Intestinal and other worms can become a problem for horses under modern management systems. Many worms pass their eggs in the faeces of horses, and the eggs or larvae of the worms are then ingested as the horses graze. Other parasites such as bot flys lay their eggs on the horses legs and body where they are ingested by the horses as they groom.

Preventing worm infestation

By looking at the way worms infect horses it makes sense that to decrease the chance of our horses getting worms we should follow the following steps:

  1. Reduce contact with infective larvae by collecting faeces from pasture at least weekly if possible.
  2. If it is not possible to collect faeces pasture management such as cross grazing pasture with cattle or sheep (most worms are species specific i.e. horses will not get cattle worms and cattle will not get horse worms), grazing young horses on ‘cleanest’ pasture, use of faecal spreaders and taking advantage of climatic changes such as cold winters or hot dry summers to kill worm eggs by cold or drying.
  3. Place feed and hay in bins, not on the ground.
  4. Reduce fly or insect born worms by good fly managment- keeping areas where there are horses kept clean, use of short or long acting fly repellants, use of rugs.
  5. Reduce bot infestations by regular removal of bot eggs from the horses (using a bot knife or similar).
  6. Do not keep horses with donkeys unless the donkeys are known to be free of lungworm.
  7. Note this is at the bottom of the list- targeted use of anthelmintics (wormers).

Assessing worm infestation

The eggs of many of the worms are spread in faeces, so one of the most valuable tests available to us for determining worm infestation in horses is the use of a faecal egg count. Faecal egg counts invole diluting a sample of faeces in a solution and allowing the worm eggs to float to the top of the sample. Part of the sample is then transferred to a microscope slide and examined for worm eggs. The eggs of some of the worms appear very similar, so it is also possible to allow the eggs to hatch and then examine the larvae to determine the worm type (for this a sample needs to be sent to an external pathology lab).

If you would like a faecal egg count for any of your horses please call us and we can give you directions on collecting the sample from your horse(s). If you bring the sample to the clinic we can run the test for you.

If no worm eggs are found your horse may not need worming.

The faecal egg count is also a simple way of assessing whether your worming program is effective. If a faecal egg count is performed prior to worming, then 2 weeks after worming there should be a significant decrease in the number of worm eggs found after drenching compared to before.

What worm product should I use?

There are a lot of worm products available on the market, however most wormers can be broadly placed into groups:

– The ‘BZ’s’: the benzimadazole group includes oxfendazole, oxbendazole, fenbendazole. There are multiple strains of resistant worms in Australia to the ‘BZ’s’, especially small cyathastomes.

– The ‘BZ’s’s are often combined with Morantel or Pyrantel to reduce the level of resistant worms to treatment, and to treat tapeworm infection.

– The ‘Mectins’ (macrocyclic lactones): the ‘mectins’ were released approximately 20 years ago and had a significant effect on controlling worms in horses and other species. Unfortunately there is evdience of resistant strains in some worms in Australia. The ‘mectins include: ivermectin, abamectin and moxidectin.

– Praziquantel: effective against tapeworm

Wormers are often rotated to try and reduce the development of resistance. It is important to realize that there is no one worm program suitable for all horses. Please feel welcome to contact us for advice on worm control management for your horses.

What are the different types of worms that can affect horses?

Often referred to as redworms or strongyles. There are several species of strongyle that may affect horses. The may be broadly divided in large and small strongyles. The adults live in the large intestine, they lay eggs in the faeces which hatch and develop to the infective larval stage on the pasture and horses ingest them as they graze. Adult worms may be seen in the faeces of affected horses. Diagnosis is made by use of a faecal egg count.

The larvae of the large strongyles migrate through the liver and blood vessels in the abdomen, and if large numbers buildup they may cause liver damage or block the blood vessels leading to the abdominal organs leading to colic symptoms. Fortunately modern worm control programs have substantially reduced the incidence of large strongyle infestation in Australia.

The larvae of the cyathastomes, otherwise known as small strongyles migrate through the intestinal walls and may become encysted in the wall. Very large numbers of encysted strongyles may cause weight loss due to reduced uptake of nutrients by the intestine. Of more concern is the possibility that the larvae erupt from their cysts en mass causing damage to the intestinal wall and diarrhoea. Unfortunately small strongyles are still a problem in horses in Australia.

Often referred to as Bots. The adult is a fly which lay small yellow coloured eggs on the legs and body of horse. The eggs are ingested when the horse grooms itself. They hatch and migrate through the oral mucous membranes until they reach third stage larval stage, which attach to the stomach lining and mature until they release from the stomach to be passed out in the faeces and become adult flies. Bots are not thought to be highly pathologic for horses however there may be some association between stomach ulceration and bot larvae.

Often referred to as ascarids. These are large white worms up to 30 cm long. They mainly affect young horses under 2 years of age. Adult worms live in the small intestine and the female releases eggs into the intestinal contents to be passed in faeces. The mature egg is ingested from pasture by horses and hatches, and the larvae migrate through the lungs eventually passing into the small intestine. The worms may cause illthrift, however the main concern with these worms is the possibility of large numbers of adult worms causing intestinal blockages requiring surgery to correct. They may also cause coughing and lung irritation as they migrate through to lungs. Diagnosis is made by use of a faecal egg count. Adult worms may sometimes be seen in faeces, especially after worming.

Often referred to as Pinworms. The adults live in the terminal colon and  the female lays eggs around the rectum. The egg laying may cause irritation around the rectum and affected horses may rub and itch at their tail. The eggs may stick to objects as the horse rubs at their tail. Larvae are ingested from the pasture. The larvae pass through the intestine as they mature to adult worms. The main pathology associated with pinworms is scratching and hair loss in the tail region. Diagnosis is made by applying sticky tape to the perianal region and looking for eggs stuck to the tape under the microscope.

Often referred to as threadworm.  These worms mainly affect foals and may be associated with diarrhoea in the first few weeks after a foal is born. They live in the small intestine. Adult horses are rarely infected with strongyloides, however mares may have a larval form which is activated around the time of foaling and is spread through the mammary gland to foals as they nurse. Diagnosis is made by faecal egg count.

Often referred to as tapeworm. The adult worms live in the intestine of the horse and are usually found in the region of the ileocaecal junction. The worms may cause thickening of the ileocaecal region resulting in colic even after the tapeworm infestation has been treated. They may also be associated with intestinal perforation. Eggs are passed in faeces and are ingested by a species of mites found in the pasture. These mites are ingested by horses as they graze and develop to adult tapeworm. Diagnosis may be made by faecal egg count or faecal sedimentation, however as eggs are released sporadically they may not always be found. There is a blood test available for diagnosis but I have not found the test commercially available in Australia.

The microfilaria (juvenile forms) live in the skin and the adults in the ligaments in the neck that support the head (nuchal ligaments). Onchocerca microfilaria may also be found in the tissues around the eyes of some horses. Most horses are infested with Onchocerca however only some horses show clinical signs, which are thought to be a hypersensitivity (allergic) reaction to the juvenile worms. Worming horses with a worm product effective against Onchocerca may temporarily increase the symptoms of affected horses as the microfilaria die. Symptoms include pruritis (itching) and a variety of skin and eye lesions. It is usually non seasonal. A skin biopsy is needed for diagnosis, although even if Onchocerca are found on biopsy they may not be causing the clinical signs as normal horses may have Onchocerca microfilaria in their skin. Worm products will kill the microfilaria but do not affect the adult worms so periodic flareups may be anticipated.

Habronema are nematodes. The adult worms live in the intestine and larvae are passed in the faeces, fly maggots ingest the larvae which mature as the maggots become flys. The flys deposit larvae on wounds or wet areas. If deposited around the mouth the larvae are swallowed and develop to adults in the intestine. If laid on a wound or on some tissues of the eye the larvae may cause skin lesions that usually appear as no healing ulcerated wounds. They may also be itchy. Diagnosis is usually by cytology and biopsy of skin lesions.

Also referred to as lungworm. Lungworm are more commonly found in donkeys than horses. Donkeys do not usually get symptoms with lungworm infestation but they can pass the worms to in contact horses who may show symptoms such as a chronic cough. The adult worms live in the lungs where they lay eggs which are transported up the trachea (windpipe) and are swallowed by the horse. The eggs are passed in faeces, mature into larvae and are eaten by the horses as they graze. Diagnosis often relies on history of exposure to donkeys, and response to worming. Larvae may be found in samples of fluid collected from the lungs of affected horses and donkeys.

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