#worminghorses

Horses & Worms

Parasites are a part of our horses world, and they are here to stay. Almost all horses carry some level of parasites all of the time and the horse-parasite relationship seems to be an important part of the overall ecosystem. And the parasites, like most pathogens in the microbiome, are not a problem if they are in balance. In fact, they help support our horses’ immune system. However, should parasites over-produce to the point where the horse is carrying a heavier load, health problems can easily develop, some of them quite serious. Horses with heavy loads lose nutrients and blood and suffer reduced reproductive and athletic performance.

We refer to our horses’ worms as parasites which they are. But the term parasites actually encompasses many other types of organisms that live on or in a host and gets its food at their expense. Technically, worms are “helminths” which describe them as intestinal worms and helps to distinguish them from the many other types of parasites which can reside within our horses. The average horse is home to thousands of different kinds of protozoa, molds, fungi, bacteria, and viruses; these pathogens are mostly not yet scientifically identified and the ones that are known, are often not detected since they don’t always cause clinical symptoms until the imbalance is extreme.

Worms, both adults and their encysts, can be responsible for a lot of different symptoms. We are familiar with common signs of parasite overgrowths such as a rough hair coat, pot bellies, weight loss, and digestive issues including poor appetite, diarrhea, and colic. But parasites are responsible for many other symptoms as well because they produce different chemicals and toxins including ammonia, which have adverse effects on our horses’ chemistry and physiology.

Parasites frequently cause anemia (by eating blood), alter the metabolism, reduce circulation, decrease energy levels, and compromise immunity by disturbing the intestinal immune system and contributing to leaky gut. Horses with parasite loads therefore often have an increase of allergies, skin problems, chronic infections, generalized stiffness, and difficulties in recovering from illness. And what is not often discussed is the effect of parasites on hoof health; laminitis is exacerbated through chemicals and toxins in the blood which circulate through the feet causing inflammation and damage to the tissues.

Encysted parasites are the larvae which become enclosed in a protective sac. Encysts frequently migrate from the large colon to other parts of the body as they seek to hibernate. Encysted parasites can be found in the intestinal membranes, the mesenteric artery, other blood vessels, liver, kidneys, pancreas, lungs, and even the heart. These encysts can scar the affected organs and also interfere with important body functions.

Why Do Horses Get Parasites?

Of all domestic livestock, horses have the largest numbers of parasites including large strongyles, small strongyles, roundworms, and pinworms. Horses graze close to the ground and, more than other livestock, are always smelling, nibbling, and licking, whereby they can pick up large numbers of eggs or infective larvae that live on the grass blades. Therefore, horses that graze on grass in a domesticated living environment are the most susceptible. In fact, studies have determined that grazing horses are far more likely to host a parasite load than those horses housed on dry lots with hay diets.

Grazing horses are also at risk if they are allowed to overgraze their pasture, because overgrazed, nutrient-poor grass favours higher larval populations. Insufficient space for multiple horses is also a risk factor because in tight quarters they are more likely to pick up eggs shed by one another. And horses kept in crowded facilities are generally not as healthy and have weaker immune systems.

Horses can be susceptible to a wide variety of internal parasites. The types of parasites and the parasite load is dependent upon grazing, soil conditions, diet, age, overall health and immunity, and exposure to other horses.

So, let’s find out what culprit worms are the most common and likely to cause the horses a variety of health problems.

What Type of Common Worms Can Infect Horses?

The life cycles of the different parasites are very similar and rotate between the external environment and the inside of our horses. Eggs are passed through the manure and onto the grass or barn floor as eggs and are then inadvertently ingested by the horses. Eggs grow into the larval stage either before ingestion or in the hindgut, intestinal membranes, and other organs. And then after several months the larvae move back into the hindgut where they mature into adult worms and lay thousands of eggs which are then passed back out again into the pasture to start the cycle all over again.

Large Strongyles (Strongylus vulgaris)

Strongyles, both large and small, are the most common of parasites and also the most destructive. There are three species of large strongyles (S. vulgaris, S. endentatus, and S. equinus) with S. vulgaris being the most common. Strongyle vulgaris eggs are shed into the manure and hatch out into the larval stage outside in the grass or soil in as little as three days. Strongyles can survive in a freezing environment, but a hot, dry environment will often kill them. The infective larvae survive up to 30 weeks at winter temperatures, compared to up to 7 weeks in the summer.

The horses ingest the larvae through grazing, the larvae drop their protective coating, and then in the fall, prompted by changes in daylight, they attach themselves to the intestinal wall in the hindgut. From here they migrate through the arteries to the mesenteric artery. After 6-8 months the larvae travel back to the large colon where they mature into adult worms and lay several thousand eggs each day. In the spring the eggs are passed into the manure and back onto the pasture starting the cycle all over again. The entire life cycle is 6-7 months.

S. vulgaris are often referred to as blood worms or red worms because they travel in the bloodstream and because they can rough up the artery walls with lesions that cause inflammation, thickening, and the formation of blood clots. These clots can break away from the wall and block blood flow in the vessels, including the mesenteric artery which is the artery that flows to the intestines. If the mesenteric artery itself becomes damaged it can cause “verminous arteritis” where a segment of the intestines can become inflamed or die due to loss of blood supply. This is known as thromboembolic colic (the blocking of a blood vessel by a blood clot particle) and can be fatal.

Adult large strongyles can also bite off pieces of the intestinal membranes leading to colic, diarrhea, fevers, and anemia from the bleeding bite wounds.

A parasite load of large strongyles can cause weight loss, poor circulation, a dull hair coat, pot bellies, diarrhea, constipation, liver toxicity, anemia (worms feed off blood), immune problems, skin conditions, damage, or ruptures to the artery walls, mild or severe colic, and laminitis. Encysted strongyles can be found in the blood vessels, the liver, kidneys, the pancreas, and the heart.

S. endentatus, and S. equinus have a similar life cycle, but their larvae migrate through the liver. This migration damages the liver but not to the extent that S. vulgaris can cause to the arteries. They also return to the large colon with a life cycle of 8-11 months.

Small Strongyles (Cyathostomes)

Small strongyles have similar life cycles to the large strongyles. They pass their eggs into the grazing pastures during the summer which then hatch into larval stages 1 and 2 in the grass. Larval stage 3 is the infective larvae which are encased in a protective sheath that enables them to withstand winter climates very well. Once they are ingested, they lose their protective sheath in the small intestine and then move into the large colon where they encyst into the colon membranes and stay there. This damages the intestinal lining and so the intestines launch a defense strategy by forming scar tissue to envelop each larva. These little capsules or nodules are usually filled with blood that the larvae can feed off.

The larvae can live here for several weeks and even up to several years. The larvae develop into the 4th stage in late winter-early spring when the weather warms up and then emerge from the cyst and enter the large colon. However, the majority of the larvae remain encysted in the intestinal walls which is why fecal tests are often negative for small strongyle eggs.

As they emerge, they release toxins from the accumulated larval waste products. If too many encysted larvae emerge all at once they can cause diarrhea, recurring colic, blood loss, anemia, loss of appetite, weight loss, conjunctivitis, depression, dehydration, edema (water retention in the lower legs), and laminitis. This hindgut overload is more likely to occur with immune compromised horses, horses with a history of infrequent deworming, and exposure to other pasture horses with high worm loads.

If a very large number of small strongyles emerge all at once a health crisis called “larval cyathostomosis” can occur. This causes an immune reaction with inflammation and swelling of the intestinal wall. It results in damage to the hindgut membranes as well as a shutdown of peristalsis (normal gut action) which may lead to severe colic, and even death. A larval cyathostomosis is most often caused by using the wrong dewormers. As we shall see in Part II if adulticide dewormers are used in horses with a large load of small strongyles it will act as a trigger for an excess of encysted L3 larvae to come out of the intestinal membranes and dump into the colon in large numbers. Small strongyles are also much more resistant to chemical dewormers than large strongyles.

In some horses, small strongyle larvae continue to encyst and fail to erupt at all. Therefore, massive amounts of cysts build up causing an increase of scar tissue and therefore reducing gut absorption capacity. This usually occurs in late summer after significant exposure to contaminated pastures with L3 stage larvae, the stage that is also immune to chemical dewormers. Symptoms of this “adult cyathostomosis” could include weight loss, poor condition, anemia, fatigue, elevated liver enzymes, persistent diarrhea, and intestinal ulcers.

Roundworms (Parascaris equorum)

Roundworms are also known as ascarids, and the eggs are ingested by the horses, then migrate to the liver and lungs and back to the small intestine in the foregut to become adults. This process takes about 10-12 weeks. Adult worms can produce 200,000 eggs per day which are then passed into the manure. The eggs have a thick shell and are very resilient; roundworm eggs can survive in the environment for many years.

Roundworms are large white worms about the size of a pencil and can grow to be a foot long. Roundworms mostly affect youngers horses under 2 years of age and foals may have an overload of roundworms as early as 2-3 months. Many horses develop immunity by the age of 2 although they occasionally can be found in adult horses with poor intestinal health.

Symptoms of the migrating larvae can cause coughing and nasal discharge.  Adult round worms can cause a pot belly, digestive problems, weight loss, poor appetite, and poor nutrient absorption. Roundworms can become very large and block the intestines resulting in colic including severe colic. They can also scar the liver and migrate through the lungs resulting in coughing, fevers, shortness of breath, and respiratory infections such as pneumonia.

Pinworms (Oxyuris equi)

Pinworm eggs are ingested from contaminated pastures. They can also cling to stall walls and other surfaces. Once ingested the larvae hatch out in the hindgut and grow into adults. The female pinworms lay their eggs around the anus producing a sticky yellowish secretion. Pinworms can cause considerable irritation, itching, and tail rubbing; but do not usually cause digestive problems. Many horses on pasture itch a lot from the sugars in the grass that affect their overall immune system and skin health. So be sure and confirm the presence of pinworms before treatment.  

Tapeworms (Anoplocephala)

Tapeworms are more common in older and younger horses. They can grow to 4 feet in length and attach themselves to the blood vessels in the ileo-cecal junction just inside the cecum between the small and large intestines.

They release segments which contain eggs into the feces and the eggs are eaten by forage mites on the pasture The horses then eat the mites while they are grazing. Tapeworms are usually found in areas with lush green pastures and are very uncommon in dry arid areas or dry lots. The ingested tapeworm eggs mature in 6-10 weeks when they attach to the intestinal linings and absorb nutrients. Then in the spring the segments full of eggs break off from the tapeworm and are passed into the manure where they are ingested by pasture mites to start the cycle over again. However, because the eggs are housed in these segments, they are difficult to detect in fecal counts and are most often missed.

Symptoms of adult tapeworms are not clearly defined but can include rough hair coat, slow growth, nutrient deficiencies, and hindgut ulcers. If there’s enough of them they can cause an impaction colic in the region of the ileocecal junction.

Bots (G. intestinalis, G. nasalis)

Bots are not technically worms. The eggs are laid by the big yellow bot flies in late summer and are typically found on the shoulders, front legs, neck, mane, and flanks which must be removed with a bot knife or serrated knife. Very often they are worse on the left side of the horse. The eggs are dormant until the horse licks the eggs off and the moist warmth inside the mouth causes the eggs to hatch, and the larvae burrows into the gums and tongue. Eventually they migrate to the stomach and attach to the stomach wall causing inflammation and ulceration. The eggs will release from the stomach wall within 8-12 months and are passed into the manure and will hatch into another bot fly continuing the cycle. A bot infection can cause chronic colic, infections, changes in appetite, stomach pain, and other digestive problems.

Stomach Worms (Habronema spp)

These parasites reside in the stomach. They lay eggs here which are passed through the manure where they adhere to maggots. When the maggots develop into flies they can land on open wounds and deposit the larvae where they cause irritation and impede healing.

Neck Threadworm (Onchocerca cervicalis)

This parasite is transmitted by gnats or midges and the larvae live underneath the skin where they can cause severe dermatitis and thickening of the skin. Adult threadworms live in the tendons and ligaments.

Lungworm (Dictyocaulus arnfieldi)

The horse lungworm is long and slender measuring up to 1-3 inches. These parasites travel from the intestinal walls through the blood stream to the lungs where they mature. When large numbers of larvae are present the lining of the bronchioles may become stressed resulting in persistent coughing, difficulty breathing, and loss of appetite. Older horses are more immune, but foals are very susceptible. Nevertheless, foals as well as donkeys may not show any clinical signs.

Healthy Horses

Parasites are not interested in killing their hosts, rather they are interested in using their hosts as a pathway in continuing their life cycle to increase their numbers. However, they are more likely to cause harm to horses who are in a weakened condition and who have a poor intestinal immune system. The naturally healthy horse with a high level of resistance is a horse with efficient digestion, low sugar diets, free forage, strong immunity, adequate levels of exercise, regular cleanses, and well-selected remedies, including the smart use of chemical dewormers.

The effective prevention and treatment of parasites is discussed in detail in Part II: How Should I Deworm My Horse?