Young horses are more vulnerable to internal parasite infection and damage than adult horses, because they have less immunity.
Dr. Ray Kaplan, Professor of Parasitology, College of Veterinary Medicine, University of Georgia (UGA) says the most important issue today in the battle against internal parasites is surveillance, checking for eggs in the feces. Deworming should only be done when needed, and with a drug that works effectively to kill the targeted parasite.
With foals, however, don't wait until you see a problem (signs of worm burden or high egg counts) before you start treating. Kaplan recommends that foals be treated on a regular basis. "However, if you are just treating without doing fecal sample surveillance, you won't know whether the drugs are working or not," he says.
With drug resistance issues today, we don't want to be treating foals unless they need it or this will compromise our ability to control worms in the future. At the same time, if we are not treating correctly, with drugs that work, we may think everything is fine because we are deworming on a regular basis - and not realize we have a disaster brewing because the drugs aren't working anymore.
"Doing egg count surveillance for both strongyles and roundworms (ascarids - the number one problem in foals) is critical, to make sure your program is working," said Kaplan.
Ascarids are the predominant parasite in foals between 4-8 months old. This is when we tend to see disease from heavy worm burdens if the foals are in an environment heavily contaminated with these worms.
"Once the foals are eight months or older they start to develop immunity to ascarids," he explained.
At that point, which is usually around weaning time and during the foal's first winter, small strongyles become more of an issue. It takes several years for the young horse to develop some immunity to small strongyles.
"Weanlings and yearlings remain highly susceptible to these, whereas by about eight months of age they are developing immunity to ascarids and rid themselves of these, even without treatment. They may still be infected, but usually not with pathogenic levels," said Kaplan.
Every farm is different however, the worm burden in the horses - whether it's high enough to be a problem depends on the management system and how contaminated the environment is. Good nutrition and good overall health also play a role in management. All of these things can have an effect. But if horse owners or farm managers are doing egg count surveillance they can keep track of what's happening and know if control programs are working.
Most veterinarians recommend starting treatment of foals for ascarids at two months of age.
"Some people want to start treatment at two-four weeks of age, but this will just encourage drug resistance. Those treatments are not very effective. The early stages of this parasite are the hardest to kill; you have to wait until the worms are older to have an effective kill rate," explained Kaplan.
In ascarids, resistance to ivermectin and moxidectin is becoming common. The early larval stages are the most difficult to kill, even when the worms are susceptible. If the worms are already in the process of becoming resistant, the first ones that will resist being killed are those early larval stages.
"By treating foals sooner than two months of age, all you are doing is putting more selection pressure on those worms for developing drug resistance, without accomplishing much in parasite control," he said.
After that initial treatment, Kaplan recommends treating the foal every two months. Prior to the second treatment, however, do an egg count. If more than a few eggs show up in that check, he suggests doing another egg count two weeks after treatment - to see if the drug is working.
"To find out if the drugs are effective against ascarids, it's best to do this test when the foal is in the four-six months of age. Once foals get to be more than six months old, you may see reductions in egg counts that have nothing to do with the drugs, just because foals at that age are starting to build their own immunity," he explained. "We see a lot of resistance to benzimadazole drugs and to pyrantel in small strongyles. The most resistance in ascarids is to ivermectin and moxidectin. In some cases, depending on a particular farm's worms (and resistance), it may be necessary to treat with two different drug classes at the same time - because you are targeting two completely different parasites and want to make sure you kill both."
Rotating drug classes every time you deworm (every two months) would not be a good idea in this situation because then you could have a four-month period in which you are not adequately treating a certain parasite if one of those drugs doesn't work. This could set foals up for severe worm burdens if the parasites have resistance to one of the two drugs you are using.
We are trying to keep the animals dewormed without compromising future effectiveness of available drugs. At this point in time, there are no new drugs being developed, for horses.
Managing the Environment
More important than using chemical dewormers is trying to keep the horses' environment less contaminated with worm eggs and larvae, reducing the incidence of transmission. The use of deworming drugs should be an adjunct to good management, rather than thinking we can depend on these drugs alone.
"Some horse owners in other countries are managing fine without dependence on dewormers. I was recently in Brazil, on a farm that has resistance to all drugs, in all the parasites, but the horses looked great - mainly because of good management and diligent egg count surveillance, just selectively treating the individuals that need to be treated. Even though there is drug resistance, this doesn't mean the drugs are useless. You just don't get a high kill rate," explained Kaplan. You may still help the horse by giving him a little assistance when he needs it.
If you are only getting 80-90 percent kill rate with a certain drug, then technically you have a resistance problem. If you keep using that drug frequently, in all your horses, it will keep losing efficacy until it is useless.
Kaplan points out that if a drug has only 80-90 percent efficiency and you start using that drug in a targeted manner, you can still get good use from it and the worms won't get any more resistant to it than they already have, because you are not using it that often.
The farm in Brazil that he visited uses cattle in a pasture rotation to help break the life cycle of the worms. The adult horses were grazing in one pasture, the young horses in another, and cattle in a third pasture and these groups were then rotated among the pastures.
This grazing strategy can help tremendously, in contrast to having the foals always in the same pasture, constantly in a worm-contaminated environment - which would greatly increase transmission of parasites. But if you can move them into a pasture that was just grazed by cattle, the cattle have vacuumed up many of the horse parasites (which won't complete their life cycle in the cattle). Also, the extra time between occupancies by horses allows some of the parasite larvae to die naturally. This tactic can greatly diminish worm numbers.