Answer – MYTH (according to one recently published study)
Small strongyles (cyathostomins) have become the most prevalent and important internal parasite of the horse. Emergence of large numbers of the larvae from the intestinal wall during late winter/spring has been linked to issues such as acute diarrhea, fever, colic and sometimes death, with the most severe complications being seen most often in horses less than 5 years of age. Frequent long term use or misuse of dewormer products (anthelmintics) has led to resistance by these small strongyles to many of the anthelmintics available today. An FEC (Fecal Egg Count) is the method most often used to identify and quantify the type and amount of eggs shed by a particular horse. FECs are also used after a dewormer is given to detect anthelmintic resistance. In one study recently published in JAVMA, 120 mares from 21 states all currently living on the same farm had an initial FEC done. Eighty-four mares had a FEC > 200 eggs/gram and were randomly divided into 2 groups, one group receiving a single dose of moxidectin and the other group receiving the 5 day treatment of fenbendazole. Follow up FECs at day 14 showed reduction in egg counts in both groups (moxidectin did better than fenbendazole) BUT by day 45, the group that received the fenbendazole had FECs that were HIGHER that the FECs recorded before they were dewormed. The FECs of the moxidectin group remained below their initial FEC numbers for the duration of the 90-day study. The conclusion from this study was that resistance to fenbendazole by small strongyles is fairly widespread and that moxidectin remains an effective treatment for the control of these very important parasites.
Scientific article: Mason ME, Voris ND, Ortis HA, Geeding AA, and Kaplan RM, Comparison of a single dose of moxidectin and a five-day course of fenbendazole to reduce and suppress cyathostomin fecal egg counts in a herd of embryo transfer-recipient mares. Journal of the American Veterinary Medical Association, vol. 245, No. 8, Oct. 15, 2014: 944-951