[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 9 The Filariae
TABLE 9-1
Parameter
Size range
Sheath
Arrangement of nuclei in tail
characteristic that helps distinguish it from other
sheathed microfilariae.
Adults
The adult Wuchereria bancrofti worms are white
and assume a threadlike appearance. The females
are typically larger than the males, measuring 40
to 100 mm and 20 to 40 mm, respectively.
Laboratory Diagnosis
Wuchereria bancrofti
Microfilaria: Typical
Characteristics at a Glance
Description
240-300 µm long
Present
Tip of tail free of nuclei
The examination of fresh Giemsa-stained blood
for W. bancrofti microfilariae serves as the laboratory
diagnostic method of choice. A more sensitive
method for microfilariae recovery involves
filtering heparinized blood through a special
filter, known as a nuclepore filter, and then staining
and examining the filter contents. The Knott
technique may also be used. Light infections may
be diagnosed by immersing 1 mL of blood in
10 mL of a 2% solution of formalin, which lyses
the red cells. Microscopic examination of the
stained sediment is then performed. In all these
methods, the optimal sample is collected at night
because this organism generally exhibits nocturnal
periodicity. Peak hours for specimen collection
are between 9:00 p.m. and 4:00 a.m. which
correlates with the appearance of its vector, the
mosquito. However, subperiodic organisms are
sometimes detected throughout the day. They are
more prevalent in the late afternoon. Serologic
tests, including antigen and antibody detection
and PCR assays, have been developed. The sensitivity
and specificity of these tests vary widely.
With all these techniques available, it is interesting
to note that in endemic areas, clinical
symptoms and patient history serve as the
primary means of diagnosis.
Life Cycle Notes
The Culex, Aedes, and Anopheles spp. of mosquitoes
serve as the intermediate hosts and
vectors of W. bancrofti. In the human host, the
adult worms take up residence in the lymphatics,
where they lay their microfilariae. These microfilariae
live in the blood and lymphatics.
Epidemiology
W. bancrofti may be found in the subtropical
and tropical areas of the world. These include
central Africa, the Nile Delta, India, Pakistan,
Thailand, the Arabian sea coast, the Philippines,
Japan, Korea, and China in the Eastern Hemisphere
and in Haiti, the Dominican Republic,
Costa Rica, and coastal Brazil in the Western
Hemisphere. Mosquito breeding occurs in contaminated
water in these areas. It is interesting
to note that indigenous inhabitants of the endemic
areas are at a greater risk of contracting W. bancrofti
than are non-indigenous individuals living
in these areas.
Clinical Symptoms
Asymptomatic. Adult patients, who as children
were most likely exposed to W. bancrofti,
may become infected and experience no symptoms.
Microfilariae are usually recovered in blood
samples from these patients. Eosinophilia may
also be noted in these samples. Physical examination
reveals only enlarged lymph nodes, particularly
in the inguinal region, the groin area.
Infections of this type are self-limiting because
the adult worms eventually die and there are no
signs of microfilariae being present. A patient may
undergo the entire process and not even know it.
Symptomatic Bancroftian Filariasis. A wide
variety of symptoms may be experienced by
patients infected with W. bancrofti. In general,
they develop a fever, chills, and eosinophilia. The
invasion of the parasite may result in the forma-