Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs.
Authors: Lloyd S
Journal: The Veterinary record
Summary
Lloyd's year-long investigation examined how previous anthelmintic strategies influenced strongyle egg-shedding patterns in 267 untreated horses across 18 stables, categorised by historical treatment intensity: no anthelmintics for years (group 1), twice-yearly dosing (group 2), or five to seven times annually (group 3). Faecal samples collected at four intervals throughout the study revealed that whilst 50.2% of individual samples contained fewer than 200 epg overall, the proportion of low egg-shedders (LES; <200 epg consistently) varied dramatically by previous management: only 35.6% overall, but significantly higher in group 3 stables (those previously treated most frequently) compared to groups 1 and 2. Paradoxically, the most intensively treated horses (group 3) developed the most favourable shedding profiles once treatment ceased, suggesting that frequent anthelmintic use may have prevented the establishment of naturally resistant parasite populations or altered the susceptibility profile of resident worm burdens. For practitioners, these findings imply that strategic, targeted deworming protocols may build population-level resistance to anthelmintics without compromising long-term pasture contamination control, whereas sporadic treatment regimens may fail to select for either resistant parasites or naturally low-shedding phenotypes. Faecal egg counting remains essential to identify truly low-shedding individuals and inform evidence-based herd treatment decisions rather than relying on blanket or interval-based programmes.
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Practical Takeaways
- •A history of frequent anthelmintic use paradoxically results in more low egg-shedding horses when treatment stops, suggesting selection for lower-shedding phenotypes or potential parasite population changes
- •Not all horses require regular anthelmintic treatment; identify and monitor low egg-shedders to reduce unnecessary drug use and potential resistance development
- •Faecal egg count monitoring should inform anthelmintic programmes rather than routine treatment, particularly in stables with established control programmes
Key Findings
- •35.6% of untreated horses were low egg-shedders (LES), with 50.2% of individual faecal samples containing <200 epg
- •Previous anthelmintic programme history significantly affected both mean egg count and proportion of LES horses
- •Stables with high-frequency anthelmintic use (group 3) showed markedly higher proportion of LES and lower mean egg counts when treatment was withdrawn compared to stables with no or low-frequency use
- •Continuing anthelmintic treatment at previously treated stables resulted in lower egg counts than untreated horses, though differences were usually not statistically significant