Chemotherapy and delivery systems--helminths

Anthelmintics in the environment: Their occurrence, fate, and toxicity to non-target organisms

Anthelmintics are drugs used for the treatment and prevention of diseases caused by parasitic worms (helminths). While the importance of anthelmintics in human as well as in veterinary medicine is evident, they represent emerging contaminants of the environment. Human anthelmintics are mainly used in tropical and sub-tropical regions, while veterinary anthelmintics have become frequently-occurring environmental pollutants worldwide due to intensive agri- and aquaculture production. In the environment, anthelmintics are distributed in water and soil in relation to their structure and physicochemical properties. Consequently, they enter various organisms directly (e.g. plants, soil invertebrates, water animals) or indirectly through food-chain. Several anthelmintics elicit toxic effects in non-target species. Although new information has been made available, anthelmintics in ecosystems should be more thoroughly investigated to obtain complex knowledge on their impact in various environments. This review summarizes available information about the occurrence, behavior, and toxic effect of anthelmintics in environment. Several reasons why anthelmintics are dangerous contaminants are highlighted along with options to reduce contamination. Negative effects are also outlined.

Evaluation of three strategic parasite control programs in captive wild ruminants

Parasite control in wild ruminants is based mainly on anthelmintics. The objective of the present study was to evaluate three parasite control programs in seven herds of captive wild ruminants in 3 consecutive years. In the first year, a biannual spring-summer treatment regime with fenbendazole at 7.5 mg/kg body weight p.o. for 3 days was applied. The next year, an early-season treatment program with three administrations of fenbendazole at the same dosage at 3-wk intervals was used. In the third year, an early-season treatment program with ivermectin (0.2 mg/kg p.o. for 3 days), applied three times at 5-wk intervals, was evaluated. Effectiveness of these control programs was assessed by fecal egg counts and by scores of body condition and fecal consistency at weekly intervals. Involved animal species were Arabian oryx (Oryx leucoryx), scimitar-horned oryx (Oryx dammah), slender-horned gazelle (Gazella leptoceros), Soay sheep (Ovis aries soay), ibex (Capra ibex), red deer (Cervus elaphus hippelaphus), and Nelson's elk (Cervus elaphus nelsoni). With the spring-summer regime, fecal egg counts remained low during the first 5 mo, but from September onward, they slowly increased to significant levels in all seven herds. The early-season program with fenbendazole resulted in fecal egg counts near zero during the entire year in four herds but a significant egg shedding from August onward was noted in Arabian oryx, scimitar-horned oryx, and Soay sheep. The early-season program with ivermectin resulted in very low to zero egg shedding in gazelle, adult Soay sheep, ibex, red deer, and Nelson's elk during the entire grazing season, but failed to prevent high shedding in October in Arabian oryx and scimitar-horned oryx. High contamination of grassy pastures with infective larvae/eggs in the previous year or before the first treatment may explain failure of the treatment regimes in some herds.

Targeted selective treatment for worm management—How do we sell rational programs to farmers?

Seriously escalating global anthelmintic resistance in gastrointestinal nematodes of small ruminants has spawned a variety of alternatives to anthelmintics for worm management, based on the need for sustainable Integrated Parasite Management (sIPM). Pivotal to the sIPM approach is the concept of refugia, the proportion of a given parasite population that escapes exposure to control measures. By balancing drug applications with the maintenance of refugia, the accumulation of anthelmintic resistance alleles in worm populations can be considerably delayed, while still providing good levels of control. The over-dispersed nature of parasitic infections provides an opportunity to achieve this balance, by targeting treatments to the members of a flock or herd that are least tolerant to nematode infection. However, implementation of this strategy has only recently become feasible, with the development of the FAMACHA((c)) system for clinical evaluation of anaemia due to haemonchosis. Subsequently, the use of milk yields has proven an effective indicator in dairy goats infected predominantly with nematodes other than Haemonchus contortus. In addition, short-term weight changes and perhaps also body condition scoring may provide indices of parasitism, permitting the rapid identification of animals likely to benefit from treatment. However, sIPM and refugia-based approaches are more complex than whole-flock treatments in conventional programs, and adoption by farmers is most likely where the theoretical basis is understood. As close communication with informed advisors is generally limited, there is a danger that sIPM will remain a theoretical concept without alternative modes of communication. The development of computer-based decision support programs, which use epidemiological, seasonal and clinical information to provide recommendations for specific situations, should be accorded high priority in the future development of worm management systems.

From discovery to development: Current industry perspectives for the development of novel methods of helminth control in livestock

Despite the extraordinary success in the development of anthelmintics in the latter part of the last century, helminth parasites of domestic ruminants continue to pose the greatest infectious disease problem in grazing livestock systems worldwide. Newly emerged threats to continuing successful livestock production, particularly with small ruminants, are the failure of this chemotherapeutic arsenal due to the widespread development of anthelmintic resistance at a time when the likelihood of new products becoming commercially available seems more remote. Changing public attitudes with regards to animal welfare, food preferences and safety will also significantly impact on the ways in which livestock are managed and their parasites are controlled. Superimposed on this are changes in livestock demographics internationally, in response to evolving trade policies and demands for livestock products. In addition, is the apparently ever-diminishing numbers of veterinary parasitology researchers in both the public and private sectors. Industries, whether being the livestock industries, the public research industries, or the pharmaceutical industries that provide animal health products, must adapt to these changes. In the context of helminth control in ruminant livestock, the mind-set of 'suppression' needs to be replaced by 'management' of parasites to maintain long-term profitable livestock production. Existing effective chemical groups need to be carefully husbanded and non-chemotherapeutic methods of parasite control need to be further researched and adopted, if and when, they become commercially available. This will require veterinary parasitology researchers from both the public and private sectors to work in close co-operation to ensure 'sustainability' - not only of the livestock industries that they service - but also for their very own activities and enterprises.

Evaluation of the chemoprophylactic efficacy of 10% long acting injectable moxidectin against gastrointestinal nematode infections in calves in Belgium

The chemoprophylactic efficacy of a single dose of the 10% long acting (LA) injectable formulation of moxidectin on nematode infections in calves, was evaluated. Two similar groups of 11 female, first grazing season Holstein calves were turned out in early May on separate plots of a single, naturally infected pasture. Until 56 days post-treatment (pt), the percentage reduction in faecal egg output was 100%, remaining above 90% during the entire trial, except for day 126 pt. More than 90% of the larvae in the treated group were identified as Cooperia until 140 days after treatment and more than 70% during the rest of the trial, whereas in the control group Cooperia was the most abundant species until day 84 pt and Ostertagia from 126 days pt onwards. The reduction in faecal egg output in the treated group was reflected in the mean pepsinogen levels being below the pathogenic threshold at the end of the grazing season (1.8 units of tysrosine (U tyr)) and the absence of diarrhoea during the second half of the grazing season. In the control group pepsinogen levels remained high (mean: 5.5 U tyr) and prolonged diarrhoea occurred in the second half of the grazing season. Furthermore, the weight gain for the treated group at the end of the grazing season was 41.9 kg higher than for the control group. At necropsy, the reduction in O. ostertagi worm burden in the treated group was 97.5% compared to the control group, while the reduction in C. oncophora worm burden was 57%. An additional benefit of the long acting parasitological control, was reduced pasture contamination.

Formulation and biopharmaceutical issues in the development of drug delivery systems for antiparasitic drugs

The development of really new antiparasitic drugs to market level is a very rare event. A large number of lead structures have already been screened and discarded, the market is large but poor, and the administrative barriers are increasingly high and costly. Novel antiparasitics must not only be better, they must also be substantially safer than the existing repertoire. There are two major aspects to drug development. One is the strategy of pathogen-specific biochemical intervention, the other the strategy of optimal formulation and application. This review focuses on the latter. In finding and adapting innovative and "intelligent", i.e. parasite- and disease-specific formulations and delivery systems, established but deficient drugs might be optimised, enhancing their efficiency and reducing negative side effects at relatively low cost. Further, many promising new ideas are severely hampered by the low water solubility of the antiparasitic drug. Here as well, some of the innovative drug formulation and delivery systems discussed below might offer highly efficient, while technologically simple, solutions.

Delivery strategies for antiparasitics

Optimisation of drug carrier systems and drug delivery strategies that take into account the peculiarities of individual infectious agents and diseases are key elements of modern drug development. In the following, different aspects of a rational design for antiparasitic drug formulation will be reviewed, covering delivery systems such as nano- and microparticles, liposomes, emulsions and microemulsions, cochleates and bioadhesive macromolecules. Functional properties for each carrier system will be discussed as well as their therapeutic efficacy for parasitic diseases, including leishmaniasis, human African trypanosomiasis, human cryptosporidiosis, malaria and schistosomiasis. Critical issues for the application of drug carrier systems will be discussed, focusing on biopharmaceutical and pathophysiological parameters such as routes of application, improvement of body distribution and targeting intracellularly persisting pathogens.

Loperamide-induced enhancement of moxidectin availability in cattle

Moxidectin (MXD) is a milbemycin endectocide compound active at extremely low dosages against a wide variety of nematode and arthropod parasites. Different pharmacological approaches are currently being tested to delay the bile-faecal elimination and to obtain increased systemic availability for endectocide molecules in ruminants. Loperamide (LPM) is an opioid derivative, whose main pharmacological action is to abolish intestinal propulsive peristaltic waves. The influence of LPM on the pattern of faecal excretion of MXD and on its plasma disposition following intravenous (i.v.) and subcutaneous (s.c.) administrations to cattle was evaluated in the current work. Parasite-free calves were treated with MXD given either alone at 200 microg/kg by i.v. (Experiment 1) and s.c. (Experiment 2) administrations or coadministered with LPM subcutaneously injected at 0.4 mg/kg. Blood and faecal samples were collected over a period of 20 (Experiment 1) and 40 (Experiment 2) days post-treatment. The recovered plasma and faecal samples were extracted and analysed by high-performance liquid chromatography (HPLC) using fluorescence detection. Significantly higher MXD plasma concentrations were obtained after the coadministration of MXD + LPM compared with treatments with MXD alone by both routes. The higher MXD plasma concentration profiles measured after the coadministration with LPM accounted for the significantly higher AUC values obtained following the i.v. (> 46%) and s.c. (> 38%) treatments. A reduced MXD body clearance was observed in the presence of LPM. The appearance of MXD in faeces was significantly delayed after the i.v. and s.c. coadministrations of MXD with LPM (T(1/2app)=5.87 and 10.6 h, respectively) than that observed after the treatment with MXD alone (T(1/2app)=3.48 and 5.12 h). A delayed MXD peak concentration in faeces collected from MXD + LPM-treated animals compared with those receiving MXD alone, was observed. The delayed intestinal transit time caused by LPM and a potential competition between MXD and LPM for the P-glycoprotein-mediated bile/intestinal secretion processes, may account for the enhanced MXD systemic availability measured in cattle in the current work.

Plasma achiral and chiral pharmacokinetic behaviour of intravenous oxfendazole co-administered with piperonyl butoxide in sheep

Co-administration of piperonyl butoxide (PB) potentiates fenbendazole (FBZ) in small ruminants. The resultant increase in bioavailability of FBZ and its metabolite oxfendazole (OFZ) has important implications for the efficacy of these drugs against benzimidazole (BZD)-resistant strains of Teladorsagia circumcincta. This study evaluated the racemic (achiral) and enantiomeric (chiral) plasma disposition kinetics of OFZ and its metabolites after the co-administration of PB and OFZ in sheep. Six 6-8-month-old, parasite-free, female Dorset sheep (30-40 kg) were used in a two-phase crossover experiment. In phase I, three sheep received 30 mg/kg PB orally, followed by a single intravenous (i.v.) injection of OFZ at 5 mg/kg. The other three animals were treated similarly except that 5 mL of water replaced PB. In phase 2, treatments for the two groups were reversed and were given 14 days after the initiation of phase I. Three analytes OFZ, FBZ and fenbendazole sulphone (FBZSO(2)) were recovered in plasma up to 48 h post-treatment in both experimental groups. Achiral and chiral pharmacokinetic (PK) profiles for OFZ, after the co-administration of PB, were characterized by a significantly greater area under the concentration--time curve (AUC) and a longer mean residence time (MRT). Chiral OFZ distribution ratios were comparable in both treatment groups. Piperonyl butoxide treatment markedly influenced the plasma PK profiles for FBZ and FBZSO(2) following OFZ administration. Production of FBZ was enhanced as reflected by increased (> 60%) AUC, delayed T(max) and a significantly delayed (> 45%) elimination (t(1/2)(el)). Although AUC values for FBZSO(2) were not significantly different between groups, this metabolite was depleted more slowly from plasma (t(1/2)(el) > 60% and MRT > 42%) following PB treatment. This study demonstrated that PB co-administration is associated with an inhibition of OFZ biotransformation, as evidenced by the significantly higher plasma concentrations of OFZ and FBZ, and this could have important implications in terms of anti-parasite therapy against BZD-resistant parasite strains.

Ivermectin disposition kinetics after subcutaneous and intramuscular administration of an oil-based formulation to cattle

Slight differences in formulation may change the plasma kinetics and ecto-endoparasiticide activity of endectocide compounds. This work reports on the disposition kinetics and plasma availability of ivermectin (IVM) after subcutaneous (SC) and intramuscular (IM) administration as an oil-based formulation to cattle. Parasite-free Aberdeen Angus calves (n = 24; 240-280 kg) were divided into three groups (n = 8) and treated (200 microg/kg) with either an IVM oil-based pharmaceutical preparation (IVM-TEST formulation) (Bayer Argentina S.A.) given by subcutaneous (Group A) and intramuscular (Group B) injections or the IVM-CONTROL (non-aqueous formulation) (Ivomec, MSD Agvet) subcutaneously administered (Group C). Blood samples were taken over 35 days post-treatment and the recovered plasma was extracted and analyzed by HPLC using fluorescence detection. IVM was detected in plasma between 12 h and 35 days post-administration of IVM-TEST (SC and IM injections) and IVM-CONTROL formulations. Prolonged IVM absorption half-life (p < 0.05) and delayed peak plasma concentration (p < 0.001) were obtained following the SC administration of the IVM-TEST compared to the IVM-CONTROL formulation. No differences in total plasma availability were observed among treatments. However, the plasma residence time and elimination half-life of IVM were significantly longer after injection of the IVM-TEST formulation. IVM plasma concentrations were above 0.5 ng/ml for 20.6 (CONTROL) and 27.5 days (IVM-TEST SC), respectively (p < 0.05). The modified kinetic behaviour of IVM obtained after the administration of the novel oil-based formulation examined in this trial, compared to the standard preparation, may positively impact on its strategic use in cattle.

Integrated control of nematode infections in cattle: a reality? A need? A future?

Helminth infections are a major cause of production loss in cattle. Great progress has been achieved in the design of control strategies for these infections. Control is based mainly on the use of anthelmintics, and these have become more potent and easier to administer. However, the most effective control is possible only through the integration of different approaches. Moreover, an increasing number of disadvantages of chemotherapy/prophylaxis--biological, economical and environmental--have been suggested. In sheep, the high incidence of anthelmintic resistance has simply forced veterinarians/producers to adopt alternative control strategies; in cattle, no real need for deviation from the actual control programmes seems to exist. Therefore, the following questions are discussed: (1) Based on the distribution of cattle worldwide, what are the target parasites? (2) Can we continue to rely on control based mainly on the use of (highly effective) anthelmintics? (3) What are the prospects for non-chemical control? (4) Who will develop and implement integrated control systems? (5) In the case of parasite control in Western Europe, has it been efficient and can/need it be changed? (6) How can we integrate helminth control in the general design of herd disease control?

Gastrointestinal nematode infections of first-grazing season calves in Western Europe: general patterns and the effect of chemoprophylaxis

Research on the prevention of gastrointestinal nematode infections of cattle has mainly concentrated on comparing a specific chemoprophylactic treatment system to an untreated control group on a particular farm. Here, the results from analysis of 85 studies involving over 2000 first grazing season (FGS) calves put onto pasture for at least 4 months from late spring/early summer over a 26-year period in 13 countries in Western Europe are presented. Both control and chemoprophylactic treated FGS calf groups were considered. All chemoprophylactic systems (slow- and pulse-release boli, strategic treatments) were given early in the grazing season. Two general infection levels emerged--'sub-clinical' (32 studies) and 'clinical' (53 studies). The 'sub-clinical' infections were characterised by no clinical symptoms of parasitic gastroenteritis (PGE) being observed in the control groups. Mean faecal egg counts in the 'clinical' control groups were significantly higher than those for 'sub-clinical' control groups for almost the entire season with overall peaks of 275 and 100 EPG respectively. Maximum pasture larval counts were also significantly higher in the 'clinical' control groups with 44% of 'clinical' pastures > 10,000 L3 kg(-1) dry herbage by the end of the FGS, compared to only 15% of 'sub-clinical' pastures. There was a significant positive relationship between log transformed worm burdens from tracers put onto pastures for 2 weeks and the corresponding pasture larval count. No evidence of density dependence in tracer worm burden was observed. Weight gains in the 'clinical' control groups (375 g/day) were significantly lower than those of the 'sub-clinical' control groups (530 g/day). No symptoms of PGE were observed in any of the chemoprophylactic treated groups, but in those studies with an outbreak of PGE in the control group, the treated groups had significantly higher faecal egg and pasture larval counts than treated groups in 'sub-clinical' studies. The overall weight gain in chemoprophylactic treated calves in 'clinical' studies (600 g/day) was significantly lower than the chemoprophylactic treated calves in 'sub-clinical' studies (690 g/day), and was not significantly different from the weight gain of control calves in 'sub-clinical' studies. These results indicate that on heavily infected pastures, chemoprophylaxis will prevent PGE, but calves will still suffer production losses.

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

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

Anthelmintic resistance

Since the first reports of resistance to the broad spectrum anthelmintics were made some three decades ago, this phenomenon has changed from being considered merely as a parasitological curiosity to a state of industry crisis in certain livestock sectors. This extreme situation exists with the small ruminant industry of the tropical/sub-tropical region of southern Latin America where resistance to the entire broad spectrum anthelmintic arsenal now occurs. In contrast, the cattle industry does not appear to be threatened--or so it seems. Although field reports of resistance have been made to the range of broad spectrum anthelmintics in nematode parasites of cattle, it appears that the evolution of resistance in cattle parasites is not as dramatic as for sheep worms. However, one cannot remain confident that this state of affairs will remain static. Concern is shared amongst parasitologists that we have not looked closely enough. In regions of the world where internal parasites are considered a problem in cattle and drenching occurs frequently, no widespread surveys have been carried out. It appears that because of the very high costs and risks associated with taking a new active drug down the development track to marketing, that the pharmaceutical industry has, in general, turned away from this activity. By implication, the international small ruminant industry is too small for these companies to make the necessary investment. This begs two questions: What is the fate of the sheep (and goat) industries in those parts of the world where resistance is rampant and immediate ameliorative parasite control options are required? What will be the response if significant resistance is found in cattle parasites? There is a body of opinion which suggests that if resistance becomes an issue in the control of cattle parasites then the pharmaceutical industry will find it commercially attractive to re-enter the anthelmintic discovery and development business. This is based on the simple commercial fact that by far the greatest anthelmintic sales are associated with the cattle industry. However, this market is specific and sectoral, with by far the greatest sales in North America and Western Europe, where the prevalence of resistance is likely to be low and remain so, more-or-less indefinitely. So the chances of the above scenario occurring must be considered low. Remarkable developments have recently occurred in non-chemotherapeutic parasite control options, for example worm vaccines, host selection and biological control. Also, there seems to be greater acceptance of various grazing management practices designed to reduce the frequency of anthelmintic treatment. However, they collectively cannot be expected to offer immediate salvation to farmers now faced with chemotherapeutic failure to control nematode parasites in their flocks. The future for these farmers must be considered bleak, because compounded with these problems are the poor commodity prices for sheep and goat meat and fibre, resulting in relentless reductions in funding for research to support these industries. Perhaps the major social issues associated with re-structuring and possibly abandonment of sheep and goat farming in affected areas may precipitate action? As veterinary parasitologists, who in general have an interest and expertise in parasite control, we must promote the importance of the problem of anthelmintic resistance and ways to tackle it.

Efficacy of morantel sustained release trilaminate bolus against gastrointestinal nematodes in grazing dairy calves in Kenya

The efficacy of morantel sustained release trilaminate (MSRT) bolus against gastrointestinal nematodes was evaluated under field conditions over a 10-month period. Twenty weaner calves were randomly divided into 2 groups of 10 calves each and grazed from March to December on adjacent, similarly contaminated paddocks. Group 1 calves (T-1) served as untreated controls while group 2 calves (T-2) were dosed at turnout with MSRT bolus designed to release morantel tartrate continuously for 90 days. The efficacy of MSRT was assessed by comparison of parasitological data (faecal worm egg counts, herbage larval counts, worm counts from tracer calves and set-stocked trial calves, determination of haematological parameters and pepsinogen levels), weight gains and clinical status of the animals. Faecal egg counts from the treated group (T-2) were reduced by 100% (P < 0.001) following treatment and remained significantly (P < 0.05) lower than counts from T-1 calves up to trial termination. The use of MSRT bolus resulted in a reduction of 92% (P < 0.001) in the number of gastrointestinal nematodes in set-stocked calves at the end of the study and a 55 to 85.7% reduction in herbage larval infectivity as reflected in lowered parasite burdens in tracer calves. At the trial termination, the control calves had gained on average (+/- s.d.) 59.4 +/- 4.8 kg (200.0 +/- 7.4 g day-1) and the treated ones on an average 128.6 +/- 10.5 kg (530.0 +/- 13.1 g day-1).

Gastrointestinal nematode infections of first-season grazing calves in Belgium: general patterns and the effect of chemoprophylaxis

Comparative analyses of the patterns of gastrointestinal nematode infections of first-grazing season cattle in Belgium are presented. The analysis involves 17 studies covering a 10 year period on 13 different farms in Flanders, Belgium. In all studies the calves were divided into an untreated control group, and one or two groups treated with chemoprophylactic systems. Two general infection levels emerged-'sub-clinical' (14 studies) and 'clinical' (three studies). The 'sub-clinical' infections were characterised by no clinical signs of parasitic gastroenteritis in the untreated control groups. Mean faecal egg counts remained low (less than 200), maximum pepsinogen levels only reached about 3500 mU tyrosine, and very small reductions in overall daily weight gain were observed compared with calves given chemoprophylaxis (less than 40 g day-1). Based on these results, on these 'sub-clinical' farms, chemoprophylaxis may not have been needed. In contrast, multiple salvage treatments of the control calf groups were required in the 'clinical' infections. Even with these salvage treatments mean faecal egg counts were high (more than 300), maximum pepsinogen levels were over 5500 mU tyrosine and there was a very large reduction in overall daily weight gain (more than 300 g day-1). However, it was not possible to predict either at turnout, or during the first month afterwards whether an infection on a particular farm would develop into a 'clinical' infestation. With the present data this prediction was possible from 8 weeks (Day 56) onwards, based on faecal egg counts and pasture larval contamination. It was also possible to predict using serum pepsinogen levels on Day 84. Therefore, one possible strategy for the effective control of gastrointestinal nematode infections of calves in temperate regions would be to evaluate faecal egg counts 2 months after turnout, and then only start treatment (i.e. metaphylaxis) if required.

The efficacy of anthelmintic treatment on the parasite abundance of free-ranging snowshoe hares

We examined the efficacy of an anthelmintic agent (ivermectin, Merck–Agvet) on the parasitic nematodes of free-ranging snowshoe hares (Lepus americanus) during 27 months in southern Manitoba. Marked hares on six 25-ha sites were injected bimonthly with ivermectin (treatment, n = 134) or propylene glycol (control; n = 149) and carcasses were necropsied to determine prevalence and intensity of Obeliscoides cuniculi, Nematodirus triangularis, Trichuris leporis, Dirofilaria scapiceps, and Protostrongylus boughtoni. We supplemented the control sample with hares (n = 175) captured off the study sites and not given propylene glycol. Mean prevalence (percentage of hosts infected) and intensity (number of parasites among infected hosts) of each nematode species decreased after treatment and were 35 – 85% lower than controls after 20–40 days. Treatment significantly reduced prevalence of D. scapiceps and P. boughtoni for about 50 days and of O. cuniculi and N. triangularis for longer periods. Overall intensity of O. cuniculi, N. triangularis, and P. boughtoni was significantly lower in treated than in control hares, and intensity of D. scapiceps was marginally (P < 0.10) lower for 50 days post treatment. Total reductions in nematode abundance (prevalence × intensity) ranged from 29 to 88% among the five species. Neither prevalence nor intensity of ticks (Haemaphysalis leporispalustris) was reduced following treatment. We conclude that ivermectin is effective at controlling nematodes in snowshoe hares but that its efficacy is variable among species. We use our results as a basis for discussing the applicability of anthelmintics to the conservation and management of free-ranging wild mammals.