Black flies are feared in many regions because of the relentless and intolerable nature of their attacks, not only on man, but on livestock, poultry, and wildlife. Bites can not only provoke severe reactions in many bitten individuals, but often have serious social and economic effects. Black flies can reduce the milk output of dairy cows. When a comparison of the expected and actual milk yields was made on a California dairy farm where cows were under attack, a clear loss of production was found to be correlated with high levels of biting.
In other countries, black flies are also carriers of the filarial disease Onchocerciasis, commonly referred
to as River Blindness. A blackfly carrying larvae of Onchocerca volvulus bites a
victim and transmits the infection. The larvae grow into groups of string-like worms that
live wrapped up in a bump under the skin. Adult female worms produce thousands of tiny
microfilariae for approximately 8-12 years. Unlike the adults, the microfilariae migrate
throughout the victim's skin, causing skin disfigurement and intense itching. The worms
can cause permanent damage when they reach the eyes, impairing sight and finally causing
blindness. When another blackfly bites the victim and then bites an uninfected person, it
begins the cycle of infection again.
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