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Blood (Intracellular) and Other Tissue Protozoa 103

A clinical diagnosis may be complicated by the in- in courtship displays, but the differences are more subtle
ability to isolate microfilaria from a patient’s blood speci- in the filaria of W. bancrofti. The male worm is 40 mm
men. Even individuals who exhibit all of the signs and long and 100 μm wide with a curved tail, whereas the fe-
symptoms of having been infected by microfilaria may male is much larger than the male. The female worm is
not contain the organisms in their blood when stained 6 to 10 cm long and 300 μm wide or nearly three times
blood smears are microscopically examined. These indi- longer and larger in both diameter and length than the
viduals are considered amicrofilaremic in the absence of male. Females are ovoviviparous, meaning reproduction
directly demonstrated infection. In this case, in addition by the hatching of eggs inside the mother organism, a
to clinical observations, serological tests designed to de- process that can produce thousands of juveniles known
tect circulating antigen in the blood may be effective in as microfilariae. The microfilariae of W. bancrofti are
obtaining a definitive laboratory result. approximately 245 to 300 μm and retain the egg mem-
brane as a sheath and are sometimes considered embry-
onic stages.
WUCHERERIA BANCROFTI AND
W. TIMORI Symptoms

The Wuchereria bancrofti organism is named for a Early in the course of the infection for Bancroftian fi-
German physician, Otto Wucherer (1820–1873), for a lariasis, fever and chills may be accompanied by lymph-
genus of filarial worms of the class Nematoda (round- adenitis and eosinophilia. Inflammation of lymphatic
worms) and is commonly found in the tropical regions. vessels (lymphangitis) or inflammation of the lymph
W. bancrofti is one of the most important causative or- nodes (lymphadenitis) may impact the lower extremities
ganisms for the condition of elephantiasis. Adults of this and sometimes the genitals and breasts. Lymph engorge-
species inhabit the lymphatic ducts and the larger nodes, ment by the worms may become fibrotic and distended,
plugging up this important component of the body for producing considerable pain. The hardening and thick-
draining tissue fluids and preventing swelling. Females ening of the skin follows with skin abscesses that drain to
give birth to “sheathed” microfilaria, and which remain the outside of the body.
in various internal organs of the host during the day. But
at night they circulate in the blood, when mosquitoes Life Cycle
most often feed on the blood of the infected humans. The
development of the W. bancrofti organisms continue for W. bancrofti was the first organism discovered to be
about two weeks, when the larvae become infective, and transmitted through an arthropod vector. The organism
are then passed to other uninfected humans through the carries out its life cycle in two different hosts. The defini-
bites of infected mosquitoes. tive host is a human and mosquitoes are the intermediate
host. Adult parasites live in the lymphatic system of the
Morphology human host, but the microfilariae found in the circulatory
system are transported throughout the host. W. bancrofti
The adult worms of these nematodes are long and is a periodic strain that exhibits nocturnal periodicity by
slender, and smooth with rounded ends. It has a short- residing in the deep veins during the day and during the
ened head (cephalic) region and its nuclei are arranged night they migrate to the peripheral circulation between
throughout the length of the body except for the caudal 10 pm and 2 am. Next, the microfilariae are transferred
or tail region. W. bancrofti is characterized by consider- into one of three common vectors which are of the mos-
able sexual dimorphism. Sexual dimorphism means that quito genera: Culex, Anopheles, and Aedes, depending on
there are systematic differences in the morphology be- the geographic location. Inside the mosquito vector, also
tween members of the two genders of the same species. known as the intermediate host, the microfilariae mature
Examples of sexual dimorphism include such factors as into motile larvae called juveniles.
color, size, and the presence or absence of parts of the The microfilariae obtained by the feeding mosquito
body. Some males of other species are even more dimor- penetrate the stomach wall and then migrate to the tho-
phic, with displays of ornamental horns or feathers used racic region of the mosquito. In the thoracic musculature