Parasites In Humans
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Wuchereria Bancrofti - Lymphatic Filariasis - Elephantiasis

Lymphatic filariasis is a parasitic disease caused by thread-like worms called Wuchereria bancrofti. The parasite is carried from person to person by mosquitoes. 120 million people are infected in subtropical and tropical Asia (mostly in India), Africa, the Pacific and the Americas (mostly in Brazil, Haiti, Guyana and the Dominican Republic). Lymphatic filariasis is the leading cause of permanent disability worldwide. Out of the 120 million more than 30 % are severely incapacitated by the disease. Over one billion people in over 80 countries are at risk of getting infected.

The life cycle of Wuchereria bancrofti starts, when a male and a female mate inside lymphatic vessels of an infected human. The female releases thousands of microfilariae (prelarval eggs) into the bloodstream. When the host is awake, the microfilariae tend to stay in deep blood vessels. During the sleep they travel near the surface in peripheral blood vessels. This behaviour enables them to get ingested by the night biting mosquito. When ingested by the mosquito, the microfilariae migrate through the wall of the proventriculus and cardiac portion of the midgut eventually reaching the thoracic muscles. Within 1–2 weeks they mature into first-stage larvae and eventually into infective third-stage larvae which migrate through the hemocoel to the mosquito's prosbocis. When the mosquito bites another person, the larvae are injected into the human skin. They migrate to the lymph vessels and mature into adults within six months. Adult females can live up to seven years.

Wuchereria bancrofti life cycle

Repeated mosquito bites during several months are usually needed to develop lymphatic filariasis. In some cases lymphedema (swollen tissue caused by obstruction of the lymph fluid) may develop within six months and elephantiasis within a year. Citizens of tropical and subtropical areas have the biggest risk whereas tourists have a very low risk.

Wuchereria bancrofti infection is usually asymptomatic. Some people can develop lymphedema, swelling, which is prevalent in the legs, but sometimes also in the arms, genitalia and breasts. The swelling and decreased flow of the lymph fluid will expose the body to skin and lymph system infections. Over time the disease causes thickening and hardening of the skin, a condition called elephantiasis which can be fatal. Filarial infection might also cause pulmonary tropical eosinophilia syndrome, which is mostly found in patients living in Asia. Pulmonary tropical eosinophilia syndrome can cause: cough, shortness of breath, and wheezing. In addition to eosinophilia there might be high levels of IgE (Immunoglobulin E) and antifilarial antibodies.

Diagnosis for lymphatic filariasis is traditionally done from a blood sample by microscopic examination. The sample has to be taken during the night to ensure the microfilariae are present in the bloodstream. The blood can also be studied to check for the presence of antibodies (antifilarial IgG4) that the human body develops to fight against antigens excreted by adult female Wuchereria bancrofti worms. A new method of a highly sensitive "card test" has been developed to detect antigens without laboratory equipment using finger-prick blood droplets taken anytime of the day. Molecular diagnosis by polymerase chain reaction (PCR) is possible, too.

Treatment for infected patients is usually done using a drug called diethylcarbamazine (DEC). The medicine kills the microfilariae in the bloodstream and sometimes adult worms in the lymph vessels. It has some side effects which include: dizziness, fever, headache, nausea and muscle and joint pain. DEC should only be used, if Wuchereria bancrofti has been identified. This is because most people with lymphedema are not infected with parasites. DEC can worsen Onchocerciasis (an eye disease caused by Onchocerca volvulus) and can cause encephalopathy (brain disease) and death in people who are infected with Loa loa. Another drug, ivermectin, can also be used, although it only kills microfilariae. In some cases lymphedema can be prevented from getting worse by exercising the swollen leg or arm to improve the lymph flow. The swollen skin is vulnerable to bacterial infections because immune defences cannot work properly due to the impaired flow of fluids. That is why the skin must be kept clean.

According to some new studies Wolbachia bacteria are in symbiosis with Wuchereria bancrofti. The bacteria live inside the worm. If the bacteria are killed with antibiotics, Wuchereria bancrofti dies, too.

To prevent new infections, avoid infective mosquitoes between dusk and dawn (the time when they mostly feed). A mosquito net can be applied all around your bed. Mosquito repellent applied on your skin or the use of long trousers and sleeves might keep the mosquitoes away. Mass treatments are given to whole communities in some endemic countries. Programs to eliminate lymphatic filariasis in more than forty countries are decreasing the risk of infection.

Wuchereria bancrofti adults - male on the left, female on the right An Adult female Wuchereria bancrofti is about 80–100 mm long and 0.24–0.30 mm in diameter, whereas a male is about 40 mm long and 0.1 mm in diameter.

 

 

  

A microfilaria is about 240–300 µm (micrometers) long and 7.5–10 µm thick. It is sheathed and has nocturnal periodicity, except the South Pacific microfilaria which does not have marked periodicity. It has a gently curved body, and a tail that is tapered to a point. The nuclear column (the cells that constitute its body) is loosely packed. The cells can be seen individually under a microscope and do not extend to the tip of the tail.

mosquito A mosquito is the intermediate host and carrier. The most common vectors/carriers are:

  • in Africa: Anopheles species
  • in the Americas: Culex quinquefasciatus
  • in the Pacific and in Asia: Mansonia and Aedes species.

Also check out the Wuchereria bancrofti pictures and videos.

Wuchereria bancrofti Quiz

To reveal the answer you need to click the correct option.

What is the life cycle route inside mosquitoes?

+ A) L1 larva --> L3 larva
+ B) L1 larva --> L3 larva --> Microfilaria
+ C) Microfilaria --> L1 larva --> L3 larva

What is the life cycle route inside humans?

+ A) L3 larva --> Adult --> Microfilaria
+ B) L1 larva --> L3 larva --> Adult --> Microfilaria
+ C) Microfilaria --> L1 larva --> L3 larva --> Adult --> Microfilaria

What is elephantiasis characterized by?

+ A) Thickening and hardening of the skin
+ B) Increased body size due to masses of worms all over the body, especially in the nose
+ C) Eosinophilia, heart failure and breathing difficulty

What causes elephantiasis?

+ A) Decrease of blood flow due to worms inside blood vessels
+ B) Blockage of lymph fluid due to worms inside lymph vessels
+ C) Masses of microfilaria in skin tissue