Encysted small redworm can remain dormant inside a horse for up to 2 years, but usually develop and emerge from the gut wall all at the same time in the early spring. It is most important to use a wormer containing moxidectin or a 5 day fenbendazole course licensed to treat encysted small redworm. It is important to remember that there is now widespread resistance to fenbendazole in parasite populations4, whereas moxidectin has been shown to be effective against benzimidazole resistant worms. Treating with products that do not specifically treat for encysted small redworm can increase the risk of larval cyathostominosis6.
(with thanks to Zoetis for their input and guidance)
3 Love S. et al (1999) Veterinary Parasitology 85, 113-122
4 Matthews JB (2008) An update on cyathostomins: Anthelmintic resistance and worm control. Equine Vet. Education 20 552-560
5 Dowdall S. et al (2002) Veterinary Parasitology 106, 225-242
6 Craig R. Reinemeyer and Martin K. Nielsen. Handbook of Equine Parasite Control