Colorado Tick Fever

Abstract

Summary and Conclusions Dog ticks obtained from Long Island, New York were suspected of harboring the virus of Colorado tick fever, based on the following evidence: Hamsters injected with serum from the animal that fed the adult ticks showed a splenic reaction characteristic of Colorado tick fever, as did animals injected with ground larvae. Hamsters infected with Rocky Mountain spotted fever, Q fever, and tularemia failed to show a similar reaction. Serial passage of serum in hamsters resulted in low white blood cell counts, consistent with Colorado tick fever infection. Male guinea pigs injected with the suspect material or with virus from established laboratory strains of Colorado tick fever failed to elicit any reaction, unlike animals that are inoculated with Rocky Mountain spotted fever, Q fever, or tularemia. The agent passed a 24 mμ gradocol membrane, as do the laboratory strains of Colorado tick fever virus. Two additional strains were isolated from another lot of Dermacentor variabilis, also from Long Island. Evidence of infection was based on lowered white blood cell counts. Cross immunity experiments in which recovered animals infected with one minimal infective dose of virus and challenged three weeks later with three minimal infective doses of both eastern and western strains showed the animals to be completely protected. Neutralization tests using a human immune serum and hamster immune serums were carried out. In the first instance all the available strains of virus were tested. The eastern showed a lower titer than the western strains. When random selections of eastern and western strains were tested against hamster immune serums to these same viruses, no difference in titer was detectable. The reason for this difference is unknown. The virus of Colorado tick fever, or a very closely related virus immunologically has been isolated from Dermacentor variabilis on Long Island. The disease has not been reported from this area.