Evaluation of exclusive use of ivermectin vs alternation of antiparasitic compounds for control of internal parasites of horses
Authors: Lyons E. T. , Drudge J. H. , Tolliver S. C. , Granstrom D. E. , Stamper S. 
Published: January 2024

A study for about a 30-month period was done to compare strongyle control programs, using per os treatments of ivermectin (ive) paste exclusively or alternation of 4 antiparasitic paste compounds: ive, oxfendazole (ofz), oxibendazole (obz), or pyrantel pamoate (prt). Every 8 weeks, 1 group of horses (barn C; n = 14 to 16) was given ive paste exclusively, and a second group (barn E; n = 16) was given the 4 antiparasitic pastes on an alternating schedule. Worm eggs and larvae per gram of feces (epg and Ipg, respectively) values were determined every 2 weeks during the investigation. This study in grazing horses (mares and fillies), naturally infected with internal parasites, was conducted during the period between Oct 22, 1987 and Feb 8, 1990, with an additional observation on Mar 28, 1990.

For barn-C horses, treated exclusively with ive (200 µg/kg of body weight) 14 times, 2-week posttreatment mean strongyle epg and lpg (small strongyle) values were reduced 99 to 100%. Mean strongyle epg and Ipg (small strongyle) values for each 2-week sample period remained low (<20) throughout the study period, except for 1 moderate transient increase in July 1988. For the entire study period, the aggregate mean strongyle epg value was 12 and the lpg value was 6.

Two-week posttreatment mean strongyle epg and lpg (small strongyle) values for barn-E horses, treated alternately with therapeutic (approx) dosage of ive (200 µg/kg; 4 times), ofz(10 mg/kg; 5 times), obz (10 mg/kg; 4 tunes), or prt(6.6 mg base/kg; 2 times), varied within and between compounds. Posttreatment (2-week) mean epg values were reduced 100% by ive, 0 to 100% by ofz, 74 to 100% by obz, and 92 to 100% by prt. Mean small strongyle Ipg values at 2 weeks after treatment indicated reduction of: 100% for ive, 0 to 76% for ofz, 43 to 100% for obz, and 97 to 100% for prt. For the entire study period, the aggregate mean strongyle epg value was 54 and the lpg value was 64. The epg and Ipg reduction values for the 2 benzimidazoles indicated an increase in the benzimidazole-resistant segment of small strongyles. Fecal cultures for horses in both groups contained larvae of large strongyles (Strongylus vulgaris and S edentatus) only at the time of initial treatment.

Treatment program evaluations included necropsy of 9 foals (56 to 203 days old), 7 born to barn-C mares and 2 born to barn-E mares. Generally, low numbers of bots (Gasterophilus intestinalis) and Habronema muscae were found in foals of both groups. Intestinal stages of immature and mature ascarids (Parascaris equorum) were also found in foals born to both groups of mares. Mature pinworms (Oxyuris equi) were found in 1 barn-C foal. Only barn-E foals had Strongyloides westeri. Small strongyles were detected in foals of both groups, up to several thousand in some. The latter finding indicates, in particular, that although barn-C mares had low small strongyle epg and Ipg values throughout the study, eggs and larvae built up on pasture in sufficiently high numbers for major transmission to foals born there. Small strongyles in foals were composed of 3 genera and 10 species for barn-C foals and of 3 genera and 8 species for barn-E foals. Seasonal transmission of parasites also was observed.