Treatments for the Main Equine Internal Parasites

Equine Parasites - A look at the main equine internal parasites, their symptoms and treatment

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Types of Internal Equine Parasites
There are four main types of internal parasites that pass through the horse’s body during their life cycle. Some may cause extensive damage and in the paragraphs below, I have listed them, the symptoms that the horse may show, and the recommended treatment for each.

The most harmful of these are the large and small strongyles (Redworms and Bloodworms). They are approximately 1/2 inch long (1cm) and are reddish in color. The larval forms of these parasites may damage the blood vessels and other organs. Young horses are particularly susceptible to large infestations. Equine worming programs are mostly aimed at the control of large and small strongyles.


  • Loss of condition.
  • Anaemia.
  • Dry, staring coat
  • Diarrhoea
  • Worms may be visible in droppings

A sample of faeces may be sent to your veterinarian to perform a faecal egg count on a regular basis to monitor the levels of eggs. An untreated horse will become extremely debilitated.


  • Oxibendazole, Dichlorvos, Pyrantel Pamoate and Ivermection are effective against large and small strongyles.
  • Phenothiazone and Thiabendazole are not as effective against small strongyles as they were, due to resistance build-up.
  • Thebenzimidazole anthelmintics are effective against large strongyles.

Ascarids, or Round Worms, are stiff, white and up to a foot long (30cm) in the adult stage.


  • In large numbers, they may cause a loss of condition, irregularity of the bowels or colic.
  • In small numbers, they rarely cause any symptoms.
  • Young foals from 12 weeks to one year are most vulnerable.


  • Piperazines are effectie against ascarids. These are best administered by a veterinarian by means of a stomach tube (known as “tube-worming”)
  • Foals should be de-wormed at eight weeks. It is not necessary to tube worm foals, Telmin paste works well.
  • Ivemectin paste is also effective against ascarids but should not be given to young foals.
  • All horses over two months olf should be de-wormed every 4 to 8 weeks.

Pinworms are about 1 3/4 inches long (4.5 cm) and are long and thin and occur in the rectum.


  • Pinworms cause irritation – the horse will rub his tail.
  • A discharge will be present and pinworms may be visible around the anus.


  • Piperazine, Fenbendazole, Mebendazole (Telmin) and Ivermectin are all effective against pinworms.
  • Use paper towels or disposable “wipes” for cleansing the dock area, rather than sponges, to prevent the spread of eggs and infection.

Bots (gasterophilus) are the larvae of botflies. The yellow eggs are laid on the hairs on the front part of the horse, within reach of his tongue. The eggs hatch in 10 to 14 days and the horse will lick them off his coat and transfer them to his mouth, where the larvae burrow into the mucous membranes of the lips and gums. There they remain for a short growing period and then they pass on to the intestine where they attach to the stomach wall. They remain there for almost a year before passing out with the faeces to pupate in the soil. Adult botflies then emerge from the pupal cases in 3 to 9 weeks, depending on the temperature.


  • Large quantities in the gut will cause a loss of condition along with a dy, staring coat.
  • Temperature and pulse rate may rise.
  • The horse may shows signs of colic – restlessness and kicking at belly.
  • Intermittent diarrhoea or constipation may be apparent.


  • Keep horses off pasture when botflies are laying their eggs. Turn horses out after dark and bring in early in the morning.
  • Pick or clip the eggs of the coat as soon as they are seen.
  • Frequent grooming removes some eggs.
  • Provide pasture kept horses with a darkened shelter during the botfly season.


  • Dichlorvos, trichlorfon and carbon disulphide, administered by stomach tube, are effective in controlling bots. This should be done in early spring when the larvae are in the stomach and before they pass out of the horse in the faeces.
  • Ivermectin paste and mebendazole-trichlorfon (Telmin B) paste are also effective against both the stomach bots and the earlier migrating stages in the mouth tissues.

Rotating wormers
It is a well-known fact that large and small strongyles develop resistance to wormers and therefore it is generally recommended that wormers be rotated by chemical class, not just brand name, in an effort to lessen the occurrance of resistance. With the help of your veterinarian, who will perform a faecal egg count to determine the numbers and types of parasites you are dealing with, you can formulate an effective program of parasite control.

Pasture Management
When I lived in England and looked after horses there, one of our regular tasks was to go around the pasture with a wheelbarrow and remove all the droppings. We did this every other day, as the climate in England rarely produced the hot, dry weather needed to kill the eggs, which would stay viable within the moist droppings until the flies were ready to hatch and start their life cycle again.

  • Other pasture management techniques are to graze other animals on the pasture. Only horses are affected by redworms. Other animals, such as cattle, will ingest the eggs and interrupt the life cycle, thereby reducing contamination.
  • Ploughing and cropping for a year or two with a straw crop

This story was published in December 2002.

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