Harvest
mites, harvest bugs or bracken bugs are the names popularly
given to the larvae of the mite Trombicula autumnalis. This six legged larva feeds on tissue fluid and may
cause considerable skin itch and discomfort to both humans
and cats in certain areas of the country during late summer
and autumn. The large orange/yellow larvae are widely distributed
in the UK and are particularly abundant on chalk upland. Heavy
infestations may be sharply localised - even to the extent
of being abundant in one garden and absent from others in
the same village. It is also found in town gardens and parks.
The
larva is about 0.2 mm long and is just visible to the naked
eye - it swells to about three times this size when it has
finished feeding.
Harvest mite larvae
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Life
cycle
The
first active stage in the life cycle of Trombicula is the six-legged
larva (see picture) and this is the only stage which
attacks animals. These larvae are present on vegetation and
are active during the day, especially when it is dry and sunny.
When they come into contact with any warm blooded animal they
swarm on and congregate in areas where there is little hair
and the skin is quite thin. This usually occurs around the
end of June, but can be earlier, and persists through the
summer until the end of September.
The
larva feeds by thrusting its small hooked fangs into the skin
surface layers of the skin. It then injects a fluid which
breaks down the cells underlying the horny layers of the skin.
The liquid food resulting from this process is sucked back
into the digestive system of the larva. It will inject and
suck for two to three days at the same site until it is replete
and has increased in size three to four times.
The
larva then drops to the ground to complete its life cycle.
It descends into the soil and after about six weeks becomes
an eight-legged nymph and then an adult which eats plants
and small insects. Eggs laid by the adult in the spring and
summer hatch into the six-legged larva known as harvest mites
and the cycle starts again.
Dermatitis caused by harvest mites
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Signs
of infestation
When
the larva injects fluid into the skin this can cause a skin
reaction in sensitive individuals. In people this can show
as small inflamed pimples. In cats the irritation can cause
reddening of the skin, papules and crusted areas. The areas
most likely to be affected are the base of the ears (Henry's
pocket - the little pocket of skin on the side of the pinna
of the ear), foreface, chin and around the mouth, the neck
and shoulders, under the front legs, mid abdomen and around
the nipples, vulva and scrotum and between the toes - areas
where there is only a thin covering of hair. Scratching which
may dislodge the mites (until more attach), can result in
injury to the skin and in severe cases raw areas can develop.
These areas can become infected with bacteria.
Diagnosis
Diagnosis
is made by observation and identification of the larvae through
the microscope. A veterinary surgeon will have to take a skin
scraping to make a definitive diagnosis. It is best to present
the cat to the vet at the end of the day rather than the next
morning when the mite may have released and dropped off.
Treatment
There is no licensed treatment for harvest mites available in the UK. However, some particular flea treatments available from your vet may be effective. Treatment should be carried out under direction of your vet. A cat which appears to be hypersensitive to
the mite may also need glucocorticoid therapy for a short
period until the mites have reduced in number. A very sensitive
cat may have to be confined during the mites' active period
and an Elizabethan collar used to prevent further self-inflicted
injury to the itchy parts. If a run is provided for the cat
during this period the grass must be kept short (or preferably
be concrete based) and the cat let out in the early morning
or during dull or wet days when the mite is less active.
Updated November 2008
