Ovicidal efficacy of fenbendazole after treatment of horses naturally infected with cyathostomins

- Significant decrease in egg hatch viability for three days after fenbendazole treatment in resistant cyathostomins.
- Significant reduction in the numbers of eggs and larvae, compared to baseline, of BZ resistant cyathostomins after fenbendazole treatment.
- Ovicidal effect of a five consecutive day course of fenbendazole in feed is no longer than a single oral dose per os.

The ovicidal activity of benzimidazole (BZ) anthelmintics is unique and not seen in other drug classes. Such ovicidal efficacy is not widely reported for equine cyathostomins, nor has this activity been tested in the face of BZ resistance. Although the product label states that fenbendazole is for use against BZ-susceptible cyathostomins, susceptibility testing is rarely performed. In this field-based study, the ovicidal efficacy of fenbendazole in horses (n = 39) harbouring BZ-resistant cyathostomins was compared when dosed at 7.5 mg/kg body weight (BW) orally, as a single dose per os (n = 21) or daily for five consecutive days in feed (n = 18). Suppression of egg hatch rate was observed in the single and five- day treatment groups; a significant difference between pre- and post-treatment egg hatch rates (P < 0.05) was observed for three days after treatment with a single dose of fenbendazole (on premises with BZ-resistant cyathostomins), and for three days after treatment for five consecutive days with fenbendazole (on premises with BZ-resistant cyathostomins). Post treatment numbers of eggs and larvae remained significantly lower (P < 0.05) than pre-treatment levels to the end of the trial. We conclude that in the face of BZ-resistant cyathostomins the ovicidal effect of fenbendazole persist for three days after both a single oral dose of 7.5 mg/kg per os and after treatment orally for five consecutive daily doses at 7.5 mg/kg in feed.