Diagnosteq - How worms cause disease

Worms would not be a problem in horses if they lived in perfect harmony with their hosts. At low levels of infection, parasitic worms are well tolerated and cause no detectable harm. However, a small proportion of horses is very susceptible to worm infection and will develop large burdens. It is these horses that are at risk of suffering from disease due to their parasites. The diseases caused by heavy parasite infection can be divided into different categories:

Horse rolling with Colic

Spasmodic colic

Research work by vets and parasitologists over the past fifteen years has demonstrated that both small redworms and tapeworms are associated with colic. One study in the United States investigated the incidence of colic on five different establishments each using a different worming programme. One conclusion from this study was that the better the worm control programme, the lower the incidence of colic.

Recent research has also highlighted the role of tapeworms in spasmodic colic. A study using data from 100 horses with colic and 200 normal horses living close to the colic cases, demonstrated a significantly increased risk of colic in horses with tapeworm infection. This study used the tapeworm ELISA test and showed that the risk of colic increased with tapeworm burden.

Graph illustrating the risk of spasmodic colic increasing with tapeworm infection intensity
as measured by the tapeworm antibody test

Large Colon infarction
due to large redworm
damage

Colic that requires surgery

The story of the large redworm was first discovered by researchers at Glasgow University. They established that when the larvae (immature stages) of this parasite reach the large intestine of the horse, they migrate through the gut wall. The migratory route is then along the arteries of the intestine to a site where the major intestinal artery branches off from the aorta. Here the larvae congregate in the wall of the artery, thus damaging the normally smooth wall and causing blood clots to form. These clots can, at some time in the future, dislodge and travel down the artery until they become stuck in a small branch. When this happens the gut, which should receive blood from the blocked artery, becomes starved of oxygen. If this situation is prolonged for more than a few minutes the gut wall begins to die, a process called infarction. This is painful for the horse and also causes leakage of potentially fatal bacteria and toxins into the abdominal cavity.

ileal impaction

Tapeworms can be the cause of a very specific obstruction of the gut, impaction of the ileum (the last three feet of small intestine) at the point where it joins the large colon. This type of colic problem has to be corrected surgically. Research using the tapeworm antibody test has demonstrated that horses with high levels of infection are up to twenty times more likely to suffer this type of colic.

Intussusception of the
caecum into itself

A further colic problem related to worms is that of "intussusception." This is a technical term describing a situation where one piece of gut telescopes into an adjacent piece of gut. Both small redworms and tapeworms have been implicated in this condition which most frequently occurs around the caecum. This problem has to be corrected surgically, and only then with considerable difficulty.

Diarrhoea

Diarrhoea

Many horses that suffer from diarrhoea, particularly young horses in the spring time, do so because of small redworm infection. In particular, it is the larval stages of this parasite that cause problems. During the grazing season horses can accumulate tens of thousands of larvae within their gut walls. These larvae go into a state of arrested development or "hibernation." At a later time, often in the spring, the larvae will re-activate or wake up from hibernation and erupt out of the gut wall. If tens of thousands of larvae all do this at the same time it causes massive damage to the gut wall leading to diarrhoea, weight loss and sometimes death. This is a particularly difficult condition for veterinary surgeons because:

It is difficult to diagnose

The drugs available are not very effective at treating it

It is difficult to detect the build up of larvae in the gut wall

A significant proportion of horses with this condition will die in spite of intensive veterinary treatment.
Some horses with heavy small redworm burdens will suffer from weight loss, poor condition and failure to grow without developing diarrhoea. These problems can arise without the larvae erupting from the gut wall. The exact pathological mechanism by which this occurs is poorly understood at present.

Masterclass

Masterclass - Population aspects of parasite infection

Distribution of parasites within a population of horses

Given a field containing 10 horses, each exposed to infective larvae on the pasture, how many would you expect to be heavily infected with worms? Most people assume that they will all be infected. This is an incorrect assumption. Scientific data, and our own experience of testing large numbers of horses for parasites, indicates that parasites are unevenly distributed throughout the host population. This has been modelled mathematically and for most host-parasite relationships the model that fits best is the negative binomial distribution.

Distribution of parasites

There are considerable practical consequences of this distribution:

Within a group of horses, only a small proportion will be heavily infected. If you can recognise which ones, by using diagnostic tests, treatment efforts can be concentrated on these animals.
It is only the heavily infected animals which are likely to suffer from parasite-related disease. Identify these animals, and you can treat the parasites in order to prevent disease.
A simplified way of thinking of this distribution is that 80% of the parasites live in 20% of the horses!

Individual Predisposition

A further fascinating feature of parasite infection is that animals are not equally susceptible. Certain horses will repeatedly become infected with parasites, whereas others will never acquire high burdens even if they are not treated. Scientists are undecided about the reason for this phenomenon. It is consistent with immunity. Some animals develop a good immune response and others do not. It is also consistent with behaviour. Some animals may show behavioural differences that make them ingest higher levels of infective larvae.

There are very practical implications to this phenomenon. If the heavily infected animals, which are predisposed to infection can be identified with diagnostic tests, treatment efforts can be concentrated on these high risk animals. Also, these animals are the ones likely to develop high levels of infection in the future and therefore more likely to suffer from parasite related disease. Information from diagnostic tests has implications for the future management of individual animals.