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Loa loa

By Michael Harris

Geographic Range

Loa loa inhabits rainforests in West Africa and equatorial Sudan. (Roberts and Janovy, 2000; Thompson, et al., 2000)

Habitat

Loa loa live in areas with hot, wet climates, such as rainforests and swamps. They are transfered by tabinid flies to their human hosts, where they live in subcutaneous tissues, although they have been known to migrate deeper into the body. (Gardon, et al., 1997; Roberts and Janovy, 2000)

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

Physical Description

Loa loa is cylindrical, has a cuticle with three main outer layers made of collagen and other compounds. The outer layers are non-cellular and are secreted by the epidermis. The cuticle layer protects the nematodes so they can invade the digestive tracts of animals. The worms molt four times, the first two before hatching, and then before their adult stage.

Nematodes have longitudinal muscles along the body wall. The muscles are obliquely arranged in bands. Dorsal, ventral and longitudinal nerve cords are connected to the main body of the muscle.

Loa loa adults are small, thin worms ranging in length from 20-70 mm long and 350-430 micrometers wide. Females are typically larger than males. The head of Loa loa is simple and lacks lips. The tail is blunt. Loa loa juveniles look similar to adults, but are much smaller. (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, 2000)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    20 to 70 mm
    0.79 to 2.76 in

Development

After mating, female Loa loa give birth to microfilariae. The microfilariae migrate into the blood stream. At this stage, they can be ingested by any number of deer fly species that feed on the infected host. In the fly, the microfilariae develop in the fat body until reaching the juvenile stage. The infective juvenile Loa loa move to the mouthparts of the fly so that when it slices open a human to feed, the Loa loa may again invade a human host. After a year of development in their definitive host, the Loa loa are adults that may begin the cycle anew. (Roberts and Janovy, 2000)

Reproduction

Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella. (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, 2000)

  • Parental Investment
  • pre-fertilization
    • provisioning

Lifespan/Longevity

Adults may live up to 15 years or more. (Roberts and Janovy, 2000)

Behavior

Adult worms migrate throughout the subcutaneous tissues of their host at all times of the day. When they linger in one area long enough, a swelling in the shape of the worm can be seen on the skin. Microfilariae live the bloodstream during daylight hours when the host is most likely to be bitten by flies. At night, they retreat into the lungs. In their fly intermediate host, Loa loa microfilariae live in the fat body, while the infective juveniles inhabit the mouthparts of the fly. (Roberts and Janovy, 2000)

Communication and Perception

Nematodes within the Secernentea have phasmids, which are unicellular glands. Phasmids likely function as chemoreceptors. Females may produce pheromones to attract males.

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, 2000)

Food Habits

Loa loa is an obligate endoparasite, feeding on fluids in the tissues of humans. Pharyngeal glands and intestinal epithelium produce digestive enzymes to feed on the hosts’ body fluids. Extracellular digestion begins within the lumen and is finished intracellularly. Tabanid flies serve as intermediate hosts. (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, 2000)

  • Animal Foods
  • body fluids

Predation

These parasites are usually not preyed on directly, but are ingested. Larval mortality is high as most of the parasites do not reach appropriate hosts. (Barnes, 1987; Brusca and Brusca, 2003; Roberts and Janovy, 2000)

Ecosystem Roles

Loa loa is an obligate endoparasite, feeding on fluids in the tissues of humans. Tabanid flies serve as intermediate hosts. (Roberts and Janovy, 2000)

Species Used as Host

Economic Importance for Humans: Negative

Loa loa is a serious parasite of humans in rural areas of West Africa. When Loa loa migrate into deeper tissues of a host, they can cause encephalitis, sometimes leading to death. Other pathogenic effects include joint pain caused by swelling when a worm stays near a joint for a period of time and damage to the eyes as the worm crawls through the cornea and conjunctive tissues. (Blum, et al., 2000; Chippaux, et al., 1996; Gardon, et al., 1997; Roberts and Janovy, 2000)

Contributors

Renee Sherman Mulcrone (editor).

Michael Harris (author), University of Michigan-Ann Arbor, Teresa Friedrich (editor), University of Michigan-Ann Arbor.

Glossary

Ethiopian

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

World Map

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.

internal fertilization

fertilization takes place within the female's body

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

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.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

sedentary

remains in the same area

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.

References

Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.

Blum, J., A. Wiestner, P. Fuhr, C. Hatz. 2000. Encephalopathy following Loa loa treatment with albendazole. Acta Tropica, 78: 63-65.

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Carlo Denegri Foundation, 2000. "Blood and Skin Parasites, Nematoda: Order: Filariata, Loa loa" (On-line). Atlas of Medical Parasitology. Accessed September 27, 2004 at http://www.cdfound.to.it/HTML/loa1.htm.

Chippaux, J., M. Boussinesq, J. Gardon, N. Gardon-Wendel, J. Ernould. 1996. Severe adverse reaction risks during mass treatment with ivermectin in loiasis-endemic areas. Parasitology Today, 12: 448-450.

Gardon, J., N. Gardon-Wendel, Demanga-Ngangue, J. Kamgno, J. Chippaux. 1997. Serious reactions after mass treatment of onchocerciasis with ivermectin in an area endemic for Loa loa infection. Lancet, 350: 18-22.

Ohio State University, 2001. "Loa loa" (On-line). Parasites and Parasitological Resources. Accessed September 27, 2004 at http://www.biosci.ohio-state.edu/~parasite/loa.html.

Roberts, L., J. Janovy. 2000. Foundations of Parasitology 6th ed.. Boston: McGraw Hill.

Thompson, M., V. Obsomer, M. Dunne, S. Connor, D. Molyneux. 2000. Satellite mapping of Loa loa prevalence in relation to ivermectin use in west and central Africa. Lancet, 356: 1077-1078.

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