[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 9 The Filariae
Continuous row of posterior nuclei
Sheath
Size range: 248-300 m long
FIGURE 9-6 Loa loa microfilaria.
TABLE 9-3
Parameter
Size range
Sheath
Arrangement of nuclei in
tail
(Fig. 9-6; Table 9-3). Nuclei fill the organism and
are continuous to the tip of the pointed tail. This
characteristic helps distinguish it from the other
sheathed microfilariae.
Adults. The adult L. loa worms are typically
white in color and exhibit a cylindrical threadlike
appearance. The adult females are large, relatively
speaking, measuring 38 to 72 mm in
length. The adult males are significantly smaller,
measuring 28 to 35 mm in length.
Laboratory Diagnosis
Loa loa Microfilaria: Typical
Characteristics at a Glance
Description
248-300 µm long
Present
Distinct continuous row
of nuclei; extend to
tip of tail
The specimen of choice for the recovery of L. loa
microfilariae is Giemsa-stained blood. The Knott
technique may also be used. These samples yield
the best recovery rate when collected during the
midday hours, between 10:15 AM and 2:15 PM,
because this organism displays diurnal periodicity.
The migrating adult worms may be extracted
from a variety of body locations, including the
eye. Residence in an endemic area and the presence
of eosinophilia and Calabar or transcient
subcutaneous swellings also aid in diagnosis. As
with the other microfilariae discussed, serologic
testing is also available.
Life Cycle Notes
Human infection of L. loa is initiated by the bite
of an infected Chrysops fly. Adult worms take up
residence and multiply throughout the subcutaneous
tissues. The microfilariae are present in the
blood but not until years after the initial infection
making the diagnosis more difficult.
Epidemiology
As with all the filarial organisms, the endemic
regions of infections correlate with the areas
where the vector flourishes. In the case of L. loa,
the Chrysops fly inhabits Africa especially the
rainforest belt region. It is estimated that infection
rates may be over 70% in the areas in which
a large vector population exists. A less than 10%
infection rate occurs in regions in which minimal
numbers of vectors reside.
Clinical Symptoms
Loiasis. After the initial bite, individuals
infected with L. loa may experience pruritus or
itchiness and localized pain. Development of
Calabar swellings at the site of initial discomfort
usually follows. This localized subcutaneous
edema may occur anywhere on the body and is
thought to result from the migration and death
of the microfilariae. It is interesting to note that
the presence of circulating adult worms in the
subcutaneous tissues usually causes no discomfort.
The adult worms may only be noticeable
when seen migrating under the conjunctiva of the
eye or crossing under the skin of the bridge of
the nose.