Piperazine resistance in population-B equine strongyles: a study of selection in Thoroughbreds in Kentucky from 1966 through 1983.

The research study looks into the impact of piperazine resistance in a specific kind of horse parasite (equine strongyles) within a population of Thoroughbred horses in Kentucky. Data was collected over 18 years, revealing a trend of higher counts of worm eggs and larvae in the horse feces, suggesting increasing resistance to certain drugs.

Study Context and Methods

• The context for the research was a Thoroughbred horse farm which bred its own foals and had a history of benzimidazole-resistant small strongyles (a type of intestinal nematode) emerging from 1962 to 1965. This farm operates a closed, non-boarding system.
• The researchers monitored the strongyles infections in around 15 mares and 11 to 24 yearlings annually over an 18-year period (1966 to 1983).
• Every two weeks, they counted the number of worm eggs per gram of feces (EPG) and larvae per gram of feces. This was done to monitor the infections and effectiveness of the antiparasitic treatments given every eight weeks, either through stomach tube or food (dichlorvos pellets).

Experimentation and Results

• The research experiment involved different drugs including thiabendazole (TBZ), phenothiazine (PTZ) + piperazine (PPZ)-carbon disulfide (CS2) complex, PTZ + PPZ + trichlorfon (TCF), dichlorvos, and pyrantel pamoate. These were used selectively in different subgroup of horses (usually three each year).
• Despite different treatments in subgroups, the horses shared the same pasture which may limit or rule out efficacy comparisons between drugs.
• The annual average EPG and larvae counts in mares and yearlings suggested an increase over time, indicating drug resistance. For yearlings treated specifically with TBZ + PPZ and PTZ + PPZ-CS2, this increase was statistically significant (with P less than 0.05).

Study Implications

• The increase in EPG and larvae counts over the years points towards growing resistance to the treatment drugs by the equine strongyles.
• Resistance to antiparasitic drugs among horse parasites could potentially have health implications for horses and require a change in parasite management strategies.