Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

The larvæ live parasitically in various parts of the body of mammals, such as the stomach (horse bot-fly), the subcutaneous connective tissue (warble-fly of cattle), or the nasal passage (sheep bot-fly or head maggot).

There are on record many cases of the occurrence of the larvæ of Oestridæ as occasional parasites of man. A number of these have been collected and reviewed in a thesis by Mme. Pètrovskaia (1910). The majority of them relate to the following species.

Gastrophilus hæmorrhoidalis, the red tailed bot-fly, is one of the species whose larvæ are most commonly found in the stomach of the horse. Schoch (1877) cites the case of a woman who suffered from a severe case of chronic catarrh of the stomach, and who vomited, and also passed from the anus, larvæ which apparently belonged to this species. Such cases are exceedingly rare but instances of subcutaneous infestation are fairly numerous. In the latter type these larvæ are sometimes the cause of the peculiar "creeping myasis." This is characterized at its beginning by a very painful swelling which gradually migrates, producing a narrow raised line four to twenty-five millimeters broad. When the larva is mature, sometimes after several months, it becomes stationary and a tumor is formed which opens and discharges the larva along with pus and serum.

Gastrophilus equi is the most widespread and common of the horse bot-flies. Portschinsky reports it as commonly causing subcutaneous myasis of man in Russia.

Hypoderma bovis (= Oestrus bovis), and Hypoderma lineata are the so-called warble-flies of cattle. The latter species is the more common in North America but Dr. C. G. Hewitt has recently shown that H. bovis also occurs. Though warbles are very common in cattle in this country, the adult flies are very rarely seen. They are about half an inch in length, very hairy, dark, and closely resemble common honey-bees.

They deposit their eggs on the hairs of cattle and the animals in licking themselves take in the young larvæ. These pass out through the walls of the œsophagus and migrate through the tissues of the animal, to finally settle down in the subcutaneous tissue of the back. The possibility of their entering directly through the skin, especially in case of infestation of man, is not absolutely precluded, although it is doubtful.

For both species of Hypoderma there are numerous cases on record of their occurrence in man. Hamilton (1893) saw a boy, six years of age, who had been suffering for some months from the glands on one side of his neck being swollen and from a fetid ulceration around the back teeth of the lower jaw of the same side. Three months' treatment was of no avail and the end seemed near; one day a white object, which was seen to move, was observed in the ulcer at the root of the tongue, and on being extracted was recognized as a full grown larva of Hypoderma. It was of usual tawny color, about half an inch long when contracted, about one third that thickness, and quite lively. The case resulted fatally. The boy had been on a dairy farm the previous fall, where probably the egg (or larva) was in some way taken into his mouth, and the larva found between the base of the tongue and the jaw suitable tissue in which to develop.

Topsent (1901) reports a case of "creeping myasis" caused by H. lineata in the skin of the neck and shoulders of a girl eight years of age. The larva travelled a distance of nearly six and a half inches. The little patient suffered excruciating pain in the place occupied by the larva.

Hypoderma diana infests deer, and has been known to occur in man.

Oestris ovis, the sheep bot-fly, or head maggot, is widely distributed in all parts of the world. In mid-summer the flies deposit living maggots in the nostrils of sheep. These larvæ promptly pass up the nasal passages into the frontal and maxillary sinuses, where they feed on the mucous to be found there. In their migrations they cause great irritation to their host, and when present in numbers may cause vertigo, paroxysms, and even death. Portschinsky in an important monograph on this species, has discussed in detail its relation to man. He shows that it is not uncommon for the fly to attack man and that the minute living larvæ are deposited in the eyes, nostrils, lips, or mouth. A typical case in which the larvæ were deposited in the eye was described by a German oculist Kayser, in 1905. A woman brought her six year old daughter to him and said that the day before, about noontime, a flying insect struck the eye of the child and that since then she had felt a pain which increased towards evening. In the morning the pain ceased but the eye was very red. She was examined at about noon, at which time she was quiet and felt no pain. She was not sensitive to light, and the only thing noticed was a slight congestion and accumulation of secretion in the corner of the right eye. A careful examination of the eye disclosed small, active, white larvæ that crawled out from the folds of the conjunctiva and then back and disappeared. Five of these larvæ were removed and although an uncomfortable feeling persisted for a while, the eye became normal in about three weeks.