In contrast to most tropical and subtropical countries, Canada has relatively few diseases transmitted by biting flies that affect man and livestock.
Mechanical transmission is much less important in Canada than is biological transmission.
Deer flies, horse flies, and stable flies, which tend to feed on small amounts of blood from several animals in succession, are believed to transmit the bacteria that cause tularaemia and anthrax by carrying these bacteria on their mouthparts from an infected host to an uninfected one. The flies themselves are not necessary steps in the life cycle of the bacteria. Canadian arboviruses are primarily pathogens of wild mammals and birds and their occurrence in man and livestock is sporadic, that is to say, only when populations of the vector mosquito rise to abnormally high levels. Four of these arboviruses, transmitted biologically by mosquitoes, cause encephalitis in man. Another strain of the California group, Powassen virus, is transmitted by ticks.