Reduction of the nematode egg reappearance period in horses after anthelmintic therapy

Anthelmintics are used to control equine nematodes. However, helminth resistance to regularly used drugs is a well-known challenge. Among tests to assess effective control and monitor resistance, the most common is the fecal egg count reduction test (FECRT). In the absence of reliable FECRT results, the nematode egg reappearance period (ERP) is taken into account. This study aimed to examine horses from farms around the Moscow Region to assess nematode resistance through ERP after therapy.

In the first stage, fecal samples from 280 horses were examined by the flotation method with a sodium nitrate solution. The eggs per gram (EPG) in feces were counted using the modified McMaster technique. One hundred and forty out of 280 horses were selected for further work. Five groups were formed: Two groups of horses infected with strongyles (n = 50) and three groups with Parascaris equorum (n = 90). Therapy against strongyles was performed with albendazole and ivermectin. Therapy for parascaridosis was performed with fenbendazole, ivermectin, and aversectin C. Samples from the horses in each group were taken on the 14^th day (2 weeks), 28^th day (4 weeks), 42^nd day (6 weeks), 56^th day (8 weeks), and 84^th day (12 weeks) after treatment, and the amount of EPG in each sample was determined.

Overall, nematodes were found in 65% of the horses examined. P. equorum was most frequently recorded (42.1%) followed by Strongylidae gen. spp. (27.9%). The strongyles ERP after therapy with albendazole and ivermectin was estimated on 42 days (6 weeks). The growth of P. equorum eggs in the feces was observed from the 56^th day (8 weeks) after therapy with fenbendazole, from the 42^nd day (6 weeks) after therapy with ivermectin, and was observed from the 84^th day (12 weeks) after the use of aversectin.

Our study shows widespread reductions in nematode ERPs across the Moscow Region after ivermectin therapy in horses, suggesting that additional monitoring of these farms is needed for effective control of anthelmintic resistance.