Effect of early season ivermectin and pyrantel treatments on strongylid infections in young Shetland ponies in The Netherlands.

This research article investigates the effectiveness of using ivermectin or pyrantel, two types of deworming drugs, on young Shetland ponies in the Netherlands to treat strongylid infections, which are types of intestinal parasitic worms. The study found that ivermectin was more potent than pyrantel in reducing both egg output in feces and the worms’ burdens at the end of the study. However, one type of worm, Strongylus vulgaris, was found in all test subjects despite the treatments, suggesting that some worms may be resistant or that the larvae may survive colder weather on pastures.

Study Design

• The researchers used two groups of three Shetland ponies each to test the effectiveness of the treatments.
• Each group was treated with either ivermectin or pyrantel three times at five week intervals at the start of the grazing season.

Results of Pyrantel Treatment

• Each pyrantel treatment led to a reduction of more than 95% in fecal egg counts for the initial three weeks.
• However, high counts of larvae were seen in pastures from the start of August onwards, and the ponies had significant cyathostomine (a type of small strongyle) burdens as determined at necropsy (a post-mortem examination) in December.

Results of Ivermectin Treatment

• The ivermectin treatments provided a more significant reduction in fecal egg output.
• The pasture larval counts and cyathostomine burdens at necropsy were considerably lower in the ivermectin-treated group compared to the pyrantel-treated group.
• The proportion of early L3 (larval stage 3) cyathostomines that were inhibited or ‘stunned’ was lower in the ivermectin group compared to the pyrantel group.
• Fecal egg output of large strongyles (a type of intestinal worm) was completely suppressed in the ivermectin group.

Possible Resistance

• Despite the successful treatment, Strongylus vulgaris larvae were found in all treated ponies in both groups, suggesting possible resistance to the deworming drugs or the capability of the larvae to survive over winter on the pasture.
• In the pyrantel group, egg output from Strongylus edentatus and to a lesser degree Strongylus vulgaris, were not completely suppressed, indicating that pyrantel might not be fully effective against these worms.