Horse Worming

Signs of worm infestation in horses can be extremely variable, ranging from a barely detectable reduction in growth or performance through to a severe disease, colic, and sometimes death. High worm burdens can also cause permanent damage and scarring to the intestinal lining.

Signs of worms:

  • poor growth
  • weight loss
  • tail rubbing
  • scouring
  • coughing in young foals
  • colic
  • death

There are a number of worms that infest horses; the most important are large strongyles, small strongyles or cyathostomes, threadworms and large roundworms.

Large and small strongyles produce eggs which are passed in the manure. These eggs hatch and develop within the manure into immature worms known as ‘infective larvae’. The speed at which these worm eggs develop into infective larvae is dependent upon temperature and moisture. In warm, moist conditions development is rapid, whereas in cool, dry conditions development is slower. Eggs and larvae can survive for considerable periods in cool weather with adequate moisture. Often the infective larvae move away from the manure to nearby pasture, ready to be eaten by a horse. A horse becomes worm-infested by ingesting infective larvae. In favourable climates, where conditions are cool and moist, pasture contamination can increase steadily; however, heat and dryness can reduce larvae numbers.

Large roundworms produce large numbers of eggs which tend to stick around the anal area of the horse, as well as to objects in the horse’s environment. Large roundworm eggs develop into an infective stage, and when ingested, hatch and continue development inside the horse. Horses usually develop an immunity to large roundworms at approximately 6 to 9 months of age, so these worms are generally only a problem in foals. Foals usually ingest infective eggs while suckling from their dam.

The more horses in a given area, and the heavier the worm burden, the greater will be the level of environmental contamination.

Due to the variety of conditions under which horses are used and housed, horse owners should develop a worm control program for their own situation in consultation with their veterinarian.

Worm eggs are too small to be seen without a microscope, so normal faeces and a horse in good condition does not mean the horse is worm free.

Worm control program

An effective worm control program consists of drenching to remove worms from the horse; limiting reinfestation by removing manure, harrowing paddocks, or grazing management; and monitoring worm burdens using Faecal Egg Counts.


Drench are designed to kill the worms inside the horse’s body without harming the horse. There are many different drenches on the market today.


Foals should be drenched from 6 weeks of age every 4 weeks until 6 months of age.

Pregnant mares

Pregnant mares should be treated just before foaling to control roundworm. Make sure the drench is safe to use in pregnant mares.

Other horses

Drenching may be required as often as every 6 to 8 weeks, or as little as four times a year, depending on a number of factors including paddock size, horse numbers, weather, terrain and a number of other factors.


A basic drenching program for adult horses is:

Spring and Summer - Strategy T (or equivalent)

Autumn and Winter - Equimax (or equivalent)

August – Panacur is a drench that targets a stage of worm development called the ‘encysted’ stage when worms are resistant to all other drenches due to having a fibrous capsule and being in the lining of the stomach. Panacur is given once daily for 5 consecutive days.


Correct dosage is essential. Never under-dose. If you don’t have access to large animal scales, consider floating your horse to the local weighbridge. Alternatively a girth weigh tape may be used.

There is a formula to work out a horses weight

[girth cms  x girth cms] x body length (shoulder – bottom cms)

11 877

Limiting reinfestation

The number of worm eggs or infective larvae on the pasture or in the horse’s environment can be physically reduced by several management practices.

Remove manure regularly from stables, yards and paddocks – daily to every 3 days.

Harrow the paddock and spell it during hot, dry conditions for 6 to 8 weeks.

Alternate grazing with another species such as cattle or sheep.

Wash the perineum and udder of pregnant mares prior to foaling.

Clean and disinfect foaling boxes and stables.

Avoid feeding horses on the ground; use feed bins and hay nets or racks.

Foals and young horses should be given priority for low-worm pasture as they are most susceptible to significant disease from worms. If possible, segregate horses by age. Do not put mares and foals on paddocks previously grazed by weanlings or yearlings as these paddocks are likely to be high-risk for worms.

Faecal Egg Counts

This test measures the number of worm eggs being shed in the faeces which provides an indication on the number of worms currently infesting a horse.

Faeces need to be collected fresh with no contamination from the environment. Once collected they should be dropped into the clinic as soon as possible to limit the development of eggs in the faeces. Samples can be dropped at the clinic Monday-Thursday and are sent to an external laboratory for testing.  Results are usually received within 1-2 days.