The amount of trouble they cause varies very greatly in different people. Children and women with soft skins suffer, as a rule, most; but, as happens in the case of other biting insects, certain individuals seem to be almost immune, whilst others suffer very considerably. The trouble is caused by the mite implanting its mouth-parts in the skin—preferably in the hair-follicles or the sweat-glands. When it is once fixed it rarely moves. The body remains, of course, on the surface of the skin as a little reddish-orange point, scarcely perceptible unless many of them are congregated in the same position. The effect of their presence is to produce a swelling in the skin, which may be as large as a split pea, accompanied by an intense itching and a smarting which banishes sleep. This leads to the patient scratching, and this scratching is the departure-point of many troubles. Scoriated papules appear and eczematous patches, and when the mites are very numerous an erythema, named by Rubies Erythema autumnale, supervenes. The skin near the point of puncture swells, becomes red, sometimes almost purple, and irregular patches, which when confluent, appear a centimetre in diameter.
These skin troubles, which may end in a kind of generalised eruption, are accompanied by a rise of temperature and a certain—sometimes a high—degree of fever. Besides men, dogs and cats suffer from these pests; and in these domestic pets the parasites give rise to a miliary eruption. Domestic cattle—sheep and horses—are also attacked. And, according to some authorities, poultry are not only attacked but killed by these parasites. The larvae apparently only lives a few days in the skin of the victim.
As far as is known at present the larvae of Trombidium convey no protozoal disease; but there is a terrifying little creature, known as the Kedana mite, which in some districts of Japan causes a serious illness, with a mortality of some 70 per cent. Apparently, it does not act as an inoculating agent itself, but the papule, surrounded by the red area which forms as a result of its bite, changes to a pustule, and this lesion becomes the point of entrance of bacteria which produce the so-called ‘river’ or ‘flood’ fever. If these mites be carefully removed the patient suffers no harm.
Another species of mite, Pediculoides ventricosus, lives in stalks of cereals, and is very apt to attack labourers who are dealing with grain. Their bites cause severe irritation, local swellings, reddening of the epidermis, and fever. In this particular species the female before she is fertilised has an elongated form 0·2 mm. in length and 0·07 mm. in breadth; but when fertilised the ovaries increase to such an extent that the posterior end of the body becomes spherical. In this respect it resembles that remarkable flea, the chigo or jigger. The larvae are exceptional in being born with four legs instead of the usual three, and they pair almost immediately after emerging from the egg-shell.
Fig. 33.—Pediculoides ventricosus. Male, ventral view (× 250). Female, before fertilisation (× 225). A, after fertilisation; the abdomen has begun to swell (× 250). B, with abdomen fully swollen (× 40). (After Laboulbène and Mégnin.)
CHAPTER VIII
MITES
Part II
ENDO-PARASITIC MITES (Demodex, Sarcoptes)