Filariasis (or philariasis ) is a parasitic disease
caused by an infection with roundworms of the
Filarioidea type. [1] These are spread by
blood-
feeding black flies and mosquitoes . This
disease belongs to the group of diseases
called helminthiasis .
Eight known filarial nematodes use humans as
their definitive hosts. These are divided into
three groups according to the niche within the
body they occupy:
Lymphatic filariasis is caused by the worms
Wuchereria bancrofti , Brugia malayi, and Brugia
timori . These worms occupy the lymphatic
system, including the lymph nodes; in chronic
cases, these worms lead to the disease
elephantiasis .
Subcutaneous filariasis is caused by Loa loa
(the eye worm), Mansonella streptocerca , and
Onchocerca volvulus. These worms occupy the
subcutaneous layer of the skin, in the fat layer.
L. loa causes Loa loa filariasis, while O.
volvulus causes river blindness .
Serous cavity filariasis is caused by the
worms Mansonella perstans and Mansonella
ozzardi , which occupy the serous cavity of the
abdomen .
The adult worms, which usually stay in one
tissue, release early larval forms known as
microfilariae into the host's bloodstream.
These circulating microfilariae can be taken up
with a blood meal by the arthropod vector; in
the vector, they develop into infective larvae
that can be transmitted to a new host.
Individuals infected by filarial worms may be
described as either "microfilaraemic" or
"amicrofilaraemic", depending on whether
microfilariae can be found in their peripheral
blood. Filariasis is diagnosed in microfilaraemic
cases primarily through direct observation of
microfilariae in the peripheral blood. Occult
filariasis is diagnosed in amicrofilaraemic
cases based on clinical observations and, in
some cases, by finding a circulating antigen in
the blood.
Signs and symptoms
The most spectacular symptom of lymphatic
filariasis is elephantiasis—edema with
thickening of the skin and underlying tissues—
which was the first disease discovered to be
transmitted by mosquito bites. [2] Elephantiasis
results when the parasites lodge in the
lymphatic system.
Elephantiasis affects mainly the lower
extremities, while the ears, mucous
membranes, and amputation stumps are
affected less frequently. However, different
species of filarial worms tend to affect
different parts of the body; Wuchereria
bancrofti can affect the legs, arms, vulva ,
breasts, and scrotum (causing hydrocele
formation), while Brugia timori rarely affects
the genitals. [ citation needed ] Those who
develop the chronic stages of elephantiasis are
usually amicrofilaraemic, and often have
adverse immunological reactions to the
microfilariae, as well as the adult worms. [2]
The subcutaneous worms present with rashes,
urticarial papules , and arthritis , as well as
hyper- and hypopigmentation macules .
Onchocerca volvulus manifests itself in the
eyes, causing "river
blindness" ( onchocerciasis ), one of the leading
causes of blindness in the
world. [ citation needed ] Serous cavity filariasis
presents with symptoms similar to
subcutaneous filariasis, in addition to
abdominal pain, because these worms are also
deep-tissue dwellers.
Cause
Human filarial nematode worms have
complicated lifecycles, which primarily consists
of five stages. After the male and female
worms mate, the female gives birth to live
microfilariae by the thousands. The
microfilariae are taken up by the vector insect
(intermediate host) during a blood meal. In the
intermediate host, the microfilariae molt and
develop into third-stage (infective) larvae.
Upon taking another blood meal, the vector
insect injects the infectious larvae into the
dermis layer of the skin. After about one year,
the larvae molt through two more stages,
maturing into the adult worms.
Diagnosis
Filariasis is usually diagnosed by identifying
microfilariae on Giemsa stained , thin and thick
blood film smears, using the "gold standard"
known as the finger prick test. The finger prick
test draws blood from the capillaries of the
finger tip; larger veins can be used for blood
extraction, but strict windows of the time of
day must be observed. Blood must be drawn
at appropriate times, which reflect the feeding
activities of the vector insects. Examples are
W. bancrofti , whose vector is a mosquito;
night is the preferred time for blood collection.
Loa loa's vector is the deer fly; daytime
collection is preferred. This method of
diagnosis is only relevant to microfilariae that
use the blood as transport from the lungs to
the skin. Some filarial worms, such as M.
streptocerca and O. volvulus, produce
microfilarae that do not use the blood; they
reside in the skin only. For these worms,
diagnosis relies upon skin snips, and can be
carried out at any time.
Concentration methods
Various concentration methods are applied:
membrane filter, Knott's concentration method,
and sedimentation technique.
Polymerase chain reaction (PCR) and antigenic
assays, which detect circulating filarial
antigens, are also available for making the
diagnosis. The latter are particularly useful in
amicrofilaraemic cases. Spot tests for antigen
[3] are far more sensitive, and allow the test to
be done any time, rather in the late hours.
Lymph node aspirate and chylus fluid may also
yield microfilariae. Medical imaging, such as
CT or MRI, may reveal "filarial dance sign" in
chylus fluid; X-ray tests can show calcified
adult worms in lymphatics. The DEC
provocation test is performed to obtain
satisfying numbers of parasites in daytime
samples. Xenodiagnosis is now obsolete, and
eosinophilia is a nonspecific primary sign.
Treatment
The recommended treatment for people outside
the United States is albendazole (a broad-
spectrum anthelmintic) combined with
ivermectin .[4][5] A combination of
diethylcarbamazine and albendazole is also
effective. [4] All of these treatments are
microfilaricides; they have no effect on the
adult worms. Different trials were made to use
the known drug at its maximum capacity in
absence of new drugs. In a study from India, it
was shown that a formulation of albendazole
had better anti-filarial efficacy than
albendazole itself.[6]
In 2003, the common antibiotic doxycycline
was suggested for treating elephantiasis. [7]
Filarial parasites have symbiotic bacteria in the
genus Wolbachia , which live inside the worm
and seem to play a major role in both its
reproduction and the development of the
disease. Clinical trials in June 2005 by the
Liverpool School of Tropical Medicine reported
an eight-week course almost completely
eliminated microfilaraemia.
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