Ivermectin should not be drug of choice for large roundworms in foals says Swedish Study

Ivermectin has been found to be ineffective in the control of roundworms in foals.
Ivermectin should not be the drug of choice for controlling Parascaris equorum (large roundworm) in foals, a recent Swedish study indicates.

The authors also emphasised the growing importance of non-chemical strategies in the control of parasites.

Dr Eva Osterman-Lind and Dr Dan Cristensson of the Department of Virology, Immunobiology and Parasitology, at the National Veterinary Institute, in Uppsala, investigated the occurrence of Parascaris equorum infection on nine stud farms in Sweden and assessed the effectiveness of three commonly used dewormers on faecal egg output.

A faecal egg count reduction test (FECRT) was used to assess the effectiveness of the three deworming agents (ivermectin, fenbendazole and pyrantel) against P equorum in groups of weaned foals, with an average age of 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 their faeces 14 days after ivermectin treatment. On only one farm where ivermectin was used did the post-treatment egg count fall by more than 90 per cent compared with pre-treatment levels.

In contrast, the FECRT 14 days after treatment was 100 per cent in the fenbendazole treated group, and more than 90 per cent 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
doi: 10.1186/1751-0147-51-45