Fig. 98.—Male (a) and female (b) of Filaria bancrofti. Natural size. (From Greene after Manson.)
The sheath is simply the egg membrane which from being oval at first becomes stretched by the developing embryo to finally become a long, narrow sac encasing the fully developed embryo as it exists in man. From the lymph stream they reach the general circulation. In a case of a man with filarial embryos in his peripheral circulation, who committed suicide one morning, Manson found the embryos, in large part, contained in the vessels of the lungs. There were 675 embryos per slide in blood from the lungs for one from blood from the spleen or liver. It would thus appear that during the day, when the embryos are absent from the peripheral circulation, they retire to the lungs. In the case of the filarial embryo of persons in the Pacific Islands there does not appear to exist any periodicity. Bahr thinks this absence of nocturnal periodicity to be connected with the habits of its principal intermediary host, Stegomyia pseudoscutellaris, which feeds by day. Culex fatigans feeds at night.
With the filarial embryos found in patients in the Philippines there is also a lack of nocturnal periodicity. In the opinion of Ashburn and Craig the Philippine filarial worm is a new species, Filaria philippinensis.
Walker, however, recently examined four adult filarial worms in the Philippines and was unable to note any differences from F. bancrofti.
Pathology and Morbid Anatomy
The adult worms may exist in numbers and over long periods of time give off great numbers of embryos into the peripheral circulation without there being any evidence of disease in the patient. There is apt to be at such time a marked eosinophilia. The process by which the fibrosis of lymph channels with obstruction to the flow of lymph occurs is unknown. Some think that with the pouring out of embryos inflammatory processes, bacterial or otherwise, may be set up. We know that there is a tendency for these adults to die and become calcified, in this way bringing about lymphatic obstruction.
Bahr notes the influence of adult filariae in producing an increase in connective tissue in glands and considers such glands as less resistant to bacterial infection.
Manson has an idea that some factor may cause the female to give off immature embryos, which being oval, and of considerable width, may block the lymphatics.
It has often been claimed that various cocci were the exciting factors in the lymphangitis associated with filariasis. Recently Dutcher has reported the isolation of an organism resembling B. subtilis as the cause of filarial lymphangitis (Bacillus lymphangiticus).