Population-S benzimidazole- and tetrahydropyrimidine-resistant small strongyles in a pony herd in Kentucky (1977-1999): effects of anthelmintic treatment on the parasites as determined in critical tests.

The research article focuses on the study of population-S small strongyles in a closed Shetland pony breeding herd. The study, carried out in central Kentucky from 1974 to 1999, evaluated the effects of various anthelmintic treatments on the parasites and the relative changes in the composition of small strongyle species over this period.

Research Methodology

• The study examined small strongyles in a Shetland pony herd from 1974 to 1999, wherein the ponies were periodically treated with different anthelmintics: cambendazole (1974-1978), oxibendazole (OBZ) (1978-1992), or pyrantel pamoate (PRT) (1992-1999).
• The effects of these treatments on the small strongyle species were analyzed based on data from critical tests.
• The treatment effects were evaluated in terms of (1) the numbers of gravid worms passed in feces and recovered at necropsy, (2) reductions in egg and larva counts per gram of feces before and after treatment, and (3) reduction percentages in egg counts versus worm removal.
• The study also tracked the changes in the composition of small strongyle species over the 22-year period.

Key Findings

• The researchers observed that the posttreatment reductions in egg counts were greater than those in larva counts with OBZ, but not with PRT. Further, the reductions in egg counts were also greater than the actual worm removal rate for both anthelmintics.
• The results indicated that egg count reductions post treatment suggested a higher drug activity than what was actually reflected in worm removal – underscoring the value of critical tests for properly assessing anthelmintic activity.
• Over the course of the study, a total of 28 species of small strongyles were identified. Of the 20 most prevalent species, two decreased, three initially increased and then plateaued, five increased but subsequently decreased, five continuously increased, while five had no significant change over time.
• The highest numbers of small strongyles were recorded in 1987 and 1999.

Implications

• The study provides important insights into the relative efficacy of different anthelmintic treatments on small strongyles and the need for more precise or additional measures beyond egg and larva counts to assess treatment effectiveness.
• The findings can guide future research and development of more effective treatments for gastrointestinal parasites in equine health.