Sweet Itch
is an allergic reaction and therefore an immune system problem.
Unfortunately these are notoriously complicated and difficult to
deal with.
The
disease is a delayed hypersensitivity to insect bites and results
from an over-vigorous response by the animal's immune system.
In the process of repelling invading insect saliva (which actually
contains harmless protein) the horse attacks some of its own skin
cells 'by mistake' and the resulting cell damage causes the symptoms
described as Sweet Itch.
In
the UK several species (of the 1,000 or so that exist) of the
Culicoides midge and, to a lesser extent, the larger, hump-backed
Simulium Equinum, a member of the blackfly family, are responsible.
Each has a preferred feeding site; Culicoides tend to be
body feeders and the Simulium earfeeders.
Culicoides
adults mainly rest among herbage and are most active in twilight,
calm conditions. Breeding sites are commonly in wet soil or moist,
decaying vegetation. They are tiny, with a wing length less than
2 mm and able to fly only a short distance (100 metres or so).
Male
Culicoides are nectar feeders, but soon after hatching
the females mate and require a blood meal to mature their eggs.
They do not fly in strong wind, heavy rain or bright, clear sunshine.
They dislike hot, dry conditions. The grey light at dusk and dawn
suits them well, and they are at their most active at these times.
However, as they are poor fliers, if there is too strong a wind,
or rain during early morning they will simply wait until later
to feed. Likewise they may feed at any time during humid days
with cloud cover.
Culicoides
are on the wing and breeding from as early as late March until
the end of October, depending on geographical location. There
is only a short breeding season each year in the north of Scotland,
while in the south of England larvae will hatch throughout the
spring, summer and autumn, depending on weather conditions. Seasonal
variations in the weather can have an impact - recent winters
have been milder and damper allowing breeding to start earlier.
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
numbers are the critical factor.
Culicoides
larvae are able to survive severe frosts but they do not survive
prolonged drought conditions.
Research
into how Culicoides locate their prey indicates that they
primarily do so by sight.
