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    Worms / Internal Parasites in horses

    This article looks at the most common worms that affect horses, how they reproduce, what affects they have on a horse, what signs to look for, and what natural remedy in the Natural Pet Eponacare range has to effectively help your horse with worm burden.

    Worms can cause many problems with horses, some can be mild while others can be severe and even life threatening.  Below are detailed some of the most common worms:

    Small Strongyles Cyathostomin spp

    The most common intestinal worm found in horses. Adults of this worm produce eggs in the intestine where they are passed out in the faeces, 1 to 2 days later they hatch into larvae, they will stay here and grow in the faeces till they are at stage 3 where they can infect the next horse that ingests it. Once ingested they travel to the lower intestine where they burrow into and become encysted in the intestinal wall.  This process involves them developing a fibrous capsule around themselves so they can developed from stage 3 to stage 4, which can take several months.   After this stage they emerge as adults and travel back in to the intestinal space where they begin to lay eggs, finishing the cycle.

    If the horse is over burdened with encysted larvae it can reduce absorption of nutrients.  The larvae can also go into a dormant stage called hypobiosis, which can last for months to years, waiting for a trigger usually, environmental to wake up. 

    Small strongyles can cause severe damage to the wall of the intestines.

    Clinical signs can be: diarrhoea, weight loss, shock and even death.

    Large Strongyles Strongylus edentatus, Strongylus vulgaris, Triodontophorus spp

    Also known as the blood worm, palisade worms, sclerostomes or red worms are a blood sucking parasite that lives in the cecum of horse’s digestive systems.  

    The eggs from an infected horse are passed out in the faeces, where 1 to 2 days later they hatch into larvae, it will stay here and grow in the faeces till it is at a stage 3 where it can infect the next horse that ingests it.  If they are ingested before this stage they will simply be passed through the horse causing no damage.  Once the larvae are ingested they end up in the intestine mainly the cecum, ileum and colon, where they burrow into the blood vessels, in particular the tiny arteries that supply the intestine with blood, here they cause inflammation and blockage of blood supply which results in the symptoms of colic.  Once in the vessels they molt and develop into stage 4. 

    They work their way along the arteries until they get to a major junction branching from the aorta.  When they arrive at these junctions they are still only 1-2mm long, but here they grow to about 15mm long where they fill up the artery and cause blood clotting or thrombus, where blood can’t get through.  If the blockage is to a large blood vessel to the intestine, the horse may develop peritonitis, which can be fatal. 

    After several months the larvae return to the intestine where they can further develop in to adults and start the cycle again. 

    Clinical signs can be fever, loss of appetite, emaciation, depression, abdominal pain and discomfort, colic and occasionally the animal can have diarrhoea.

    Tapeworm Anoplocephala perfoliata

    A cestode flat worm that consists of a head and segments.  It attaches onto the intestine of it host and absorbs nutrient through the skin.  They can grow from between 70cm to 10 metres long depending on species. 

    Each tapeworm is made up of many segments, the head has 6 rows of teeth with suckers and muscular grooves which is how it attaches to the intestine of the horse.  They do not contain a digestive system, but simply absorb nutrients from their host through their skin.  Each new segment grows from the head end, with the more mature ones being at the end of the tapeworm.   Each segment contains its own female and male reproductive organs, so can produce large amount of eggs.  Segments break off from the tapeworm that contain eggs, these eggs are passed through the faeces, where they are eaten by forage mites that live on the pasture, the horse is then reinfect and the cycle starts over again.

    Clinical signs include: failure to grow or do well, irritability, increased appetite, dull coat, stomach upset / colic and diarrhoea.

    Roundworm Parascaris equorum

    More common in young horses, 2 years and under, before their natural immunity has matured. The adult round worm live in the small intestine of the horse, where they produce their eggs, a single female can lay anything from 100,000 to 200,000 eggs a day. These eggs are passed out through the horses faeces where they can survive for up to 10 years in the environment.

    The eggs are eaten by a horse, once in the intestine they hatch into larvae which penetrates the intestinal wall.  They travel through the hepatic vein to the horse’s liver, where they spend the next 7-10 days feasting. They then continue their journey to the lungs, where they spend another 14-21 days feasting there. It is here where the most damage is done, the liver can regenerate but the lungs can become permanently damaged, as when they heal they produce scar tissue.

    Once they have finished here they penetrate the lung wall into the air sack.  The body’s immune response to this attack is to destroy the alveoli, which can lead to young horses developing pneumonia. Once in the alveoli they travel to the bronchioles, then on to the bronchi, and then the trachea, here they cause irritation and make the horse cough, so they are thrown into the back of the throat, where they are re-swallowed back into the stomach as mature adults to start the cycle again.

    Clinical signs are coughing and nasal discharge, weight loss, a rounded belly and also colic.

    Pinworm Oxyuris equi

    Is a less common problem in horses.  The adult pinworm lives in the lower intestine of the horse, once it is relaxing the female pinworm emerges out of the rectum and lays her eggs with a sticky yellow substance around the anal area of the horse. 

    The pinworm don’t cause any harm to organs or the health of the horse, the main result is irritation to the rectum area, and because they live their entire cycle in the lower intestine they have very little clinical signs – itching and rubbing of tail area.

    Bots fly Gasterophilus intestinalis and Gasterophilus nasalis

    These are a fly rather than a worm, but their life cycle causes them to be treated under the worming arena.  An female adult bot fly has no mouth, so cannot feed, she lives on internal reserves, so only long enough to lay her eggs on to horses, mainly around the mouth, eyes and legs.  The moisture from the skin and also saliva allows the eggs to hatch into larvae.  With the horse licking these areas, they ingest the eggs and larvae, which they travel to the stomach where they attach themselves to the mucus lining.  After about 10 months they detach from the lining and exit the horse via faeces, where the larvae burrow in to the ground to develop into a mature bot fly over a 3-10 weeks period to start the life cycle again.

    Clinical signs can be minimal but can include an inflamed mouth and stomach irritation.  You can also see sign of bot fly egg on the horse’s hair.


    With many different commercial drugs out there for eliminating worms from your horse, there is becoming the problem of resistance.  This is where a horse is treated with a commercial wormer, and there ends up being a small group of worms that are treated but not killed, they go on to reproduce and slowly develop a resistance to the worming treatment, so unfortunately the commercial worming treatment have to become stronger and more dangerous for your horse.

    What can you do to help?

    Do not over stock paddocks.  Cleaning up poo 1-2 times a week will help interfere with the life cycle also spread your rotation out between paddocks if you can. Regularly grooming your horse, looking for signs of worm infestations.

    Natural Pets equine range Eponacare has a homeopathic worming remedy called Horswrm, which has been designed specifically for horses internal parasite complaints.  It has the common worm repelling remedies plus also worm species specific nosodes.  It can be individually feed or trough treated.

    All worming should be done around the full moon, as this is when the worms are at their most active and you will get a better expulsion rate.


    Worms / Internal Parasites in horses

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