Colorado Tick Fever

Abstract

Summary and Conclusions Wood ticks were collected from three areas near Denver where cases of Colorado tick fever had presumably been acquired. These ticks were divided into ten pools, nine of which showed evidence of infection with the virus of Colorado tick fever on the basis of lowered white blood cell counts. Neutralization tests were done on five of these and compared to two established laboratory strains of the virus. On the basis of the neutralizing power of one human immune serum, there was but little difference between our laboratory strains and field isolations. The wood tick has thus been shown experimentally to be a vector of Colorado tick fever. Laboratory reared adult wood ticks were carried through a complete cycle using hamsters to feed the various stages. The original ticks were considered disease-free, since none of the animals used to feed them or injected with the various stages of ground ticks showed lowered white blood cell counts. Some of the disease-free adult ticks and their larvae and nymphs were allowed to feed on hamsters infected with an established laboratory strain of Colorado tick fever virus, and were then carried through the cycle to the next generation of adults. Each stage was tested either by grinding the ticks and injecting hamsters with the supernatant or testing the hamsters on which the ticks fed, or both. Only the eggs failed to elicit evidence of infection on grinding and injecting, although the progeny from these eggs were infected. Both male and female adult ticks were found to be infected. The original virus used to infect the ticks was recovered from the progeny in an unmodified form as measured by the minimal infective dose and neutralizing power of the immune serum. The virus of Colorado tick fever is transovarially transmitted in the wood tick. Virus acquired in either the larval or nymphal stages is likewise transmitted to the adult.