RED DEER – Horse owners must change the way they deworm their horses to make it more effective and to slow drug resistance, said a veterinarian specializing in parasite research.
Instead of deworming horses every eight weeks, horse owners should deworm twice a year, in the spring and fall, or only if a fecal egg count shows the horses need to be dewormed, said Dr. Ray Kaplan, a veterinarian with the College of Veterinary Medicine at the University of Georgia.
Veterinarians and horse owners wouldn’t administer antibiotics to a barn full of horses if they didn’t show signs of disease, but they routinely administer dewormer without any signs, Kaplan told the Alberta Horse Breeders and Owners Conference.
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“We need a new approach to parasite control,” said Kaplan.
“The frequent administration of dewormer to all horses is no longer a viable approach.”
Kentucky veterinarians trying to eliminate bloodworms in horses developed the frequent deworming schedule in 1966. The program was so successful, bloodworms are now considered rare, but other parasites developed resistance to the frequent deworming medication.
Equine small strongyles is the main disease-carrying parasite in horses. The eggs drop to pastures through horse feces and develop into larvae that are ingested by horses while grazing.
Horses around the world are infected with this parasite. In low numbers it does no harm. Only when the horses are infected with large numbers are there problems, he said.
“The truth is that most adult horses will remain quite healthy with much fewer treatments.”
While small strongyles are the main parasite in adult horses, roundworms are most significant parasite in foals.
The eggs are hardy and can survive for many months.
Damage from other parasites like tapeworms is “very overrated,” said Kaplan. Recent studies show there is little correlation between tapeworm and colic and Kaplan recommends treating for tapeworm only once a year.
Bots, the small egg laid on horses’ legs and face, are more a cosmetic problem than a serious horse health problem.
“The bot parasite doesn’t cause much harm at all, but they look like they should.”
Threadworms, which are transmitted from mare to foal, are also considered rare.
Many species of parasites are becoming resistant to drugs. In the United States, 20 percent of the farms with sheep and goats are totally resistant to drugs.
“It’s reached a crisis situation and equine is not far behind,” he said.
“We’re at risk of having no effective dewormers.”
There are three main classes of dewormers: benzimidazoles, avermectin and pyrimidines. There hasn’t been a new class of drug introduced in 28 years.
“Drug resistance is the inevitable consequence of drug use.”
Kaplan said horse owners can’t stop drug resistance, but can control how rapidly resistance occurs.
The key is to maintain a portion of the worm population that isn’t resistant to drugs. By treating only horses that need deworming, horse owners will help maintain a population of parasites susceptible to drugs.
“The goal for parasite control is not to eliminate all parasites.”
In 1999 in Denmark, dewormers could only be obtained through a veterinary prescription with a diagnosis. The amount of dewormer used decreased and so did the number of parasites.
Sample deworming program for adult horses
April
- Egg transmission begins.
- Treat and perform fecal egg count on all horses.
- Use Quest for moderate and high egg-shedding horses. Use ivermectin for low egg-shedding horses.
July
- Treat moderate and high egg-shedding horses.
- Treat with oxibendazole or pyrantel.
- Do fecal count to see if treatment is effective.
August
- Treat moderate and high egg shedders with ivermectin or moxidectin.
October
- End of worm season.
- Treat all horses with ivermectin or praziquantel to kill tapeworm, bots, stronglyes or other miscellaneous worms.
- Check fecal egg count.
November to May
- Do not treat. Only treat horses with obvious parasite problems.
