Wuchereria bancrofti

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Geographic Range

Wuchereria bancrofti is a common human parasite found in tropical regions worldwide. Approximately 106 million people in 76 countries are infected, with one billion people thought to be at risk for infection. This nematode is found in Central Africa and the Nile delta, South and Central America, and the tropical regions of Asia including southern China and the Pacific. The major factor limiting the geographic range of W. bancrofti is likely the climate requirement of its intermediate host, the mosquito. (CDC, 2009; "Nematode and Neglected Genomics", 2008; de Almeida and Freedman, 1999)

Habitat

Mainly this parasite is found where mosquitoes are found: in and around temporary pools or standing water. Humans are the only known definitive host of Wuchereria bancrofti. Although monkeys have been infected artificially, they are not hosts in the wild. Adult worms live in regional lymphatic vessels, particularly in the region of the groin. Juveniles circulate in the blood and are ingested by mosquitoes when the latter takes a blood meal from a human. Once inside the mosquito the juveniles migrate into the thoracic muscles and eventually to the mouthparts, allowing transmission to the next human host when the mosquito feeds. (Ash and Schacher, 1971; Baron and Cross, 1996)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams
  • temporary pools

Physical Description

Eggs: The eggs average 40 by 25 μm in size. They lack a true shell, instead being enclosed in a membrane which stretches to form a sheath over the microfilaria (embryo).

Microfilariae: The embryo itself averages 290 μm by 6-7 μm, though the sheath surrounding it is slightly larger. The cuticle of the embryo has well-marked striations and the tail tapers gradually to a rounded tip.

Infective Larva (3rd stage): The head of the infective larva is truncate-conic, trapezoidal when viewed laterally. The oral aperture is circular with prominent papillae ringing the outer edge. The tail is blunt with three caudal papillae, two latero-ventral and one dorso-terminal. In females the tail is relatively longer. As the larva grows the head loses its truncate shape and is well rounded by the time it develops into a 4th stage larva.

Adults: Wuchereria bancrofti is a translucent white worm with a smooth cuticle. The head is rounded and separated from the body by a neck-like constriction. In females the tail tapers gradually and is rounded at the tip, while in males the tail curves ventrally. Males and females live coiled together. Females vary in length from 30-100 mm with a diameter ranging from 100-300 μm. Males are smaller, typically half the size of females. (Ash and Schacher, 1971; Baron and Cross, 1996; Napier, 1994)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    30 to 100 mm
    1.18 to 3.94 in

Development

Wuchereria bancrofti enters a human host when a mosquito infected with third stage larvae takes a blood meal. The third stage larvae are infective to the human host and after entering through the bite-wound they migrate to the lymphatic tissues and develop into adults. Adults sexually reproduce to give rise to sheathed microfilariae which migrate into lymph and blood channels. When a new mosquito takes a blood meal it inadvertently ingests the microfilariae, which then shed their sheaths, penetrate the mosquito's midgut and migrate to the thoracic muscles. The microfilariae then develop to the third larval stage and migrate to the mosquito's head and proboscis, where they are able to infect a human host the next time the mosquito feeds. (CDC, 2009)

Reproduction

Wuchereria bancrofti is dioecious and copulation between individuals of opposite sexes is required for reproduction. Males possess a coiled tail region which allows them to grasp females during copulation. Additionally, the male cloaca is ringed with sensillae and copulatory spicules which serve to hold open the vulva and vagina during sperm transfer. There is evidence, both chemical and behavioral, for the use of sex pheromones involved in attracting mates. Additionally, there is some evidence that males possess a chemosensory apparatus required for recognition of such signals. (Nisbet, et al., 2004; Nutman and Scott, 2000)

Once the third stage infective larvae enter a human host, they require 3 months to a year of development to become mature adults. Most adults have a fecund lifespan of approximately 5 years, although they may live as long as 10 years. Males and females live coiled together in pairs. Females exhibit a circadian periodicity in the release of microfilariae. Peaks in production vary depending upon the geographic strain of the worm and usually correlate to the peak feeding period of the local mosquito vectors. ("Nematode and Neglected Genomics", 2008; Baron and Cross, 1996; Napier, 1994)

  • Range age at sexual or reproductive maturity (female)
    3 to 12 months
  • Range age at sexual or reproductive maturity (male)
    3 to 12 months

No information was found concerning the parental investment of this species.

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

The average fecund lifespan of Wuchereria bancrofti is approximately 5 years, although some studies suggest that adult worms can live for as long 10 years. ("Nematode and Neglected Genomics", 2008; Vanamail, et al., 1996)

  • Typical lifespan
    Status: wild
    5 to 10 years
  • Average lifespan
    Status: wild
    5 years

Behavior

Wuchereria bancrofti is a parasite of humans and uses mosquitoes as intermediate hosts. This species migrates through both hosts and molts several times to reach an adult stage. Like all nematodes, it lacks circular muscles and therefore must move in a serpentine pattern by contracting the longitudinal muscles on the sides of its body. Apart from chemical signaling to detect mates, no current studies have found social behavior for this species. (CDC, 2009; Napier, 1994)

Communication and Perception

Wuchereria bancrofti lacks visual abilities and instead relies on chemosensation to detect chemicals in its environment and pheromones released by other members of its species. Additionally, papillae, both oral and elsewhere on the body, allow for tactile communication with the environment and food. The worm operates on a diurnal rhythm which matches the lifestyle patterns of its human host. It uses its chemosensory abilities to detect the difference in oxygen levels between arterial and venous blood vessels. A smaller difference between venous and arterial blood oxygen is indicative of decreased oxygen intake and lessened host activity, which are conditions associated with nighttime and the feeding time of mosquito vectors. Thus, when Wuchereria bancrofti senses a smaller difference in oxygen content between arterial and venous blood vessels, it emerges to the peripheral circulation where it is likely to be ingested by feeding mosquitoes, allowing dispersal to other hosts. (Ash and Schacher, 1971; Cox and Chappell, 1993; Napier, 1994)

Food Habits

Wuchereria bancrofti uses its mouth to feed on the body and tissue fluids of its human host. (CDC, 2009)

  • Animal Foods
  • body fluids

Predation

No information was found concerning predation of Wuchereria bancrofti, nor are there any free-living stages which would provide the opportunity for predation.

Ecosystem Roles

Humans are the only vertebrate host of Wuchereria bancrofti and experimental attempts to infect other animals have been unsuccessful. Wuchereria bancrofti uses mosquitoes, including species of Culex, Anopheles and Aedes, as intermediate hosts. (Cox and Chappell, 1993; Edeson and Wilson, 1964)

Species Used as Host

Economic Importance for Humans: Positive

No information was found concerning any benefits Wuchereria bancrofti may provide for humans.

Economic Importance for Humans: Negative

Wuchereria bancrofti is the cause of the human disease bancroftian filariasis which usually targets the genitalia and lower extremities. The disease is rarely fatal but can cause great pain to its victims. Furthermore, many of the effects of filariasis incapacitate the victim, reducing overall quality of living. Acute infection includes repeated episodes of lymphadenitis and lymphangitis, accompanied by fever and malaise. Chronic infection includes hydrocele and lymphoedema, which can lead to the condition known as elephantiasis in which the patient experiences massive swelling of the infected body part, enlargement of fibrous tissues and thickening and depigmentation of skin. There is no cure for this condition once it has manifested itself in the patient and often surgery is required to remove surplus fibrous and calcified tissues. Furthermore, pulmonary eosinophilia (asthma-like symptoms) can also result when the microfilarial migration through the blood and lungs triggers an IgE-mediated allergic response. Rarely chyluria occurs if lymph fluids leak into the kidney, ureter or bladder. (Napier, 1994; Rajkumar, et al., 2005)

Conservation Status

This is a common parasite which infects humans in tropical regions worldwide. (CDC, 2009)

Contributors

Karen Guy (author), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

chemical

uses smells or other chemicals to communicate

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

freshwater

mainly lives in water that is not salty.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

suburban

living in residential areas on the outskirts of large cities or towns.

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

year-round breeding

breeding takes place throughout the year

References

2008. "Nematode and Neglected Genomics" (On-line). Wuchereria bancrofti. Accessed February 22, 2010 at http://www.nematodes.org/fgn/pnb/wuchban.html.

Ash, L., J. Schacher. 1971. Early life cycle and larval morphogenesis of Wuchereria bancrofti in the jird, Meriones unguiculatus. The Journal of Parasitology, 57 (5): 1043-1051.

Baron, S., J. Cross. 1996. Medical Microbiology: Parasitology. Galveston, Texas: The University of Texas Medical Branch of Galveston. Accessed May 17, 2011 at http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mmed&part=Section4.bxml.

CDC, 2009. "Parasites and Health" (On-line). Filariasis. Accessed May 17, 2011 at http://www.dpd.cdc.gov/DPDx/HTML/Frames/A-F/Filariasis/body_Filariasis_w_bancrofti.htm.

Cox, F., L. Chappell. 1993. Modern Parasitology: A Textbook of Parasitology. Cambridge, MA: Blackwell Science Ltd. Accessed May 17, 2011 at http://books.google.com/books?hl=en&lr=&id=jj18axV3TTAC&oi=fnd&pg=PA157&dq=Wuchereria+bancrofti+sensory+organs&ots=0Y8tpBxDEN&sig=mLq7OIOiUt98bZFR2cX4S-f6tiM#v=onepage&q=&f=false.

Edeson, J., T. Wilson. 1964. The epidemiology of filariasis due to Wuchereria bancrofti and Brugia malayi. Annual Review of Entomology, 9: 245-268.

Napier, L. 1994. Filariasis due to Wuchereria bancrofti. Medicine, 23 (2): 149-180.

Nisbet, A., P. Cottee, R. Gasser. 2004. Molecular biology of reproduction and development in parasitic nematodes: progress and opportunities. International Journal for Parasitology, 34 (2): 125-138.

Nutman, T., A. Scott. 2000. Lymphatic Filariasis. Covent Garden, London: Imperial College Press. Accessed May 17, 2011 at http://books.google.com/books?hl=en&lr=&id=sH_YL0aXvkkC&oi=fnd&pg=PA5&dq=wuchereria+bancrofti+copulation&ots=s72Na3MSus&sig=EcEIF96ZGylcibPOXwIJH8cDkn0#v=onepage&q=&f=false.

Rajkumar, M., S. Wright, J. Aslanzadeh. 2005. Wuchereria bancrofti and Onchocerca volvulus co-infection in a refugee from Sierra Leone. Annals of Clinical and Laboratory Science, 35: 199-201. Accessed May 17, 2011 at http://www.annclinlabsci.org/cgi/content/full/35/2/199.

Vanamail, P., K. Ramaiah, S. Pani, P. Das, B. Grenfell, D. Bundy. 1996. Estimation of the fecund life span of Wuchereria bancrofti in an endemic area. Transactions of the Royal Society of Tropical Medicine and Hygiene, 90 (2): 119-121.

de Almeida, A., D. Freedman. 1999. Epidemiology and immunopathology of bancroftian filariasis. Microbes and infection, 1 (12): 1015-1022.