[Elizabeth_Zeibig]_Clinical_Parasitology__A_Practi(z-lib.org)

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CHAPTER 9 The Filariae
microfilariae. The adults usually appear creamy
white and assume a threadlike appearance. Adult
males may measure from 20 to 500 mm in
length, which is often half that of typical adult
females. The microfilariae are slender and may
range in size from just under 150 µm to 350 µm
in length. The distribution of nuclei within the
tip of the tail, as well as the presence or absence
of a delicate transparent covering known as a
sheath, are the two key characteristics helpful in
speciating the microfilariae forms of these
organisms.
The basic life cycle is the same for all members
of the filariae. Only one to four infective larvae,
injected by an infected arthropod at the feeding
site, are required to initiate human infection.
Once inside the body, the larvae migrate to the
tissues, where they complete their development,
a process that may take up to 1 year. The resulting
adult worms may reside in the lymphatics,
subcutaneous tissue or internal body cavities.
Fertilized adult female worms lay live microfilariae,
which take up residence in the blood or
dermis. The microfilariae exit the body via a
blood meal by the appropriate arthropod vector.
The arthropod serves as the intermediate host for
the parasite. Larvae development into the infective
stage takes place in the insect host. Once the
infective stage is reached, the parasite is ready to
be transferred into an uninfected human, thus
initiating a new cycle.
The location of the adult worms and microfilariae
in the body and specific arthropod vector
all vary by individual species. The specifics of
these differences are addressed on an individual
basis.
Quick Quiz! 9-1
Speciation of the microfilariae can be done by recognition
of the distribution of nuclei in the tip of the tail
and the presence or absence of a delicate transparent
covering known as the: (Objective 9-1)
A. Flilariform
B. Cuticle
C. Sheath
D. Nucleus
LABORATORY DIAGNOSIS
Some species of filarial parasites exhibit periodicity,
a phenomenon whereby the parasites are
present in the bloodstream during a specific time
period; this feature is helpful in selecting the
appropriate time for specimen collection. There
is evidence to suggest that this periodicity, which
may be nocturnal (occurring at night), diurnal
(occurring during the day), or subperiodic (timing
of occurrences not clear-cut) is connected to the
corresponding vector’s feeding schedule. The
periodicity for each of the filarial parasites is
described on an individual basis.
The primary method of filarial diagnosis is
microscopic examination of the microfilariae in
a Giemsa-stained smear of blood or a tissue
scraping of an infected nodule. Whole blood
samples may also be collected. Processing of
these samples consists of lysing the cells followed
by concentrating and examining the sample for
microfilariae using the Knott technique. Although
a number of serologic tests have been developed
and are available, there is some concern as to
their specificity, and thus they are not universally
considered as viable diagnostic techniques. Representative
laboratory diagnostic methodologies
are found in Chapter 2 as well as in each individual
parasite discussion, as appropriate.
Quick Quiz! 9-2
What type of periodicity is exhibited if microfilariae
appear in the blood of an individual at 2:00 p.m. each
day? (Objective 9-1)
A. Nocturnal
B. Diurnal
C. Subperiodic
D. Biannual
PATHOGENESIS AND CLINICAL
SYMPTOMS
The clinical symptoms experienced by persons
infected with filarial organisms vary, depending
on the species. Such symptoms range from
involvement of the lymphatics, with subsequent
granulomatous lesions, eosinophilia, fever, chills,