EQUINE INTERNAL PARASITES AND THEIR CONTROL
The information provided in these sources should be used for guidance only. If you suspect your horse to be suffering from any medical condition you should always seek professional help from your Veterinarian.

ALL horses have internal parasites and a  high percentage of deaths amongst horses may be related to internal parasites. It is possible for one horse to pass up to 24 million worm eggs daily. Therefore it is important to ensure you use the correct worm control programme for your circumstances, to maintain your horses health and reduce the risk of colic, diarrhoea, loss of condition or poor performance. Many instances of colic may be related to damage to blood vessels caused by the migrating larvae of Large Redworms although colic may also be brought on by Small Redworms and Tapeworm infestation.  

Unfortunately horses can look perfectly healthy but, left untreated can pass millions of eggs onto the pasture each day. These rapidly develop into infective larvae in warm, moist weather conditions. Signs to watch for include - sudden weight loss, onset of diarrhoea, lethargic or sluggish behaviour, recurrent bouts of colic, loss of appetite, filled / puffy legs, abdomen or sheath, coughing, dull coat and / or poor growth rate (in the case of youngstock).

Inappropriate use of worming products can be ineffective, is costly to the owner, prevents the development of some immunity by the horse and hastens the development of worm resistance to the currently available anthelmintic (worming) products. Worms of particular importance in the adult horse are small redworms (small strongyles/Cyathastomes) and tapeworms.

Internal parasites have adapted themselves to the internal environment of their host animal and have become host-specific. Horse parasites can only exist in horses, this means that cross grazing with either cattle or sheep can actually assist to reduce the worm burden on your pasture. Although Cattle and Sheep are also affected by worms, those which affect horses do not affect cattle or sheep and if ingested can not mature, thereby breaking the life cycle. This can be a useful factor when considering parasite control. 

Major Parasites 

Small Redworms (Small Strongyles / Cyathostomes)
The most common and most destructive of all internal equine parasites in the horse are the Small Redworms. This group consists of around 50 different species though only 10-12 are commonly present. They can be found in horses of all ages, except in very young foals. 

Adults living in the large intestine lay eggs that are passed out in the faeces. The worm eggs hatch and develop through 3 stages. The 3rd stage is the infective stage. If a grazing horse eats the infective stage the larvae will migrate to the intestinal lining where it can remain dormant (inhibited) for long periods of time (usually during the winter months, but can lie dormant for years). Vast numbers of larvae can potentially build up in this area. At a later date the larvae will re-activate and erupt out of the gut wall. If large numbers of larvae all do this at the same time it can cause extensive damage to the gut wall. Larvae emerge into the large intestine, develop into adults which lay eggs that are passed out in the droppings thus completing the cycle.

It is because of this Inhibited (dormant) stage that the use of Panacur Equine Guard is recommended around November time, as this is the only wormer, at this time, to be 92% effective against the removal of the Inhibited larvae in the gut wall. Removal of the Inhibited larvae in Nov will reduce the risks of damage to the gut wall as fewer will be left to further develop.

It is also recommended to use either Panacur Equine Guard or a moxidectin based product in Feb, as these products are both effective against the developing larvae in the gut wall.

Signs of larval cyathostomosis (the clinical name for when the larvae begin emerging from the gut wall in large numbers) may include :- bouts of colic, weight loss, diarrhoea, fluid swellings in the legs, on the belly or round the sheath. 

Remember that worm egg counts will NOT show up that your horse has these inhibited larvae.

Large Redworms (Large Strongyles)
The large redworm differs from small redworms in size and life cycle. Horses become infected by ingesting larvae as they are grazing. They migrate through the blood vessels to the arteries of the intestinal tract where they can cause severe damage and restrict blood supply. After about six months they return to the intestine as egg-laying adult worms. Adult worms vary in size between 1.5 and 5 cm.

This used to be a high risk worm but with the use of effective anthelmintics (wormers) the numbers of horses identified with this problem at post mortem has dramatically reduced.

All chemical groups with the exception of Praziquantel are effective against this parasite.

Large Roundworms (Ascarids)
These worms are usually only problematic for foals and younger horses as older horses develop immunity. They can  range from 12 to 40 cm, reach up to 2cm in diameter when mature and look like round earth worms

A female roundworm can produce as many as 200,000 eggs per day. Roundworm eggs can remain viable in the soil for many years and horses usually ingest as they graze. The eggs hatch inside the intestine and the emerging larvae migrate through the walls of the small intestine and into the veins. Larvae are then transported via the blood to the lungs where they are coughed up, re-swallowed and passed into the small intestine. Once in the small intestine the larvae develop into adult roundworm. The adults, which can grow up to about 50cm long, lay eggs which are passed out in the faeces.

Heavy infections can cause digestive problems and can block the gut, causing fatal ruptures.

All chemical groups with the exception of Praziquantel are effective against this parasite.

Tapeworm
The equine tapeworm is flat, triangular and relatively short compared to tapeworms found in dogs, cats and humans. It has a small, round head which attaches to the lining of the gut by four suckers. The equine tapeworm can grow to about 8 cm long by about 1.5 cm wide. It lives in the mid-part of the gut at a junction between the small and large intestine known as the ileo-caecal junction. Infected horses pass mature segments of the tapeworm on to the pasture, which disintegrate releasing the eggs. These eggs are eaten by tiny oribatid mites which are present in their thousands in every square metre of grass. Once inside the mite, the eggs hatch and develop into an intermediate infective stage. Grazing horses inadvertently eat mites with almost every mouthful of grass. Very few of the mites will be infected but those that are will release the larvae, which continue to develop into adult tapeworms inside the horse. The adults attach in clusters to the lining of the gut at the ileo-caecal junction and continue to release mature segments containing eggs in to the pasture, thereby completing their lifecycle. 

At one time it was thought that the tapeworm was a relatively harmless inhabitant of the horses' intestine. However, recent research has revealed that tapeworm are associated with certain types of colic in the horse.

Control of tapeworm is recommended during the months of March/April and again during September/October. There are now several options for tapeworm control, double dosing with a Pyrantel Embonate based wormer or single dosing with a Praziquantel based wormer.

Pinworms 
 Pinworms are not thought to be harmful but can provoke irritation around the tail resulting in tail rubbing. Pinworms inhabit the large and small colon and have a relatively simple life cycle. The females lay their eggs around the anus of the horse using a sticky substance, which is irritating to the horse. The eggs are dislodged as droppings are passed and fall onto the pasture where they are eaten by horses. 

If you spot a discharge at the anus, burn anything you use to clean it up.

All chemical groups with the exception of Praziquantel are effective against this parasite.

Bots 
Bots are the larvae of the bot fly. Small, sticky, yellow eggs are deposited on the horses coat by the female bot fly. The eggs hatch and the larvae get into the horses mouth when the horse licks its coat. Larvae in the mouth are then swallowed. Once in the gut the bot larvae attach to the stomach lining. They are eventually passed through the horses digestive system and are expelled in the droppings. Once expelled the larvae develop into the adult bot flies and the life cycle begins again. 

Bots can cause mild damage to the mouth and gums and also to the stomach lining. 

Control of Bots is best done in December after the first frost by using an Ivermectin based product or by the removal of the offending eggs from the coat prior to ingestion.

Intestinal Threadworm
Only a major problem in foals, passed via mares milk. Killing this worm in the adult horse will prevent transfer to the foal. They are up to 1cm long, slender hair-like worms. 

Symptoms include:- diarrhoea, anorexia, dullness, loss of weight, reduced growth rate.

Moxidectin and Ivermectin are effective against this parasite.

Lungworm
Primarily found in Donkeys, horses are typically only infected if grazed alongside donkeys. These are up to 8cm long, slender thread-like worms and would be located in the lungs.

Symptoms include:- persistent cough and increased respiratory rate. May cause secondary pneumonia.

Ivermectin is effective against this parasite.

Additional Tips On Reducing Worm Burdens

1. Weigh your horse to ensure accurate dosing
2. Control programmes should be tailored and developed to suite your needs with the assistance of your Vet or a Suitably Qualified Person
3. Keep a record of when you worm and what you use
4. Treat all horses sharing pasture at the same time and with the same chemical
5. Worm new arrivals with unknown worming history with a 5 day Panacur Equine Guard course, followed immediately by a Tapeworm control product.
6. Worm pregnant mares before and after foaling.
7. Take care to rotate your routine worming schedule with a different chemical group each year. Using a different named product does not mean that you are rotating the chemical group. By using the same chemical group all the time you risk the worms in your pasture becoming resistant to that chemical meaning that they will no longer be killed off by any product containing that chemical.

Internal parasite control is a continual battle and there are some obvious Stable/Paddock Management routines that can help: 

1. Regular cleaning of stables and paddocks - removal of dung from pasture on a regular basis will mean that less larvae are left on the grass for the horse to ingest.
2. Feeding hay in mangers and nets - feeding from the floor, although more natural, will mean that even in the stable your horse may be ingesting worm larvae. 
3. Resting pasture - to be part way effective this would have to be for 6 months at a time, however some worm larvae can live for years both on pasture and in the horse so this does not even guarantee a worm free pasture.
4. Don't overcrowd / over graze pasture - ideally no more than one or two horses per acre. 
5. Harrowing pasture - this is only effective during warm weather as this allows the sun to dry up and kill the exposed larvae. Harrowing during wet weather will only succeed in spreading the worms further across the pasture. 
6. If possible the grazing of cattle or sheep in horse pastures decreases exposure as equine parasites do not mature in cattle or sheep and the parasites life cycle is broken. Known as the biological vacuum cleaning effect.

The proper treatment program using the right wormers at the right time is essential for good control. There are many excellent wormers sold under brand names with the three main active chemicals in wormers purchased in the UK are Ivermectin/Moxidectin, Mebendazole/Fenbendazole and Pyrantel Embonate. A good program can be established with the above. It is not always appreciated that these chemicals cause various degrees of short-term toxic shock and it is important to get your horse over these effects as quickly as possible. 

Horse Connections recognise the important need to worm horses correctly with the correct chemical, at the correct time and the correct dose. To help you we now run a comprehensive worming register for our clients. If you would like to join the worming register or learn more about internal parasites, their life cycle and their effects upon the horse please contact our SQP Susan - susan@horseconnections.co.uk