition, daily rainfall data were used to calculate the translocation rate of the L3 from the faecal pat into the herbage. The model produced results for the development and survival of the free-living stages that were comparable to previous observations. Temperatures below 6 °C or above 35 °C resulted in a low estimate of developed L3, which in between increased and peaked at an optimal temperature estimate of 25.6 °C. Provided sufficient rainfall the model predicted that the developed L3 would be able to translocate from the faecal pat into the herbage. When validating model output for the herbage contamination with C. oncophora infective stage larvae against results of a two year field experiment, the comparison indicated that the model was able to reproduce the observed contamination pattern. Further, detailed examination of different model components helped to identify possible factors causing the decay of larval herbage contamination during winter-spring as occurred in the field experiment.

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.

Managing anthelmintic resistance-Variability in the dose of drug reaching the target worms influences selection for resistance?

The concentration profile of anthelmintic reaching the target worms in the host can vary between animals even when administered doses are tailored to individual liveweight at the manufacturer's recommended rate. Factors contributing to variation in drug concentration include weather, breed of animal, formulation and the route by which drugs are administered. The implications of this variability for the development of anthelmintic resistance was investigated using Monte-Carlo simulation. A model framework was established where 100 animals each received a single drug treatment. The 'dose' of drug allocated to each animal (i.e. the concentration-time profile of drug reaching the target worms) was sampled at random from a distribution of doses with mean m and standard deviation s. For each animal the dose of drug was used in conjunction with pre-determined dose-response relationships, representing single and poly-genetic inheritance, to calculate efficacy against susceptible and resistant genotypes. These data were then used to calculate the overall change in resistance gene frequency for the worm population as a result of the treatment. Values for m and s were varied to reflect differences in both mean dose and the variability in dose, and for each combination of these 100,000 simulations were run. The resistance gene frequency in the population after treatment increased as m decreased and as s increased. This occurred for both single and poly-gene models and for different levels of dominance (survival under treatment) of the heterozygote genotype(s). The results indicate that factors which result in lower and/or more variable concentrations of active reaching the target worms are more likely to select for resistance. The potential of different routes of anthelmintic administration to play a role in the development of anthelmintic resistance is discussed.

Managing anthelmintic resistance in Parascaris spp.: A modelling exercise.

A previously described model for the dynamics of the parasitic stages of Parascaris spp. was modified to include eggs outside the host and the genetics of anthelmintic resistance before being used to address questions regarding the development of resistance. Three broad questions were addressed; i) How sustainable is the current common practice of treating foals monthly for their first year of life (i.e. 12 treatments/year)? ii) Does the timing of treatments have an effect on resistance development? (i.e. do certain treatments select for resistance more strongly than others?), and iii) How sustainable is the currently recommended strategy of targeting ascarid infections in foals with two treatments applied during the first five months of life? A range of variations within these broad questions were considered, such as the value in rotational deworming, whether larvicidal treatments are more selective for resistance, and whether combination anthelmintics should be introduced. Twelve anthelmintic treatments at monthly intervals resulted in the development of resistance to all the anthelmintics used, regardless of how they were used, indicating that such intensive treatment frequency is unlikely to be sustainable. The timing of a single annual treatment influenced resistance development with treatments at 3 and 4 months of age being more selective than treatments at other times. Treatments administered to foals older than 6 months of age did not select for resistance within the timeframe of these simulations. Treatments with activity against migrating third stage larvae (ivermectin and a programme of 5 daily treatments with fenbendazole) were more selective for resistance than those which only killed worms in the intestine. Restricting the number of treatments to young foals to two, administered at 2 and 5 months of age slowed the development of resistance by allowing a small contribution from susceptible genotype worms to subsequent generations. If the interval between treatments was reduced, resistance developed more rapidly demonstrating the importance of allowing some susceptible worms to reach patency before the second treatment is administered. Under a reduced treatment schedule with a clearly defined 'refugium' of susceptibility, the use of effective actives in combination appears to offer advantages for delaying resistance development. The model offers insights into more sustainable drug use strategies and has identified some priority questions for future research.

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.

A model for the development and growth of the parasitic stages of Parascaris spp. in the horse.

Literature documenting the growth and development of Parascaris spp. infections was used to develop a model describing worm dynamics in the young horse. The model incorporates four main variables; the rate at which larvae migrate through host tissues to return to the small intestine, the proportion of migrating larvae which succeed in returning to the small intestine, the rate of growth in size of maturing and adult worms and the survival rate of maturing and adult worms. In addition, the number of eggs laid each day by adult female worms is calculated as a function of worm size (length) and is used to calculate faecal egg output of the foal. Published data describing the rate of migration through host tissues, and the growth of worms following their return to the small intestine, was used to derive relationships describing these processes. However, only limited data exists relating the survival of migrating larvae and mature worms in the intestine to host age and experience of infection. Therefore, relationships and coefficients describing these variables were modified so that output aligned with published experimental results. As a consequence, the model has not yet been evaluated against an independent data set, and so remains as the best 'current hypothesis' for the dynamics of this parasite. Hopefully, future experiments designed to test specific assumptions and outputs of the model will lead to a better understanding of the biology of this important parasite. For example, the most influential variable in determining model output is the survival rate of worms in the small intestine. In the model, worm survival declines in response to both the increasing age of the horse and the increasing cumulative length of worms in the intestine (used as a proxy for crowding). Given the importance of this variable to model behaviour and the paucity of experimental data on this topic this would seem a priority for future study. Initial experiments using the model suggest that a single anthelmintic treatment, administered soon after patency of initial infection, may effectively control environmental contamination with Parascaris spp. eggs while allowing a small 'refugia' of susceptibility to delay the emergence of anthelmintic resistance. Further evaluations of the practicality of this approach may be worthwhile.