The Canadian study made clear that no single deworming can be assumed to control all species of worms in foals.
Parascaris equorum, the large roundworm of horses, is a common parasite of foals.
Older foals develop immunity to it, and it rarely causes problems in adult horses. Under optimum conditions, P. equorum eggs become infective within about two weeks of being passed in the faeces. At lower temperatures the eggs may survive for many years in stables and on pasture.
Infective eggs contain larvae, which hatch within the foal's intestines. The larvae penetrate the gut wall and migrate through the liver and lung before passing up the trachea and back to the intestines. They may cause a mild cough and nasal discharge during the migratory phase. A heavy intestinal infection leads to failure to thrive, and may cause intestinal impaction or rupture.
Professor Owen Slocombe of the Ontario Veterinary College and others, in a report in Veterinary Parasitology, describe how they found ivermectin - and moxidectin - desistant P. equorum on stud farms in Canada.
Ivermectin has been used for deworming horses for more than 20 years. Its efficacy against a wide range of equine internal parasites has made it a popular choice. It is highly effective against migrating large strongyles.
Early reports suggested that it was effective against P. equorum. Ivermectin has been widely used in foals, especially since the problem of cyathostomin resistance to fenbendazole has been recognised.
The problem was first suspected in 2001 by one of the authors of the report, Dr Rolph de Gannes.
He had been monitoring faecal worm egg counts on various horse farms in Ontario for more than 20 years. These routine tests alerted him to the apparent failure of ivermectin to remove P. equorum.
Over the next two years, in conjunction with Professor Slocombe and technician Mary Lake, he carried out a series of trials on two thoroughbred farms and one standardbred farm.
Faecal worm egg counts were monitored before and after treatment with one of four anthelmintics. Some foals were left untreated to act as controls.
Overall, they found that ivermectin reduced the Parascaris equorum faecal worm egg count by only 33.0% and moxidectin by 47.2%. In contrast, fenbendazole and pyrantel pamoate were highly effective, reducing the faecal egg count by 97.6%.
In fact, many foals had no P. equorum eggs in the faeces after treatment with fenbendazole or pyrantel.
It was a different story with strongyle eggs. All strongyle eggs look similar.
It is only possible to tell them apart by culturing them and examining the larvae. However, resistance to anthelmintics is a particular feature of the cyathostomins (or small redworms).
The scientists found that neither pyrantel nor fenbendazole was fully effective at reducing the number of strongyle eggs in the faeces. All foals treated with pyrantel still had strongyle eggs in the faeces after treatment.
Foals on the standardbred farm had eggs in the faeces after treatment with fenbendazole, suggesting that resistant cyathostomins were present. However, no foals had strongyle eggs in the faeces after treatment with ivermectin or moxidectin.
This study emphasises that a single dewormer cannot be assumed to control all species of worms in foals. It may well be necessary to use more than one type of dewormer to control all the potential parasite problems in foals.
For more details see:
Macrocyclic lactone-resistant Parascaris equorum on stud farms in Canada and effectiveness of fenbendazole and pyrantel pamoate. J OD Slocombe, RVG de Gannes, Mary C Lake. Veterinary Parasitology (2007) 145, 371 - 376.