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Roundworms (a.k.a. ascarids)

​Roundworms are more prevalent in young horses, mainly foals, weanlings, and yearlings.  Colic is a huge risk factor of roundworms since adult roundworms are several inches long and almost the width of a pencil.  These worms can easily cause blockages/impactions of the intestines.  Additionally, the larvae of roundworms will travel to the lungs where they are coughed up and swallowed back into the digestive tract.  Due to the migration during these larval forms, roundworms can also lead to damage of the liver or lungs.  

It is important that expectant mares be wormed about a month before her due dates and/or immediately following birth to reduce exposure to the new foal.  Infections of roundworms in young horses can cause poor body condition, poor growth, coughing, pot belly, rough coat, and colic.  Colic is most likely in foals over 3 months of age with a heavy roundworm infection after they are dewormed for the first time.  By this time, larvae have matured into adults that easily cause impactions.  Because of this situation, as well as roundworm resistance to many dewormers, it is a good idea to have Dr. Matt recommend a deworming plan for your foals.

Triple Oaks Equine LLC

Equine Parasites


​Dictyocaulus arnfieldi is the specie of lungworm that affects donkeys, this is different from the cattle lungworm (Dictyocaulus viviparous) which causes severe lung damage.  The first thing to realize is that these are two different lungworms, cattle lungworms do not infect horses and donkeys, and vice versa.  Lungworms are 'host-adapted' meaning that horses can only be infected if they live with infected donkeys.  Horses can ONLY get this parasite from donkeys.  Once a horse is infected it is a 'dead-end host' meaning the lungworms cannot reach their adult reproductive stages, so horses cannot pass this parasite to other horses.

If a donkey or horse is infected, lungworms are usually restricted to the bronchial tree (the part of the windpipe just before and after it branches out to each lung) so damage to the actual lung itself should not occur.  Lungworms do, however, cause inflammatory airway disease (IAD) which can be difficult to manage.  

After the worms reproduce in the donkey bronchia, the lungworm larvae are coughed up, swallowed and passed through with the feces.  This infects the pasture where horses can ingest the larvae.  Donkeys do not develop clinical signs of lungworm infections, however horses often suffer from coughing, respiratory irritation, and/or bronchitis.  Since lungworms do not reach reproductive stages in a horse, eggs and larvae will not be present in their feces, making a lungworm infection difficult to identify as a cause of respiratory disease in horses.  A larval culture of donkey feces is the main definitive diagnostic tool for lungworms.

Internal parasites are silent thieves and and can be killers.  Before you even realize your animal is infected they can cause extensive internal damage. The effects on your horse can range anywhere from a dull haircoat and unthriftiness to severe colic and death.  Internal parasites can also lower a horse's resistance to infection, they rob the horse of valuable nutrients, and sometimes cause permanent damage to the internal organs.  It is important to realize that in many case only 20% of a horse herd harbors about 80% of the infection.  Once these carriers are identified through fecal egg counts and treated appropriately, the pasture becomes less contaminated and the herd becomes healthier.  Many of the symptoms for these internal parasites are similar.  It is near impossible to diagnose a parasite infection by symptoms alone.  A fecal exam is needed to diagnose an infection, determine the type of parasite, and assess the severity of infection.

There are more than 150 species of internal parasites that can infect horses. The most common and troublesome are the following:

​​Most internal parasites have a lifecycle that involves eggs, immature worms known as larvae, and mature worms.  Eggs or larvae are usually passed through a horse's digestive tract in the manure to infect a pasture.  While the horse is grazing they ingest the larvae which mature into adults usually in the stomach or intestines.  With some parasite species, the larvae migrate out of the intestines to other tissues or organs, before returning to the intestine to lay eggs. 

There are many, many dewormers available to combat equine parasites.  Unfortunately, not every dewormer works effectively on every type of parasite and many parasites have grown resistance to certain dewormers.  Contact Triple Oaks to set up an effective parasite control program today!


Tapeworms can cause colic ranging from mild cramping to severe colic that requires surgical treatment.  The equine tapeworm is different than many other species of tapeworms for a few different reasons.  Equine tapeworms, rather than being composed of long chains of segments, are pumpkin-seed-shaped about 1 inch long and 1/2 inch wide.  Each one has four separate suckers which enable them to attach to the intestinal lining where they are easily able to absorb nutrients and damage adjacent tissues.  The life cycle of the equine tapeworm is also unique as it involves a tiny forage mite ingesting the tapeworm eggs.  This mite serves as an intermediate host for the immature tapeworm to develop into it's infectious stage.  When a horse eats this mite in grass or hay, the larvae is released and within 6-10 weeks it develops into an adult tapeworm and attaches to the horse's intestinal wall.  As eggs are passed within manure, the forage mite ingests them and the cycle continues.  

Although we commonly use fecal tests to diagnose roundworms and strongyles, these tests are not effective in diagnosing tapeworm infections.  The only test thus far which has been successful in detecting the presence of tapeworms is a blood test.  Contact Dr. Matt for advice on the best deworming product for use in your situation.

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  • Bots
  • Strongyles (bloodworms or redworms)
  • Lungworms
  • Pinworms
  • Roundworms (ascarids)
  • Tapeworms​


During the summer months, female bot flies lay their eggs on the haircoats of horses.  These flies and their eggs are known as bots.  Bots lay hundreds of tiny white-yellowish eggs and 'glue' them on the hair of horses, mainly on their legs and shoulders, but also around their mouth and bottom part of the lower jaw and throat.  As a horse itches these spots with their teeth, they ingest the tiny eggs which hatch into larvae.  Bot eggs are not only unpleasing to look at, but they can cause irritation in the mouth and the larvae can cause numerous problems in the GI tract.  

Horses with a bot infection may suffer from ulcers, colic, and/or discomfort when eating.  Often though, infected horses show no clinical signs and fecal egg tests do not show signs of an infection.  If you see the presence of these eggs on your horse, you can usually guess the presence of bot larvae in your horse.  Positive identification of bot larvae in feces is another way to diagnose a bot infection.

In the mouth, the larvae can bury themselves between the molars and around the tongue causing pain and irritation.  In the stomach the bot larvae will develop for nine to twelve months attaching to the stomach lining and causing ulcers. before they move throughout the rest of the intestines and eventually pass through in the feces.  

Prevention includes removing bot eggs with a tool such as a pumice stone or a bot knife.  Insect control is also an important measure when considering prevention of insect spread diseases and infections.  Contact Dr. Matt for information about diagnosing and treating a bot infection.

Strongyles (a.k.a. bloodworms or redworms)

Strongyle eggs are passed through the digestive tract with the feces of an infected horse.  Once passed, the eggs hatch and must develop through three larval stages before they can be infective.  When a horse ingests an immature worm in it's third larval stage through grazing or drinking contaminated water, it becomes infected.  The life cycle of large and small strongyles are identical outside of the horse, but once ingested their approach to infection differs greatly.  

Large strongyles leave the intestine shortly after infection.  They migrate for 6-11 months through various tissues including the arteries lining the gut, the peritoneum, liver, and pancreas before returning to the gut as mature adults to lay their eggs.

Small strongyles take a much different approach.  Instead of migrating, these larvae set up home immediately in the lining of the gut wall.  These larvae become 'encysted' with a tough capsule of scar tissue resistant to any attacks from the horse's immune system or dewormers.  While encapsulated, the third stage larvae can either undergo transformation into fourth stage larvae or they can remain in limbo for up to two years waiting to develop.  Once fourth stage larvae emerge from their cysts and mature into adults they can lay hundreds of thousands of eggs in their lifetime, continually infecting the pasture.  

Strongyles are a concern through out the horse's life, beginning as early as six weeks old when they begin to graze.  Infected pastures can remain infected for years, even if no horses graze on them because third stage larvae survive the winter months extremely well, ready to become infectious again in the spring.  

Symptoms of a strongyle infection may include rapid weight loss, loss of appetite, poor growth, anemia, lethargy, dull coat, poor performance, pot belly, diarrhea, constipation, colic, and in severe cases, death.  Fortunately, strongyles can be effectively controlled by most available dewormers for horses after a fecal egg count determines the size of the infection.



Although pinworms usually cause little actual damage to the horse, they are certainly a pain in the butt.  Literally.  Adult pinworms crawl out of the horse's rectum to lay their eggs, which causes intense itching.  An obvious sign that a horse could have pinworms are severe itching and hair loss at the tailhead.  Although the normal fecal egg tests generally do not show pinworm eggs, since eggs are not passed through with feces, occasionally you can make a diagnosis by actually seeing adult pinworms around the rectum.  A test to visualize the actual eggs is the 'scotch tape test.'  This requires an owner or veterinarian to touch the sticky side of the tape around the horse's anus and search for eggs under the microscope.  

Prevention can include keeping your horse's rear end clean and decontaminating areas where infected horse's rub and leave eggs (fence posts, walls, etc.)  Treatment includes a positive diagnosis since many other conditions can cause tail itching, such as a dirty sheath.  Once a diagnosis of pinworms has been made Dr. Matt will recommend a deworming protocol for your situation, since resistance in pinworms is becoming a problem.