Ivermectin has been a popular dewormer for foals, but there has been an increasing number of reports from around the world of its lack of efficacy against the large roundworm (Parascaris spp).
Migrating larvae may cause a mild cough and nasal discharge. Adult worms live in the small intestine, and a heavy infection leads to failure to thrive and may cause intestinal impaction or rupture. Older foals develop immunity, and the parasite rarely causes problems in adult horses. But deaths have been reported in youngsters up to 4 months of age.
A recently published study, by Maria Studzińska and colleagues at the University of Life Sciences in Lublin, is the first to report ivermectin resistance to Parascaris in foals in Poland.
A total of 225 foals, from seven stud farms in south-eastern Poland, were treated orally with ivermectin paste. The research team collected fecal samples one day before and two weeks after deworming.
Foals with fecal worm egg counts lower than 150 eggs per gram before treatment and farms with fewer than 6 eligible foals were excluded from the statistical analysis.
The research team found that, before treatment, 40% of foals had Parascaris spp eggs in their feces. Two weeks after ivermectin treatment, 28.4% of foals were still excreting Parascaris spp eggs.
Fecal egg count reduction (FECR) ranged from 44% to 97%. Overall average efficacy was 49.3%.
The FECR test is the standard method of assessing resistance in small redworms (cyathostomins) but has not yet been validated for use in Parascaris. In general, for cyathostomins, if the FEC is not reduced by 95% or more after treatment with a macrocyclic lactone such as ivermectin, that is interpreted as evidence of resistance.
The researchers found that ivermectin was least effective against Parascaris spp. infection on farms with the largest herd sizes. Reduced efficacy was more common in older foals compared with those of three months of age. In this study, ivermectin was more effective in male foals than in females.
A survey of ivermectin resistance in Parascaris species infected foals in south-eastern Poland. Maria Bernadeta Studzińska, Guillaume Sallé, Monika Roczeń-Karczmarz, Klaudiusz Szczepaniak, Marta Demkowska-Kutrzepa & Krzysztof Tomczuk. Acta Veterinaria Scandinavica (2020) vol 62, Article number: 28. https://doi.org/10.1186/s13028-020-00526-2