Scrub itch is the irritation caused by the bite of a small orange-red mite called a Trombiculid larva. This animal is the same animal order as other mites and ticks (Acarina). Various species are found around the world and are known under various names as Harvest Mite, Chiggers or Velvet mites.
Around Brisbane the mite is active in the summer months in all rainforest areas, particularly above the 300 metre altitude, where rain is more frequent during this period. The parasite is located as far south as the Dorrigo Plateau and northwards along the Queensland coast into New Guinea. North of Cairns and into New Guinea, the parasite carries Scrub Typhus, a potentially life threatening complication of the bite. To date, no one bitten south of Townsville has suffered this problem.
Eggs are laid in the soil and, on hatching, the larvae, which are 0.2mm long and scarcely visible to the human eye, climb up grass or other low vegetation. When an animal, including man, brushes past the plant the larvae drop and, if successful, land on fur or clothing whence they scramble to secluded areas or high perspiration zones.
Typically, humans are bitten at sites where the mite can get entry: the collar line, belt line and tops of the socks. The favoured sites are the armpits, pelvic region and calves of the legs. The mite secretes a saliva which dissolves the upper layers of skin – the resulting fluid being sucked up. This process rapidly produces a very itchy, angry, red lump with the mite continuing the process for several days.
Itching, or abrasion, can cause secondary infection, or possibly it causes the mite to relocate to another site and the process recommences. After a few days of feeding, the larva drops off and for the rest of its life cycle is no longer parasitic on warm-blooded animals.
Prevention is the best recourse and our local pharmacist recommends using Citronella Oil or Tea Tree Oil, available in over-the-counter Preparations of “Bushmans Extra” and “Walkabout”. The liquids must be applied to the skin at places where the mite can gain access. Putting these fluids on clothing seems to be ineffective, but a past recommendation of soaking clothing in Dibutyl Phthalate was considered an effective repellent.
If you have been bitten, then an over-the-counter lotion is Ascaboil, which will kill the mite, as will dabbing with kerosene, petrol or alcohol, but these fluids might produce their own reactions. If the itching is really severe, then medical advice might be necessary and the best relief will be to take some antihistamine orally. Rubbing anti-itch creams on the lumps is more likely to aggravate the skin reaction.
Finally, the best procedure is not to go into rainforest when the mite is active, but, if you do, wear a broad-brimmed hat, do not get off the path, do not brush against vegetation and do not sit on the ground! Enjoy walking in the rainforest but be prepared to repel these mites over the summer period.