Handbook of Medical Entomology

by William A. Riley; O. A. Johannsen

Filariasis and Mosquitoes—A number of species of Nematode worms belonging to the genus Filaria, infest man and other vertebrates and in the larval condition are to be found in the blood. Such infestation is known as filariasis. The sexually mature worms are to be found in the blood, the lymphatics, the mesentery and subcutaneous connective tissue. In the cases best studied it has been found that the larval forms are taken up by mosquitoes and undergo a transformation before they can attain maturity in man.

The larvæ circulating in the blood are conveniently designated as microfilariæ. In this stage they are harmless and only one species, Filaria bancrofti, appears to be of any great pathological significance at any stage.

Filaria bancrofti in its adult state, lives in the lymphatics of man. Though often causing no injury it has been clearly established that they and their eggs may cause various disorders due to stoppage of the lymphatic trunks ([fig. 119]). Manson lists among other effects, abscess, varicose groin glands, lymph scrotum, chyluria, and elephantiasis.

The geographical distribution of this parasite is usually given as coextensive with that of elephantiasis, but it is by no means certain that it is the only cause of this disease and so actual findings of the parasites are necessary. Manson reports that it is "an indigenous parasite in almost every country throughout the tropical and subtropical world, as far north as Spain in Europe and Charlestown in the United States, and as far south as Brisbane in Australia." In some sections, fully 50 per cent of the natives are infested. Labredo (1910) found 17.82 per cent infestation in Havana.

The larval forms of Filaria bancrofti were first discovered in 1863, by Demarquay, in a case of chylous dropsy. They were subsequently noted under similar conditions, by several workers, and by Wücherer in the urine of twenty-eight cases of tropical chyluria, but in 1872 Lewis found that the blood of man was the normal habitat, and gave them the name Filaria sanguinis hominis. The adult worm was found in 1876 by Bancroft, and in 1877, Cobbold gave it the name Filaria bancrofti. It has since been found repeatedly in various parts of the lymphatic system, and its life-history has been the subject of detailed studies by Manson (1884), Bancroft (1899), Low (1900), Grassi and Noé (1900), Noé (1901) and Fülleborn (1910).

The larvæ as they exist in the circulating blood, exhibit a very active wriggling movement, without material progression. They may exist in enormous numbers, as many as five or six hundred swarming in a single drop of blood. This is the more surprising when we consider that they measure about 300µ × 8µ, that is, their width is equal to the diameter of the red blood corpuscle of their host and their length over thirty-seven times as great.

Their organs are very immature and the structure obscure. When they have quieted down somewhat in a preparation it may be seen that at the head end there is a six-lipped and very delicate prepuce, enclosing a short "fang" which may be suddenly exserted and retracted. Completely enclosing the larva is a delicate sheath, which is considerably longer than the worm itself. To enter into further details of anatomy is beyond the scope of this discussion and readers interested are referred to the work of Manson and of Fülleborn.

One of the most surprising features of the habits of these larvæ is the periodicity which they exhibit in their occurrence in the peripheral blood. If a preparation be made during the day time there may be no evidence whatever of filarial infestation, whereas a preparation from the same patient taken late in the evening or during the night may be literally swarming with the parasites. Manson quotes Mackenzie as having brought out the further interesting fact that should a "filarial subject be made to sleep during the day and remain awake at night, the periodicity is reversed; that is to say, the parasites come into the blood during the day and disappear from it during the night." There have been numerous attempts to explain this peculiar phenomenon of periodicity but in spite of objections which have been raised, the most plausible remains that of Manson, who believes that it is an adaptation correlated with the life-habits of the liberating agent of the parasite, the mosquito.