Battle of the Bugs: Parasites Affecting the Horse

Last Updated on November 5, 2020 by Allison Price

brown and white horse with brown leather strap

In horses, internal parasites and worms are a prevalent health problem and they may be at risk. Eggs, immature worms and adults or mature worms… are part of the lifecycle of most internal parasites. Eggs or larvae in the manure of an infected horse are being collected on the grass. When the horse is feeding, they are being swallowed. And the larvae mature inside the digestive tract of the horse. Or even in other tissues or organs into adults.

Internal parasites can decrease the immunity of a horse. And its use of nutrients during this period. And, in some cases, can cause permanent damage to the internal organs. Young and older horses, because of their underdeveloped or weak immunity… are the ones who are at risk of higher worm burdens. There are variable symptoms of worm infestations. They can be subtle or very serious and will depend on the form and size of the worm burden.

Common Parasites

The most significant worms, including the following… are a variety of internal parasites that can infect your horse:

Large strongyles

They are often referred to as bloodworms. Around 10 to 20 mm in length and they are usually red or grey in color. They are successful blood feeders and have a long migratory lifecycle of 6 months. Usually causing anemia in horses. When they accumulate in the cranial mesenteric arteries, they are pathogenic. Arterial blockage (thrombosis) can occur here. Destroying the intestine, resulting in potential colic and death.

Small strongyles

Small redworms are also known as small strongyles or cyathostomins. They are usually 5-15 mm long and have a short lifecycle of 6-8 weeks. This makes them very prevalent and popular in Australian horses. In the large intestine, they feed on gut lining and material. And can cause mild ulceration, weight loss and diarrhea.

Part of their effectiveness is that, for many months… their larvae (L3) will encyst in the intestinal wall. Shielding themselves from several worm treatments. When large numbers emerge from the intestine (called larval cyathostominosis) … small starchy larvae cause serious damage. This can lead to symptoms of diarrhea, colic, poor condition, and likely death in horses. Only Moxidectin is registered. And has been shown to be effective in single dose therapy. Against encysted small starches.


Ascarids are often referred to as large roundworms. And are critical for younger horses younger than 2 years of age. In general, they are white in color and can be up to 30cms long. They have a 10-16week migratory lifecycle. Passing as adults across the bloodstream, liver and lungs before settling in the gut.

Respiratory symptoms, lung failure and, more generally, poor growth… and often colic and death are typical clinical signs. Ascarids have very durable shells. But from one season to the next, it is possible for foals to get infected. As the shells remain viable for years in the climate.


Pinworms can range in color from 10 mm long (males) to 100 mm long (females) and are white to grey. They are peculiar in that females migrate to the rectum. To lay their eggs in a gelatinous layer outside the horse’s anus. When the horse seeks to seek some relief by scratching itself… this can cause itchiness and restlessness. Resulting in hair loss and potential wounds at the base of the tail.

Younger, stable horses are primarily affected by pinworms. Primarily as horses will rub their tails (and so eggs) in feed bins where they can eat them later.


Three species of tapeworm exist. But the most dangerous is Anoplocephala perfoliata. Usually, they are 8-25cms long, but they can be larger and are located in the small intestine and stomach. As an intermediate host, the orbatid mite is a need for their life cycle. The horses ingest this when grazing on grass, grain or hay.

A tapeworm infection in high numbers can cause signs of weight loss. Also, ulceration, anemia, and colic. Praziquantel is the most potent anthelmintic agent targeting all three tapeworm species.

Stomach bots

In its climate, the bot fly is not only an inconvenience to horses. But it can also cause damage to horses internally. Adult female bot flies usually lay their eggs on the horse’s coat in the warmer months of August to May. And the eggs are then eaten by the horse. In the mouth of the horse, moisture causes the eggs to mold into larvae. Causing damage to the tongue and gum tissue. Before migrating to the stomach, where they develop for 8 to 12 months.

They can cause ulceration and lesions. And colic in large numbers here. When temperatures begin to rise in late spring, larvae detach and pass out in the feces. To pupate for many weeks in the soil before they develop into adults.

Management of Paddock

To break their life cycle for an effective worm control program… internal parasites spend a significant part of their lifecycle in the environment. So, it is important to cut pasture eggs and larval contamination. Some guidelines for paddock management include:

  • Regularly pick up manure, especially in smaller yards and paddocks
  • Graze horse paddocks of sheep, pigs, goats, or deer in rotation. This helps reduce the pollution of all equine parasites. In the pasture where horses graze
  • Harrow paddocks and rest them to expose eggs to warm weather under hot, dry conditions for 6-8 weeks
  • Keep the rate of stocking as low as possible; less horses-less worms-less egg
  • Clean and disinfect boxes and stables for foaling
  • To lessen the number of eggs that bind to the skin and pass to the foal, wash mares before foaling
  • Using feed bins and hayracks instead of feeding straight off the ground
  • Remove daily bot eggs from the horse’s hair coat

In managing paddocks, since they are most vulnerable to worm infections… it is advised to target foals and young horses for low-worm infected pasture. As these paddocks are potentially high-risk worm paddocks… avoid placing mares and foals on paddocks grazed before by weanlings or yearlings. It is recommended to maintain paddocks according to the age of the horse. Since you can apply the same treatment program to meet their requirements.


The treatment of horses with anthelmintics (wormers) to kill worms internally… is the other component of internal parasite control. A variety of treatments are available, consisting of the following chemical groups;

  • Benzimidazoles (white drenches) – Usually known as rotation group
  • Praziquantel – particularly for tapeworm control
  • Macrocyclic lactones – it provides broad spectrum safety
  • Tetrahydropyrimides (clear drenches) – smaller spectrum

Worming treatments in a range of different formulations. Including pastes and gels, liquids, granules and stomach tube drenches… may consist of one or more of these chemical groups.

Resistance management

Resistance is an inherited feature in which worms have the ability… to prevent drug effects that are usually successful against them. In Australia, anthelmintic resistance is being documented in horses. So, it is critical that we use these treatments so that we do not allow worms to develop resistance. To decrease the production of resistance… there are three significant steps we should take:

  • To assess when treatment is a need to prevent excessive drenching, use of fecal egg tests
  • Ensuring the right dosage is being administered
  • Rotating procedures and using mixed products. To ensure that one chemical category is not over-used

Fecal egg tests

The number of worm eggs found in a new sample of manure… representing the egg-laying adults found in the horse… is determined by the fecal egg count (FEC) tests. This can be used to estimate the level of worm burden. And species present to determine when treatment is a need for a horse. And also, to determine whether a treatment program is effective. With FEC checks, but there are some limitations. They do not assess larvae or encysted parasite stages. Nor those parasites that do not shed eggs in manure.

But, determining what you are best to treat and if resistance can occur is still a useful test. A fecal egg count of less than 200epg indicates a light load of the parasite. Where a high fecal egg number of 500-1,000epg indicates a high load of the parasite. A medium to high FEC means that the worming time is too long and needs care.

As other variables such as age, clinical signs, immunity, seasonal conditions, paddock management… and stocking rate must also be taken into account. We like to suggest a veterinarian or animal health specialist. This is to provide guidance about how to interpret FEC findings in a correct way.

Right Dosage

It is important to ensure that the correct dosage is being administered. As an under-dose can result in a sub-lethal dosage being given in the procedure. The treatment may not be 100 percent successful in this case. And this can encourage the growth of resistance. By exposing worms to the treatment. Without actually killing them.

Using a girth weigh tape or large animal scales if these are available… is a helpful way to ensure that you are providing the right dosage. You may otherwise measure the girth and length of the horse (from the point of the shoulder). Using a measuring tape. And use the weight measurement formula to determine the body weight of the horse:

Weight (kg) = (girth (cm) x girth (cm) x length (cm))/ 11877

Once you are aware of the body weight of your horse… always round up at least 50 kg to decide the correct dose of treatment needed.

Rotation strategies

A significant step in delaying the onset of resistance… is creating a rotation program. On the market, there are several similar items. Often containing the same active ingredients. So, it is important to be knowledgeable of what is in a product and when to use it best. Unfortunately, revolving brands doesn’t mean that a treatment is always revolving. When the season favors high worm pressure, use your all wormers. And use your narrower range or rotational treatments at times. Especially when worm pressure is lower.

Bayer has assembled a year-round rotation plan recommendation for you. To discern the challenge of finding out which item you can use when. This is an easy-to-follow treatment calendar. Designed to target, according to their seasonal prevalence… the most prevalent parasites at the right time.

To decide how often you should be worming and to decide your resistance status… FEC is often recommended to do. If you have found a problem of resistance on your farm… it is best to get guidance on a personalized program from a veterinarian. Or from animal health advisor.

Fleas on Horses

Fleas and ticks are not pleasant and nobody wants them.

But no matter how we dislike it, horses can get fleas.

Yet, it is unusual. Most of the time, if your horse is healthy, he can resist flea infestation. But as pet owners, we must still prepare to deal with these pests. In this article, we will discuss the improbability of horses getting fleas. Tips on how to get rid and avoid them will also be given.

Brown Horse Lying on Ground

Problems with Fleas

Fleas are more than just an itchy irritation to our pets, horses to be specific. These are blood-sucking parasites that can cause inflammations and irritations.

This can even cause open sores on your pets that have sensitive skin or flea and tick allergies. Severe infestations of fleas can also lead to fur loss and patchy coats, as well as anemia. With these risks, it is best to take every possible step to get rid or prevent them from recurring.

How to Get Rid of and Prevent Fleas?

Fleas are a common problem with any animal that spends time outdoors. Horses are not exempted.

To keep the fleas away from your horse, use a routine that is safe to get rid of the fleas. And of course, without harming your horse.

After the initial treatment, follow a preventative routine. It will stop the fleas from attacking the horses in the future.

1. Fill a container with 1 gallon of water and ¼ cup dish soap. Mix them together until the soap dissolves.

2. Brush over your horse using a large flea comb. It will take a while but ensures a thorough cleaning of your horse. Dip the brush into the container of soapy water as needed when you collect fleas. The soap will trap and smother the fleas.

3. Comb the mane of your horse as well because fleas can often linger in these areas.

4. Fill a spray bottle with water and vinegar. The water and vinegar must be equal. Mist this over your horse. Keep away from eyes, nose, mouth and genitalia. The vinegar emits a bitter scent and taste that drives the fleas away. Reapply the spray every two days.

5. Add 1 teaspoon of white vinegar to your horse’s water trough per gallon of water used. This is too little for your horse to notice. But it will be effective in getting vinegar in your horse’s bloodstream, thus repelling fleas.

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!