Many flaviviruses are transmitted via arthropod vectors to humans.
Dengue, yellow fever, and Japanese encephalitis are mosquito-borne, and
tick-borne encephalitis and Kyansanur Forest disease are
tick-borne. Arbovirus transmission allows flavivirus
to cross species barriers since the same arthropod may bite birds,
reptiles and mammals that rarely come in contact with one another
naturally. The virus persists in insects through transovarial transmission
and in vertebrates such as birds, pigs and
monkeys through host amplification. In these animals, the virus causes
only subclinical infection.
Except in the case of dengue and yellow fever, humans are not
involved in the primary transmission cycles. Humans only
get involved when they come into close contact with infected arthropods or
drink milk from infected animals. In certain areas, arboviruses are
enzoonotic and disease is endemic, but in most areas arthropod
transmission and flavivirus infection only reaches epidemic proportions at
the end of the wet season or after a sudden population movement which
disturbs the arthropod habitat. Irrigation, deforestation and
long-distance air travel also increase flavivirus transmission.
Hepatitis C, a non-arbovirus flavivirus, is transmitted
parentally, sexually and vertically. Transfusions were once the
major cause of infection, but today they account for only 10% of the
cases. The incidence of HCV infection among IV drug users and the
sexually promiscuous is rising sharply despite public health
campaigns advocating the use of clean needles and condoms. In rare cases,
vector-borne transmission of HCV has been reported and investigated.