[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 9 The Filariae
TABLE 9-4
Parameter
Size range
Sheath
Arrangement of the nuclei
in tail
microfilariae is that it does not possess a sheath.
The body contains numerous nuclei that extend
from the rounded anterior end, almost to but not
including the tip of the somewhat pointed tail.
Another distinction that helps in its identification
is the location of the microfilariae. Those of O.
volvulus are found in the subcutaneous tissue
and not blood specimens.
Adults. The adult O. volvulus worms are thin
and wirelike in appearance. They typically coil
up in knots inside infected skin nodules. The
adult females may measure up to 500 mm in
length, whereas the adult males are 25 to 50 mm
long.
Laboratory Diagnosis
Onchocerca volvulus
Microfilaria: Typical
Characteristics at a Glance
Description
150-355 µm long
Absent
Do not extend to tip
of tail
Multiple Giemsa-stained slides of tissue biopsies,
known as skin snips, collected from suspected
infected areas are the specimens of choice for the
recovery of O. volvulus microfilariae. The skin
snips should be obtained with as little blood as
possible. The reason is to avoid contamination
of the sample with other species of microfilariae
that may be present in the blood. Adult worms
may be recovered from infected nodules. Organisms
residing in the eye are best seen by ophthalmologic
examination using a slit lamp. As with
L. loa, patient history, particularly travel and
residence, as well as the presence of eosinophilia
and ocular discomfort, may be helpful in the
diagnosis of O. volvulus. Serologic methods are
also available. PCR can successfully detect low
level infections.
Life Cycle Notes
The blackfly genus Simulium is responsible for
transmitting O. volvulus. On entrance into the
human host and following maturation, the resulting
adult worms encapsulate in subcutaneous
fibrous tumors. It is here that the adults become
coiled and microfilariae emerge. The microfilariae
may migrate throughout infected nodules,
subcutaneous tissues, and skin and into the
eye. The microfilariae are rarely seen in the
peripheral blood making this a poor specimen
for diagnosis.
Epidemiology
O. volvulus is distributed primarily in equatorial
Africa and Central America. Specific endemic
areas include East Africa, Zaire, Angola, parts of
Mexico, Colombia, Brazil, and portions of Venezuela.
All these areas harbor the vector, the
Simulium blackfly. This insect breeds in running
water, particularly along streams and rivers.
Persons entering these areas are at risk for becoming
infected via a vector bite. There are known
animal reservoirs.
Clinical Symptoms
Onchocerciasis: River Blindness. Infection with
O. volvulus usually results in a chronic and nonfatal
condition. Patients typically experience
localized symptoms caused by the development
of infected nodules. Some patients may also
suffer severe allergic reactions to the presence of
the microfilariae. Scratching leads to secondary
bacterial infections. When the eye becomes
involved, lesions, due to the body’s reaction to
the microfilariae, may lead to blindness. Blindness
has proven to be a significant complication
for many infected adults. The specific symptoms
associated with O. volvulus infection, particularly
changes in overall skin appearance, such as
loss of elasticity, and location of nodules on the
body, vary based on whether the patient contracted
the parasite in the Eastern or Western
Hemisphere.