Daniels also found these embryos, along with those of F. demarquayi, in the blood of natives of British Guiana.
FILARIA BANCROFTI
Geographical Distribution
This parasite has been found in almost all tropical and subtropical countries. It is quite prevalent in the West Indies and has been found in some of the Southern states of the U. S. It is very common in some of the South American countries as well as in Central America. The infection is widespread in Arabia, India and China. Africa, especially the West Coast, and Australia, particularly in Queensland, are parts of the world where the infection prevails.
It is especially in some of the Pacific islands, as Samoa and Fiji, that it is extraordinarily prevalent. Bahr has stated that Fijians in the proportion of 27% show filarial embryos in their blood. In 25% of these natives clinical manifestations of the disease exist but the embryos are absent from the peripheral circulation. In other words more than one-half of the population show absolute evidence of infection.
Life History
It is a well-known fact that filarial embryos may be present at night in the peripheral blood of persons not showing a single symptom of filariasis and again, in those with marked elephantiasis, varicose groin glands or chyluria there may be an entire and permanent absence of embryos in the blood. When certain mosquitoes bite persons having embryos in the blood they take into their stomachs the sheathed embryos of F. bancrofti. Flu states he has succeeded in infecting the following mosquitoes: C. fatigans, S. scutellaris, M. ludlowi and M. rossii. The most suitable carriers proved to be C. fatigans and S. scutellaris. Large numbers of larvae perish in anopheline mosquitoes.
The following developmental cycle has been demonstrated for Culex fatigans and Stegomyia pseudoscutellaris. Bahr has found that if there are too many embryos taken up by the mosquito the insect is apt to die, as the result of too heavy an infection; so that a person harboring many filarial embryos may be less dangerous than one with a smaller number. Upon reaching the stomach of the mosquito the sheath of the embryo becomes fixed in the viscid blood contents and the embryo itself by active motions is able to force itself from its sheath. This escape usually occurs within two hours but may take longer. The free embryo then bores its way through the stomach walls and within twenty-four hours has reached the thoracic muscles of the mosquito. Within forty-eight hours the embryo begins to broaden and the anterior and posterior V spots to become more prominent. About the end of the first week there commences the formation of an alimentary canal, by which time the developing larva is about 0.5 mm. long. When the larva is about 0.6 mm. long an ecdysis apparently takes place. Later on these larvae develop 3 or 4 terminal papillae and make their way to the fleshy labium of the mosquito’s proboscis. An occasional larva may enter other structures than the labium but in such case they would be unable to effect an entrance to their definitive host, man. These larvae in the proboscis are about 1.5 mm. long and about 20 microns broad.
The mosquitoes have two terminal processes, the labella, separated from the labium by a thin membranous partition called Dutton’s membrane. The larvae, having completed their developmental cycle in the mosquito, which takes about three weeks, and moving down the labium, break through this membrane when it is put upon a stretch by the wide separation of the labella at the time of feeding on the part of the mosquito. It was formerly supposed that the larvae entered man through the puncture made by the biting parts of the mosquito, but Bahr has shown by experiments that they effect an entrance through the intact pores of the skin as does the ancylostome larva.
These larvae upon entering the human host reach the lymphatic vessels or glands and in this definitive host (man) the females are fertilized by the males and give off sheathed larvae from the uterine opening near the anterior end of the worm.