Red worms are the most important type of worm to consider when planning you worming since these are the most likely to cause disease. These are a problem during the grazing season as the number of larvae on the pasture can increase if proper worm control is not undertaken

 

Large redworm (strongyles) – the larvae migrate through the blood vessels causing blood clots to form. Pieces can break off these clots leading to obstruction of the blood vessels supplying an area of gut. This leads to a very serious type of colic which can only be treated by surgery.

 

Small redworm (cyathostomes) – this is the most common horse parasite. The larvae migrate into the wall of the gut and lie dormant. In this stage many commonly used worming drugs will not kill them. In the spring they emerge into the gut and in large numbers can lead to serious damage to the gut wall causing serious diarrhoea and weight loss. Small redworm are now reistant to many of the older worming products

 

Pasture management especially dropping removal can be effective in controlling redworm along with the use of wormer products, in particular a dose of moxidectin the winter months to kill the over-wintering larvae.

 

 

Round worms or Ascarids are normally only a problem in young animals as natural immunity develops in the older horse. The larvae migrate through the lungs which can lead to pneumonia. The adults are very large 6-12 inches long and in large numbers can cause impaction of the gut.

Some resistance to ivermectin in ascarids has been reported so a specific dose of pyrantel is recommended to control them in foals and young horses.

 

Bots are the larvae of the Gad fly. The gad fly lays sticky yellow eggs on the hair of the horse’s legs. The eggs can be swallowed during grooming and they go on to develop into larvae which attach to the stomach lining, where they can cause superficial ulceration.

The eggs can be cleared off the horse’s legs using a knife blade and a dose of ivermectin or moxidectin in the winter will kill any larvae in the stomach. It is worth remembering though that the adults can cause extreme upset to horses so by controlling bot larvae we are limiting the adult population.

 

Lungworm is only a problem in horses kept with donkeys. This is because donkeys carry large numbers without showing any clinical signs. The larvae migrate to the lungs where they develop to adults, who lay eggs which are coughed and swallowed to pass out in the droppings. The signs of lungworm infection are coughing and weight loss.

 

 

 

Horse tapeworms are small compared to those in other species, only 8cm long, they attach at the junction of the large and small intestine. Tapeworm eggs on the pasture are eaten by the microscopic forage mite horses eat these mites while grazing, they then develop into adults in the gut. Tapeworms were for a long time thought to be harmless, but recent research has showed that they are a risk factor in some types of colic. Tapeworm levels can be assessed using a blood test or treatment can be given once annually with praziquantel.

 

These are uncommon in the UK and are not harmful but can cause itching and tail rubbing. The adults lay eggs around anus which sticky “glue” that can be very irritating to the horse.

 

 

 

There are several strategies for management of worm populations, which of these strategies you employ depends upon your individual situation. It is important to seek independent advice to help plan your worming program, this may be best done by your veterinary surgeon

 

  

 

Wormers must be used at the correct interval for the type of drug used and dosed accurately for the weight of the horse. Always use the correct wormer for the type of worm you are trying to target and at the correct time of year.

 

The following table outlines the wormer types, the dose interval they should be used and the types of worms they are effective against.

 

 

 

 

Worm egg counts give an estimate of the number of redworm eggs per gram of faeces. They cannot tell us about tapeworm or bot levels.  All that is needed is a fresh dropping ball.

We are happy to tolerate low levels (less than 200 eggs per gram) because they will not cause disease. A reduction in the use of wormers helps to prevent resistance developing and avoids the use of unnecessary drugs. Worm egg counts can also help in the detection of resistance to wormers.

In situations with good pasture management through the grazing season, worm egg counts can allow you stop using wormers through the grazing season safe in the knowledge that your horse has a low worm burden. Worm egg counts are less useful in large yards with lots of horses grazing a small area as counts are likely to be high and interval wormer dosing may be more appropriate.

 

Here is an example of a worming program using worm egg counts.