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EPIC's guide to equine parasites

Ponies and Parasites

Most visitors to the New Forest enjoy the opportunity to see “wild” ponies running free in Britain. However, the inquiring visitor will soon learn that the ponies are not truly wild but that they are all owned by someone. Like any other pony owner these people are concerned about the welfare of their ponies, but along with all the usual problems, they experience some which are specific to the New Forest. For example, it is not always easy to catch an unbroken pony in a largely unfenced area of approximately 150 square miles. Also, many groups of people are interested in the way the Forest is managed. For example, for wildlife conservation, tourism, or forestry. Reconciling the needs of these groups with the needs of the ponies may produce difficulties. Most ordinary pony owners do not expect to have thousands of visitors camping, or playing cricket on their pasture every year.

Are infective larvae washed into water courses? (photo by Tim Mackintosh)

Owners of ponies on the Forest face other problems too. One of the most important threats to the health of all free-living ponies comes from parasitic infection. If an animal becomes ill, or dies, parasites may often be overlooked as a contributing factor, because they are relatively invisible. For example, around 90% of colics are due to worm damage, but these are often put down to a change in diet. Next time you look at a horse in a field try to remember that you are not just looking at one animal, but a whole zoo-full when you consider all the parasites living inside that horse as well.

Controlling worms in domestic horses is not difficult, if the horses are routinely wormed, and their pasture is managed to prevent the build up of infection. But try to imagine the problems faced by owners of ponies grazing freely on large areas such as the Forest. Many of the ponies can only be caught once a year, if at all, so the opportunities for worming are limited. Anyone who has collected droppings from their paddocks will appreciate it would be impossible to do this for an area the size of the New Forest.

Like most horses, New Forest ponies suffer from an alarming number of parasites. They can have ticks, mites and lice feeding of their skin, while inside their bodies there may be redworms, roundworms, pinworms and tapeworms, to name but a few. The parasites associated with most disease and damage in ponies are the nematode worms belonging to the Strongyle family. These are commonly known as the large and small Redworms.

In order to begin to understand how to control these Redworms it is important to understand their life-cycle. Adults of all species live in the gut, attached by their mouths to the gut walls on which they feed. They lay their eggs into the gut contents and these are passed out with the droppings onto the pasture. The eggs hatch to release larvae which feed on soil bacteria, and moult twice to reach the infective larval stage. These larvae are resistant to desiccation and to freezing; they can even be freeze-dried and will still be infective when rehydrated. However, as the larvae do not feed when they have reached the infective stage they must survive on limited food reserves until they find a suitable host to infect.

To help them find a host they can wriggle away from droppings when the grass is damp, as ponies prefer not to graze next to pony droppings. Each time it rains a new wave of larvae are washed out onto the pasture. When the larvae are eaten they moult again, and if they are small Redworm larvae they burrow into the wall of the gut, and can remain there for many months before emerging as adults into the gut to begin the cycle again. The larvae of large Redworms also burrow into the gut walls, but then they go on long migrations which, depending on the species, may take them to the liver, right flank, or along the blood vessels towards the heart. The larvae then return to the gut to reach the adult stage.

To look for ways of reducing the damage caused to New Forest ponies by these parasites I began a three year study, based at Southampton University. The study was jointly funded by the Home of Rest for Horses, and the British Horse Society. It aimed to assess the level of parasitic infection to which the ponies were exposed, to study the effects of parasites on the ponies’ health and their ability to maintain good body condition, and to develop ways of controlling the transmission of parasites under the special conditions of the New Forest. All wild populations of mammals in Britain have an annual cycle of body condition. They lay down fat over the summer and autumn and use up these reserves over the winter and spring. But on the Forest concern had been raised about the number of ponies losing too much condition in the late spring, when many were heavily pregnant or had just foaled.

To determine the rate that infection was being passed onto the pasture I collected dropping samples from known groups of ponies. Samples were collected every month for three years to count the numbers of parasite eggs being passed in the ponies’ droppings. This showed a clear cycle in the rate of egg production through the year. Counts were highest in the late autumn and lowest in late spring. Herbage samples at a number of sites in areas inhabited by the study group ponies were also taken every month. These pasture samples also reflected the annual cycle of egg production. Infective larvae were most abundant on the pasture in September/November and fewest in late April and early May. This meant that if it were possible to worm just once a year, worming in the autumn would be more effective as it would result in a greater reduction in the numbers of eggs and therefore infective larvae being introduced to the grazing.

Doubt has been cast on the benefits of worming ponies, if it is only possible to worm once a year. This is because egg counts are reduced to nil for only 4 to 10 weeks, depending on the wormer used. By comparing worm egg counts from groups of ponies that had, or had not been wormed, I was able to show that even annual worming significantly reduced the numbers of eggs produced for up to a year following worming. The wormed ponies as a group were also in better condition than their unwormed companions at key times in the year. During the autumn when ponies are laying down fat reserves for the winter, wormed ponies were in better condition, and also in late spring when the pony population as a whole was in its lowest average condition during the year. This demonstrated the benefits of even annual worming as ponies were not having to feed a population of parasites through the winter, as well as themselves. Worms did not affect all ponies within the population equally. Particularly at risk of infection were youngsters under the age of two, and mares that were in foal or feeding a foal at foot. Once infected these groups also suffered more severe worm damage.

To control the parasites on the Forest it is not possible to rely completely on wormers, as the majority of the ponies can only be dosed once a year, if at all. The pony Drifts (round-ups) can only be held in the autumn, because of the dangers involved in drifting heavily pregnant or newly foaled mares. At each Drift up to 200 ponies could be caught, and it is simply not possible to worm that many wild ponies in a day, so it is important to target those groups which will benefit most from treatment. Therefore young ponies and lactating mares should be treated if they are to remain on the Forest for the winter. Unfortunately it is not possible to identify newly pregnant mares at the autumn drifts.

As wormers alone will not control worm infection it is necessary to develop ways of controlling the parasites in the environment, before they infect the ponies. This will benefit all of the ponies, whether they can be wormed or not, and reduce overall infection rates. It is well known that the juvenile Redworms cause more damage than the adults, but 2 out of the 3 most commonly used wormers do not affect juveniles worms at normal dosage levels. It would be much better to reduce the chance of ponies being infected by controlling pasture levels of infection, than to wait until the damage is done, and then rely on wormers to remove the adult parasites.

If the Redworm larvae are exposed to conditions which induce high levels of activity, their food reserves are rapidly exhausted and they will die. High temperatures and high levels of light quickly kill larvae. On pony pastures larvae are kept safe inside the piles of droppings in dark, moist conditions, waiting for the next rainfall to be washed out onto the pasture. There are two ways of preventing this; either the droppings must be removed, or they must be broken up to expose the larvae to lethal levels of light and temperature. Luckily ponies do not defecate at random. Anyone who has looked at a field grazed by ponies will have seen that a pattern of toilet areas

and grazing areas develops. This is also true on the grassy lawns of the New Forest. Studies of the feeding behaviour of New Forest ponies have shown that they spend the majority of their grazing time on these lawns, but they make up only 6% of the total area of the New Forest. This behaviour also results in large amounts of droppings accmulating on the lawns. Therefore the majority of the parasite eggs are also there, ready to hatch out in the areas where they are most likely to be eaten by a grazing pony.

This conveniently concentrates the majority of the next generation of parasites in a relatively small area of the Forest, and within these lawns they are inside the pony latrine areas. Control exercised at this level will also benefit the entire pony population, not just those which can be caught and wormed.

It should be possible to find ways of removing or killing enough larvae in the lawn latrine areas to bring about a reduction in the overall rates of pasture infection on the Forest. Methods could include harrowing latrines on sunny dry days, to break up the droppings and expose the larvae to high levels of light and temperature. Although it would not be possible to remove the droppings manually, this could be done using the pasture “vacuums” which are now available and which can be fitted to a Landrover. There is even a fungus that preys on nematode larvae, but any other possible ecological implications of such a biological control method would have to be thoroughly researched, before this particular method could be attempted on the New Forest.

Reducing the high levels of parasitic infection that New Forest ponies were found to carry, should also reduce the numbers of ponies in poor condition, particularly during the late spring. The New Forest will never be a parasite free environment, but by combining the use of wormers and pasture management it should be possible to limit the rate of parasite transmission, and reduce the impact of parasitic infection on the health of the ponies.

by Dr. Debbie Goodwin, who was awarded her Doctorate in 1992 for her studies of the New Forest Pony parasites.

(This article first appeared in Going Native Autumn 1992)

Tim Mackintosh(©)