Worms in horses
S. Smith, Biosecurity, DPI&F
Internal parasites are one of the most common cause of disease in the horse. A properly instituted drenching programme will effectively control the problem and improve health, stamina and appearance. Significant savings on feed costs should result.
Common internal parasites and their effects
Common internal parasites fall into two groups, worms and bots.
Worms
This group consists of three main types.
1. Redworms (Strongylus group) are the most common internal parasite of the horse. Strongylus vulgaris is the most significant worm in the group. Larvae invade the blood vessels which supplies the gut. Damage to blood vessels and the gut can cause severe attacks of colic. The adult worm lives in the large intestine and sucks blood.
Horses with redworms will show anaemia (low blood count which can be seen as pale mucous membranes of the lips and inside of the eyes), loss of appetite, a dull rough coat and lack of endurance. Some deaths may occur due to anaemia and occasionally a blood vessel may rupture leading to the rapid death of the horse.
2. Roundworm (Parascaris equorum) live in the small intestine and grow to the thickness of a pencil and 20 to 25 cm long. They cause illness mainly in foals. Mares are the main source of infection for foals. Foals with heavy burdens will become unthrifty, weak and depressed. Occasionally, obstructions and bowel ruptures will occur.
3. Pin worms (Oxyuris equi) live in the horse and crawl out at night through the anus to lay eggs under the tail area. The irritation they cause can lead to tail rubbing.
Bots (Gasterophilus sp.)
Adult bot flies lay the eggs on their hair of the legs or under the jaw of the horse during late summer and autumn. At hatching, larvae find their way to the tissues of the tongue and mouth. They are then swallowed and attach to the stomach lining over winter, upsetting digestion and reducing appetite. Heavy burdens of bots have been known to cause stomach ulcers and ruptures.
In the late spring \ early summer, the larvae detach themselves and pass out in the dung. After further development in the dung pat, the adult fly emerges ready to begin the cycle again.
Treatment
There are a number of ways of drenching horses for internal parasites, and many different drugs are available.
Stomach tubing is the procedure favoured by veterinarians as it ensures that the correct dose is delivered to the stomach, but is requires skill on the part of the operator, quiet horses or good restraining stalls and crushes. It should be noted that incorrect administration can result in the death of the horse.
Paste formulations are probably the easiest and safest forms of drench for horse owners to use.
The paste syringes are graduated accordingly to body weight allowing the correct dose to be pre-set.
Drenching programmes
To achieve maximum benefit from a drenching programme, all horses should be drenched every six to eight weeks. This ensures that all horses have small worm burdens at any one time and that pasture contamination is reduced to a minimum.
Under extensive conditions where horses are held mainly in large paddocks, four drenches a year at the turn of each season are usually sufficient.
Bot control is aimed at killing the larvae while in the stomach. In most districts, one worming in mid winter and a second in late winter or early spring will give good control.
Correct dosage is essential; never underdose as it can promote resistance of the internal parasites to the chemicals used. If cattle scales are unavailable, consider floating your horse to the local weighbridge. By using the girth of the horse and its length ( measured along the animal's side, from point of shoulder to the rear extremity) in a simple formula, an estimate of bodyweight can be calculated.
Body weight (kg) = girth (cm) 2 x length
(cm)
11 000
Summary
A program of regular drenching using the most efficient drenches is the single most important aspect of worm control.
Where horses are stabled or kept in yards, daily manure removal is very important. Wherever horses are hand fed, feeders should be positioned off the ground and designed so that spillage is minimal.
Keep stocking rates as low as possible. Alternatively grazing horses with sheep or cattle may be beneficial, but do not rely on pasture spelling as it is unlikely sufficient time will elapse to significantly reduce the level of contamination. Pasture renovation may have mixed results.
If possible, practise age segregation. Drench all horses in each mob at one time and always give young stock the benefit of the least contaminated pastures.
Further information
Contact your private veterinarian or other supplier for further information on suitable drenching pastes.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries and Fisheries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
Last updated 10 June 2005
