Searching for evidence of resistance development of Parascaris spp. against fenbendazole in the Netherlands.

Abstract

Background information: Worldwide Parascaris spp. are common infections in foals that can lead to respiratory signs due to hepato-tracheal migration of larvae and impaction due to adult stages in the small intestines. Relevance of the study: It is vital to have suitable treatment options for foals infected with Parascaris spp., since impactions can have fatal consequences. There are a number of studies that reported reduced efficacy of fenbendazole in other countries, however the current state of resistance development of Parascaris spp. against fenbendazole in the Netherlands is unknown. Aim of the study: To investigate the current efficacy, and thereby to determine if there currently is reduced efficacy, of fenbendazole against Parascaris spp. in foals aged 3 to 12 months on farms in the Netherlands. Materials and methods: In order to find foals that were infected with Parascaris spp., 297 fecal egg counts for individual foals have been performed at 11 farms in the Netherlands. All detected eggs were counted during the process, not only Parascaris spp. eggs. To investigate the current state of resistance development of Parascaris spp. against fenbendazole, fecal egg count reduction tests were performed with 24 foals which were treated with Panacur®. Additionally, there was a questionnaire send to the 11 farms about their anthelmintic policy and general management. Results: Only 72 of 297 foals tested positive for Parascaris spp., of which 49 foals had a fecal egg count of 50 Parascaris spp. eggs per gram (EPG) or more. Of the 24 fecal egg count reduction tests performed with fenbendazole; one foal had a reduction of 66% while the other 23 foals had a reduction of 100% of Parascaris spp. eggs. Twenty foals had strongylid type eggs in their feces with an average count of 347 EPG. The reduction of fecal egg counts of strongylid type were on average 62,6%, with a standard deviation of 49,7%. Remarkable data from the questionnaire was the amount of anthelmintics that are used on the visited farms. Discussion: Fourthy-nine out of 297 foals tested positive for Parascaris spp. which is lower than prevalences reported in earlier studies. A possible explanation is the extent of deworming that the foals are exposed to and / or building-up of immunity to Parascaris spp.. The individual foal with a reduction of 66% of Parascaris spp. eggs meant that there was still one egg detected in the feces, two weeks after treatment. Conclusion: There is no clear evidence for existing fenbendazole resistance development, monitored with Fecal Egg Count Reductions Test of Parascaris spp. in 23 foals. However, as previously has been reported in other studies, there was reduced efficacy of fenbendazole against strongyles.