Insect Bite Hypersensitivity (Sweet Itch)

By Dr Dave Mullins: Equine Veterinary Services 

 

This is the most common cause of summer pruritis. It results in an immune mediated hypersensitivity to the salivary antigenic chemicals of Culicoidesspp.midges.

There is a strong seasonal relationship with the occurrence of Culicoides midges. ie: late Spring to late Autumn. Horses affected are usually over 5 years of age as it takes time for the hypersensitivity to build up. There is a link to a specific gene resulting in heritability to the disorder. Pony breeds seem to have a predisposition.

Symptoms.

The clinical signs are derived from the effect of the itchiness caused by the hypersensitivity to the bites of the midges. The degree of itchiness can vary between horses and is usually show progressive increase each year
Papules usually involve the head, neck, dorsum and ventral midline. The itchiness leads to self-inflicted trauma of the head, neck and base of the tail.

Diagnosis

Confirmation of the diagnosis can be assumed from:

The seasonality of the condition coupled with its recurrence in subsequent seasons. Improvement is usually rapid following movement of the horse from midge contact ie: stabling, repellents, blanket covers. Skin biopsies can be helpful but not always diagnostic.

Management

The only really safe and effective method of management is to separate midges from the horses! Stabling in an insect proof environment from mid-afternoon to mid-morning as midges to do not enter the stable environment. Windy hillsides and possibly fan assisted ventilation is effective because the midges are relatively weak fliers. Total body covers can be used effectively and permit the affected horses to be grazed outside. Topical pyrethrins achieve control by application of the inspect repellent to the horse and should include treatment of the dorsum and belly of the horse. This must be done daily before horses are put out. Remember to consult your Veterinarian before using a spray as some of them can be harmful and result in a another form of Hypersensitvity. Control of itchiness can be achieved using systemic cortico steroids.  The safest in this respect is prednisilone at 1mg/kg by mouth administered in the morning.  As complications can occur with cortisone such as laminitis, this should be carefully monitored in consulation with your veterinarian. Antihistamins have little effect as they have a short half life in equine blood so repeated high doses would be required and is impractical. Allergens desensitization can be attempted but currently this carries a poor outlook.

Conclusion

Currently there is no definitive advice that can be given apart from the basic concept of separation of the horse from the allergen. There is no long term cure for insect bite hyper sensitivity and one is directed to achieve control of the condition rather than cure. Daily spraying with pyrethroids also serves to reduce the incidence of African Horse Sickness. Ultimately most case are treated with Cortisones to eliminate the itchiness and allow time for recovery.

 

 

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