WUCHERERIA AND BRUGIA : PARASITOLOGY
The two agents most commonly responsible for lymphatic filariasis are W. bancrofti and B.malayi. Both are thread-like worms that lie coiled in the lymphatic vessels, male and femaletogether, for the duration of their decade-long lifespan. The female W. bancroftimeasures 100 mm in length, and the male 40 mm. B. malayi adults are approximately half these sizes. The gravid females produce large numbers of embryonated eggs. At oviposition, the em-bryos uncoil to their full length (200 to 300μm) to become microfilariae. The shell of the egg elongates to accommodate the embryo and is retained as a thin, flexible sheath. Al-though the offspring of the two species resemble each other, they may be differentiated on the basis of length, staining characteristics, and internal structure (Table 56 – 2).
The micro-filariae eventually reach the blood. In most W. bancrofti and B. malayi infections, they accu-mulate in the pulmonary vessels during the day. At night, in response to changes in oxygen tension, they spill out into the peripheral circulation, where they are found in greatest num-bers between 9 PM and 2AM. A Polynesian strain of W. bancrofti displays a different period-icity, with the peak concentration of organisms occurring in the early evening. Periodicity has an important epidemiologic consequence, because it determines the species of mosquito to serve as vector and intermediate host. Within the thoracic muscles of the mosquito, mi-crofilariae are transformed first into rhabditiform and then into filariform larvae. The latter actively penetrate the feeding site when the mosquito takes its next meal. Within the new host, the parasite migrates to the lymphatic vessels, undergoes a series of molts, and reaches adulthood in 6 to 12 months.