Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs.

This study examines the impact of past anthelmintic treatment programs on the proportion of horses shedding low quantities of strongyle-type eggs. It concluded that past treatment had a significant effect on the number of eggs shed, with horses that received more frequent treatments generally having lower numbers.

Introduction and Methodology

• The authors of the study conducted research on horses at 18 different stables. The horses were divided into three groups based on their history of anthelmintic (anti-parasitic) treatments.
• Group 1, from three stables, consisted of horses that had not received any anthelmintic treatments for many years. Group 2, from 10 stables, included horses that had received anthelmintic treatments about twice a year. Group 3, from the remaining five stables, comprised horses that had been treated with anthelmintics five to seven times per year.
• The study then withdrew anthelmintic treatments from between 30% and 70% of the horses at each stable for a whole year. During this period, the researchers collected four faecal samples from each horse and noted the number of strongyle-type eggs in them, a category of parasitic worm that often infects horses.

Results and Findings

• Of all the collected samples, 50.2% had fewer than 200 eggs per gram (epg). Individual horses’ epg counts from all four samples showed that 35.6% of these horses were identified as low egg-shedders (LES), meaning they excrete fewer eggs through their faeces.
• Notably, the study found that previous control programs had a significant effect on both the mean epg and the proportion of horses classified as LES. There were more LES and fewer epg, indicating less parasite infection when the horses had received more frequent previous treatments (group 3).
• The proportions of LES in groups 1 and 2 were similar, though horses in group 2 (who had a history of biannual treatment) had a significantly lower epg count. This implies that even infrequent treatment could potentially reduce the number of shed eggs.
• The study also compared horses that were left untreated for a year with those that continued with the previous treatment regimen. The result showed that horses continuing with the treatment regimen generally had lower egg counts, although the difference was often not statistically significant.

Conclusions

• The data suggest that the frequency of prior anthelmintic treatment, even at biannual levels, influences the number of eggs horses shed. More frequent treatment leads to a lower epg and a higher proportion of LES, indicating lower parasitic load and potentially less transmission of parasites.
• It should be noted, however, that even in the cases where horses continued their usual anthelmintic treatments, there was not always a significant difference in epg count compared to the untreated group. This could have implications for anthelmintic resistance and needs further study.