Page:Popular Science Monthly Volume 81.djvu/554

548 for the transmission of a parasitic disease of the tropics known as filariasis. The direct cause is a nematode worm belonging to the genus Filaria which is present in the circulation and lymphatics of the infected person. In the late stages of the disease the microscopic larval worms occur abundantly in the blood. For some unexplained reason they remain in the deep-seated blood vessels during the day, but usually appear more abundantly in the peripheral circulation during the night. Here they are readily obtained by mosquitoes with their meal of blood. In the alimentary canal of the mosquito the larval Filaria discards a sheath-like envelope which has previously invested it, and works its way through the wall of the stomach into the thoracic muscles where it increases greatly in size and finally migrates to the base of the proboscis. From two to three weeks are necessary for this metamorphosis, and for some time longer the Filaria may remain in the proboscis awaiting its opportunity to enter another person through the wound occasioned by the mosquito's beak. Once they have been transferred to their human host, the parasites enter the lymphatics where they attain sexual maturity and give rise to the abundant microscopic larval Filarias that reenter the circulation to await ingestion by another mosquito.

Filariasis is most common in equatorial regions, but extends less commonly into the subtropics. The parasites themselves do not ordinarily cause great inconvenience, but their presence in the lymphatics may clog these vessels to such an extent that secondary swellings may be developed in the limbs or other parts of the body.

Several insects have been associated with a peculiar tropical disease of the old world, variously known as kala-azar, dum-dum fever and leishmaniosis. In this case the organism is a flagellate protozoan, Leishmania donovani, of which there are possibly two forms, one producing a children's disease termed infantile kala-azar and the other the true leishmaniosis of adults. It has been shown that among domestic animals the dog at least is susceptible, and other animals may be also. In 1907 Patton discovered various stages of the Leishmania parasites in bed-bugs that had fed on persons suffering from kala-azar and this insect has been considered to be one, if not the exclusive, carrier. Very recently, however, some doubt has been expressed regarding the role of the bed-bug and a certain reduviid bug has been suspected. There seems to be no doubt, however, that kala-azar is insect-borne.

The diseases which we have enumerated are the more important insect-borne ones that affect man. A number of others of greater rarity or of minor nature are known to be carried regularly or occasionally by various insects, and many others are now being investigated in the light of present knowledge to ascertain if they, too, may not be spread by insects. It seems probable that flies may take some part in the dissemination of the bacilli of leprosy, although to how great an extent