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
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
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 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.
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.
Information taken from: http://www.fabcats.org/owners/skin/harvest_mite.html
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