Medical Entomology

MEDICAL ENTOMOLOGY Although medical entomology includes in its scope venomous and parasitic arthropoda that inflict minor hurts upon the indi vidual human body, its main concern is with arthropoda which, either as the necessary intermediary hosts or as the fortuitous mechanical disseminators of other pathogenetic organisms, stand in the main line of causation of serious endemic and epidemic disease.

Manson's Discovery.

Such insect-borne disease, though from of old a topic of popular belief and even of rational speculation, was first clearly demonstrated by Manson, who discovered, in 1879, how the embryos of the worm that causes a common filarial disease of the tropics are extracted from one human host by a female mosquito and then pass through their larval development within that insect's body before they can find their way into another human host. The value of this discovery, however, was not generally appreciated until 1898, when Ross discovered how the parasites of malaria, after an analogous dependence on the mosquito for transport and sustenance, are injected into their human host by the insect intermediary, it being, however, "Man son's theory, and no other, which actually solved the problem" of the mode of transmission of this disease.

In these and similar instances the insect is a go-between, safe guarding the transfer of a specifically "human" parasite from one human being to another. But many blood-sucking insects are normally infected with seemingly innocuous endoparasites of their own, and some maintain that such insects may inject their own parasites into the human body where they may not remain in nocuous ; and it is to be noted that surmise of the same still un satisfied kind has been awakened by the recent discovery in the normal intestine of certain insects, particularly bugs and lice, of bodies similar to the minute Rickettsia-bodies isolated, in 1916, by Rocha Lima from typhus fever cases and regarded by him as specifically morbific.

Recent developments in medical entomology may be sum marized thus: Diseases Specifically Attributable to Mosquito Agency.— To filariasis, malaria and yellow fever, the parasite of which last was in 1918 determined by Noguchi as a Leptospira, must be added dengue fever, shown to be transmitted in Australia (Cleland and others) by Stegomyia f asciata, and in Syria (Graham) by Culex fatigans.

Diseases Specifically Attributable to Sandfly Agency.— In entomological company with phlebotomus fever will probably have to be placed oriental sore (Sergent brothers: Aragao), and also kala-azar, the natural history of which seems to be definitely associated with Phlebotomus argentipes (see KALA-AZAR and SAND FLY FEVER).

Diseases Attributable to Gadfly (Tabanidae).

The de tails of the post-embryonic development of Filaria (Loa) loa in the course of its transmission from man to man by Chrysops dimidiata and salacea were described in 1921 by Dr. and Mrs. Connal. (Tularaemia, a recently recognized septic fever of Amer ican rabbits, may be imparted from rabbits to man by Chrysops discalis, but merely mechanically, since the infection may also pass to man without any insect intervention.) Diseases Specifically Attributable to Tsetse-fly Agency.— Besides the notorious Glossina palpalis and morsitans several other common species of Glossina are now known to be intermediary in the spread of the trypanosomes of African sleeping sickness.

Diseases Specifically Attributable to Flea Agency.—Hirst, in Ceylon, and Cragg, in India, have produced good evidence that not all the three species of rat-fleas of the Xenopsylla genus are equally formidable in spreading bubonic plague, and that plague does not rage as an epidemic in places where Xenopsylla astia is the only flea infesting the local rats. The evidence of the inter mediation of the dog-flea in the transmission of infantile kala azar is still obscure.

Diseases Attributable to Bugs (Rhynchota).

The parasite of South American trypanosomiasis (Chagas's disease) is now known to be transmissible by several species of reduviid bugs— and also, experimentally, by other blood-sucking insects—and is believed to be imparted to man in the insects' excrement. An original natural source of infection is said to exist in armadillos and opossums.

Diseases Attributable to Lice.

In company with typhus and relapsing fever the war disease "trench-fever" is to be in cluded. In all three instances the virus is inoculated by con tamination. The lousy person as he scratches himself rubs the infective juice or excrement of the infected insects into the abrasions of his skin. Another and unpleasant complaint thus engendered is Pediculosis.

Diseases Attributable to Ticks.

Several species of Ornitho dorus are now suspected of transmitting the spirillum of relapsing fever, and 0. talaje in tropical America has been shown to be as dangerous in this respect as the African 0. moubata. The virus of Rocky Mountain spotted fever, conveyed to man by Dermato centor venustus, is declared by Wolbach to be a Rickettsia. An other tick of this genus, D. andersoni, has been found infected with the bacillus of tularaemia. In these tick-borne infections the parent tick, becoming infected, transmits the infection to its offspring.

Diseases Specifically Attributable to the Agency of Harvest Mites (Trombidiid larvae).—Besides Japanese river fever, the virus of which is inoculated by the akamushi mite, there are other local and seasonal epidemic fevers that have a similar causation. Among them is the pseudotyphus of Sumatra.

Diseases Specifically Attributable to Crustacean Agency. —Small freshwater copepods of the genus Cyclops have been known from the time of Fedshenko to be intermediary hosts of the guinea-worm (Dracunculus medinensis) ; • and in 1917 Janicki and Rosen discovered, in Cyclops strenuus and the near-related Diaptomus gracilis, the long-sought first intermediary hosts of the larval ribbon worm. (Botlariocephalus latus). Japanese workers have shown that species of river crabs play an intermediary part in the post-embryonic development of the lung-fluke.

Diseases Attributable to Fortuitous Insect Agency.— We must consider also the odious assistance lent by domestic in sects—houseflies, cockroaches, cellar-beetles, ants, fish-insects and by the ubiquitous fauna—moth and beetle-larvae, booklice, food mites—of granaries, provision stores, warehouses etc., in the in discriminate and mainly mechanical dissemination of morbific material of various kinds. Houseflies are the most assuredly dan gerous of these disturbers of the public health. Fibiger found that in rats infected with a filarial worm of the genus Gongy lonema, through feeding on infected cockroaches, cancerous growths started from the lesions caused in the rat by the worm.

Although the insects here very briefly reviewed are dangerous to public health, it is beginning to be realised that the peril is conditional. In an environment unmodified by sanitation, such as prevails in extensive areas in the tropics, these insects are a constant danger and, at the present time, must be attacked with energy and with resource based upon a study of their life-history, seasons and habits. But in countries having settled standards of personal and domestic hygiene and an efficient sanitary adminis tration these insects have ceased to be a standing menace to the public health, as is exemplified by the history of plague, typhus and malaria in England since the establishment of a sanitary regime.

See A. Alcock, Entomology for Medical Officers (192o) .

(A. W. A.)