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A practical approach to the itchy horse

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01 October 2009, at 12:00am

MARION McCULLAGH reports on the opening dermatological session at the BEVA congress

THE BEVA congress, held in Birmingham last month, was another great success with a substantial number of papers highly relevant to those of us involved with horses on a regular basis. 

Rob Pilsworth chaired the opening session on dermatology, which started with “A practical approach to the itchy horse” given by Rod Rosychuk of Colorado State University. 

Skin diseases tend to mimic each other so that similar-looking lesions can have different causes. For example, atopy can mimic insect bite hypersensitivity (IBH) caused by Culicoides. The horse can have itchy areas in the mane, tail and distal extremities in either syndrome and these lead to alopecia and self trauma. 

It is important to takeafull clinical history and performathorough clinical and dermatological examination and also to know the geographical and seasonal patterns of disease. For example, lice are more of a winter problem while IBH is a warm weather one. 

It is also important to determine the sequence of events. Did the itch become a rash or did the rash become itchy? Urticaria can be associated with pruritus but urticaria and wheals due to atopy are usually non-pruritic. 

Atopy can give rise to focal areas of eosinophilic dermatitis which appear as alopecia, inflammation and crusting without being infected.

Food sensitivity is rare in horses; it is usually a response to sweet feed or commercial grain-containing feed and the onset is usually not related to the time of year. It may present as urticaria of the mane, tail and ventrum. 

Restrict feed 

The suspect food should be removed and the feed restricted to one sort of hay given for four weeks before reintroducing the suspect food. 

It is more difficult to establish a direct causal link in Malassezia dermatitis. This condition manifests itself as a dermatitis of the prepuce, groin, axillae and udder with a greasy, waxy, smelly skin. The affected horse may rub its tail. 

Malassezia is recovered from the cytology of the areas, but it occurs in these sites in normal horses, so the condition may be due to hypersensitivity. 

The diagnosis of common parasitic conditions such as pediculosis, chorioptes (usually found in heavily feathered legs, diagnosed by combing and taking skin scrapes) and Trombiculidiasis (chiggers, found in autumn) is suggested by the location of lesions, season and management. 

The age of onset of the condition is also significant: IBH occurs at around two to four years while atopy can show at one to six years, though the mean age of onset is around five to six years. It is always worth checking the health of the horse’s companions. 

Contact hypersensitivity often involves the distal extremities and it can be complicated by photosensitisation. Trigger factors can include plants, bedding, topical applications and tack. 

Eruptions which follow the therapeutic use of drugs for nonrelated conditions can mimic every sort of skin lesion. Finding one’s way out of this maze can be helped by trial treatment. 

Routine cytology 

This should be preceded by cytology (which is best used routinely and early), fungal culture and scrapings.Acrusty area is best investigated by lifting up the crust to expose the moist area underneath it and making an impression smear from this. 

Lesions may involve resistant bacteria, trial treatment with systemic antibiotic may not control them. IBH, food sensitivity and atopy are steroid responsive (starting at 0.5 to 1mg/kg/day prednisolone or 0.05 to 0.1mg/kg/day) and may also respond to antihistamine. 

These conditions all cause eosinophilic dermatitis so they cannot be separated out by histology alone. However, multiple biopsies are necessary in investigating any unclear diagnosis. At the end of the day, rely on the clinical picture when laboratory tests appear to give contradictory results. 

“If a disease has many names, it is a problem.” Marianne Sloet of Utrecht University is well-known for her research on Culicoides hypersensitivity (IBH) which is also known as sweet itch or summer eczema. 

IBH is a huge welfare problem of horses worldwide. It occurs when genetics, geography, management and insects all combine. Of the 400,000 horses of 27 different breeds that inhabit the Netherlands, 8 to 18% suffer from IBH, which makes a conservative estimate of 30,000 itchy animals, and the incidence of the disease is increasing, probably due to global warming and the reduced use of insecticides in agriculture. 

Dutch research 

The threat of midge-borne diseases such as bluetongue and African Horse Sickness has fuelled research in the Netherlands. 

There are 4,000 species of biting midge and more than 1,000 species of Culicoides. They need moisture and an ambient temperature of over 5ºC to carry out the life-cycle and must have a blood meal before they can reproduce. They like fields near cities and forests and they do not cause a problem on the Dutch offshore islands, or for that matter in Iceland (Iceland ponies only suffer IBH when they are exported). 

They do like decaying vegetable matter such as horse faeces so mucking out paddocks is helpful. The larvae can overwinter here, but midges do not fly long distances though they are widely dispersed on the wind. 

A new study, carried out in 2005- 06, found that the midges attacking the horses were Culicoides obsoletus (94.1%), C. pulicaris (5.81%), C. stigma and a single C. vexans.

Extract of midges 

A whole body extract of these “local” midges was made and gave consistent results when it was injected intradermally into pairs of horses.

Each pair had one hypersensitive horse and one unaffected one. Both had been kept at the same location for at least one year. 

The wheal diameter and firmness was evaluated at 30 minutes,1 hour, 4 hours and 24 hours after injection. Reactions read at 1 hour and 4 hours gave the most reliable response. 

There was a clear correlation between test hypersensitivity and the clinically affected horse. A similar trial carried out in 2004 using an extract of Culicoides nubeculosis obtained from the USA failed to distinguish affected horses from control horses. 

Work is progressing on improving the specificity and sensitivity of the intradermal test and it is hoped to develop a programme to hyposensitise horses to Culicoides. The genetic background of individual affected horses is being investigated with a view towards finding a genetic marker and developing a breeding programme to limit the numbers of susceptible horses. 

It would be good if a reliable blood test could be developed but for the present prevention is the key to managing this condition. 

Well-known strategies include stabling through dawn and dusk when the midges feed most actively, wearing fly-proof protective clothing when turned out, the application of fly repellents and choosing grazing areas that the midges find less hospitable. The use of electric fences gives the horse nowhere to rub and cause self-trauma but it does not treat the itch. 

Overall, in spite of the progress made so far, IBH remains a big welfare problem for horses throughout the world.