27 April 2016
The Danger of Ascarids in Foals
Brought to you by Merck Animal Health

While all intestinal parasites are concerning, one of the most lethal threatens horses during their first 18 months of life. Ascarids, also known as roundworms, are migrating parasites that travel through the bloodstream to the horse’s liver and lungs during their juvenile stage before returning to the small intestine to develop into a mature parasite. Because of this migration, clinical signs of an ascarid infection can vary from respiratory disease to impaction colic, both of which can threaten the foal’s health and potentially his life.

Not only are ascarids a serious concern, but they also have developed widespread resistance to ivermectin and moxidectin1-7, two deworming compounds commonly used to control them. The larvicidal dose of fenbendazole is widely recognized as one of the safest and most efficacious products for treating ascarids, including ivermectin-resistant ascarids.8

Ascarids can also threaten horses older than 18 months that were not effectively dewormed as foals and have harbored an ascarid infection into their adulthood9. In addition to foals, fenbendazole should also be considered as part of a veterinarian-guided deworming regimen for adult horses.

Work with your veterinarian to protect your foals from this potentially fatal parasite and to create a deworming regimen based on fecal egg count tests for every horse in your barn. For more information, visit GetDewormingRight.com.

Consult your veterinarian for assistance in the diagnosis, treatment and control of parasitism. Do not use in horses intended for human consumption.

Copyright © 2016 Intervet Inc., d/b/a Merck Animal Health, a subsidiary of Merck & Co., Inc.

1 Reinemeyer CR. Diagnosis and control of anthelmintic-resistant Parascaris equorum. Parasites & Vectors 2009, 2(Suppl 2):S8.
2 Craig TM, Diamond PL, Ferwerda NS, et al. Evidence of ivermectin resistance by Parascaris equorum on a Texas horse farm. J Eq Vet Sci 2007; 27:67-71
3 Hearn FP, Peregrine AS. Identification of foals infected with Parascaris equorum apparently resistant to ivermectin. J Am Vet Med Assoc. 2003; 223(4):482-485.
4 Boersema JH, Eysker M, Nas JW. Apparent resistance of Parascaris equorum to macrocyclic-lactones. Vet Rec 2002; 150(9):279-281
5 Schougaard H, Nielsen MK. Apparent ivermectin resistance of Parascaris equorum in foals in Denmark. Vet Rec 2007;160: 439-440
6 Lyons ET, Tolliver SC, Ionita M and Collins SS. Evaluation of parasiticidal activity of fenbendazole, ivermectin, oxibendazole, and pyrantel pamoate in horse foals with emphasis on ascarids (Parascaris equorum) in field studies on five farms in central Kentucky in 2007. Parasitol Res 103(2): 287-291, 2008.
7 Reinemeyer, C. Anthelmintic resistance in non-strongylid parasites of horses. Veterinary Parasitology. 2012; 185(1): 9-15.
8 Reinemeyer CR, Vaala, WE. Larvicidal efficacy of fenbendazole against a macrocyclic lactone-resistant isolate of Parascaris equorum in foals. 2010 AAEP Proceedings.
9 AAEP Parasite Control Guidelines. Revised 2016.