In a recent study conducted in Brazil, it was discovered that there is widespread resistance to ivermectin among cyathostomins (small redworms) in most of the properties evaluated. This finding is concerning, as it has been increasingly recognised that important equine worms are developing resistance to commonly used anthelmintics.
The severity of this issue is underscored by the fact that resistance to all currently available classes of anthelmintics has been reported not only in cyathostomins but also in ascarids (large roundworms). Compounding the problem is the lack of new drugs on the horizon to combat this resistance.
Brazil is home to one of the largest horse populations globally, estimated to range from 5 to 6 million. In light of this, Giordani Mascoli de Favare and colleagues undertook a year-long research study in the western region of São Paulo state. The study aimed to assess the effectiveness of ivermectin as an anthelmintic in naturally infected horses on 12 breeding farms, involving a total of 123 horses.
To evaluate resistance in cyathostomins, the standard faecal egg count reduction (FECR) test was employed. If the FECR does not show a reduction of 95% or more after treatment with a macrocyclic lactone such as ivermectin, it is indicative of resistance. Before the study, the horses had not received any anthelmintic treatment for at least 60 days. Each horse was orally administered the recommended dose of ivermectin paste based on its weight. Faecal samples were collected at the beginning of the treatment and 14 days later.
The results of the study revealed that in five of the properties, the FECR was below 90%, indicating significant cyathostomin resistance to ivermectin. Additionally, three properties showed a FECR between 90% and 95%, further indicating the presence of resistance. Only on four of the 12 properties did the faecal egg count reduction exceed 95%.
The authors of the study explain that in Brazil, the control of equine gastrointestinal parasites typically involves treating the entire herd without prior diagnosis and regularly rotating anthelmintic drugs. However, these findings highlight the urgent need for alternative strategies and interventions to effectively manage anthelmintic resistance in horses.
Anthelmintic resistance of horse strongyle nematodes to ivermectin in São Paulo state, Brazil. Giordani Mascoli de Favare, Isabela de Almeida Cipriano, Tábata Alves do Carmo, Mateus Oliveira Mena, Gabriel Jabismar Guelpa, Alessandro Francisco Talamini do Amarante, Ricardo Velludo Gomes de Soutello. Veterinary Parasitology: Regional Studies and Reports (2023) Jun;41:100864.
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