Equine Worming
Equine Worming
There are many types of intestinal parasites (worms) which can cause disease in your horse and the various methods of their treatment and control can be baffling.
Each horse and management system is different and will need different treatment and control measures – we can help you to draw up a targeted control plan for your horse.
First, let’s have a look at what worms are and how they cause problems…..
The adult forms of these worms live in the horse's large intestine where they lay eggs that are passed out in the dung. The eggs hatch and develop into infective larvae. When these larvae are eaten by the horse during grazing, they migrate to the lining of the intestine. These larvae can hibernate here for long periods and a horse can accumulate large numbers, called "encysted cyathostomes". When in this hibernation phase, they are not affected by most wormers and are not detectable by any dung or blood tests.
At a later stage, these larvae will emerge into the intestine and develop into adults which lay eggs again. When large numbers of these encysted larvae emerge all at the same time (usually in late winter or early spring), they can cause considerable damage to the intestinal wall. This can result in diarrhoea, weight loss, failure to grow and even death.
Small redworms can also be involved in cases of spasmodic and surgical colic.
Large redworms (strongyles)
These are also present as adults in the horse's intestine and lay eggs passed out in dung. When infective larvae are taken in by the grazing horse, they migrate through blood vessels for about six months before returning to the intestine to become adults. During this migration, they can cause serious damage to the intestinal arteries. This damage can result in blood clots blocking the small arteries of the intestinal wall, causing the intestine to die and leading to colic requiring surgery.
Tapeworms
These are present in the horse's intestine at the ileo-caecal junction, attached to the intestinal wall by suckers. The adults lay eggs which are passed out in dung. These eggs are eaten by forage mites where they develop into an infective stage. These forage mites are eaten by the grazing horse, releasing the larvae which go on to develop to adults in the horse's intestine.
Tapeworms are associated with spasmodic colic and with colic requiring surgery. Only two types of wormer are effective against tapeworms and dung tests are not always reliable at demonstrating infection. A blood test is available that indicates the level of tapeworm infection present.
Bots
Bots are not really worms, rather they are fly larvae. Bot flies lay eggs on the horse, especially on the legs, chest and muzzle. The eggs hatch into larvae and are taken in to the horse's mouth during grooming. They are swallowed and attach themselves to the stomach lining. After some time, they pass through the horse and out in the droppings. Here they develop into adult bot flies. Bot larvae can cause irritation to the mouth and the stomach lining.
Large roundworms (ascarids)
The adults of these worms can be very large (up to 50cm long) and are present in the small intestine. They lay eggs which are passed out in the droppings and which can survive for many years in soil. The eggs are taken in by the grazing horse and hatch into larvae in the small intestine. These larvae migrate though the wall of the intestine, via the blood vessels to the lungs. Here, they are coughed up by the horse and swallowed to return to the small intestine to develop into adults.
Large roundworms commonly cause problems in foals and young horses.
Pinworms
Pinworms are present in both the large and small intestine. The adults lay eggs around the horse's anus and can cause irritation and rubbing of the tail head.
Management and Control
Pasture management and de-worming drugs are the mainstay of preventing large worm burdens accumulating in your horse. Let's look at each of these methods of control…..
Pasture management
Reducing pasture contamination will reduce the amount of infective larvae taken in by the horse. There are lots of ways to reduce pasture contamination, such as:
- Removing droppings from pasture - twice weekly removal is very effective at keeping worm burdens low.
- Keep stocking densities low ' ideally no more than one or two horses per acre.
- Rest pasture from grazing for a minimum of three months ' this will reduce the level of infection on the pasture.
- Graze sheep or cattle alongside horses ' by eating the eggs and larvae, they will reduce the level of infection on the pasture.
Worming drugs
Drugs can be used to kill adult worms thereby reducing egg outputs and limiting pasture contamination, however their indiscriminate use can increase the pressure on worms to become resistant. It is important to remember certain principles when worming your horse:
- Dosing intervals are different for different types of drug.
- Dosing intervals are based on high risk periods such as summer grazing and may be different during lower risk periods such as winter or when horses are stabled for much of the time.
- Not all drugs are effective against all types of worms. It is important to use targeted treatments for specific infections.
- It is very important to give the correct dose of wormer for your horse's weight. A weigh tape is a simple and convenient way of checking your horse's weight.
- Testing horses before treatment is a valuable way of safely reducing the number of wormer treatments needed. By counting the number of worm eggs in your horse's dung, we can check whether or not he needs to be wormed. It is important to remember that this test will not detect encysted cyathastome stages and we recommend that all horses are treated with a product effective against these stages at least once during the winter months.
- A blood test to check for tapeworm antibodies is the best way of checking tapeworm infection levels and is recommended every 1-2 years.