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Controlling Parasites in Foals and Horses

Pet Column for the week of January 18, 1999

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
By Sarah Probst
Information Specialist

Devising a parasite control program for your horses can be a confusing task, especially
when recommendations advise alternating the de-wormer used. "Rotating your
de-wormer-not by brand but among classes of drugs-can prevent parasites from
developing resistance, thus preventing development of a 'superworm,' " explains Dr. Doug
Hutchens, veterinary parasitologist formerly at the University of Illinois College of Veterinary
Medicine in Urbana. Depending on your location, your veterinarian may suggest rotating
your de-wormer in an interval program, a seasonal program, or a daily feeding program to
harness your parasite problem.

"Parasite programs for adult horses aim to control bots and large and small strongyles; for
horses less than 2 years of age programs need to control Parascaris equorum
(roundworms), Osyuris equi (pinworms), Strongyloides westeri, too," explains Dr.
Hutchens. If large strongyles are not controlled, the parasite could enter the migratory stage
in your horse, causing symptoms ranging from damaged abdominal arteries and colic to
edema and roughened hair coats.

Non-migratory small strongyles damage mainly the large intestine and can cause larval
cyathostomosis-seasonal (spring in northern states and fall in southern states) onset of
diarrhea and weight loss. Damage from small strongyles decreases the healthy surface area
of the intestine, thus decreasing the horse's ability to absorb food, causing poor feed
conversion and physical condition. Moxidectin and fenbendazole are effective against small
strongyle larvae.

Roundworms, also known as ascarids, are problems in young horses. Roundworm larvae
migrate through the lungs and liver and mature in the intestine. Larvae cause a summer cold
syndrome and nasal discharge that may get better with antibiotics but return when
antibiotics are discontinued. Treatment with an effective anthelmintic (such as ivermectin,
pyrantel pamoate, moxidectin, or daily feeding of pyrantel tartrate [Strongid C�]) will
prevent this problem.

If a heavy, severe infection is suspected, your veterinarian may suggest a de-wormer that
kills the parasite slowly to give the foal's body a chance to eliminate the worms. Piperazine,
a fast ascarid kill product, should not be used in animals with a high ascarid load, which
could result in an ascarid-impaction colic or even perforation of the small intestine.
Moxidectin (Quest �), a good broad-spectrum anthelmintic, should not be used in foals
under 4 months.

Even before the roundworm treatment for foals, the parasite Strongyloides
westeri-especially a problem on farms with a high concentration of horses-needs to be
bridled. It is best controlled by dosing the mare with ivermectin less than 24 hours after
foaling. "Milk-borne infections may occur, leading to enteritis and "foal heat" diarrhea in
foals," says Dr. Hutchens. Foals can be treated at 3 weeks of age with ivermectin or

Yearlings and two-year-olds often suffer from pinworms, which cause intense itching
around the tail and lead to tail rubbing. Most de-wormers are effective against this parasite.

Younger horses, because they are exposed to roundworm eggs year-round, need an
interval de-worming program. "Interval de-worming for foals may begin at about eight
weeks of age, with earlier treatment for Strongyloides if needed," says Dr. Hutchens. In
adult horses, an interval de-worming program should include boticide treatments tailored to
the botfly life cycle in your part of the country. Ask your veterinarian to recommend a
boticide treatment plan against the oral and gastric stage. Avermectins, including ivermectin
and moxidectin, are good examples. Veterinarians base their recommendations on the
parasites prevalent in your area and on the number, age, type, and movement of horses on
your property.

"Seasonal programs, which target parasites when the anthelmintic-susceptible,
egg-producing adults peak, can also be used for adult horses," clarifies Dr. Hutchens.
"Strategically planned de-wormings based on seasonal rises in these populations have
successfully controlled parasite loads and tend to be less expensive than interval programs."
Seasonal programs also decrease the likelihood of resistant strains of parasites developing.
Because bots and strongyles are the main targets in seasonal programs, seasonal or
strategic programs are suggested for horses over 2 years old.

Instead of interval and seasonal programs, some owners choose to use daily anthelmintic
feeding programs. Such programs aim to control stongyles, pinworms, ascarids, and
tapeworms. "Daily treatments with feeding programs kill adult parasites and kill larvae
before their migration in the intestine," explains Dr. Hutchens. So far resistance has not been
a problem with daily de-wormers such as pyrantel tartrate.

Environmental control will aid the de-worming program in controlling horse parasites:

1. Remove feces routinely to decrease transmission of eggs and larvae; do not dispose of
feces on pastures or near water or feed supply.

2. You may harrow pastures to break up fecal pats and kill larvae only in hot, dry weather
or below-freezing weather.

3. Quarantine all new additions to your herd and perform fecal examinations on them.

4. De-worm all horses at the same time.

5. Evaluate parasite control via fecal examination of 10 percent of horses two or three times
per year, 14 days after de-worming. "If environmental control is adequate, less than 10
percent of samples obtained should contain eggs," says Dr. Hutchens.

The best way to develop an effective parasite control program is to contact your local
equine veterinarian, who will know the prevalent parasites and transmission patterns in your