Strategic deworming key for parasite control

WT.GRAZINGFall is here and it’s time to deworm your horse again — or is it?

Traditional deworming protocols with a zero tolerance for any parasites may not be the best option for your horse, says Dr. Fernando Marqués, a board-certified specialist in large animal internal medicine at the Western College of Veterinary Medicine’s (WCVM) Veterinary Medical Centre.

There’s no one-size-fits-all protocol for every horse, but the principles of strategic deworming will help you develop a more effective deworming protocol for your situation and reduce your impact on the development of parasite resistance to deworming products.

“Strategic deworming takes into account all factors involving the parasite life cycle, the environment and the horse,” says Marqués.

There are many types of parasites, each with their own life cycles. While we primarily think about parasites living in the horse’s gut, they can migrate to the lung, liver, and other organs to cause disease. Typical clinical signs of a severe parasite infection are colic, weight loss, poor growth, ill thrift and a dull hair coat. Parasites can even become deadly if they are left unchecked.

Because parasites have the potential to harm horses, the traditional way of thinking was to kill them all by deworming periodically throughout the year. However, parasites are capable of developing resistance to deworming products just like bacteria develop resistance to antibiotics.

The more dewormers are used, the more likely parasites are to develop resistance.

“For you as clients and owners and for us as veterinarians we need to be careful,” says Marqués. “We don’t want to develop resistance [to these drugs].”

While dewormers are necessary for controlling parasites, the horse’s immune system is also a vital defence. Parasites will try to reduce the immune response to get the upper hand in the battle, so it’s important to help your horse develop a strong immune system.

“Good nutrition, good water source, good vaccination protocol, good dentition,” says Marqués. “All these things will help to keep the immune system strong.”

As well, Marqués points out that horses are individuals and each animal’s immune response to parasites varies.

“If you look at a herd, only 20 per cent of those horses are high shedders,” he says, adding that those 20 per cent of horses are possibly shedding 80 per cent of the parasite eggs in a herd. A horse that tends to be a high shedder is most likely to be one for life.

A strategic deworming protocol will target these problem horses. By conducting fecal egg count tests on each member of your herd, your veterinarian can determine which of the animals are shedding higher levels of parasite eggs. The number of parasite eggs present in a horse’s fecal sample reflects its parasite burden.

Since some parasites will always be present in horses, Marqués says veterinarians would much rather deal with parasite populations that still respond to drugs. Refugia are parasites that are susceptible to deworming products, are present in an equine herd and are maintained by limiting the use of dewormers.

 But parasites don’t just live in your horses: they also live in pastures where they develop from eggs in the feces to larvae that are capable of infecting your horse. Trying to break the life cycle of parasites before they get to your horse is an effective way of controlling infection.

“There are many factors that we can, or we should, assess before deworming and deciding when to deworm with regards to the environment,” says Marqués. Parasites require specific temperature, humidity and other circumstances to develop into infective larvae.

Here are some environmental control methods that can help to control parasite development on your farm or acreage:

  • pick up manure and remove it from your horse’s environment
  • keep the number of horses per acre to a minimum to prevent overgrazing and to reduce pasture contamination
  • avoid spreading manure in a pasture as this spreads parasite eggs
  • rotate pastures with other species to break parasites’ life cycles
  • house young animals separately from adult animals as they carry different parasites

There’s no standard recipe for controlling parasites in every horse, stresses Marqués. The key point is to think about how to break the parasite life cycle with more than deworming products. A strategic deworming protocol will reduce your contribution to the development of parasite resistance and, more importantly, it will be more effective for your horses.

“Assessing the place, assessing the horses, assessing the type of parasites,” says Marqués.  “Remember the triangle of parasites, environment, and horses to assess the whole picture and to make a good strategy and plan for your specific situation.”

Hayley Kosolofski is a fourth-year veterinary student from Sherwood Park, Alta., who is the undergraduate student representative for the WCVM’s Equine Health Research Fund.

 Reposted with permission from Canadian Horse Journal ( 


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