Fenbendazole in equids: Further controlled tests with emphasis on activity of multiple doses against naturally occurring infections of migratory large strongyles
Authors: Lyons E. T. , Drudge J. H. , Tolliver S. C. 
Published: February 2025

Two controlled tests (experiments D and E) were done to evaluate a fenbendazole (fbz) paste dosage regimen (10 mg/kg once a day for 5 days) for activity against naturally occurring infections of migrating Strongylus vulgaris and S edentatus in the mesenteric arteries and ventral abdominal wall, respectively. Data were also obtained on other internal parasites of the gastrointestinal tract and eyes in experiment E. Eight pony yearlings were used in experiment D (4 treated and 4 nontreated) and 6 horse weanlings were used in experiment E (3 treated and 3 nontreated). Intervals, expressed as days between treatment and necropsy of animals, were 42 (n = 1) or 52 (n = 7) for experiment D and 52 or 53 for experiment E. Animals were on pasture during all or a portion of post-treatment intervals of both experiments.

Efficacious killing of retroperitoneal forms of S edentatus in the ventral abdominal wall was observed in both experiments. The average numbers of live 4th-stage forms in equids were 0.0 (treated) and 2.25 (nontreated) for experiment D and 2.0 (treated) and 1.7 (nontreated) for experiment E. The average numbers of 5th-stage S edentatus were 0.5 (treated) and 4.0 (nontreated) for experiment D and 1.3 (treated) and 23.3 (nontreated) for experiment E. Fragments of dead specimens of S edentatus were also found in nodules in treated animals in both experiments, but not in nontreated animals. Consistency of the retroperitoneal lesions provided additional evidence of the killing action of the FBZ treatment. Lesions in treated animals were nodules which varied from soft to hard in contrast with the edematous, cystic-type lesions characteristic for this parasite in nontreated animals.

A similar killing effect on S vulgaris in cranial mesenteric arteries was recorded in both experiments. The average numbers of 4th-stage forms in equids were 3.0 (treated) and 12.0 (nontreated) for experiment D and 2.0 (treated) and 77.7 (nontreated) in experiment E. The average numbers of 5th-stage S vulgaris in arteries were 0.0 (treated) and 3.0 (nontreated) in experiment D and 0.0 (treated) and 36.0 (nontreated) in experiment E. Arterial lesions also provided an index of the effect of treatment. In treated animals, evidence of larval activity was slight compared with the moderate-to-high levels found in nontreated controls. Severity of lesions in treated animals was generally less extensive with evidence of healing, compared with the more extensive, nonhealing lesions found in the nontreated animals.

For experiment E, additional antiparasitic efficacy of FBZ was evaluated. Eyeworms (Thelazia lacrymalis) and 1 species of stomach worm (Draschia megastoma) were present in low numbers, but some removal activity was evident. Intestinal stages of ascarids (Parascaris equorum) and S vulgaris were present in lower numbers in treated than in nontreated animals; some may have entered the lumen of the intestine during the long (about 2 months) posttreatment period.

Transient toxicosis possibly appeared after treatment of 2 weanling horses; 1 had colic after the 2nd treatment, and the other, hyperperistalsis after the 4th treatment. Toxicosis was not evident in the treated ponies.