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Blood (Intracellular) and Other Tissue Protozoa 97

bancrofti, Brugia malayi, and B. timori. Other species on the tissue types each of the species typically in-
of filarial that infect the subcutaneous tissues of the body vades. Organisms infect humans and other vertebrates
and those that invade the serous cavities will also be through the bites of mosquitoes, gnats, and flies, de-
presented. pending upon the geographic location where each is
found. Human filarial nematode worms have a compli-
cated life cycle, which primarily consists of five stages.
FILARIAL WORMS AND After the male and female worm mate, the female gives
LYMPHATIC FILARIASIS birth to live microfilariae by the thousands. The micro-
(ELEPHANTIASIS) filariae are taken up by the vector insect (intermediate
host) during a blood meal. In the intermediate host, the
A discussion of the basics of filariform larvae and the dis- microfilariae molt and develop into third-stage (infec-
eases they produce and the impact of these organisms on tive) larvae. Upon taking another blood meal, the vector
humans is provided first in this section. It is followed by insect injects the infectious larvae into the dermis layer
specific organisms and details of those particular organ- of our skin. After approximately one year the larvae
isms and identification of them. A number of significant molt through two more stages, maturing into the adult
pathogens include small “worms” called microfilaria. worm. Adult worms can survive in the lymphatic sys-
The superfamily (a group that falls between an order tem for 5 to 15 years.
and a family) Filarioidea contains a number of nematodal Infected individuals do not always exhibit microfi-
organisms that reside in tissues of the human body. Some laria in their blood. When microfilaria are demonstrated
species of microfilaria are are found only in animals, but in stained blood smears, the victim of the infection is said
three members of superfamily Filarioidea are commonly to be microfilaremic, a term indicating a condition of
found in humans and will be presented in this section. the blood where the parasite can be visualized. A term,
Filariasis is the general name for a group of tropi- occult filariasis, may be used when an individual dem-
cal diseases caused by various threadlike parasitic round onstrates the signs and symptoms of being infected by a
worms (nematodes) and their larvae. The larvae trans- microfilarial organism but no organisms are found dur-
mit the disease to humans most often through mosquito ing an examination of a blood sample. In some cases, as
bites, but the disease may also be less frequently trans- serological test for antigens (proteins) of the organism
mitted by sand gnats, tabanid flies, blackflies, and a few are found in the blood, although no visible organisms are
other miscellaneous insects. An outbreak of filariasis may observed.
be characterized by chills and fever, headache, and skin The organisms migrate throughout the body, and
lesions in the early stages. If left untreated or if treatment infect a variety of tissue types. Wuchereria bancrofti,
is delayed, the disease can progress to a state of gross limb Brugia timori, and Brugia malayi are the most com-
enlargement and often with enlargement of the genitalia. mon organisms causing filariasis in humans. Filariasis
This marked condition is commonly called elephantia- is classified into three distinct types according to the
sis and when the tissue injury has occurred, it cannot be parts of the body that become infected. Lymphatic fi-
corrected with treatment with medications. But surgery lariasis affects the circulatory system that moves tissue
can be performed to remove some of the misshapen and fluid and immune cells (lymphatic system) and may
massive amounts of skin and other tissues generated by cause gross enlargement of areas of the body due to
the disease in the affected areas. the inability to reabsorb tissue fluids into the occlude
blood vessels. Subcutaneous filariasis infects the areas
Life Cycle beneath the skin and in some cases the whites of the
eye. The third type of filariasis is that of infections of
The major species of causative organisms of filariasis in the serous cavities, where the filaria infect body cavi-
humans occupy certain niches of the body, which lead ties but does not cause serious disease. Several differ-
to their discovery and initial attempts at identification. ent types of worms, or filarial, can be responsible for
The life cycles for each of the species that cause filari- each of these types of filariasis. But the most common
asis have slight variations from each other. Table 4-1 species that infect humans include those found in the
shows the general division of the organisms based following table (Table 4-5).