Trials were conducted in ponies to evaluate the efficacy of pyrantel pamoate (Strongid-T®) and two newer anthelmintics not yet commercially available, nitramisole and avermectin B1a, against migrating Strongylus vulgaris larvae. Ponies were removed from their mares within 24-48 hr after birth and reared in isolation, worm free. Between six and 14 weeks of age they were infected with 2000 or 2500 infective S. vulgaris larvae. Subsequently, they were monitored daily for clinical signs until the experiment terminated at 28 days postinfection. All ponies showed increased body temperature and reduced appetite within the first week of infection. All anthelmintics were administered on day 7 and in addition pyrantel pamoate was given on day 8 postinfection. The anthelmintics were in liquid formulation. Nitramisole and pyrantel pamoate were given by stomach tube and avermectin B1a by subcutaneous injection.
Following administration of these compounds toxic reactions were not observed. All anthelmintics caused a reduction in body temperature and increased appetite and effected a clinical cure. In ponies which were not treated with an anthelmintic, temperatures remained elevated and appetites never returned completely to normal. These ponies also showed variable degrees of lethargy, depression, recumbency and colic and the majority died between two and three weeks postinfection. At necropsy, these control ponies showed variable degrees of adhesions involving the abdominal organs, necrosis of the ileum and cecum and severe arteritis and thrombosis of the major abdominal arteries and their branches.
Although pyrantel pamoate, used at eight times the therapeutic dose for intestinal nematodes in the horse, effected a clinical cure it did not produce a radical cure. At necropsy, ponies treated with pyrantel pamoate had arteritis and thrombosis of the cranial mesenteric artery and its major branches. Nitramisole and avermectin B1a were able to effect both a clinical and radical cure.