HOW-TO SESSION: LIFE STAGE MANAGEMENT


5 d) has been shown to be very effective against migrating ascarid larvae,47 even ML-resistant iso- lates.48 Considering the growing number of re- ports from around the world of P. equorum populations that have become resistant to MLs, a larvicidal dose of fenbendazole may be the only drug treatment still effective against migrating ascarid larvae.


Understanding the mode of action and spectrum of activity of the three major classes of anthelmintics, benzimidazoles, tetrahydropyrimidines, and MLs enables practitioners to leverage the full potential of each drug class. The mode of action of benzimidazole anthelmint-


ics (e.g., fenbendazole, oxibendazole) is to disrupt parasite energy metabolism at a cellular level. Time to kill susceptible parasites is generally slower than that of other drug classes. These dewormers provide a broad spectrum of efficacy against large strongyles, cyathostomins, Parascaris, and Oxyuris infections. Oxibendazole is effective against S. westeri when administered at 15 mg/kg. Fen- bendazole administered at an elevated dose of 10 mg/kg for 5 consecutive days exhibits larvicidal effi- cacy against migrating large strongyles and suscep- tible populations of migrating and encysted small strongyles. Although not part of the label claim, the larvicidal dose of fenbendazole has been shown to kill migrating ascarid larvae.47,48 Larvicidal treatment with fenbendazole has been suggested as a possible tool for preventing the inadvertent intro- duction of resistant ascarid isolates by foals arriving from other farms.8 This treatment also has been shown to suppress ascarid egg shedding for at least 8 weeks post treatment.4 Although benzimidazoles may be the optimal drug class to control P. equorum infections, reports of benzimidazole resistant cya- thostomin populations10–15 suggest that other drug classes might be better suited for control of drug- resistant small strongyle infections in older, post- weaning-age foals. Pyrimidine anthelmintics are marketed for horses


in North America as pyrantel pamoate, available in paste or suspension formulations and as pyrantel tartrate, available as a pelleted formulation de- signed to be used as a daily feed additive. The mode of action for these drugs is as selective acetyl- choline agonists resulting in rapid, spastic paralysis of susceptible worms that are then usually expelled by intestinal peristalsis. Given that there is no intestinal absorption of the pyrimidine anthelmint- ics, their effect is confined to only luminal stages of parasites and does not include migrating larval stages. Lack of efficacy against fourth-stage cya- thostomin larvae (L4) has been reported.49 Pyrim- idines are broad spectrum with efficacy against cyathostomins, large strongyles, ascarids, and pin worms. In addition, pyrimidines have demon- strated good efficacy against the equine cestode, Anoplocephala perfoliata. The label dose of pyrantel pamoate (6.6 mg/kg) may provide at least 80% ces-


474 2016  Vol. 62  AAEP PROCEEDINGS


tocidal efficacy50 and administration of twice that dose (13.2 mg/kg) affords greater than 95% efficacy against tapeworms.51 Confirmed and suspected cyathostomin resistance to pyrimidine dewormers has been reported in numerous countries world- wide.16–18,52 Although the pyrimidines are usually considered to be effective against P. equorum infec- tions,5,6,8 there are a growing number of reports of pyrantel-resistant ascarid populations.4 Some re- searchers have demonstrated that administering pyrantel pamoate at a higher dose of 13.2 mg/kg demonstrates good efficacy against drug-resistant P. equorum populations.4,53 The ML drug class contains two subgroups: the avermectins and milbemycins. Both drugs cause neuromuscular flaccid paralysis in susceptible par- asites by interfering with the function of glutamate- gated chloride channels found only in neurons and myocytes of invertebrates. Affected parasites can no longer ingest nutrients and die of starvation or are expelled by intestinal peristalsis if they are re- siding within the gut lumen at the time of treat- ment. Similar to the pyrimidines, onset of action is rapid and occurs within 48 hours following treat- ment. Both ivermectin and moxidectin have broad parasiticidal activities and are effective against lu- minal stages of nematodes as well as migrating lar- val stages of ascarids, large strongyles, and Strongyloides. In addition to nematodes, these drugs also exhibit efficacy against arthropods and are useful in the treatment of Gasterophilus larvae. Drugs in this class are also effective against On- chocerca larvae, Habronema and Draschia. Moxi- dectin is more lipophilic than ivermectin, which accounts for its accumulation in fatty tissues, longer elimination half-life, and prolonged ERP. Unlike ivermectin, moxidectin demonstrates efficacy against encysted cyathostomin larvae. Safety profiles also differ. Due to its lipophilic nature, moxidectin be- comes highly concentrated in serum when it is admin- istered to horses with very little body fat, such as foals or thin, debilitated individuals. The immature blood brain barrier of young foals renders them more sus- ceptible to moxidectin toxicity. Signs associated with toxicity include dyspnea, depression, weakness, ataxia, coma, and seizures.54,55 In the United States, moxidectin is not labeled for use in foals less than 6 months of age. Provided the ascarid population on a given farm is


not ML resistant, ivermectin exhibits good efficacy against P. equorum larval and adult stages. How- ever, reports of ML-resistant ascarids are wide- spread and worldwide.1–8,56 If ivermectin-resistant ascarids are identified in pre-weaning-age foals, it is the author’s experience that moxidectin will be in- effective in clearing ascarid infections in older wean- lings and yearlings on the same farm. Macrocyclic lactones continue to demonstrate good efficacy against cyathostomins at 14 days post-treatment and large strongyles remain susceptible. However, there are reports of the strongyle ERP becoming


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