While recent studies suggest that moxidectin and the five-day fenbendazole regimen may be performing at approximately the same level of larvicidal efficacy (Reinemeyer et al., 2015; Bellaw et al., 2018), moxidectin generally maintains efficacy against adult parasites within the intestinal lumen, whereas fenbendazole does not (Table 4). Consequently, though larvicidal efficacy may not differ greatly, moxidectin can be expected to yield much greater overall reductions of cyathostomin burdens and suppression of egg shedding following treatment (Mason et al., 2014).
Some veterinarians and horse owners express concerns over larvicidal treatment of horses with perceived large encysted larval burdens. However, a series of studies of inflammatory reactions to treatments with moxidectin, the five-day fenbendazole regimen as well as ivermectin (non- larvicidal) in heavily parasitized horses only found very subtle responses, which were most pronounced in the untreated control groups (Nielsen et al., 2015; Steuer et al., 2018; 2020). Thus, adverse inflammatory reactions to dying worms do not appear to be a major concern.
8. Methods of Parasite Control A summary of currently available anthelmintics can be found in Appendix B.
8.1. Environment-based approaches Equine strongyle parasites begin life as eggs in a manure pile, which then must develop to infective larvae in the feces, get out onto the pasture, and be ingested by a horse. Thus, infection of horses could be prevented if all feces were promptly removed from the pasture.
8.2. Environmental control A summary of development and survival of strongyle eggs and larvae on pasture at different temperatures can be found in Appendix C. Strongyle eggs hatch and develop into infective larvae under conditions of moderate temperature and moisture. Cold slows the rate of development or stops it altogether, and excessive heat kills eggs and larvae. Proper composting of manure and soiled bedding will generate relatively high internal temperatures, and strongyle larvae in manure are virtually eradicated by exposure to temperatures over 104˚F (40˚C) for a minimum of one week (Gould et al., 2012).
Non-composted horse manure should never be spread onto pastures grazed by horses as this will increase the level of parasite contamination and transmission. This practice has been associated with higher Parascaris spp. prevalence in Germany (Fritzen et al., 2010).
Leaving pastures unoccupied for several months of the year may or may not significantly reduce the levels of contamination depending on climate and time of the year. Infective strongyle larvae (L3) can survive for only a few days to a few weeks in hot weather (temperatures over 100°F), but for as many as six to nine months during colder weather (Nielsen et al., 2007). Consequently, L3 survival in the environment will vary greatly from region to region and season to season.
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