Strongylid
infections are ubiquitous in grazing horse populations.
Infections with cyathostomin (small strongyle) and strongylin (large strongyle) nematodes have long been associated with clinical disease in horses, but little is known about their subclinical impact. A masked, randomized, controlled study was conducted to evaluate the effects of daily administration of
pyrantel tartrate on body condition scores,
weight gain, fecal egg counts, and total worm counts of young horses repeatedly inoculated with strongylid larvae. Twenty eight immature horses were treated with larvicidal
anthelmintic regimens and randomly allocated to two groups. Group 1 horses were given a pelleted placebo product once daily, and those in Group 2 received
pyrantel tartrate once daily at ∼ 2.64 mg/kg
body weight. On five days during each week, ∼ 5000 infective cyathostomin larvae were administered to each horse. In addition, horses received ∼ 25 infective Strongylus vulgaris larvae once weekly. Horses were maintained on pasture for 154 days and had ad libitum access to grass hay throughout. At approximate, 14-day intervals,
body weights were measured, body condition scores were assigned, fecal samples were collected for egg counts, and blood samples were collected for measurement of S. vulgaris
antibodies and various physiologic parameters. After 22 weeks at pasture and 14-17 days in confinement, horses were euthanatized and necropsied. Nematodes were recovered and counted from aliquots of organ contents, representative samples of large intestinal mucosa, and the root of the cranial mesenteric artery. Daily treatment with
pyrantel tartrate at the recommended dosage significantly reduced numbers of adult cyathostomins in the gut lumen and early third-stage larvae in the cecal mucosa, increased the proportions of fourth-stage larvae in the gut contents, and was accompanied by significant improvements in body condition scores. Fecal egg counts of horses receiving daily
pyrantel tartrate were significantly reduced, with percentages of efficacy ranging from 84.4% to 98.9%, but egg counts of both groups increased significantly over the course of the study. Treatment also significantly reduced the numbers of S. vulgaris larvae in the cranial mesenteric artery by 99.2%. Serum
antibodies to S. vulgaris apparently persisted from pre-enrollment
infections, but ELISA values gradually declined over the course of the study. This study has provided useful insights into the effects of daily
pyrantel tartrate on the dynamics of cyathostomin
infection, and into some subclinical effects of strongylid parasitism in horses.