HORSES suffer from a variety of skin conditions which can be seasonal in nature. We are all familiar with "sweet itch" in summer as just such an example.

There are however, a group of skin diseases which tend to occur mostly in the winter months, some of which can be relatively easily prevented.

Louse infestation

This insect is prevalent throughout the year but is most commonly obvious during the winter. This is because lice becomes sterile above a certain temperature (38C) and are killed altogether above 50C. Although these temperatures are unlikely to be achieved as ambient air temperatures, they are quite commonly achieved in the deep skin folds of horses where the lice tend to congregate and this limits their reproductive capacity. Once the air temperature drops in winter, the lice breed prodigiously, each generation producing many hundreds of eggs which go onto produce adults, which do the same , leading to a rapid increase in population. The disease is highly contagious and usually affects a group of horses synchronously, often after the introduction of a new horse. Although many horses show intense itchiness with louse infestation which gives an indication of the problem, there also exist sub clinical carriers which can be crawling with lice and yet show no itchiness at all. It is these animals which are probably the culprits for the continued spread of lice.

Louse infestation on its own is not usually injurious to health but stressed or malnourished horses, or horses suffering from other diseases can be particular susceptible to being "pulled down" by it.

Most of the clinical signs are the result of self inflicted trauma resulting from the horse being 'on fire'. All of the areas where horses can easily reach to bite at or rub on something begin to lose their hair and the coat assumes a moth-eaten appearance (Fig 3). Hairs often break after repeated rubbing giving a stubbly feel to the residual hair stumps.

If you are looking for lice infestation on a horse a good tip is to look under the rug immediately you remove it. Very often the lice will become too warm under a New Zealand rug and migrate to the outer layer of the hair coat. As soon as the rug is removed and the cold air contacts the lice they will move deeper into the coat and this can be a good time to see them as they move. It is usually easier to find the eggs than the adults. These are the shape of small, grey skittle-shaped structures which are attached to the hair shaft as if by the neck of the skittle. Usually a magnifying glass is necessary to see them. The best places to look are along the base of the mane and under the forelock, where lice seem particularly keen to lay eggs.

The clinical signs of lice infestation are not specific to lice and can follow any cause of pruritus (itchiness) in the horse. However, most of the other insect related pruritic skin conditions occur mainly in summer, so a horse showing a sudden onset of itchiness and patchy hair loss in the middle of the winter should trigger an investigation for lice. The diagnosis can only be confirmed by finding lice in hair brushings taken from the coat into something like a cat litter tray, or by finding the intact eggs. You may have to ask your veterinary surgeon to collect these coat brushings for examination under a microscope as the lice can be difficult to see to the naked eye.

Treatment of lice is relatively straightforward but unfortunately the medicine regulations currently prevalent in Europe prevent the use of many topical application products because of the horse's classification as a food animal. These have gradually disappeared from the market place. There are commercially available louse powders of variable efficacy, but you should seek your veterinary surgeon's guidance on the best currently available as these products tend to come and go fairly regularly because of EU changes in legislation. One product which seems to be available fairly predictably is Coopers Fly Repellent Plus (Schering-Plough Animal Health) which is a synthetic pyrethroid applied topically. Although primarily intended as a fly repellent, it is also recommended for, and does seem effective, at killing lice. One of the basic principles of treatment must always be a repeat of the treatment after 10 days. This is because only the adult lice are killed by the insecticide and if no further action is taken. The crop of eggs which those adults laid will hatch out and re-colonise the horse. A 10-day treatment period allows time for the eggs to hatch but also allows the newly emerged adult to be killed before they have time to lay more eggs, therefore breaking the vicious cycle of re-infestation. Three treatments 10 days apart should cover the incubation period for all eggs and be completely curative. If the horses affected are stabled then the rugs should be steam cleaned as the heat will rapidly kill any lice which are in the rug. Another basic principle of treatment is that all horses in the group should be treated, whether they are showing signs of itchiness or not, to eliminate the possibility of a continuous sub clinical carrier re-infecting the treated horses.

Ringworm

Ringworm is one of those great diseases where the name is its most misleading aspect, as this condition often does not occur in a ring and is certainly not caused by a worm!

Although ringworm occurs all year round, there does tend to be an increased incidence of the disease during the winter months as horses spend more time in stables, are not exposed to sunlight which can inhibit fungal infections, and are often moved through sales and competitions at that time. The causal organism is a fungus, similar to the types of fungus and moulds which grow on bread left in a polythene bag.

In common with most other fungi, the ringworm infection is spread by spores which are microscopic and produced in their millions by the infected lesion.

These spores are highly resistant and are able to be carried on inanimate objects such as fence posts and gates for many years, maintaining their infectivity. Again, as with most fungi, ringworm only affects the dead part of the skin (the hair and keratin layers) and will not invade healthy, living dermis.

All ages of horses are affected. The incubation period for the disease is approximately 2-3 weeks, which means that by the time the first signs of infection are clearly visible (hairs breaking and falling out leaving bald areas) the infection is already well established and has already been producing spores which will almost certainly have been carried to other horses.

Published: 03/09/2004