How to treat internal parasites in horses


Internal parasites in horses, if not kept at bay, can be silent killers. They can cause extensive internal damage and owners may not even realise that their animals are heavily infected.

At the very least, parasites can lower resistance, rob the horse of valuable nutrients and cause gastrointestinal irritation and unthriftiness. At their worst, they can lead to colic, intestinal ruptures and death.

In terms of management priorities, establishing an effective parasite control program is a critical part of the minimum standard preventative health care program, along with providing a balanced diet, regular dental assessments and treatments, vaccination programs and regular farriery or hoof care.

Identifying the enemy

There are more than 150 internal parasites that afflict horses, including several major species. Among the most common and troublesome are:

  • large strongyles (bloodworms)
  • small strongyles
  • ascarids (roundworms)
  • pinworms
  • bots
  • tapeworms
  • threadworms
  • lungworms

Historically, prior to the advent of the macrocyclic lactone (ML) products like ivermectin, moxidectin and doramectin, the large strongyles would have been considered the most dangerous on that list. However, the extensive use of the macrocyclic lactone wormers, with the tapeworm treatment praziquantel added, has markedly reduced the importance of large strongyles, bots and tapeworms in regularly treated horses.

Today, the more important issue affecting parasite control in Australia is the emergence of populations of macrocyclic lactone-resistant small strongyles and ascarids.

Parasite damage

Horses have evolved with parasites for millions of years, and therefore having some worms in the intestines is quite normal, and not necessarily dangerous. It is actually unrealistic to try to keep your horse totally free of worms.

Horses in the wild avoid accumulating large worm burdens by rarely grazing the same place repeatedly, instead ranging for 20-40km per day. Unfortunately, our horses are confined to fenced areas of land, and accumulating large worm burdens is a common occurrence.

Different parasites harm the horse in different ways. They can damage tissues and vital organs, including the major blood vessels to the intestines, lungs, liver, stomach and intestines as they migrate through the horse’s system to complete their life cycles. They can cause obstructions and ulcerations within the horse’s digestive tract, and they can also cause intense irritation as they lay eggs, such as seen with pinworms.

Signs of parasitism

Historically, the important issue was the migrating larvae of large strongyles which could damage the internal blood vessels and consequently damage the gut wall. As the use of the ML wormer has become widespread, these are less common. Nevertheless, when horses are crowded, have poor nutrition, or young horses live on intensely grazed areas, certain worms, like small strongyles and ascarids can cause lots of damage.

Signs of infestation might include:

  • dull, rough haircoat
  • lethargy or decreased stamina
  • weight loss, with or without appetite loss
  • coughing and/or nasal discharge
  • tail rubbing and hair loss
  • resistance to the bit due to mouth lesions
  • colic, diarrhoea/scouring
  • summer sores

Faecal examinations

One of the most under-utilised tools in an effective parasite control program is the faecal examination, which merely involves taking two to three fresh faecal balls to your veterinarian for laboratory analysis.

Within a herd of horses, a minority of animals harbour the majority of worms, usually based around the 20/80 rule, where 20% of the herd harbour 80% of the worms.

The objective of Faecal Egg Counts (FEC) is to identify the high egg shedding horses so that these horses can be targeted for treatment. It may take more than one Faecal Egg Count to determine that certain horses are consistent high or low shedders, but once identified, high shedders often remain that way all the time, as do low shedders. The low shedding horses tend to require much less treatment, often no more than once or twice per year. Conversely, the high shedders often need treatment every 8-10 weeks, depending on the product used. It is recommended that high shedding horses are treated with the most effective and long lasting treatment available. The low shedding horses can usually be treated with other broad spectrum treatments (like Ivermectin/praziquantal combinations) once the program has been established.

Within a herd of horses, if the high shedding horses are identified and targeted, the risk of major parasitism in low shedding horses, which are therefore receiving less treatment, is greatly reduced, as the number of worm eggs being shed on the pasture is being reduced.

It must be noted that this rule applies to adult horses, ie over 4 years old. In younger horses, egg shedding can fluctuate as the horse’s immunity develops, and therefore trying to identify high egg shedding horses in any horse under 4 years old is not reliable.

The faecal examination is also a cost effective follow up to dewormer administration, to determine whether it has worked. It is good practice to do a faecal egg count within two weeks after the use of an anthelminthic.

Parasite control programs

It is essential to follow some basic rules when planning and implementing a worm control program:

  1. Always use an effective worming product - there is substantial resistance to the benzimidazole class of drenches, so the ML wormers are still our most important type, however, beware the emergence of ML resistant small strongyles (cyathostomes). Traditional interval dose programs and rotating wormer types has been found to aid in the development of resistance, and should be avoided.

  2. Ensure the horse gets the full dose calculated for its body weight. Know your horse’s weight. Most horses are heavier than the owners think. The average thoroughbred weighs 500 kilograms. When in doubt, use a weighband, and always overestimate – an underdose of worm treatment is worse than no dose at all.

  3. Practice good pasture management. This is critical to a good worm control program. Firstly, avoid overstocking – the ideal stocking rate will vary according to location. Rotational grazing will also help reduce the concentration of worm eggs on any one paddock. Other strategies include: where possible removing faeces twice weekly; spelling paddocks for 3-4 months; alternately grazing with sheep and cattle; harrowing paddocks over summer. These practices greatly reduce the amount of worm eggs on the pasture available to reinfect the horse.

  4. Faecal examination for identification of high and low shedding horses, and to determine how effective your worm treatment has been.

Veterinary advice

Your EVA member vet can give you specific advice for the type of worming program you will need.

This content was originally published by Equine Veterinarians Australia (EVA), a special interest group of The Australian Veterinary Association.