Deworming for horses: How often it need?

Last Updated on November 6, 2020 by Allison Price

Do Horses Need to be Dewormed?

Over the years, one of the reasons why horses suffer from weight loss is because of an excessive number of internal parasites. Internal parasites rub the nutrients, and they compete in the utilization of those nutrients. Also, these parasites can damage the digestive tract tissues and can irritate. With this, maintaining weight for horses is hard. Some horses, young or old, are most susceptible to getting parasites. 

The deworming of horses aims to limit the infestation of parasites inside the body, not to eradicate them. With this, horses can remain free from clinical illnesses and be healthy. However, parasites evolve and create resistance over the deworming products. This means that parasitic worms developed genes that would make them resistant to drugs. Resistance will limit the ability of the drugs to treat horses. Furthermore, this is an issue that the developers face and have corresponding consequences. Massive numbers of the parasite inside your horses’ body will lead to a severe health condition like severe colic.

Giving your horse products for deworming without a proper series of tests is not a good practice. Horses owners and veterinarians should create a strategic plan on how to combat the parasites and control them. All aspects should be covered with strategic deworming like the life cycle of parasites, the horse, and the environment.

Moreover, refugia is another term that you should understand and become familiar with. This will help you to have more knowledge about the points of strategic deworming and prevent parasitic drug resistance. Refugia refer to the portion of parasites’ populations that do not have a resistance over drugs. These weak parasites are easy to kill, the ones that we would likely have in the pasture. But parasites would adapt and develop resistance through deworming treatments to survive. If you deworm every six to 8 weeks and use different dewormers, the population of refugia will decrease. However, the population of those parasites that have resistance will rise. Soon enough, if not strategize carefully, deworming products will lose their effectiveness.

Before you come up with the best strategy for deworming, consider first the overall health of your horse and environment. With researching, the horses that are in the same herd have different burdens with parasites. Some horses have very few numbers of worm eggs, and others have high numbers. In line with this, others who don’t have a parasitic problem will probably become infected. With this, you need to have deworming customization for each horse in your barn. Treat your horses individually or have group selection based on their age.

Let’s Get Started!

The Source of Worms Infestation

First and foremost, horses are vulnerable to have worms inside their bodies. These worms can live as larvae or an egg in feed, on horses’ coats, in mares’ milk, grazing pasture, and throughout the environment. Parasites can be spread through the contamination of pasture caused by feces, direct contact, bites of insects, laying of egg, and feeding. 

You can understand the nature of how horses can get infected when you know that parasites can lay eggs in the intestinal tract. When horses now excrete stool, these will carry over and become larvae. These larvae now will find their way to survive and stick into the grasses. Once horses eat the contaminated grasses, the worms’ larvae now hibernate in the intestinal tract before they will develop. After being developed into adults, they will lay eggs again so the cycle of infestation may continue. The infected horses may show symptoms such as fever, colic, and diarrhea. Sometimes the misconception of horse owners is that the feed is the source of infestation. For this reason, they will change the feed again and again.

Roundworms are one of the major problems when it comes with equine parasites. The most affected of these parasites are the young foals. The best treatment for this is to come up with a better prevention program. By giving your foals a proper treatment that will be set every thirty to forty days, you can control these parasites. This will start when the age of your foals reaches one month or two months. Furthermore, if you will not permit your horses to stay in one place, you might control the infestation. You should understand that putting your horses in one place will lead to multiple contamination. The cause of infesting them might include sharing of pastures, feed buckets, and direct contact. Proper hygiene is one of the needs of horses to maintain good body condition. Also, proper deworming measures are what horses need to control the infesting parasites.

Deworming for Horses

Most horses are required to have deworming once or twice a year. In the spring season, before deworming, have a Fecal Egg Count (FEC) for your horses. It is a test that will measure how many parasite eggs do each of your horses have. Upon thorough examination, your veterinarian would recommend if your horse will undergo deworming or not. 

Moreover, your horses’ deworming should be in the late fall with appropriate products for deworming. Also, it is better to conduct a second fecal egg count for your horses in the fall. However, most of the horse owners would not do the second examination. Your veterinarian will give advice about what deworming products to use based on the common parasites.

The pregnant mares should have a deworming session in spring before the foal with the chosen dewormer. The deworming is based upon the result of fecal egg count. Also, mothers should have their deworming treatment with an ivermectin product after giving birth. The treatment will begin after 24 hours, based on the time when the mother gave birth.

For the last few years, deworming for horses is set to be every two months. However, there are arguments upon the effectiveness of using the same dewormers. So, there is an alternation in using different deworming products to minimize the parasites building up resistance. There are some horses that are good to deworm twice a year – spring and fall. However, there are horses that may need continuous treatment to prevent the same problem after being treated.

Foals need frequent deworming than those other horses. They will be treated with fenbendazole when they are two months old. The treatment will repeat every two months until the foals become yearlings. Also, foals should have ivermectin type product for deworming when they reached 12 months old. Then, every three to four months with the chosen deworming product based upon the fecal egg count test result.

These are some of the factors that may affect the deworming schedule of your horses:

Age – Very old and very young horses need to have continuous deworming as they are vulnerable to contamination.

Size of Herd – If you have a lot of horses in your barn, series of deworming should be done to lessen the contamination. Unless you only have one horse, then you need to have less frequent deworming.

Health – Horses that have a better health condition may need few deworming treatments. Having a good body condition resist the effects of having a worm.

Other horse exposure – If your horses have contacts with unknown horses at boarding facilities or shows, better to desire more deworming treatments. 

Condition of the Pastures – A small and dirty pasture has a high chance of having a large number of egg worms than a large and open field.

Climate – Worms succeed in an atmosphere that is a warm and moist environment. If in cold and dry places, horses may need fewer treatments for deworming.

Some types of worms and the problems they can cause

Lungworm – Also known as Dictyocaulus arnfield. You can find this type of worm and its larvae in the lungs of horses that share places with donkeys. It can cause bronchopneumonia and parasitic bronchitis.

Bots – This type of worm is the adult bots’ larvae. The scientific name of this worm is Gasterophilus spp. In the droppings, the larvae could be passed and will develop inside the body of horses, specifically in the stomach. The adult flies laid eggs in the coats of horses or in their legs where it will hatch. Once the horses lick their legs, they can swallow the eggs of these flies. The medical problem that your horses can encounter is the mild ulceration of their stomach. With this, there is a possible stomach ruptures once the number of the eggs become alarming and hatch inside the stomach. But today, cases like this is rare to cause a certain problem.

Tapeworms – Anaplocephala perfoliate is the scientific name of these worms. They live at the intersection between the cecum and small intestine. The intermediate host, that is responsible for the transmission of this worm, is a mite. They are also responsible for the lifecycle of the worms to be completed. In these recent days, tapeworms are responsible for the spasmodic colic disease of the horses. Furthermore, they bring problem like intussusception that needs surgical correction cause by colic severeness. You can see other symptoms such as diarrhea and weight loss.

Ascarids – Also known as for its scientific name as Parascaris equorum. The most affected of these worms are the younger horses, such as foals. Young horses could pick up the eggs of these worms and could be swallowed while they are grazing. Moreover, the worm eggs will develop into larvae and migrate to other organs like lung and liver. Afterwards, they will return to the small intestine to develop and become adults. If these worms are not controlled, they will lay eggs again that may lead to massive number of parasites. Also, they can stunt growth, damage the lungs, and can cause colic and diarrhea.

Stronygloide westeri – These worms can cause harm to young foals by affecting their small intestines. The transmission of the worms’ larvae, from mares to foals, is through the milk. Though there could be an infection, through the developed immune system of foals as they aged, it could be under control. However, if they swallowed large numbers of larvae, it will result to severe diarrhea. The immune system of the foals should not be compromised, so place them where they are not overcrowded.

Large redworms – There scientific could be Strongylus vulgaris, S. equinus and S. edentatus. As others, horses could swallow these worms through the contaminated grasses or pasture. The worms’ larvae will migrate through the blood vessels of the liver and intestine. It can cause obstruction of blood vessels and inflammation where it leads in damaging the intestines. The symptoms such as diarrhea, colic, and ill thrift are visible in your horses.

Small redworms – These worms are also called Cyathostomes. Through grazing, horses can swallow the larvae of these small redworms. As usual, they are found on the blades of grasses that are contaminated. These larvae stick into the lining of large intestines to develop and become adults. Now, these adults will lay eggs and will pass to other horses through horses’ droppings. Larvae that picked up by horses at autumn season could hibernate into the wall of intestines. They can survive for a certain amount of time, and develop into adults when the atmosphere is right and becomes suitable for them. When horses consume massive numbers of small redworms larvae, this will result to severe diarrhea. Other symptoms may include mild intermittent colic, weight loss, and lethargy.

Manage the Available Grazing

Horses can be contaminated again by consuming the infested pasture. Before this happens, you need to control it or have some plans in avoiding such problem. The following methods might help you and your horses:

1. All horses should undergo deworming treatment to reduce their output of larvae and eggs of worms.

2. Those new arrived horses should have a separate place. You should not bring them immediately to clean pasture.

3. Avoid overcrowding the pasture to have less contamination.

4. Manure of the horses should be regularly removed from the pasture to avoid further contamination. 

5. It is recommended that the paddocks should have a regular grazing with other animals such as sheep and cattle. These kinds of animals cannot acquire the equine parasites and their parasites also cannot affect horses. Also, their manures are good fertilizer for grass to grow.

For you to broaden your knowledge about the deworming of your horses, watch this video:

Allison Price
Allison Price

I’m Allison, born and raised in San Diego California, the earliest memory I have with horses was at my grandfather’s farm. I used to sit at the stable as a kid and hang out with my Papa while he was training the horses. When I was invited to watch a horse riding competition, I got so fascinated with riding!