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Abstract
Strongylus vulgaris is the most pathogenic nematode parasite of horses. Its extensive migration in the mesenteric blood vessels can lead to life-threatening intestinal infarctions. Recent work has shown that this parasite is still identified among managed horse populations. A serum enzyme-linked immunosorbent assay (ELISA) has been developed for the detection of migrating larvae of S. vulgaris. Previous work has documented an increase in ELISA values following larvicidal treatment with ivermectin and suggested that the target parasite antigen is primarily produced by the later larval stages. The aim of this study was to experimentally inoculate cohorts of foals with S. vulgaris, and then compare ELISA responses to early or later ivermectin treatments. Fifteen foals were held in confinement and infected orally with ~25 S. vulgaris third-stage larvae on Days 0, 7, 14, and 21. Foals were weaned on Day 43 and turned out to a pasture not previously grazed by horses. Foals remained at pasture continuously until the study was terminated on Day 196. On Day 55, foals were randomly allocated to three treatment groups of five each. Group 1 received ivermectin on Day 56, Group 2 received ivermectin on Day 112, and Group 3 foals served as untreated controls. Serum and fecal samples were collected at 28-day intervals throughout the study. Serum samples were analyzed with the S. vulgaris-specific ELISA and fecal samples were processed for fecal egg counting. The ELISA values of Group 1 foals were significantly lower than Groups 2 or 3 on Days 140–196. Both treated groups exhibited increased ELISA values following ivermectin treatment. Results indicate that the target diagnostic antigen is produced throughout the course of arterial infection with S. vulgaris, but that an early ivermectin treatment can reduce the cumulative antigen produced over the course of an infection.
Highlights
Strongylus vulgaris is considered to be the most pathogenic helminth parasite of horses due to the extensive migration of its larval stages
For the ascarid egg counts, no reduction was observed after the Day 56 ivermectin treatment in Group 1, whereas the Day 112 treatment in Group 2 reduced the counts by 100%
This study generated useful new information about elaboration of S. vulgaris-specific antibodies in response to ivermectin treatment of an established infection with migrating larvae
Summary
Strongylus vulgaris is considered to be the most pathogenic helminth parasite of horses due to the extensive migration of its larval stages. Upon infection, exsheathed third-stage larvae penetrate the large intestinal mucosa, where they molt to the fourth larval stage (L4) within the first 5 days. The L4s migrate proximally toward the CMA, where they arrive about 11–14 days post infection (PI). In the CMA, fourth-stage larvae (L4) molt to the immature adult stage (L5) at about 90 days PI, and migration back to the large intestine begins around Day 120 PI [1]. Migrating larvae cause fibrinous endarteritis with pronounced thrombosis, formation of aneurysms, and other pathologic alterations of the vessel structures [2]. Thrombo-embolism caused by L4s and immature adults has been associated with a painful and often fatal colic syndrome, characterized by ischemia and non-strangulating infarction of intestinal segments [3,4,5]
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