Characterization of tick salivary gland factor(s) that enhance Thogoto virus transmission

Abstract

Thogoto (THO) virus is transmitted from infected to uninfected ticks co-feeding on an uninfected guinea pig, even though the guinea pig does not develop a detectable viremia. We call this “saliva-activated transmission” (SAT). Ten-fold more ticks became infected when feeding on guinea pigs inoculated with THO virus plus salivary gland extract (derived from uninfected, partially fed female Rhipicephalus appendiculatus ticks), than those which fed on guinea pigs inoculated with virus alone. To investigate the physicochemical properties of the SAT factor, guinea pigs were infested with 50 uninfected R. appendiculatus nymphs and then inoculated with THO virus plus treated or untreated salivary gland extract, or with THO virus alone. Virus enhancement was measured by the number of uninfected ticks that acquired virus. Maximum enhancement of THO virus transmission was observed when salivary glands were derived from female R. appendiculatus ticks that had fed for 6 days and inoculated at a concentration of 40 µg of salivary gland protein per guinea pig. The salivary gland extract retained its biological activity after repeated freeze/thaw cycles, at pH 5, 6, and 7, and at temperatures ranging from 4–37 °C. Enhancement of virus transmission was not observed when salivary gland extract was treated with pronase or proteinase-k, indicating that either a protein or a peptide is involved. Complete characterization of the SAT factor will provide insights into the mechanism of this novel mode of tick-borne virus transmission.