Experimental Studies on the Transmission Cycle of Thogoto Virus, a Candidate Orthomyxovirus, in Rhipicephalus appendiculatus

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Abstract
Thogoto (THO) virus, a candidate orthomyxovirus, replicated in and was transmitted by larvae, nymphs, and adults of the brown ear tick, Rhipicephalus appendiculatus. Larvae fed on viremic hamsters (107–8 PFU/ml blood) acquired an average of 102.5 PFU per tick. Following engorgement the titer dropped to 101.9 PFU on day 2 but increased by day 6 to 103.3 PFU. Virus survived transstadially in these ticks as demonstrated by the fact that, on day 10, newly moulted nymphs contained, on average, 103.5 PFU/tick. When 10 such infected nymphs were placed on a hamster a fatal infection of the animal developed involving a viremia of 106.7 PFU/ml blood. Another group of 6 infected nymphs did not elicit a detectable viremia in a hamster, or cause death. However the animal seroconverted to THO, virus indicating that virus transmission had occurred. Following acquisition of THO virus at the larval stage, virus was detected in adult ticks 138 days later. Uninfected nymphs fed on viremic hamsters acquired an average of 104 PFU/nymph. No virus was detected in the nymphs 4 days post-engorgement. Virus was, however, recovered by 6 days post-engorgement (104.7 PFU/nymph). Virus persisted transstadially as shown by the presence of an average of 103.4 PFU in newly moulted adults. Three groups of these infected adults (5–6 ticks/group) induced viremia in hamsters with blood titers of the order 102.8–3.5 PFU/ml. Virus persisted in engorged adults for up to 66 days following nymphal engorgement. Uninfected nymphs that partially engorged on viremic hamsters transmitted virus to a second host (uninfected hamster) when completing their bloodmeal. Uninfected male and female adult ticks that fed on viremic hamsters acquired on average 102.8 and 104.8 PFU/tick, respectively. Virus persisted in the infected adult female ticks for at least 20 days post-engorgement. Virus was not detected in larvae that hatched from eggs laid by adults either infected in the larval stage or infected perorally, and these larvae did not transmit virus to uninfected hamsters.