Enhancement of virus transmission by tick salivary glands

Previous studies have demonstrated that Thogoto (THO) virus is transmitted from infected to uninfected ticks cofeeding on an uninfected guinea-pig, although the guinea-pig does not develop a detectable viraemia. To investigate this mode of transmission, guinea-pigs were infected with uninfected Rhipicephalus appendiculatus nymphs prior to inoculation with either a mixture of THO virus and tick salivary gland extract, or with THO virus alone. The number of ticks that acquired the virus from feeding on animals inoculated with a mixture of virus and salivary gland extract was 10-fold greater than the number that became infected by feeding on animals inoculated with virus alone. The increase in the number of ticks that became infected was greatest when the salivary glands used in the inoculum were derived from uninfected ticks, which had partially fed for a period of 6 days. Viraemia was not detected in any of the guinea-pigs tested during the experiments. These results indicate that THO virus transmission is enhanced by factor(s) associated with the salivary glands of ticks, and that these factor(s) may facilitate 'non-viraemic' transmission between infected and uninfected ticks.