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Last updated on April 16, 2014 at 21:24 EDT

Wuchereria bancrofti

Wuchereria bancrofti is a species of roundworm in the Nematoda phylum. This species is spread through a mosquito vector, which means that it is transferred through mosquitos. This species infects over 120 million people in South America, Africa, and other tropical and subtropical areas. It is one of three species of parasitic worm that can cause lymphatic filariasis, which can lead to elephantiasis. The disease is wrongfully named, because the term translates to “a disease caused by elephants.”  It has been known since ancient times, with notes in Roman and Greek texts. This species was named after Joseph Bancroft, a parasitologist, and a physician named Otto Wucherer. Both experts studied filarial infections caused by this species and others. It is thought that this worm was transferred into the New World during slave trade. It survived in the coastal areas around Charleston, South Carolina until the 1920’s, after which it suddenly disappeared.

Wuchereria bancrofti can reach varying lengths depending upon the sex, with females growing to be between 2.3 inches and males reaching 1.5. This species display a sexual dimorphism in both length and diameter, with males reaching one hundred micrometers and females reaching three hundred micrometers. Females can produce many thousands of advanced eggs that retain the egg membrane as a protective cover.

Wuchereria bancrofti will live in two hosts during its lifetime. The main hosts, known as definitive hosts, are human beings while the intermediate hosts, or hosts the worms spread through, are mosquitos. In the first stage of this species’ life, known as microfilariae, larvae will live in the circulatory system of humans, using deep veins during the day and peripheral veins during the night. The second stage of life occurs within the intermediate hosts, typically mosquitos of the genera Anopheles, Mansonia, Aedes, and Culex, where they will develop into juveniles known as motile larvae. Once an infected mosquito ingests the blood of a human, it passes the second stage larvae into that human through the blood. These young worms will travel through the lymphatic system to lymph nodes, commonly residing in the genital area and the legs. The juveniles remain in this area for about a year and mature into adults. Adults are able to reproduce once they reach afferent lymphatic vessels, where they will breed. Females can produce thousands of microfilariae larvae that travel into the blood stream of the host, restarting the lifecycle of this species.

Wuchereria bancrofti most often occurs along the equatorial belt, in regions like India, Africa, Turkey, the Nile Delta, and the Philippines, among other areas. Humans have been found to be the only species that is naturally infected by this parasite, and it is the cause of about ninety-five percent of lymphatic filariasis cases.

Because of the locations preferred in the human body of Wuchereria bancrofti, and the sheath like covering on the eggs and larvae, it can be relatively easy to distinguish an infection.  Optimal times of locating this species in the human host occur between the hours of 10 P.M. and 2 A.M. Although the cause of this certain timing is unknown, it is thought to increase the parasite’s chances of being ingested by a mosquito. It also coincides with the human host’s sleeping habits, with studies showing that even if a human is abnormally awake during the nighttime, the mosquito will follow that pattern by moving into deeper veins. Common symptoms that humans display includes physiological changes while sleeping like lowered adrenal activity, oxygen tension, and body temperature, as well as physical changes like increased carbon dioxide tension. All of these factors involve rhythmic patterns, so if these normal patterns change, they are a good indicator of an infection.

The progression of a W. bancrofti infection depends upon the inflammatory responses and immune system of each host. There are three stages of an infection that hosts can display. The first stage, known as asymptomatic phase, is hard to detect because hosts show little signs of infection, despite having a high microfilaremia infection.

The second stage of infection, known as the inflammatory or acute phase, occurs when the female’s antigens cause inflammatory responses that are out of the ordinary. During this stage, the worms move through the lymph channels, disturbing the flow of the lymph and causing lymphedema in the host. Symptoms of an infection will begin to show in this stage, including painful lymph nodes, fever, skin infection, chills, and tender skin around the affected lymph areas. Male hosts can display other symptoms, like inflammation in the spermatic cord, known as epididymitis as well as inflammation of the testes. After to five to seven days of this stage, most symptoms will disappear.

The third stage of a W. bancrofti infection is known as the obstructive or chronic phase. Unlike the first stage of infection, there are no Microfilariae in this stage. Hosts display a number of symptoms in this stage include elephantiasis, hydrocele, where fluid builds up in body cavities, lymph varices, and chyluria, where lymph matter appears in the urine. Scars are common in the lymph areas affected.

It is relatively easy for experts to diagnose a W. bancrofti infection by using a blood smear test, but only if the sample is taken when the young worms are moving through peripheral veins. Other methods for detecting this species include ultrasonography, which studies the movements and noises produced by adult worms, a polymerase chain reaction test, which detects small amounts of DNA, and x-ray tests, which can show dead and calcified remains of this species.

Treating hosts of W. bancrofti infections typically consists of two types of treatments for both the symptoms of the infection and the worms. Symptoms can be treated by surgery, cleaning the skin, or by medicines such as diethylcarbamazine (DEC), albendazole, and ivermectin. DEC is the most favored drug in treating these worms, because it kills adult worms and the microfilariae that move through the blood stream. It has been found that using two drugs, like ivermectin and DEC, can produce a greater healing affect than one drug alone.

Protecting against mosquitos is often the best way to prevent an infection of W. bancrofti and other parasitic species that cause disease. Using a mosquito net alongside chemicals to kill mosquitos is the best form of protection, but mass chemotherapy can also be used. Using this method across an area that is home to infected mosquitos decreases the chances of spreading the worms. Education in areas where the worm is native is vital to preventing infections. The group WHO is responsible for eradicating filarisis by administering antifilarial drugs to large groups of people for at least five years. This method has proven to be successful in Sri Lanka, an area where the disease has now been eradicated.

Image Caption: Wuchereria bancrofti Microfilaria of Wuchereria bancrofti, from a patient seen in Haiti. Thick blood smears stained with hematoxylin. The microfilaria is sheathed, its body is gently curved, and the tail is tapered to a point. The nuclear column (the cells that constitute the body of the microfilaria) is loosely packed, the cells can be visualized individually and do not extend to the tip of the tail. The sheath is slightly stained with hematoxylin. Credit: Wikipedia

Wuchereria bancrofti