Q: All of the articles I've read talk about managing parasites in horses with turnout time. Like many other owners in dry locations, here in New Mexico I don't have pasture. Both stalls and turnout areas consist of sand. I purchase bales of alfalfa and grass hay to feed my horses. What are the chances of parasite infection when horses do not graze? Do horses that don't graze have less of a chance of getting/having worms? I watch out for sand colic, but I don't know how susceptible my horses are to worms. I realize that I can have a fecal count done, but I'd like to know the parasite risks associated with feeding bales of hay. What kind of deworming program should I pursue?

Jan Henfling, Corrales, N.M.

A: This is a great question. First of all, I want to bring up the issue that there are many different parasites that can infect horses, and each has a different life cycle and epidemiology of transmission. Consequently, I cannot broadly address these questions in a single response. In addition, my response to your question addresses parasites and control for adult horses only--foals require a completely different worm control program.

In most of the world, strongyles are the most important parasites of horses and are the primary target of control programs. There are two types--small strongyles and large strongyles. Large strongyles used to be a major concern, but these have become very uncommon in recent decades. In contrast, small strongyles are as common as ever and are the primary concern for horses in most areas of the world.

When you see strongyle eggs in a horse fecal, usually greater than 99% of these are small strongyles. However, strongyles are only transmitted to horses that graze pastures. So, approaching the topic of parasite control in dry areas like New Mexico is a bit different than in most other parts of the country. Unless the hay you purchase is coming from pastures where horses are grazing (highly unlikely), the hay will not have any infective strongyle larvae; thus, it is completely safe to feed (as far as parasites are concerned).

Tapeworms are transmitted by soil mites that serve as an intermediate host. These mites will be present in hay, but, once again, unless there were horses grazing the pastures from which the hay was harvested, the mites will not be infected. Thus, it will be virtually impossible for your horse to become infected with tapeworms, making it unnecessary to treat your horse for tapeworms or strongyles (unless you take him to another area of the country and he grazes). If you checked a fecal on your horse, it's unlikely that you would see any eggs.

However, there are some other parasites that flies transmit; thus, all horses, whether grazing or not, are susceptible to infection. These include bots (Gasterophilus), the stomach worms Habronema and Draschia, and Onchocerca. Habronema and Draschia are responsible for producing "summer sores," and Onchocerca causes dermatitis and has been associated with moon blindness and fistulous withers. Interestingly, in most of the United States, Habronema, Draschia, and Onchocerca have become relatively rare as a consequence of frequent dewormings over many decades--especially with ivermectin. In contrast, these parasites are still a major concern in the dry Southwest because in this region horses typically are dewormed much less frequently. Bots are not considered a major cause of disease, but they remain common throughout the country.

Lastly, pinworms are directly transmitted from one horse to another. This happens most easily in stalls and corrals because pinworms cause the horse's back end to itch, then he rubs himself against the stall, fences, etc., where the eggs stick, waiting for another horse to ingest them.

A deworming program targeting these other parasites is required. Treatment recommendations vary depending on climate, but given the climate in New Mexico and the parasites we aim to control there, one or two treatments per year is all that is required.

I would recommend the following: So long as you do not see signs of parasitic disease, treat all horses once per year in November (after a hard frost has ended fly activity) with either ivermectin or moxidectin (Quest). A second treatment might be needed in the summer if any signs of infection are seen. Ivermectin and moxidectin are recommended here because these have the best efficacy against the parasites of greatest concern in New Mexico: Habronema, Draschia, Onchocerca, and bots.

Lastly, it's important to keep in mind that in most areas where strongyles and tapeworms are the parasites of greatest importance, much damage can be caused before it becomes clinically apparent. Thus, preventive deworming is usually recommended at high transmission times of the year. In contrast, if your horses are infected with Habronema, Draschia, Onchocerca, or pinworms, you will quickly know because the signs of infection with these parasites are easily recognized. Therefore, I recommend a single yearly deworming as a preventive measure, then any additional treatments can be given on an as-needed basis. 

About the Author

Ray M. Kaplan, DVM, PhD, Dipl. ACVIM, EVPC

Ray M. Kaplan, DVM, PhD, Dipl. ACVIM, EVPC, is a professor of parasitology in the University of Georgia College of Veterinary Medicine's Department of Infectious Diseases. He served on the 2013 American Association of Equine Practitioner's Parasite Control Guidelines subcommittee.

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