FIGURE 11.8 Gametocytes of the protozoan Leucocytozoon simondi in blood cells of a mallard duck. (A) Round form; (B) elongate form. (Photo by E. C. Greiner.)

Other Parasites and Pathogens of Veterinary Importance

Black flies transmit additional parasites to wild animals (Table II). The protozoan Trypanosoma confusum is specific to birds in North America and is transmitted when infected fecal droplets from the black fly contaminate the bite. Birds of numerous families serve as hosts. Other species of bird trypanosomes (e.g., T. corvi) are believed to cause infections when the birds consume infected black flies or eat other birds that have been infected. The filarial nematodes Splendidofilaria fallisensis and Dirofilaria ursi are transmitted to ducks and black bears, respectively. The effects of these protozoan and filarial parasites on their wild hosts are poorly known.

Evidence is mounting that several North American species of sirnujiids, such as Simulium notatum and S. vittatum, naturally transmit vesicular stomatitis virus to livestock, primarily cattle, horses, and pigs (Schmidtmann etal, 1999). The virus causes lesions in various epithelial tissues, especially in the mouth. Millions of dollars can be lost during epizootics. Laboratory experiments have shown that a viremic host is not necessary for a female black fly to become infected; rather, flies can become infected by feeding on the same host with an infected black fly (Mead tt ai, 2000).

Additional parasites of wildlife have been associated with black flies. The recent discovery of minute nematodes of the family RobertdoIJfusidae in the guts of African black flies suggests that these parasites might be transmitted to wildlife (Bain and Renz, 1993). Bunyaviruses, eastern equine encephalitis virus, and snow-shoe hare virus have been isolated from several North American black flies. Minimal mechanical transmission has been demonstrated for Whataroa virus in laboratory mice in New Zealand (Austin, 1967) and for myxomatosis in rabbits in Australia (Mykytowycz, 1957). These examples suggest that much is yet to be learned about the vector potential of black flies among wildlife.


Attacks by black flies have, at times, been so massive and virulent that livestock have been killed. Many of the deaths probably result from acute toxemia and anaphylactic shock caused by the toxins introduced with the saliva as black flies are feeding. The diseased condition, either temporary or terminal, that results from the bites of black flies is known as simuliotoxicosis, a term first used to describe the toxic effects of simuliid bites on reindeer (Wilhelm et a.1, 1982). Cattle, especially calves, are vulnerable to simuliotoxicosis, but horses, mules, sheep, goats, and pigs also have been affected. Susceptible animals succumb in less than 2 hr. Some immunity is apparent in animals living in afflicted areas. The biochemical nature of simuliotoxicosis requires more investigation.

Most of the species responsible for simuliotoxicosis breed in large rivers, from which the adults emerge in astronomical numbers. They include A. pestilens in Queensland (Australia); Cnephia pecuarum in the Mississippi River Valley (United States); S. colombaschense along the Danube River in central Europe; a member of the S. arcticum complex on the Canadian prairies; and S. erythrocephalum, the 5. omatum complex, and S. reptans in central Europe.

One of the worst attacks in recorded history killed about 22,000 animals in 1923 along Europe's Danube River in the southern Carpathian Mountains (Ciurea and Dinulescu, 1924). Prodigious attacks in this region during the 1700s prompted Empress Maria Theresa of the Old Austro-Hungarian Empire to order one of the first biological studies of black flies, which eventually was published in 1795. On the Canadian prairies, thousands of livestock were killed from about 1886 into the 1970s by a member of the S. arcticum complex (Fredeen, 1977).

Massive mortality due to attacks by C. pecuarum occurred in the United States during and immediately after the Civil War when the levees of the Mississippi River deteriorated, allowing the river to overflow and create extensive breeding areas for this species (Riley 1887).

Simuliotoxicosis on a large scale is now rare, mainly because the former breeding sites of most of the responsible species have been altered by pollution, impoundment, and land development. Some of these species, however, still create nuisance problems for livestock, with occasional deaths in localized areas of their ranges.

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