Introduction

The influenza viruses of the Orthomyxoviridae family totally depend on higher vertebrates as hosts, and are transmitted by a respiratory or a faecal-oral route. In contrast, two new members of the family, Thogoto (THO) and Dhori (DHO) viruses, here called the orthoacarivirus group, replicate in both vertebrate and tick cells, and are transmitted by tick bite. This chapter compares orthoacariviruses with orthomyxoviruses (influenza viruses), and speculates on their evolutionary origins.

Natural history of Thogoto and Dhori viruses

The first reported isolation of THO virus was from a pool of ticks removed from cattle in Thogoto forest near Nairobi, Kenya, in 1960 (Haig, Woodall & Danskin, 1965). To date, the virus has been isolated from ixodid tick species collected in several countries extending across central Africa, and in Egypt, Iran, Sicily, and Portugal (Davies, Jones & Nuttall, 1986). Dhori virus was first isolated in 1971 from ticks removed from camels in north-west India (Anderson & Casals, 1973). Subsequent isolations have been made from ticks in the former USSR, Portugal, and Egypt (Jones et al., 1989). Experimental studies confirmed that both THO and DHO viruses are true arboviruses (Davies et ai, 1986; Jones et al., 1989) in that uninfected ticks become infected as they feed on THO or DHO virus-infected vertebrate hosts; the viruses replicate in the ticks, and are then transmitted by tick bite when the infected ticks take their next bloodmeal.

Both THO and DHO viruses have a wide vertebrate host range (Table 28.1).