Studies on a Thoroughbred breeding farm in Ohio were done to: (1) compare the effects of daily administration of
pyrantel tartrate feed pellets with monthly administration of a
pyrantel pamoate paste to yearling horses (21 January-3 September); (2) assess the effects of daily
pyrantel tartrate given strategically in spring/summer to foaling mares (1 April-16 August) and given for a prolonged period to barren mares (21 January-3 September); (3) determine if strategic medication of foaling mares with daily
pyrantel tartrate protected their foals until weaning. There were no differences in cyathostome egg counts, pasture larval counts, body condition scores, or
body weights of yearlings treated with daily
pyrantel tartrate or monthly
pyrantel pamoate. Both treatments failed to maintain fecal egg counts of yearlings below 100 eggs per gram (epg), and mean counts exceeded 400 epg (
pyrantel pamoate) and 700 epg (
pyrantel tartrate) in August and September, resulting in a sharp, but moderate increase in pasture infectivity in October. By contrast, prolonged or strategic use of daily
pyrantel tartrate in mature horses were each highly effective in reducing pasture contamination and 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 when mean strongyle counts exceeded 100 epg. Treatment of weanlings with
pyrantel pamoate had little effect on egg counts. A comparative
anthelmintic study with
ivermectin,
oxibendazole, and
pyrantel pamoate confirmed earlier studies showing reduced efficacy of
anthelmintics in young horses.