What is Lymphatic Filariasis?
According to the Global Alliance to Eliminate Lymphatic Filariasis, lymphatic filariasis is "a parasitic disease caused by threadlike worms living in the human lymphatic system" (Reference 1). The "threadlike worm" that can cause lymphatic filariasis is a blood-dwelling filarial nematode, also called a roundworm, from the aschelminth phyla (Reference 2). There are 3 known species of the filarial nematode that can cause lymphatic filariasis: Wuchereria bancrofti, Brugia malayi and Brugia timori.
Wuchereria bancrofti, Brugia malayi and Brugia timori: What is the difference?
Although very similar in morphology, W. bancrofti, at left, has no nuclei in the tail, whereas B. malayi, center, and B. timori, not pictured, have nuclei that extend to the tip. Nuclei in the tip of the tail can be seen as swelled areas, highlighted by arrows in the right picture. (Images 2, 3, 4)
Differences between W. bancrofti, B. malayi and B. timori reside in epidemiology, vectors, reservoirs, symptoms, but mainly species morphology. These aspects of B. malayi and B. timori will be discussed on this page. However, W. bancrofti is the primary focus of this website, as it is the most common and well-known infectious agent of lymphatic filariasis. As a result, details associated with W. bancrofti will be included elsewhere in this website (see Contents of this ParaSite on left panel).
W. bancrofti is the most well-documented and widespread cause of lymphatic filariasis. It is more common to find elephantiasis in patients affected with W. bancrofti than those affected with the Brugian filariasis, although it can occur. Brugian filariasis also does not characteristically include symptoms associated with the genitalia or chyluria, while Bancroftian filariasis often expresses these symptoms in heavily infected patients. (Reference 3)
The morphology of W. bancrofti is the most significant differentiation from other species. The microfilariae, or larval stage of W. bancrofti, are sheathed, and range from approximately 245 to 300 µm. It can take several months for the microfilariae to sexually mature, and in the adult stage they can live for several years. As adults, the males range from 2.5 to 4 cm, and the females range from 5 to 10 cm. As a roundworm, the shape of the W. bancrofti name matches its descriptive classification. One end of the round body is blunt, while the other is pointed. Nuclei do not appear at the end of the tail, which is a major difference from other microfilariae. Both Bancroftian and Brugian filariae lack a digestive system, instead absorbing nutrients from their hosts. (Reference 4).
The distribution of B. malayi is very similar to that of W. bancrofti. However, cases are concentrated in Asia, including South China, India, Indonesia, Thailand, Vietnam, Malaysia, the Philippines, and South Korea. Other differences between B. malayi and W. bancrofti is the vector and reservoir. While W. bancrofti is transmitted mainly by Anopheles, B. malayi is transmitted by Mansonia mosquitos. Since these mosquitos feed primarily during the day, B. malayi microfilaria can be found in the blood during the day, while microfilaria of W. bancrofti is found at high levels at night. The time variation in microfilarial levels is known as periodicity. Additionally, W. bancrofti has no known animal reservoir, while B. malayi has been found in Macaques, leaf monkeys, cats and civet cats. In Indonesia, human cases have been transmitted from animals, which poses a particular challenge to the control of B. malayi.
Although most of the symptoms of Brugian filariasis are identical to Bancroftian filariasis, there are some differences in clinical presentation. First, Brugian filariasis tends to have a higher occurrence of ulcerated nodules, and as mentioned above, rarely involve genital swelling or chyluria. In addition, elephantiasis is experienced alomost explicitly in the lower part of the limbs (below the knee or below the elbow). Treatment and prevention, with the exceptional control problem associated with the animal reservoirs of Brugian filariasis, is the same as Bancroftian filariasis.
The morphology, like that of W. bancrofti, is the most reliable way to differentiate species type. Generally, microfilariae range from 200 to 275 µm. Adult size is not well-known, as very few have actually been found. Microfilariae of B. malayi are sheathed like W. bancrofti, and have a very similar shape. However, the nuclei extends nearly to the tip of the tail, a characteristic not shared with W. bancrofti. (Reference 5)
B. timori is the least common, and therefore least studied species of filaria known to cause lymphatic filariasis. This species was reported on the island of Timor in 1964, and has since been found in other islands in Indonesia. In regards to vectors, periodicity and reservoirs, B. timori is more similar to W. bancrofti than to B. malayi. Transmission of B. timori is by the Anopheles barbirostris, a vector that feeds at night. As a result, high levels of B. timori microfilariae are found in the blood at night. B. timori also has no known animal reservoir.
In regards to symptoms and morphology, B. timori resembles B. malayi more than W. bancrofti. Like B. malayi, symptoms associated with B. timori are similar to W. bancrofti, with elephantiasis only expressed in the lower part of the limbs. Microfilaria of both B. timori and B. malayi have nuclei that extends to the tip of the tail, which is not true of W. bancrofti. However, at approximately 310 µm, B. timori microfilaria are slightly larger than that of B. malayi. (Reference 6).