Reduced efficacy of anthelmintics in young compared with adult horses.

The study reveals that the effectiveness of anthelmintic treatments in controlling small strongyles or cyathostomes (parasites) is significantly lower in young yearling horses compared to adult ones.

Study Overview

• Research was conducted on a Thoroughbred breeding farm in Ohio between 1982 and 1988, with a focus on the efficiency of three specific anthelmintic pastes: ivermectin, oxibendazole, and pyrantel pamoate. These medication types were tested in their ability to control cyathostomes, or small strongyles, in horses, particularly those that had developed resistance to a group of anthelmintics known as benzimidazoles.

Comparison of Drug Efficiency

• Anthelmintic effectiveness was evaluated by measuring faecal egg counts between treatments in adult horses and yearlings. The study found a notably decreased efficiency of all three drugs in yearling horses as compared with adults.
• In adult horses, the mean faecal egg counts remained generally under 100 eggs per gram (epg) of faeces when treated with oxibendazole or pyrantel pamoate on a four to five-week schedule, and ivermectin on a bi-monthly schedule. This indicated the successful reduction of parasites within this group.
• Contrastingly, the mean counts in young horses spiked to significantly higher levels in the same timeframe after treatment with each of the three drugs. The highest mean counts noted were 655 epg for oxibendazole, 729 epg for pyrantel pamoate, and 852 epg for ivermectin. Some counts even crossed 3,000 epg in treated yearlings, demonstrating the reduced efficiency of the medications in this group.

Key Reasons for Lower Efficiency

• The research identified three distinct factors that contributed to the substandard performance of anthelmintics in young horses compared to adults.
• The first factor, labelled as the ‘anthelmintic refuge’, implies that some parasites might be physically protected from the medication within the host’s body, thus escaping the drugs’ effects.
• The second factor, referred to as ‘anthelmintic resistance‘, suggests that some parasites might have evolved resistance against the medications.
• The third factor, ‘anthelmintic avoidance’, suggests that some parasites may demonstrate behaviours that enable them to avoid exposure to the medications.