Judging by the number of advertisements for horse wormers there are in magazines, one would think that we were in danger of worms taking over the economy. Are worms the premier threat to horses that this weight of advertising would suggest? It seems to me that there are a variety of management practices which are common in the UK which make worm control a lot more diffucult than it might be and contribute to the development of populations of worms which are resistant to wormers.

The worst is the livery yard system, often overstocked, youngstock and adult horses grazing together with poor pasture management and an uncoordinated worming strategy. To make a decent fist of worm control one needs to understand the life cycle of the parasites you are dealing with. All worms are acquired by the horse eating eggs or larvae, usually from the grazing. [The only exception is a small worm Strongyloides which is most trouble in foals where it may cause diarrhoea; Strongyloides larvae may penetrate the skin of the foal or there may be infection in utero.] The worm larvae then moult several times, within the horse, before becoming adult worms in the gut. During these moults some types of worm migrate around the body cavities. They then move back to the gut and develop into adult worms which lay eggs which are passed in the droppings. The eggs start developing into worm larvae, only when they have made one or two moults, on the pasture, are they infective if eaten. Thus the whole cycle starts again.

The important worms which infest horses are 1, the small redworms (cyathostomes), 2, the large redworms 3, tapeworms and 4, roundworms.

The small redworms are the most common, the worm larvae live for a period in the lining of the wall of the large intestine before emerging as adults to live in the lumen. Adult and larvae can be seen with the naked eye in the droppings. While they are living in the gut lining they are called arrested or inhibited larvae. It is possible for horses to carry huge small red worm burdens as inhibited larvae yet show no adult worms and few worm eggs in the droppings. Small red worms can cause colic but most spectacularly a sudden onset of scouring in horses, often in the Spring, due to a mass emergence of the inhibited larvae from the gut lining. Up to half the affected horses are likely to die and the survivors are left as poor specimens which are unthrifty, with poor coats and impaired digestion. Small red worms can also cause affected lengths of the gut to become gangrenous by interfering with the blood supply. Not only do these crafty disgusting little animals have such a lethal potential but most are now immune to some of the wormers.

The large red worms also live in the large intestine as adults however during their development they are a good bit more adventurous than the small red worms. The large worms travel around the peritoneal cavity and commonly settle at the root of one of the major arteries supplying the small intestine where they can also cause lengths of the gut to become gangrenous. Heavy burdens of large red worms can cause the horse to be in poor condition, anaemic and susceptible to colic.

Only the tapeworms need another animal in order to complete their life cycle. Adult tapeworms, which live in the large intestine of the horse, pass out infective segments which contain eggs which have to be eaten by a pasture mite in order to complete the life cycle. The mite is eaten by the horse and the cycle is complete for the development of a new tapeworm. Tapeworms cannot be acquired from dogs or dogs droppings! The most important tapeworm lives around the entry of the small intestine into the caecum. Tapeworms do not migrate around the horse as part of their development but stay in the gut. Large infections seem to be associated with colic. The infected segments can be seen in the droppings as flat whitish squares.

Roundworms larvae migrate around the horse's body before settling in the small intestine to live as large (18" long) adult worms. However the route the roundworm larvae choose is to penetrate the gut wall, float along in the blood to the lungs via the liver. They then burrow through into the lungs where they are coughed up and swallowed to go on to grow into adults in the small intestine. The female roundworms lay huge numbers of eggs which can survive on pasture for years. The roundworms tend to be of clinical significance in foals and youngstock where they can retard the foals growth and in some cases of heavy infection block the small intestine causing a serious colic. The larval migration through the lungs can also cause coughing in foals.

Control
There is no doubt that the best way of controlling horse worms is to keep the pastures clean and so prevent access by the horse to the infective stages. This is not only better for the horse but also the most secure method of prevention of worm related diseases. I used to work at two Thoroughbred studs which had worm related disease in some very expensive foals because the stud groom kept using the same pasture for turnout (not too far to walk) and hoped his wormers were working, which they weren't. You cannot rely on wormers to control worms without proper pasture maintenance. To be effective, all the droppings should be removed from the pasture at least twice per week. Secondly it is important not to graze youngstock either with adults, or on pastures where adults have been grazing. The adults are a fertile souce of worm larvae for the foals. It is a good idea to worm in foal mares towards the end of pregnancy to minimise the exposure of the foal to infective eggs and larvae. Foals themselves don't usually need to be wormed until about 9 months of age. The other big danger is introducing horses of unknow worming history to a pasture, they can cause gross contamination in a couple of days. Once the eggs are larvae are on the ground you cannot get rid of them, particularly roundworms. Your best chance of ridding your pastures of worms is to use sheep to eat them up, take hay from the land or to wait for a cold winter which will reduce the level of contamination. You could of course plough up the field but then you will probably end up with lots of ragwort! The next common mistake is either to worm too frequently or to underdose, both of these will encourage a population of resistant worms in the horse's gut. Always use a weighbridge or tape to check your horse's weight before dosing. Unless you are worming a new arrival at a stable always worm all the horses on the place at the same time.

If we take a couple of instances, the private owner with a few horses on their own paddocks. This is the easiest situation in which to control worms, particularly if the paddocks are big enough to rest one for a season and take hay off it. It is worth taking dung samples from the horse (take a freshly deposited sample; elderly ones may acquire non-horse worms which will give a falsely high count) at least once a year, preferably in the Spring, so your veterinary surgeon can perform a worm egg count and advise on the level of infection. On clean pasture with regular dropping collection you may only have to worm once a year against tapeworms (e.g. Equitape). With a low level of infection you may need to worm once a year against adult red and roundworms in order to reduce the number of infected eggs and larvae produced.

Now take the livery yard with limited grazing and an itinerant horse population. A nightmare for worm control. Your main aim here it to minimise pasture contamination by heavily infected horses, by performing worm egg counts and worming if necessary before new arrivals are turned out. Dropping picking in these places is extremely important. It is as well to perform a second worm egg count on all the horses after worming in order to see whether it was effective. If not you will have to change to a different type of wormer. There are only four types of wormer available and there is worm resistance to one and probably two of the four. I would not put my faith in alternative wormers (repellers), I am unaware of any evidence that they are effective.