DEVELOPMENT OF COLORADO TICK FEVER VIRUS IN THE ROCKY MOUNTAIN WOOD TICK, DERMACENTOR ANDERSONI1

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Abstract
Larvae or nymphs of D. andersoni were allowed to engorge on hamsters which had been injected with Colorado tick fever virus. Viremia in hamsters, and the amount of virus in the ticks, were measured by determining the LD50 in suckling mice. Hamsters, injected intra-peritoneally with 0.05 to 0.25 ml of a mouse-brain suspension, usually showed about 104 to 106 LD50 of virus during a 4-day postinoculation period. Nymphs ingested from 101.5 to more than 104.5 LD50 of virus. Usually, virus concentrations remained constant during the quiescent phase of the tick, but increased from 0.2 to 3.7 logs during or shortly after metamorphosis to the adult stage. In some experiments, a drop in virus concentration was noted during the quiescent period, followed by an increase of 1.5 to 3.7 logs during or after metamorphosis. Larvae which had ingested up to 102.5 LD50 of virus also showed significant increases in virus concentrations upon molting to nymphs. However, when large amounts of virus were ingested by the larvae, no increase was noted in freshly molted nymphs. Infections acquired by the larvae were passed through the nymphs to the adults. Adult ticks usually contained from about 105 to 106 LD50 of virus; there was no difference between males and females, and there was only a slight loss of virus in adults stored under ordinary laboratory conditions for a period of about a year.

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