ONE SEASON OF PASTURE EXPOSURE FAILS TO INDUCE A PROTECTIVE RESISTANCE TO CYATHOSTOMES BUT INCREASES NUMBERS OF HYPOBIOTIC THIRD-STAGE LARVAE

The development of acquired resistance to cyathostome challenge after 1 season's exposure to a cyathostome-contaminated pasture was investigated using 17 parasite-naive ponies, which were 2–3 yr of age. These were divided into 3 groups: 1 to graze a cyathostome-contaminated pasture for 4 mo (exposed ponies), 1 to graze a “clean” pasture not previously grazed by parasitized animals (nonexposed ponies), and 1 group to remain in the barn under helminth-free conditions (parasite-free ponies). After pasture exposure all ponies were housed in stalls in the barn and dewormed with ivermectin (200 μg/kg) and oxibendazole (100 mg/kg), a treatment that eliminated most cyathostomes encysted in the mucosa as well as all luminal parasites, on the basis of necropsies of 5 animals, after 17 days. Remaining ponies were challenged with 100,000 cyathostome-infective third-stage larvae (L₃) per os 3 wk after anthelmintic treatment. Necropsies were performed 7 wk after the challenge. Total cyathostome burdens (luminal plus encysted stages) were not significantly different among any of the groups. However, a significantly higher percentage of hypobiotic early L₃ (EL₃) and a lower percentage of adults were found in exposed ponies. This observation supports the hypothesis that resistance acquired through exposure promotes cyathostome hypobiosis. This increase in EL₃ in exposed ponies was associated with a significant increase in weight of cecum and ventral colon biopsies.