Ivermectin should not be the drug of choice for controlling Parascaris equorum (large roundworm) in foals a recent study warns.
Dr Eva Osterman-Lind and Dr Dan Cristensson of the Department of Virology, Immunobiology and Parasitology, at the National Veterinary Institute, Uppsala, Sweden, investigated the occurrence of Parascaris equorum infection on nine stud farms in Sweden and assessed the efficacy of three commonly used dewormers on fecal egg output.
A faecal egg count reduction test (FECRT) was used to assess the efficacy of the three anthelmintics (ivermectin, fenbendazole and pyrantel) against P equorum in groups of weaned foals (median age 6.5 months). The foals had been dewormed already two or three times over the summer. Faecal samples were examined for ascarid eggs on the day of deworming and 14 days later.
Ivermectin had very little, or no, effect on the output of ascarid eggs. On three studs all foals included in the study still had P equorum eggs in the feces 14 days after ivermectin treatment. On only one farm where ivermectin was used did the post-treatment egg count fall by more than 90% compared with pre-treatment levels.
In contrast, the FECRT 14 days after treatment was 100% in the fenbendazole treated group, and more than 90% in the small group of foals treated with pyrantel.
“The most striking result from this study was that in five studs out of six, ivermectin failed to suppress the faecal output of P. equorum eggs” Osterman-Lind and Cristensson report.
“Ivermectin resistance is now a widespread problem in Swedish stud farms.”
They advise that instead of ivermectin, fenbendazole or pyrantel are now the drugs of choice for use against P. equorum.
“It is important, however, that the anthelmintic efficacy is monitored routinely by FECRT ” they emphasise. “In the long-term it is also necessary to incorporate non-chemo-therapeutic methods to a greater extent to control parasite infections on stud farms.”
Anthelmintic efficacy on Parascaris equorum in foals on Swedish studs.
Eva Osterman Lind, Dan Christensson
Acta Veterinaria Scandinavica (2009) 51, 45