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 7.5mg/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 7.5mg/kg per os and
after treatment orally for five consecutive daily doses at 7.5mg/kg in feed.