Wuchereria bancrofti

Wuchereria bancrofti
Microfilar of Wuchereria bancrofti, from a patient seen in Haiti. Thick blood smears stained with hematoxylin.
Scientific classification
Kingdom:	Animalia
Phylum:	Nematoda
Class:	Secernentea
Order:	Spirurida
Suborder:	Spirurina
Family:	Onchocercidae
Genus:	Wuchereria

Wuchereria bancrofti or Filaria, is a parasitic filarial nematode worm spread by a mosquito vector. It is one of the three parasites that cause lymphatic filariasis, an infection of the lymphatic system by filarial worms. It affects over 120 million people, primarily in Africa, South America, and other tropical and sub-tropical countries ^[1]. If the infection is left untreated it can develop into a chronic disease called Elephantiasis. Limited treatment modalities exist and no vaccines have been developed.

History

The effects of W. bancrofti were documented early in ancient text. Greek and Roman writers noted the similarities between the enlarged limbs and cracked skin of an infected individual to that of an elephant. Since Greek and Roman times, this condition has been commonly known as elephantiasis. However, this is a misnomer, since elephantiasis literally translates to “a disease caused by elephants.”

W. bancrofti was named after physician Otto Wucherer and parasitologist Joseph Bancroft, both of whom extensively studied filarial infections.

It is speculated that W. bancrofti was brought over to the New World by the slave trade ^[2]. Once it was introduced to the New World, this filarial worm disease persisted throughout the areas surrounding Charleston, South Carolina until its sudden disappearance in the 1920s ^[3].

Etiology

W. bancrofti exhibits considerable sexual dimorphism. The adult worm is long, slender, and smooth with rounded ends. It has a short cephalic region, dispersed nuclei throughout its body cavity, with no nuclei at the tail tip. The male worm is 40 mm long and 100 μm wide, and features a curved tail. In contrast, the female is 6 cm to 10 cm long and 300 μm wide, nearly three times larger in diameter than the male. Females are ovoviviparous and can produce thousands of juveniles known as microfilariae. Microfilariae of W. bancrofti retain the egg membrane as a sheath and are often considered advanced embryos.

Life Cycle

W. bancrofti carry out their life cycle in two hosts. Human beings serve as the definitive host and mosquitoes as their intermediate hosts. The adult parasites reside in the lymphatics of the human host. They are viviparous. The first stage larvae are known as microfilariae. The microfilariae are present in the circulation. The microfilariae migrate between the deep and the peripheral circulation. W. bancrofti is a periodic strain that exhibits nocturnal periodicity. During the day they are present in the deep veins and during the night they migrate to the peripheral circulation. Next, the microfilariae are transferred into a vector; the most common vectors are the mosquito species: Culex, Anopheles, Mansonia, and Aedes. Inside the mosquito vector, also known as the intermediate host, the microfilariae mature into motile larvae called juveniles. When the mosquito vector has its next blood meal, W. bancrofti is egested via the mosquito’s proboscis into the blood stream of the new human host. The larvae move through the Lymphatic system to regional lymph nodes, predominantly in the legs and genital area. The larvae develop into adult worms over the course of a year and reach sexual maturity in the afferent lymphatic vessels. After mating, the adult female worm can produce thousands of microfilariae that migrate into the bloodstream. A mosquito vector can bite the infected human host, ingest the microfilariae, and thus repeat the life cycle of W. bancrofti.

Epidemiology

W. bancrofti is responsible for 90% of lymphatic filariasis. Recently, it was estimated that there were 120 million worldwide cases of lymphatic filariasis. W. bancrofti largely affects areas across the broad equatorial belt (Africa, the Nile Delta, Turkey, India, the East Indies, Southeast Asia, Philippines, Oceanic Islands, Australia, and parts of South America.)

The mosquito vectors of W. bancrofti have a preference for human blood and it appears that humans are the only animals naturally infected with W. bancrofti. There is no reservoir host ^[4].

Biology

Life cycle of Wuchereria bancrofti

In humans, the adult W. bancrofti reside in the lymphatic ducts and are found mostly in the lymph glands of the afferent lymphatic channels in the lower part of the body. The microfilariae produced by the female worms have a membrane "sheath". This "sheath", along with the area in which the worms reside, makes identification of the type of species of microfilariae in humans easier to determine. The microfilariae are found mainly in the peripheral blood and can be found at peak amounts from 10p.m. to 2 a.m. The cause of this periodicity remains unknown but the advantages of the microfilariae being in the peripheral blood during these hours may ensure that the vector, the nighttime mosquito, will have a higher chance of transmitting them elsewhere. Khalil's theory, in which he maintained that the adult worms are mainly found in the lower limbs, and when microfilariae are born they gravitate in the receptaculum chyli and are stored there by daytime due to the upright position of man during waking hours. During sleep at night while in supine position the microfilariae go to the circulation and appear in the peripheral blood. Reversing sleeping hours appeared to agree with this explanation.

In the South Pacific, where W. bancrofti shows diurnal periodicity it is known as subperiodic.

Pathology

An Armigeres subalbatus mosquito ingesting a blood meal from a human finger

The pathogenesis of W. bancrofti infection is dependent on the immune system and inflammatory responses of the host. After infection, the worms will mature within 6–8 months, male and female worms will mate and then release the microfilariae. These microfilariae worms can be released for up to ten years.

1. Asymptomatic Phase -Usually consists of high microfilaremia infection and individuals show no symptoms of being infected. This occurs due to the cytokine IL-4 suppressing the activity of TH1 cells in our immune system. This can occur for years until the inflammatory reaction rises again. 2. Inflammatory (Acute) Phase -The antigens from the female adult worms elicit inflammatory responses. The worms in the lymph channels disrupt the flow of the lymph causing lymphedema. The individual will exhibit fever, chills, skin infections, painful lymph nodes, and tender skin of the lymphedematous extremity. These symptoms often lessen after 5–7 days. Other symptoms that may occur include: orchitis-inflammation of the testes, which is accompanied by painful immediate enlargement and epididymitis-which is the inflammation of the spermatic cord. 3. Obstructive (Chronic) Phase -marked by lymph varices, lymph scrotum, hydrocele, chyluria(lymph in urine), and elephantiasis. Microfilariae are not normally present in this phase. A key feature of this phase is scar formation from affected tissue areas. Other features include thickening of the skin and elephantiasis, which develops gradually with the attack of the lymphatic system. Elephantiasis affects men mainly in the legs, arms, and scrotum. In women, the legs and arms are affected.

Diagnosis

A blood smear is a simple and fairly accurate diagnostic tool, provided that the blood sample is taken during the period in the day when the juveniles are in the peripheral circulation ^[5]. Technicians analyzing the blood smear must be able to distinguish between W. bancrofti and other parasites potentially present.

A polymerase chain reaction test can also be performed to detect a minute fraction, as little as 1 pg, of filarial DNA ^[6].

Sometimes infected people do not have microfilariae in their blood. As a result, tests aimed to detect antigens from adult worms can be used.

Ultrasonography can also be used to detect the movements and noises caused by the movement of adult worms ^[7].

Dead, calcified worms can be detected by X-ray examinations.

Treatment

The severe symptoms caused by the parasite can be avoided by cleansing the skin, surgery, or the use of therapeutic drugs, such as Diethylcarbamazine(DEC), ivermectin, or albendazole. The drug of choice however, is DEC, which can eliminate the microfilariae from the blood and also kill the adult worms with a dosage of 6 mg/kg semiannually or annually ^[8]. A polytherapy treatment that includes ivermectin with DEC or albendazole is more effective than each drug alone. Protection is similar to that of other mosquito spread illnesses; one can use barriers both physical (a mosquito net), chemical (insect repellent), or mass chemotherapy′ as a method to control the spread of the disease.

Control

Prevention focuses on protecting against mosquito bites in endemic regions. Insect repellents and mosquito nets are useful manners in which to protect against mosquito bites. Public education efforts must also be made within the endemic areas of the world in order to successfully lower the prevalence W. bancrofti infection.

References

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