Wuchereria bancrofti
A. Classification:
Phylum: Nematoda
Family: filaridae
Genus and Species: Wuchereria bancrofti
Common name: (bancroftian filariasis)
Common Disease name – Filariasis or elephantiasis
B. Morphology:
Adult worms are long and slender with a smooth cuticle and bluntly rounded
ends. The head is slightly swollen and bears two circles of well-defined papillae. The
mouth is small and lacks a buccal cavity. The male is about 40 mm long and 100 um
wide. Its tail is fingerlike. The female is much longer and measures around 6 to 10 cm
long and 300 um wide. The vulva is near the level of the middle of the esophagus. After
mating, the female viviparously (a method of reproduction in which the embryo develops
inside the body of the female from which it gains nourishment) produces microfilaria
(L1).
The filariform juveniles or microfilaria are 1.4 to 2 mm long and in the L3 stage
are infective to the definitive host.
Figure 1: Male left, female right.
Figure 2 and Figure 3: Infective stage of Wuchereria bancrofti, or microfilaria (L1) as
seen in a blood smear. Note the sheath and characteristic V-spot near the head and tail.
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C. Life Cycle of Wuchereria bancrofti:
An individual first comes into contact with this parasite when a human is bitten by
a mosquito infected with L3 larvae. The L3 larvae penetrate into the bite wound and go
into the blood. They migrate to the nearest lymph gland and mature into adults. It takes
anywhere from 3 months to 1 year for the larvae to develop into a dioecious adult. The
adults can live 5-10 years in the host. The adult females then produce and birth sheathed
microfilariae that migrate into lymph and blood channels of the human host.
This parasitic infection is spread when an appropriate mosquito host ingests the
microfilariae during a blood meal. Most microfilariae have nocturnal periodicity. After
2 to 6 hours in the mosquito, the microfilariae shed their sheaths and migrate through the
cardiac portion of the misquitos midgut into the thoracic muscles of the mosquito. While
within their muscles, the microfilariae develop into L1 without reproduction and
eventually into third-stage infective larvae (L3) after approximately 2 weeks. The L3
moves into the mosquito's head and position themselves in the mouth parts or poboscis of
the mosquito so that when the mosquito takes another blood meal, the infective larvae
will be able to migrate into another human host.
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Culcicine (left) or anopheline (center) mosquitoes are the main vectors of the
nocturnally periodic forms of W. bancrofti, while day biting Aedes
polynesiensis (right) transmit the subperiodic form in various pacific islands
D. Geographic Distribution:
W. bancrofti is widely distributed in tropical or subtropical regions (41N. latitude
to 31S. latitude). Nocturnally periodic forms occur indigenously in almost every
tropical and subtropical country and are very widespread. W.bancrofti and B.malayi
infect some 128 milion people, and about 43 million have symptoms.
Its distribution includes China, India, Indonesia, Japan, Malaysia, Philippines, SouthEast Asia, Sri Lanka, Tropical Africa, Central and South America, Pacific Islands.
E. Pathology and Symptoms:
There are 4 recognized stages of this disease:
1. The incubation period of 3 to 12 months in which there are no symptoms.
2. The acute symptomatic stage in which some swelling of the extremities may
occur and this may be accompanied by pain, weakness of arms and legs,
headache, insomnia. Fever is usually not present.
3. There is a period of recovery, which is permanent if re-infection does not
occur.
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4. If there is continued re-infection the cycle repeats and elephantiasis may
result.
Elephantiasis is characterized by the gross enlargement of a limb or areas of the trunk or
head. There is an abnormal accumulation of watery fluid in the tissues causing severe
swelling. The skin usually develops a thickened, pebbly appearance and may become
ulcerated and darkened. Fever, chills and a general feeling of ill health may be present.
Image 1 and 2: Elephantiasis of the
leg due to Wuchereia bancrofti
The worms in the lymphatic system cause tissue changes which restrict normal flow of
lymph and result in swelling, fibrosis and eventually secondary infections in the affected
tissues. The adult worms can live for several years. The lower extremities and groin are
the parts most likely to be affected.
Image 3 and 4: Elephantiasis of the
groin due to Wuchereria bancrofti
Following infection with third stage larvae there is usually a period of vigorous immune
response to the invading larvae. Various pathologies associated with filarial infection
arise from immune reactions to their presence. The most pronounced of these is the
damage to the lymphatic vessels, which is mediated by the immune system's response to
the adult worms living in them. These immune responses are characterized by
inflammation of the affected area, which are usually extremities, and fever.
The microfilariae in the blood and lungs can also cause an IgE-mediated allergic response
which results in asthma-like symptoms. This condition is called "tropical eosinophilia".
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F. Diagnosis:
Until recently, diagnosing lymphatic filariasis had been extremely difficult,
because it required detection of the microflariae in the blood. The “nocturnal periodicity”
of the parasites in most parts of the world restricted their appearance in the blood to only
the hours around midnight. However, the new development of a very sensitive and
specific simple "card test" is used to detect circulating parasite antigens without the need
for laboratory facilities and using only finger-prick blood droplets taken anytime of the
day.
There are a number of other methods used for diagnosis: ultrasonography has made it
possible to visualize adult filarial worms within human lymphatic vessels. Lymphatic
Imaging may also be performed. This reveals the patency in the main vessels of the
lower limbs and may show that the para-aortic vessels are dilated. Also, the detection of
filarial antigen can be done in order to indicate if there is a presence of circulating filarial
antigen in the peripheral blood, with or without microfilariae. A urine examination and
microscopy can be performed and urine should be examined macroscopically for the
milky appearance of lymph and it can then concentrated for microfilariae.
Image 5: Ultrasonography of the scrotum of a 20-year old male from Brazil with bancroftian
filariasis. B-mode (left) and M-mode (right) show dilated lymphatics containing nematodes
G. Treatment of individual
Two main drugs that are used for treatment: diethylcarbamazepine (DEC) and
ivermectin. Diethylcarbamazine and ivermectin are effective against microfilariae and
adults. Ivermectin is a potent anthelmintic with a broad spectrum of activity against
nematodes. After being taken orally, it is well absorbed in the blood. It causes muscle
paralysis in parasites through affecting the ion-channels in cell membranes. Also, the
drug albendazole is usually taken concurrently with one of the two drugs listed above.
Usually, the primary goal of treating an entire affected community is to eliminate
microfilariae from the blood of infected individuals so that transmission of the infection
by the mosquito can be interrupted. Recent studies have shown that the use of single
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doses of albendazole with DEC or ivermectin, administered concurrently, (is 99%
effective in removing microfilariae from the blood for a full year after treatment.
Besides the use of drugs, there are other means of treatment such as surgery.
Large hydroceles and scrotal elephantiasis can be managed with surgical excisions.
Surgical removal of infected tissues is done in order to improve lymph flow. However,
the correction of gross limb elephantiasis with surgery is less successful. Also, pressure
bandages are applied to reduce swelling
H. Public Health Strategies
There is no vaccine for filariasis. There are prevention centers on mass treatment
with anti-filariasis drugs to prevent ingestion of larvae by mosquitoes, public health
action to control mosquitoes, and individual action to avoid mosquito bites.
Some tips on how to avoid being bitten by mosquitoes are: stay inside between
dusk and dark when mosquitoes are most active in their search for food. When outside
wear long pants and long-sleeved shirts, and spray exposed skin with an insect repellent.
The strategy of the Global Programme to Eliminate Lymphatic Filariasis has two
primary goals: first, to stop the spread of infection by interrupting transmission, and
second to alleviate the suffering of affected individuals.
To interrupt transmission of the infection, the entire at risk population
needs to be treated for a period long enough to ensure that levels of microfilariae in the
blood remain below those necessary to sustain transmission. For the yearly, single-dose,
2-drug regimens are being advocated. This is albendazole [400 mg] plus
diethylcarbamazine [6 mg/kg]; or albendazole [400 mg] plus ivermectin [200 mcg/kg]).
The recommended period is at least 5 years, corresponding to the reproductive lifespan of
the parasite. For the treatment regimen based on the use of DEC-fortified salt, the period
has been found to be 12 months of daily fortified salt intake.
To alleviate suffering and decrease the disability caused by LF disease, the
principal strategy focuses on decreasing secondary bacterial and fungal infection of limbs
or genitals whose lymphatic function has already been compromised by filarial infection.
It is this secondary infection that has recently been identified as the primary pathogenetic
determinant of worsening lymphoedema and elephantiasis. Operationally, a regimen of
hygiene to affected areas and the creation of hope and understanding among the patients
and their communities are the principal strategic approaches.
In 1998 the global healthcare company SmithKline Beecham collaborated with
the World Health Organization in elimination efforts. This included the donation of
numerous resources, especially albendazole, one of the mainstay drugs in the elimination
strategy, free of charge, for as long as necessary to ensure success of the elimination
programme. These donations, coupled with the recent decision by Merck and Co., Inc.,
to expand its ongoing Mectizan (ivermectin) Donation Programme to include treatment
of lymphatic filariasis and the creation of additional partnerships with other private,
public and international organizations, including the World Bank, have all further
strengthened the potential for success of these elimination efforts.
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Hillary Shields
Adam Wimer
Marjilla Seddiq
(Spring 2005)
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