The Morphology and Biology of Wuchereria bancrofti

Introduction

Wuchereria bancrofti is the filarial worm that causes the disease called lymphatic filariasis, or elephantiasis. This disease causes enlargement of body organs, such as the scrotum and legs. This article discusses the general morphology and biology of the parasite.

Morphology

Adult Wuchereria bancrofti worms are slender and long, with bluntly rounded ends and smooth cuticles. Their head is slightly enlarged and bears two circles of distinct papillae. Their mouth is little, and the buccal capsule is absent.

The females are slightly longer and wider than the males, with a height and width of 6-10 cm and 300 micrometers, respectively. The males are 4 cm long and 100 micrometers wide. The vulva of the female is located near the level of the middle of their esophagus (Baker et al. 1988; Dawes 2005; Levine 1980).

Biology

W. bancrofti adults live in the lymphatic ducts of humans. They usually thrive in the ducts emanating from major lymph glands in the lower half of the human body. They rarely attack veins. Females produce thousands of juveniles called microfilariae, or advanced embryos. These microfilariae are covered with a delicate and close-fitting egg membrane sheath for protection. The sheath can be located at the anterior or at the posterior end of the filarial worm. The presence, absence, or location of the membrane sheath helps parasitologists to identify filarial worms found in the human body.

The females release microfilariae into the lymph. Some of these microfilariae may travel into adjacent tissues, but most are swept into the bloodstream through the thoracic duct. What is very interesting about the microfilariae is that their increase and decrease in number in the blood vessels follow a daytime pattern or periodicity. They are concentrated in the blood vessels of peripheral tissues of the body from 10 pm to 8 am, when people are sleeping deeply; after this period of time, the number of filarial worms in the blood vessels dramatically decreases. During the daytime, the worms thrive in deep tissues, and only few of them are found in blood vessels of peripheral tissues (Levine 1980). Biologists speculate that the increase in number of W. bancrofti in the blood vessels of periphera tissues during sleeping time is due to factors like arterial oxygen level and temperature. The concentration of W. bancrofti in the blood vessels coincides with the feeding schedule of the vector mosquitoes. This event favors the transmission of the parasites in the mosquitoes, to preserve their population and be able to infect other victims. The worms could not anticipate feeding mosquitoes during the day, so they only thrive in blood vessels of surface tissues during the night to ensure that they would be successfully transmitted in the vectors. This behavior of W. bancrofti has been proven experimentally; the sleeping schedule of a patient has been reversed (became diurnal), and surprisingly the filarial worms conformed to such change in the sleeping schedule.

Vector mosquitoes ingest microfilariae along with their blood meal. Once inside the insect’s body, they penetrate the stomach and may or may not lose their sheaths. The microfilariae will travel from the stomach towards the thoracic muscles of the insects for about two hours. In those muscles, they develop into the first stage juvenile larvae for eight days, until they molt into the second stage larvae. The second stage juveniles look like short, sausage-shaped worms; they already have complete organ systems, although their anuses are still plugged. The second stage juveniles are already feeding and may cause damage to their host. After three days the second stage juveniles develop into an elongated, slender filariform third stage juveniles. These juveniles are about 1.4 mm to 2.0 mm long and are already infective to their definitive hosts (humans). From the different organs, the juveniles travel towards the hemocoel, eventually reaching the labium, or proboscis sheath, from which they escape when the mosquito is feeding. The main entrance of the juveniles in the human body is through the small wounds in the skin created by the mosquitoes. Upon entrance, they migrate first to the peripheral lymphatics, and then they will settle in bigger lymph vessels, where they grow towards maturity (Baker et al. 1998; Dawes 2005; Levine 1980).

References

Levine ND. 1980. Nematode parasites of domestic animals and of man. 2nd Ed. Minneapolis: Burgess Publishing Co.

Dawes, Benjamin and Ben Daws. 2005. Advances in Parasitology. Academic Press.

Baker JR, Muller R, and David Rollinson. 1998. Advances in Parasitology. Academic Press.