Resistance to dewormers is a rapidly growing concern among horse owners as options to rid horses of worm infestations are declining. Large roundworms (ascarids) have proven resistant to all available classes of dewormers, and now a study from Brazil has shown that small strongyles (also called small redworms or cyathostomins) are also showing widespread resistance to anthelmintics.
Brazil is home to 5 to 6 million horses – one of the largest equine populations in the world. Horse owners in Brazil do not routinely use fecal egg counts to determine which horses need to be dewormed; they often blanket deworm the entire herd and routinely switch out deworming products – similarly to how deworming was done previously in the United States.
Dr. Giordani Mascoli de Favare, and other researchers from the Universidade Estadual Paulista School of Agrarian and Technological Sciences in Brazil, undertook a year-long study in São Paulo to determine the effectiveness of ivermectin as a dewormer on 123 horses on 12 breeding farms.
The study team completed a fecal egg count to determine baseline worm infestations. After anthelmintic administration, a second fecal egg count should show a reduction in egg load of at least 95 percent. If it doesn't, dewormer resistance is indicated.
None of the horses in the study had been dewormed for at least 60 days prior to the study. Each horse was given the recommended dose of ivermectin by mouth based on their weight. A second fecal egg count test was run on each horse 14 days later.
The researchers found that the second fecal egg count was below 90 percent on the majority of farms, indicating significant redworm resistance to ivermectin. Additionally, three properties showed counts between 90 and 95 percent, also indicating resistance. Only four of the farms showed a reduction of more than 95 percent.
The authors conclude that these findings highlight the urgent need to utilize different strategies to effectively control anthelmintic resistance in horses. There are no new deworming drugs being created.
Read more at Equine Science Update.
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