Worm Control

We can perform faecal worm egg counts here in the practice with rapid results. We currently charge £12.00 per sample.

Tape worm blood samples have to be performed at the University of Liverpool and results are available in 3 to 4 days.

Worm control for horses can be a bewildering area with shelves crowded with wormers from many different companies and conflicting advice. The information on this page will help to clarify the picture as to what your horse is at risk from and what is the best way to protect him or her.

Types of Worm

(1) Large Redworms

Thirty years ago these were the most common parasite of horses. Their lifecycle involved migration through the gut wall where larvae formed in the blood vessels supplying the gut before moving back into the gut to produce more eggs to infect the pasture. The damage to the blood vessels frequently led to weight loss, colic and death. There were 3 different species in this group: Strongylus vulgaris, Strongylus edentatus and Strongylus equinus.

This group of parasites have now become less common with the advent of modern wormers. They can however be a serious threat to horses on crowded pastures with poor parasite control.

(2) Small Redworms (Cyathostomes)

Following the demise of the large redworms this group of parasites has become the most serious threat to horses. The parasite’s larvae are ingested from the pasture in large numbers. They then become encysted in the hindgut where they overwinter. In this location they are relatively insensitive to most worm treatments. They emerge from the gut wall in the spring in waves causing severe damage leading to weight loss, severe diarrhoea and often death.

Encysted cyathostomes in the gut wall

(3) Ascarids (Parascaris equorum)

This group of parasites are mainly a problem to foals but small numbers can also be carried by adults. The adult worms are very large and can be up to 40cm long. Once the larvae of this parasite are ingested they pass through the gut wall, via the liver to the lungs. From here they are coughed up and swallowed where they mature into adult worms producing more eggs to infect the pasture. Heavy infestations can cause coughing as the larvae migrate through the lungs and the adults can cause impaction of the gut.

Adult ascarids in the small intestine

(4) Tapeworms (Anoplocephala perfoliata)

For a long time tapeworms in horses were thought to be of little significance but recent work by the University of Liverpool has shown that they do increase the risk of certain types of colic. Eggs are released onto the pasture from horse faeces. These are then eaten by forage mites present in the soil and on the pasture. These infected mites are then ingested by other horses allowing adults to then mature in the mid part of the horse’s gut. The adults attach to the wall of the caecum causing ulceration and thickening of the wall.

Tapeworms attached to the gut wall

(5) Lungworm (Dictyocaulus arnfieldi)

This parasite mainly affects donkeys and if horses are to be affected they need to be closely associated with them. Larvae are ingested, then pass through the gut wall and migrate to the lungs where they mature as adults. The adults lay eggs which contain preformed larvae. These are then coughed up and swallowed. The larvae then pass out onto the pasture in the droppings. This parasite has been associated with coughing in horses which have contact with donkeys. Foals and yearlings are more susceptible to infection.

Lungworm egg containing a preformed larvae

(6) Pinworm (Oxyuris equi)

This parasite, although quite common, rarely causes the horse major problems. The lifecycle is quite simple with adult worms lay eggs on the skin around the anus. The eggs rapidly develop into larvae which contaminate the pasture and are ingested by horses. The only problem seen with this parasite is that when the adult worm lays her eggs it can cause intense irritation, leading to rubbing and self harm.

Pinworm eggs under a microscope

(7) Bots ( mainly Gasterophilus intestinalis)

The adult flies lay eggs on the forelimbs and shoulders of the horse in late summer and autumn. The larvae are ingested when the horse grooms itself and after several weeks migrate to the stomach where they attach to its lining and remain there for the winter. The next year they are passed in the faeces and after 1 to 2 months of being on the ground, the adult fly emerges. The adult flies can be very troubling to horses when laying there eggs. The larvae are of uncertain significance. Although they cause small ulcers where they attach to the stomach, they have rarely been associated with any significant problems.

Bot larvae attached to the lining of the stomach

Worm Treatment and Control

In addition to the measures below, there are a number of simple steps that you can perform to reduce the contamination of the pasture and spread of parasites.

  • Remove droppings from pasture
  • Do not overstock your fields
  • Alternate grazing with sheep and cattle if possible
  • Treat new arrivals to your premises with both a round worm and tapeworm product

As new research emerges and products are developed, so our advice on the most appropriate worming system for your horse evolves. Bacterial infections in humans that are resistant to antibiotics regularly make the news headlines. However there are also problems with intestinal parasites in horses developing resistance to some of the worming products. Traditionally advice has been given to rotate the use of different types of wormer product. With increasing levels of resistance amongst some parasites there are concerns that the use of some wormers may inadvertently increase the proportion of resistant worms found in the environment.

Thus the current theory is that reducing the usage of wormers is the best policy to slow the speed of development of resistance amongst intestinal parasites. How, though, do we achieve this and still maintain adequate parasite control?

The answer is to worm your horse in response to positive faecal worm egg counts. The number of parasites present in your horse’s gut can be estimated by counting the numbers of parasite eggs present in the faeces. This is not a fool proof test and there are a number of drawbacks.

  • Egg numbers do vary with season
  • Bots cannot be detected in the faeces
  • Tapeworm eggs are difficult to detect with this method and it is more accurate to take a blood sample

If worm eggs are detected, then your horse should be treated with an effective product. The faeces can be checked again 12 to 14 days later to ensure the worms aren’t resistant to the product used. For a possible monitoring program see the Table 1 below.

This approach is excellent for a small number of horses or on a yard which has a fairly constant number of horses. However if it is a large yard or a stables where the horses are frequently moving on and off then a different approach is required. In this situation it is advisable to use a very effective wormer on a routine program, ideally treating all horses at the same time. This will reduce the selection of worms with resistance. The benzimidazole wormers such as Panacur (including Panacur Guard) have high levels of resistance to them (often over 50% of parasites) and are thus best avoided. There are some resistance problems with pyrantel ebonate (e.g. Strongid-P). So when used it is wise to ascertain how effective it has been by checking the faeces 12 to 14 days later. The most effective products with little or no recorded resistance problems in the United Kingdom and Ireland are ivermectin (e.g. Eqvalan) and the long acting drug moxidectin (Equest). An effective program for this method is indicated below in Table 2.

Table 1

  Minimal Wormer Usage
MARCH Worm egg count +/- tapeworm blood sample. Treat if required
MAY Worm egg count. Treat if required
JULY Worm egg count +/- tapeworm blood sample. Treat if required
SEPT/OCT Treat with either Equest/Eqvalan/Eqvalan Duo/Equimax

Table 2

  Complete Wormer Program
MAY Equest + Equitape
SEPT/OCT Equest + Equitape


We can perform faecal worm egg counts here in the practice with rapid results. We currently charge £12.00 per sample.

Tape worm blood samples have to be performed at the University of Liverpool and results are available in 3 to 4 days.

Scott Dunn's Equine Clinic is part of CVS (UK) Limited, a company which owns over 200 veterinary practices in the UK. Company Registration Number 03777473. Registered Office CVS House Vinces Road Diss Norfolk IP22 4AY