Published April, 2013

Most horse owners are familiar with deworming as a part of their parasite control programs. But did you know some strategies can help control parasites in horses' environments? At the 2013 University of Kentucky (UK) Equine Showcase, held Jan. 18 in Lexington, Ky., Mary Rossano, PhD, an assistant professor in the UK Department of Animal and Food Science, discussed recent research about controlling equine ascarids before they enter the horse's body.

Rossano explained that ascarids (Parascaris equorum) are the largest worms that infect horses and most commonly affect horses up to 18 months of age. Once a foal develops natural immunity to the parasite, they enjoy life-long protection against it, Rossano said. Foals with heavy ascarid infections often present with a pot-bellied appearance, anorexia, and coughing (from worms migrating to their lungs); in serious cases, pneumonia can develop. She also noted that large worm masses can obstruct horses' bile ducts and intestines, leading to colic or death.

To make effective decisions on environmental parasite control, it's important to understand the ascarid lifecycle. Rossano explained that affected horses shed one-celled eggs in their feces; however, these eggs can't infect horses just yet. After one to two weeks, the one-celled eggs develope into infective eggs, which the horse can ingest from a variety of sources, including feces, soil or pasture, stall floors, feed buckets, and nearly any other surface the eggs and the horse might come in contact with. Once inside the horse, larvae in the eggs hatch and migrate from the small intestine to the host's liver. Later, they migrate again, this time to the lungs. Once the larvae are in the lungs, the horse coughs them up and subsequently swallows them, returning them to the digestive tract. There, they develop into adult worms that reside in the horse's small intestine; these adults will lay eggs, which are then expelled into the environment in the horse's feces. The entire process takes about three months to complete, she said.

Rossano explained that once the eggs have been released into the environment their protective coating enables them to survive for years in soil in most climates, which can add to the challenges of environmental parasite control. Another ascarid challenge is the resistance to several common dewormers, she said. Multiple studies conducted around the world have shown that ivermectin and moxidectin are no longer effective against ascarids, she said. Further, researchers have reported some resistance against pyrantel pamoate and double doses of fenbendazole. At this point, she noted, resistance to the latter two products could be farm-dependant.

Emerging resistance to dewormers highlights the need for environmental control methods that target parasites while they are outside the horse. To that end, Rossano, graduate student Jessica Gould, and colleagues from UK performed two studies to evaluate different environmental ascarid control methods. In the first they evaluated disinfectants that could be useful for stall and stable hygiene. In the second they evaluated composting's efficacy at killing ascarid eggs, which could help prevent the spread of infected feces over pastures.

Rossano reported that in the first study, Gould and colleagues used four commercially available disinfectants—Lysol Multi-Surface Pour, Nolvasan, Liquid Amphyl Professional Disinfectant, and Aerosol Professional Amphyl II—and one control treatment (deionized water) to see which was most effective at killing ascarid eggs over the course of 20 days. They found that the aerosol Amphyl was most effective at killing ascarid eggs, followed by the Lysol product. Neither the liquid Amphyl nor the Nolvasan was effective at killing the eggs, Rossano said.

Rossano explained that while the aerosol product was most effective, most aerosol products aren't overly practical for use on farms. She suggested that liquid Lysol might be beneficial and practical to use in stalls and stables to kill ascarid eggs. She noted in the future, she'd like to carry out a longer-term study to evaluate the disinfectants' efficacy over the course of several months.

In the second study the team tested windrow composting's effects on ascarid egg viability. Rossano said the study produced evidence that windrow composting (the production of compost by piling organic matter or biodegradable waste into long rows called windrows) is an efficient and effective way of managing equine waste products while eliminating ascarid contamination. (Editor's Note: Read more about this study in "Windrow Composting for Parasite Control and Waste Management" on TheHorse.com.)

Rossano noted that because windrow composting requires an investment in time, land, and equipment, it's not a feasible option for all farms. Other composting methods—such as building and using a compost bin—are effective as well. The UK Extension publication “Composting Horse Muck” is a good resource for learning about the process and practical considerations of composting, Rossano noted. Composting bin plans can be found by searching online, she said.

Take-Home Message

In closing, Rossano stressed that measures can—and should—be taken to reduce ascarids’ prevalence in horses' environments.

"P. equorum transmission can be reduced by composting manure from infected horses" and by applying appropriate disinfectants to stalls, she said.

She said that pastures where foals live and deposit manure still remain a "significant source of infection," and that total eradication of ascarids from breeding farms is not a realistic goal, given the resilience of the parasite. Foals that experience light ascarid infections can develop natural immunity to the parasite, but the goal of environmental control is to prevent the harm caused by heavy infections, Rossano stressed.

"Future studies should investigate the benefits of keeping foals off pastures for extended periods of time—long enough to allow natural forces (sunlight, heat, and dryness) to take their toll, reducing the numbers of infective eggs in the pasture when foals are eventually returned to it," she concluded.

Erica Larson, News Editor for The Horse