Table Ii

Filarial Nematodes Transmitted by Biting Midges to Humans and Domestic Animals: Vectors Include Cuiicoides, Forcipomyia, and Leptoconops Species


Vertebrate host

Geographic area

Known or suspected vectors

Mansonella ozzardi


South America, Caribbean Basin

C. barbosai, C.furens

C. paraensis, C. Phlebotomus, Leptoconops bequaerti

M. perstans


Sub-Saharan West Africa; Central

Africa to Kenya and Mozambique Northern coast of South America; Caribbean Islands

C. austeni C. grahamii

C. inornatipennis Cuiicoides spp.

M. streptocera


West and Central Africa (rain forests)

C. austeni, C.jjrahamii

Onchocerca cervicalis


North America Australia

C. variipennis C. victoriae

Forcipomyia townsvillensis

O. gibsoni


India, Sri Lanka, Malaysia, northern Australia, South Africa

C. pungens Cuiicoides spp.

O. gutturosa



Cuiicoides spp.

O. reticulata

Horses, ponies


C. nubeculosus C. obsoletus

O. sweetae

Water buffalo



172 GetrvR. Mullen

FIGURE 10.7 Filarial nematode (Mansonella ozzardi), microfilaria! stage. (Courtesy of Lea & Febiger.)

Mansonella ozzardi

M. ozzardi is the only native New World cerato-pogonid-borne nematode of humans (Fig. 10.7). It is indigenous to the Americas, occurring in the Amazon Basin (Brazil); along the northern coast of South America (Colombia, Venezuela, Guyana, Suriname, and French Guiana); on Trinidad, Haiti, and other islands of the West Indies; Panama; and parts of Peru, Bolivia, and Argentina, It particularly affects coastal fishing communities near breeding sites of associated vectors. The infection rate among local inhabitants is highly variable, ranging from as low as 5% or less in northern Brazil and some of the Caribbean islands to over 95% among Amerindians in Colombia and Venezuela. Infection rates are generally highest among men and women in older age groups, reflecting chronic exposure to infection in endemic areas.

Infections with M. ozzardi usually do not result in significant pathological effects. The microfilariae typically remain in the capillaries of the skin and surrounding dermal tissues, where they cause relatively little harm. Surveys are usually conducted by taking skin biopsies or blood samples and examining them for the presence of microfilariae. The adult worms are found primarily in fat tissue associated with the peritoneum and various body cavities, occasionally causing conjunctivitis and swelling of the eyes. In some cases this nematode can cause more serious problems, such as severe joint pains, eosinophilia, enlargement of the liver, and blockage or inflammation of the lymphatic vessels, resulting in conditions similar to bancroftian filariasis and elephantiasis. Ivermectin has been successfully used in treatment of M. ozzardi cases, whereas the widely used filarial nematocide diethylcarba-mazine is ineffective in killing this parasite.

Vectors of M, ozzardi include both biting midges and black flies, with different taxa playing important roles in different areas. Culicoides furens and C. pblebotomus are the principal vectors in Haiti and Trinidad, respectively. Other species which support development of microfilariae to infective larvae and are generally considered to play a secondary role in transmission are C. barbosai, C. paraensis, and L, bequaerti. After ingestion by a biting midge as it feeds on an infected host, microfilariae are carried into the midgut, where they penetrate the midgut wall and make their way to the thoracic muscles within 24 hr. There they develop to third-stage larvae during the next 6 to 9 days before moving to the head and mouthparts. Infective third-stage larvae enter the bite wound when the midge subsequently feeds on another host. Typically only one to three larvae successfully complete development to the infective stage in a host insect, regardless of the number of microfilariae initially ingested.

The role of black flies as vectors of M. ozzardi remains unclear. Species in the Simulium amazonicum group and the S. sanguineum group have been found to be naturally infected with this nematode and probably play a role in transmission, particularly in the Amazon Basin, Other species incriminated as potential vectors based on field collections and experimental infection studies include S. sancbezi and S. pintoi. Despite earlier suggestions that there may be two different forms or species of nematode involved, one transmitted by biting midges and the other by black flies, recent evidence indicates that they are morphologically identical and represent a single species.

Mansonella Persians

This nematode (formerly placed in the genera Acan-thocheilonema, Dipetalonema, and Tetrapetalonema) is the most widely distributed of the three human filarial nematodes transmitted by biting midges. It is indigenous to the Old World, where it occurs in sub-Saharan Africa, extending primarily from West African countries bordering the Gulf of Guinea (Ivory Coast, Nigeria, and Equatorial Guinea) and from Gabon and Angola east through Central Africa to Kenya and Mozambique. Infection rates are commonly 50% or higher in some communities. M. perstans was introduced to Central and South America with the slave trade and now occurs along the northern coast ofSouth America (Colombia, Venezuela, Guyana, Suriname, and French Guiana), in the Yucatan area ofMexico, and on Trinidad and other Caribbean Islands. Prevalence of infection exceeding 50% has been reported among the Curripaco Indians of Venezuela. Although there is evidence to suggest that M. perstans represents a complex of species, this issue remains unresolved.

M. perstans is typically regarded as nonpathogenic. The microfilariae remain primarily in the circulating blood, whereas the adult worms occur freely in the body cavities. Some infested individuals develop problems such as joint pains, fever, fatigue, transient edema, elephantoid scrota, mild urticarial skin reactions, and eosinophilia. Various ocular problems, including swelling of the eyelids, excessive lacrimation, pruritus, and conjunctival granulomas or nodules, have been reported. The latter is the result of adult worms coiled within the connective tissue of the conjunctiva, causing a condition known as bulge-eye or bung-eye. Adult worms also have been removed from connective tissue of the pancreas, kidneys, rectum, and mesenteric lymph nodes of infested patients, with little evidence of serious harm. Mebendazole has been used successfully in treating M. perstans cases, whereas diethylcar-bamazine and ivermectin are ineffective in killing either the microfilariae or adult worms.

The principal vectors of M. perstans have not been identified, particularly in the New World, where they remain virtually unknown. In Africa several Cuiicoides species have been implicated as vectors based on natural infections and support of development to the infective stage in experimental studies. They include C. austeni, C.gra-hamii, and C. inornatipennis as probable vectors and C. hortensis, C. krameri, C. kumbaensis, C. milnei, C. pyc-nostictus, C. ravus, C. rutshuruensis, and C. vitshumbiensis as possible vectors. As in the case of M. ozzardi, the microfilariae of M. perstans move from the midgut of the biting midge to the thoracic musculature, where they complete their development to infective, third-stage larvae 8—10 days after the infective blood meal.

Mansonella streptocerca

This filarial nematode (formerly placed in the genera Dipetalonema and Tetrapetalonema) occurs only in the rain forests of West and Central Africa, extending from the Ivory Coast and Burkina Fasso to the Congo and Zaire. Little information on prevalence is available for this species, although a figure of 13—14% has been reported in certain villages of the Central African Republic based on peripheral blood smears. Although it is regarded as nonpathogenic to humans, M. streptocerca occasionally causes mild skin reactions due to activity of microfilariae in dermal tissues, usually involving the trunk and upper arms. Adult worms typically occur subcutaneously in upper parts of the body. Diethylcar-bamazine is effective as a treatment. C. grahamii is regarded as the principal vector.

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