First Report of Anthelmintic Resistance in Gastrointestinal Nematodes of Sheep from Costa Rica

Abstract

As the prevalence and severity of anthelmintic resistance continue to rise, nematode infections in sheep correspondingly reduce the profitability of the sheep industry. In Costa Rica, sheep production systems are increasing in both number and importance. A field trial study was carried out to detect the level of anthelmintic resistance to albendazole and ivermectin in gastrointestinal nematodes (GIN) of sheep from seven farms in Costa Rica. Resistance was determined using the fecal egg count reduction test (FECRT). Three treatment groups were assessed on each farm: control, albendazole, and ivermectin.Haemonchusspp. (71%),Strongyloidessp. (57%), andTrichostrongylusspp. (43%) presented resistance levels to albendazole, whereasStrongyloidessp. (43%),Haemonchusspp. (29%), andTrichostrongylusspp. (29%) were resistant to ivermectin.Haemonchusspp.,Strongyloidessp., andTrichostrongylusspp. were the most resistant GIN to both products. This study suggests that frequency of treatment, exclusive chemical control, and visual estimation of animal weight to calculate dosage may contribute to the high levels of anthelmintic resistance that were observed on the farms analyzed herein.