Causes of Sweet Itch

Sweet Itch in horses is an allergic reaction to the saliva of small midges called Culicoides and also certain horse flies. The disease is a delayed hypersensitivity to these insect bites and results from an over-vigorous response by the animal's immune system. 

Unfortunately the allergic reaction is notoriously complicated and difficult to deal with. The horse experiences intense itching at the site of the bite and s elf inflicted damage will occur as the horse scratches and tries to relieve itself of the irritation. 

Any horse, regardless of age, sex or breed can develop Sweet Itch.  Some horses appear to be much more susceptible to sweet itch than others, and the condition can be hereditary. As with most auto-immune responses, once a horse displays the symptoms of Sweet Itch it is likely to be predisposed to it for the remainder of its life and it also seems to get worse with age.

Source: nativeremedies.com

 
        
The Culicoides midges from the family Ceratopogonidae are around 2-3mm in size and are also referred to as “no seeums”. They are tiny, with a wing length less than 2 mm and able to fly only a short distance (100 metres or so). They are poor fliers and do not fly in strong wind, heavy rain or bright, clear sunshine. They dislike hot, dry conditions but they will feed at any time during humid days with cloud cover.

Midges are seasonal insects active in the spring and summer months and thrive in warm and humid environments. They breed in wet lands, coastal lagoons, mangrove swamps, tidal flats, rivers, lakes and standing water and are most active in twilight, calm conditions.

The male Culicoides midge feeds on nectar but, soon after hatching, the females mate and require a blood meal to mature their eggs.Their breeding sites are commonly in wet soil or moist, decaying vegetation. They will feed at any time during humid days with cloud cover.

Research into how Culicoides locate their prey indicates that they primarily do so by sight.

There are about 20 different species of Culicoides midge, and each one prefers to bite a different part of the horse. The bite of the fly is particularly painful due to the chewing mouth parts and this can cause significant fly worry for the horse if the midges are present in large numbers.The horse will then itch the area around where the fly has bitten. Normally Culicoides feed on the dorsal surface of the horse, predominantly along the mane and tail area and around the face and ears. The “ventral feeders” are less common, and tend to cause the itching around the horse’s face, chest, and belly.


Culicoides numbers are the critical factor. The Culicoides midge breeds during the Spring and Summer months and its level of activity is dependent upon weather conditions. Like all insects, the midge is cold blooded and is most active when the temperature is higher. Summers that are alternately sunny and rainy cause an increase in midge breeding habitats and therefore an increase in the numbers of midges that are around to bite. Under these conditions most horses will show symptoms of Sweet Itch to some degree.

Culicoides larvae are able to survive severe frosts but they do not survive prolonged drought conditions.
 
The animal's local environment can play an important role in the presence or absence of midges and therefore incidences of Sweet Itch.
 
The midge also needs pools of calm water, boggy areas or, at the very least, moist conditions to complete its life cycle. Stagnant water, marsh land, pools, calm ponds and even the seemingly harmless water trough all present the midge with the opportunity to breed, so any horse living in the vicinity of these are prone to attack from the midges.  

Ideally, a distance of 1,000 metres should be maintained between your horse's grazing land and any free-standing water.



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Finally, your horse's overall general health and disposition may contribute to the onset of Sweet Itch. If a horse has other health concerns, is in a poor condition or is unhappy, then its immune system may be impaired and is more likely to develop hypersensitivity to the bites of midges, flies and mosquitoes.