Evaluation of ivermectin resistance in strongyle nematodes of cattle in Thailand

Abstract

Gastrointestinal nematodes (GINs) are widely distributed and represent a major health concern in large ruminants worldwide. Strongyle nematodes are significant parasites in cattle that can lead to economic losses, particularly in heavy infections. Anthelmintic resistance (AR), including resistance to ivermectin (IVM), benzimidazoles, and levamisole, has been well-documented in small ruminants and is increasingly reported in cattle in several regions worldwide. However, information regarding IVM resistance in cattle in Thailand remains limited. Therefore, this study aimed to investigate IVM resistance status in strongyle nematodes. A total of 143 faecal samples were collected from cattle across 33 farms located in Lampang, Kanchanaburi, and Nakhon Pathom Provinces, Thailand. Gastrointestinal parasites (GIPs) were detected in 91 cattle (63.64%) across 30 farms (90.91%). The predominant GI parasites identified were strongyle worms (87.88%), coccidia (69.70%), and Trichuris spp. (18.18%). Ten farms were selected to evaluate IVM resistance using the faecal egg count reduction test (FECRT). The results revealed that six farms‒three each in Nakhon Pathom and Kanchanaburi Provinces‒exhibited IVM resistance, with FECRT values ranging from 58.66% to 93.06%. The larval development test (LDT) was performed on strongyle populations from four farms; IVM IC₅₀ values ranged from 4.83 × 10 − 7 µ g/ml to 5.55 × 10 − 2 µ g/ml, indicating that three farms were susceptible to IVM, while one farm showed resistance. Third-stage larvae (L3) of strongyle nematodes were randomly collected and subjected to nucleotide sequencing, identifying Haemonchus similis , Cooperia spatulata , Cooperia punctata , Trichostrongylus axei , and Oesophagostomum radiatum . Overall, IVM resistance was confirmed via FECRT in Nakhon Pathom and Kanchanaburi, while resistance was detected via LDT in Lampang. These findings confirm the presence of IVM resistance in strongyle nematodes in cattle in Thailand and demonstrate significant variation in IVM inhibitory responses among farms. The high prevalence of gastrointestinal parasites, together with the detection of IVM resistance via FECRT and LDT, highlights the critical need for continued monitoring. Furthermore, integrating both FECRT (in vivo) and LDT (in vitro) enhances the detection of AR in cattle.