Effects of alternation of drug classes on the development of oxibendazole resistance in a herd of horses.

The research study investigates how alternating between different classes of deworming drugs affects the development of drug resistance, specifically to oxibendazole, in a herd of horses already infected with benzimidazole-resistant parasites. The study concludes that alternating drug classes didn’t prevent the development of drug resistance.

Structure of the Study

• The research was a field study using a private herd of horses, all of which had been infected with benzimidazole (BZD)-resistant small strongyles and had been given the same deworming schedule for several years.
• The researchers divided the herd into two distinct groups, consisting of 20 and 21 horses respectively. They then differentiated the horse’s deworming schedule.

Experimental Design

• Group 1 was treated solely with oxibendazole (OBZ), a type of benzimidazole, six times a year for a total of 25 treatments across the study duration.
• Group 2 was on an alternating treatment schedule, given OBZ three times a year, interspersed with treatments of non-benzimidazole drugs, for a total of 16 treatments throughout the study period.

Results of the Study

• A statistical analysis revealed no significant difference between the two groups in terms of the change in their pre-treatment fecal egg count, which is a measure of the parasite load, after treatments 1, 4, 7, 10, 13, and 16. This suggests the frequency/tablet of administering OBZ or the alternation with other drug types did not have a significant difference on the development of drug resistance.
• The alternating drug regimen did not halt the development of drug resistance to OBZ.
• The efficacy of OBZ, as measured by the reduction in fecal egg count, lessened with every repeated drug treatment. By the 13th treatment, the drug’s effectiveness had dropped to below 70% in each group.

Conclusions of the Study

• The research suggests a routine of alternating dewormers may not effectively delay the development of drug resistance in horses.
• Consistent use of a single drug, or alternation between drug types, does not affect the pace at which resistance develops.
• Resistance instead seems to be tied more closely to the repeated use of a single drug, as evidenced by the decreased efficacy of OBZ with each successive treatment.