COMPARISON OF DAILY AND MONTHLY PYRANTEL TREATMENT IN YEARLING THOROUGHBREDS AND THE PROTECTIVE EFFECT OF STRATEGIC MEDICATION OF MARES ON THEIR FOALS

Studies on a Thoroughbred breeding farm in Ohio were done to: (1) comp are the effects of daily administration of pyrantel tartrate feed pell ets with monthly administration of a pyrantel pamoate paste to yearlin g horses (21 January-3 September); (2) assess the effects of daily pyr antel tartrate given strategically in spring/summer to foaling mares ( 1 April-16 August) and given for a prolonged period to barren mares (2 1 January-3 September); (3) determine if strategic medication of foali ng mares with daily pyrantel tartrate protected their foals until wean ing. There were no differences in cyathostome egg counts, pasture larv al counts, body condition scores, or body weights of yearlings treated with daily pyrantel tartrate or monthly pyrantel pamoate. Both treatm ents failed to maintain fecal egg counts of yearlings below 100 eggs p er gram (epg), and mean counts exceeded 400 epg (pyrantel pamoate) and 700 epg (pyrantel tartrate) in August and September, resulting in a s harp, but moderate increase in pasture infectivity in October. By cont rast, prolonged or strategic use of daily pyrantel tartrate in mature horses were each highly effective in reducing pasture contamination an d infectivity with cyathostome eggs and larvae respectively. Strategic medication of foaling mares provided protection of their foals until weaning and first treatment of foals was delayed until after weaning w hen mean strongyle counts exceeded 100 epg. Treatment of weanlings wit h pyrantel pamoate had little effect on egg counts. A comparative anth elmintic study with ivermectin, oxibendazole, and pyrantel pamoate con firmed earlier studies showing reduced efficacy of anthelmintics in yo ung horses.