Pictures Of Simulium Swarming Around The Head

Species of Black Flies Regarded as Significant Biting and Nuisance Pesrs of Humans, Livestock, and Poultry

Species Geographic Region


Austrosimulium unjjulutum Prosimttliutn tnixtum group Simulium amazonieum complex S. arakawae S. boncterense S. buissoni S. chohdkovskii S. decimatum S. jenningsi S. parnassum S. penobscotsnse S. posticatum S. quadrivittatum S. sanguineum S. tescorum S. venustum complex Livestock A, pestilens Cnephia pecuarum

S. arcticum complex S. chohdkovskii S. chutteri $. decimatum S. equinum S. erytlrrocephalum S. mcrustatum 5. jenningsi S. lintntum S. luggeri S. macuiatum S. omatum complex S. reptans

5. vittatum complex Poultry

C. ornitbophilia S. meridionals S. rugglesi S. slossonae

Black Fly Bite
FIGURE 11,5 Bite wounds on human legs, caused by a North American black fly of the Simulium venustum complex.

fly is known that feeds exclusively on humans. In North America, where the name "black fly" originated, fewer than 60 species have been recorded to bite humans. Less than one-third of these hold any real status as biting pests, but those that do bite regularly can be unrelenting in their attacks. Individual reactions to bites vary from a small red spot at the puncture site, often with initial streaks of oozing blood (Fig. 11.5), to an enlarged swelling the size of a golf ball (Stokes, 1914). Swelling from bites around the eyes can impede vision, and bites on the limbs can impair walking.

A general syndrome, sometimes called black fly fever, is common in areas such as northeastern North America, where biting problems can be intense. It is characterized by headache, nausea, fever, and swollen lymph nodes in the neck. Many people experience some itching, intensified by scratching the wound. Severe allergic reactions, including asthmatic responses, are infrequent; however, medical treatment, including hospitalization, is sometimes necessary (Gudgel and Grauer, 1954). No human deaths from simuliid bites have been recorded in the 20th century, although anecdotal accounts suggest that an unclothed human can be exsanguinated in about 2 hr in some areas of Russia. Exposure to fierce attacks of biting and swarming black flies can severely affect a person's emotional state and produce short-term psychological effects that reduce individual efficiency.

Many species of black flies are attracted to humans but do not bite, or they bite infrequently in proportion to the number of flies actually attracted. These species can create enormous nuisance problems. One such species is

New Zealand Eastern North America South America (Amazonian

Region) Japan Argentina Marquesas Islands Russia

Russia, northern North America Eastern North America Eastern North America Northeastern North America England Central America Northwestern South America Southwestern United States North America

Australia (Queensland) United States (Mississippi

River Valley) Western Canada Russia

South Africa


Europe, Russia



Eastern North America Europe and Russia Western Canada Russia

Europe, Russia Europe, Russia North America

Eastern North America North America North America Southeastern United States

S. jenningsi, a major pest in North America. Females of this species sometimes bite humans and occasionally cause allergic reactions, but they are more of a nuisance because of their habit of swarming about the head and entering the eyes, ears, nose, and mouth. Outdoor activities in afflicted areas, such as Pennsylvania, can become unbearable as the females ceaselessly swarm around the head. More than US$5 million is spent annually in the management of S. jenningsi.

Occasional nuisance problems have been caused by large numbers of flies attracted to incandescent lights and by mating swarms that form over bicycle and foot paths at about the same height as a person walking or riding a bicycle. These kinds of problems usually are caused by members of the North American S. vittatum species complex, which breed abundantly in human-altered habitats, such as lake outlets and polluted waters.

Human Onchocerciasis

The greatest public health problem associated with black flies is onchocerciasis or river blindness, a tropical disease caused by the filarial nematode Onchocerca volvulus, which is transmitted solely by black flies during blood feeding. In the Old World, river blindness is found in 27 countries in the central belt of Africa, with small foci in southern Yemen. In the New World, where the disease possibly was introduced during the slave trade, its distribution is patchy, with foci in northern Brazil, Colombia, Ecuador, Guatemala, Mexico, and Venezuela. The World Health Organization (1995) conservatively estimated that about 17.7 million people are infected (17.5 million in Africa and Yemen; 140,500 in tropical America), with approximately 270,000 cases of microfilarial-induced blindness and another half million individuals with severe visual impairment. Research on the disease and its vectors has generated a massive literature (Muller and Horsburgh, 1987). Excellent reviews, from which the treatment below largely is drawn, are provided by Shelley (1988b), Crosskey (1990), and the World Health Organization (1995).

O. volvulus typically is found only in humans (definitive host) and adult flies of the genus Simulium (intermediate host). Various strains of O. volvulus are recognized, such as forest and savanna strains in West Africa, and these form highly compatible parasite—vector complexes with distinct clinical facies. Once the female black fly ingests a blood meal from an infected human, the microfilariae (220—360/xm long) penetrate the gut of the fly and make their way to the thoracic flight muscles. Once in the thoracic muscles, the microfilariae lose their motility and transform to first-stage larvae, which then molt to become second-stage larvae. The final molt in the fly produces the infective third-stage larvae, which migrate to the fly's head and mouthparts. Vector incrimination is based on the presence of third-stage larvae in the head capsules of female black flies. In West Africa, DNA tests allow animal parasites and the human parasites of savanna and forest to be distinguished. Development in the black fly, which is influenced by ambient temperature, typically requires 6—12 days, but the time between successive blood meals taken by the fly is usually 3—5 days. Consequently, the infective larvae will be passed to a human host no earlier than the third blood meal when the fly is about 8—10 days old.

In humans, the infective larvae molt to the fourth larval stage within about a week. One more molt yields juvenile adults, which grow to mature adult worms over the next 12—18 months and begin reproducing. Adults typically become encapsulated in fibrous nodules that vary in size from about 0.5—10.0 cm and can be subcutaneous or deep in muscular and connective tissues; they cause no inflammatory response and no great discomfort. Mating between the small male worms (3—5 cm long) and the large females (30—80 cm) occurs in the nodules. Adult female worms can produce microfilariae for up to 14 years. These microfilariae migrate from the nodules to the skin, where they can be acquired by a vector, as well as to the eyes and various other organs (e.g., liver) of the human host. A diagnostic clinical feature of onchocerciasis is the presence of hundreds of microfilariae in skin snips.

River blindness is essentially a rural disease, afflicting those people most vulnerable to both the medical consequences and social stigmas of infection. Symptoms of the disease depend on factors such as geographical location, microfilarial transmission rates, and frequency of reinfection. Where transmission rates are low, the disease can be asymptomatic. With heavy infections, however, the classical manifestations of the disease appear, i.e., dermal changes, lymphatic reactions, nodules, and ocular disturbances. Other than the nodules in which the adults are enveloped, all symptoms are caused by the microfilariae.

Large numbers of microfilariae migrating throughout the dermis cause horrific itching that can lead to bleeding, secondary bacterial infections, inability to sleep, fever, headache, and even suicide. In addition to itching, chronic infections in Africa and Yemen can cause dermal lesions, patches of depigmentation ("leopard skin"), fibrosis, and loss of elasticity. In Yemen, the itching symptoms of the disease are known as sowda. In Central America, two unique, chronic skin conditions occur—a painful, reddish rash on the face (erisipela de la costa) and lesions associated with reddish skin on the trunk and arms {mal morado). The lymphatic nodes also can be affected, especially in the groin and thighs; combined with loss of skin elasticity, the result is a condition known as hanging groin.

Migrating microfilariae also enter the eye, resulting in a severe ocular pathology that can involve all tissues of the eye. Ocular problems are associated with the presence of both live and dead microfilariae, and they manifest themselves in many forms, including cataracts, retinal hemorrhages, corneal opacities, secondary glaucoma, sclerosing keratitis, and optic neuritis. Various forms of visual impairment occur, such as night blindness and reduction in peripheral vision, but the most severe consequence is irreversible blindness with complete loss of light perception (Fig. 11.6). Blindness usually takes years to occur; at age 20, for example, it is rare in infected people, but at 50 years of age, half of the infected victims can be blind. The incidence of blindness is highest in the savannas of West Africa, with some villages experiencing 15% blindness. At these high levels of disease, the village is often abandoned. Outside West Africa, ocular pathology is rare.

Filarial Nematode
figure 11.6 Human blindness caused bv the filarial nematode Onchocerca volvulus, transmitted by black flies of the Simulium damnosum complex in West Africa; note opacity of cornea due to damage by microfilariae. (Copyright Eric Poggenpohl.)

At least 25 species of Simulium are known vectors of O. volvulus (Table II). Most of these vectors are members of species complexes, and considerable taxonomic work is still needed to resolve all of the vector species in areas such as East Africa and the Americas. In West Africa and Yemen, all vectors are members of the 5. damnosum species complex and include S. damnosum sensu stricto and S. sirbanum, the principal vectors associated with the savanna form of the disease and ocular pathology. The vectors in East Africa are members of the S. damnosum complex, the S. neavei group, and S. albivirgulatum. In the Americas, at least 9 species or species complexes are vectors, the most important of which are members of the S. exijjuum, S.jjuianense, S. metallicum, S. ochraceum, and S. oyapockense complexes; their importance, however, varies with location. Because the vectors in South America are more widespread than is onchocerciasis, the disease is predicted to spread as humans continue to push into undeveloped areas (Basanez et al., 2000).

An understanding of the unique life history and behavior of each vector species is key to the control of onchocerciasis. Breeding sites of the vectors, for example, represent the targets for control. The immature stages of the S. damnosum complex primarily inhabit swift sections of medium to large rivers, from dry savannas to forest highlands, depending on the species. Larvae and pupae of species in the S. neavei group live primarily in perennial, shaded forest streams, where they have an obligatory phoretic relationship with river crabs. In the New World, members of the 5. metallicum and S. ochraceum complexes breed in large and small streams, respectively, that drain forested mountain slopes, whereas members of the S. exiguum and S. oyapockense complexes breed in large rivers of the rain forest. Although each species breeds in a specific habitat, the adults of some species can travel great distances beyond their natal waterways. The adults of S. sirbanum and S. damnosum sensu stricto, for example, can travel more than 500 km, assisted by seasonally changing winds. In the wet season, moist monsoon winds from the southwest move flies in a northeastwardly direction. In the dry season, winds from the northeast assist flies in their reverse, southwestwardly flights. Continual reinvasions by vectors, therefore, occur with each season in both the northern and southern parts of West Africa and must be considered in control efforts.


The filarial nematode Mansonella ozzardi is the causal agent of mansonellosis, a questionably pathogenic disease of humans. It is transmitted by at least four species of black flies in the Neotropical rain forests of Brazil, Colombia, Guyana, Venezuela, and southern Panama

51 Ways to Reduce Allergies

51 Ways to Reduce Allergies

Do you hate the spring? Do you run at the site of a dog or cat? Do you carry around tissues wherever you go? Youre not alone. 51 Ways to Reduce Allergies can help. Find all these tips and more Start putting those tissues away. Get Your Copy Of 51 Ways to Reduce Allergies Today.

Get My Free Ebook

Post a comment