Published online by Cambridge University Press: 21 August 2009
INTRODUCTION
Ticks transmit a wide variety of arboviruses (arthropod-borne viruses). Tick-borne viruses are found in six different viral families (Asfarviridae, Reoviridae, Rhabdoviridae, Orthomyxoviridae, Bunyaviridae, Flaviviridae) and at least nine genera. Some as yet unassigned tick-borne viruses may belong to a seventh family, the Arenaviridae. With only one exception (African swine fever virus) all tick-borne viruses (as well as all other arboviruses) are RNA viruses. Some tick-borne viruses pose a significant threat to the health of humans (tick-borne encephalitis virus, Crimean–Congo haemorrhagic fever virus) or livestock (African swine fever virus, Nairobi sheep disease virus). This chapter first considers the characteristics of ticks important in virus transmission and then presents an overview of the tick-borne members of different virus families.
TICKS AS VECTORS OF ARBOVIRUSES
Ticks are not insects. The significance of this statement is considered in a review of the marked contrasts between the biology of ticks and that of insects, and the consequences for their potential to transmit micro-organisms (Randolph, 1998). Interestingly, tick-borne viruses are found in all the RNA virus families in which insect-borne members are found, with the exception of the family Togaviridae. Virus–tick–vertebrate host relationships are highly specific, and fewer than 10% of all tick species (Argasidae and Ixodidae) are known to play a role as vectors of arboviruses. However, a few tick species transmit several (e.g. Ixodes ricinus, Amblyomma variegatum) or many (I. uriae) tick-borne viruses.
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