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Deworming Myths About Stalled & Dry Lot Horses

Article Archives LinkDeworming Myths About Stalled and Dry Lot Horses

Deworming Myths About Stalled and Dry Lot Horses

MYTH #1: My horse isn’t grazing, so he’s safe from all equine parasites.


When horse owners think about equine parasites, they usually envision picking up infective larvae (strongyles) or infective eggs (ascarids) from horse grazing. So, what about a horse that doesn’t graze and spends its time in a dry lot or stall? This horse still needs to be dewormed, because there are certain types of equine parasites transmitted by flies, there are parasites that can be encountered in infested hay, and there are horse-to-horse parasites.

Transmission from Biting Flies

There are some equine parasites that biting insects transmit; thus, all horses, whether grazing or not, are susceptible to infection. These include bots (Gasterophilus), and the stomach worms Habronema and Draschia, and Onchocerca.

Habronema and Draschia are responsible for producing “summer sores” or non-healing wounds. This condition occurs when flies, whose maggots become infested by eating eggs in feces and develop into adult flies carrying the immature larvae, then deposit larvae on the horses lips, nostrils, or open sores during the process of feeding. An infected horse licks these larvae, they are swallowed, and they travel to the stomach to establish egg-laying adults. This is the reason these two parasites are collectively called, “Stomach Worms”. When deposited in wounds, these wounds can become what have been commonly called “summer sores”-open sores which tend to not heal, due to the presence of live, active, feeding stomach worm larvae.

Horse-to-Horse Transmission

Pinworms are directly transmitted from one horse to another and cause irritating, itchy “broom tail”. This happens most easily in stalls and corrals, because pinworms cause irritation near the anus, the horse rubs its tail and rump against a stall or fence, where the eggs stick, waiting for another horse to lick and swallow them. Although Ivermectin is effective against pinworms, Pyrantel is the drug of choice for pinworms in horses and, interestingly enough, people too!

Onchocerca, the Equine Threadworm, lives under the skin of horses, can cause dermatitis, and has also been associated with “moon blindness” and fistulous withers. Some horses may react following deworming, due to the death of immature threadworms, called microfilariae. These reactions are characterized by swelling of the face, head, sides of the neck, chest, and ventral midline. Normally, these reactions will subside within 24 to 48 hours and are not life-threatening. Threadworms are spread horse-to-horse by biting insects, which pick up microfilariae from an infected horse, during feeding, and deposit them as they bite another horse.

MYTH #2: I Don’t Need Tapeworm protection.


If a horse is fed hay, it is highly possible and common the hay could contain grain mites. These grain mites may contain the intermediate form of the Tapeworm (Anoplocephala sp.), called a cysticercoid. By eating the infested hay, the horse may contract tapeworms.

Tapeworms are associated with several serious types of equine colic, including spasmodic colic, ileal impaction, and ileocecal intussusception. All of these conditions can be life-threatening and may require surgery. All ages of horses are susceptible to tapeworm infection.

Every area of the United States has some degree of tapeworm infestation - the level of infection depending on the concentration of grainmites, which is environment-related. In the United States, the highest level of equine tapeworm infestations is in the area immediately around the Great Lakes and the least affected areas are the arid states of the desert Southwest. However, it must be restated that ALL horses in the U.S., to some degree, are at some risk of contracting equine tapeworms.

MYTH #3: Arid Regions Don’t Have Parasite Concerns.


While many areas of the US are free of Stomach Worms (Habronema and Draschia), and Threadworms(Onchocerca), due to the use of ivermectin over many decades, these parasites are still a major concern in arid regions, where horses typically are dewormed much less frequently.

Bots are also still present in most regions of the country. Again, Pinworms are directly transmitted from one horse to another. And, Tapeworms are transmitted by the grain mite in hay, and can cause serious life-threatening forms of colic.

MYTH #4: Fecal Tests See All Parasites.


As is true of bots, there is no reliable test for infestation with tapeworms. Traditional fecal tests are only about 3% reliable in diagnosing tapeworms in horses. Since it is almost impossible to tell which horses are infested, all horses in the United States are recommended to receive a praziquantel-containing dewormer twice a year for tapeworm control.

THE TAKE-AWAY: Non-Grazing Horses Can Contract Parasites.


So, just because your horse isn’t grazing doesn’t mean he is safe from all equine parasites. The good news is that ivermectin is very effective against all of these parasites, except tapeworms. For full parasite coverage, including tapeworms, product that contains ivermectin and praziquantel is recommended at least twice per year, usually in the late fall and spring.

Dr. David Ellefson, DVM, is Director of Technical Services at Bimeda. A former practicing veterinarian and practice owner, Dr. Dave has a wealth of experience in animal health issues and a full understanding of the issues and concerns facing veterinarians and animal owners.

Bimeda is known globally for a quality range of equine products including anti-parasitics.