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Rocky Mountain Spotted Fever

Potentialities of Tick Transmission in Relation to Geographical Occurrence in the United States¹

1. This paper concerns the transmission of Rocky Mountain spotted fever by 8 species of North American ticks, with particular reference to their importance in the United States as known or possible carriers of the virus both in nature and to man.

2. Pertinent information relative to the known carriers, Dermacentor andersoni, D. variabilis and Haemaphysalis leporis-palustris, is briefly summarized. As regards D. variabilis, data of generation to generation passage of the virus is reported for the first time. As concerns H. leporis-palustris, the suggestion is made that certain ground-frequenting, migratory bird hosts, such as the meadowlark may, through transportation of this tick, serve as agents in the extensive distribution of the virus.

3. The protocols of transmission experiments with 5 additional species of ticks not thus far incriminated as natural virus carriers are given. With D. occidentalis, continuity of the virus has been shown from larval to adult ticks of the same generation over a period of 96 days and generation to generation transmission from female to larvae over a period of 64 days; with Rhipicephalus sanguineus, virus continuity was established from larval ticks of one generation through 6 successive stages to adults of the next generation over a total period of 407 days; with Amblyomma americanum (larval to adult continuity previously shown), generation to generation transmission was proved from female to larvae over a period of 162 days; with A. cajennense continuity from larvae to adults over a period of 81 days; with D. parumapertus marginatus transmission by adults infected as adults was demonstrated (continuity from nymphs to adults previously shown). These data are considered sufficient to indicate that each of these 5 species is a possible natural carrier of spotted fever virus.

4. The rabbit tick, a proved carrier of spotted fever in nature, is prevalent throughout the United States and it is possible that spotted fever virus may be present in this tick in sections in which tick carriers to man are not now known and from which cases have not thus far been reported. This fact and the combined ranges of Dermacentor andersoni and D. variabilis, the 2 proved agents of infection in man, indicate that spotted fever cases could originate in all but a very small portion of the country. It is also shown that one other tick, D. parumapertus marginatus, probably acts as a virus carrier in nature.

5. Four other species, D. occidentalis, R. sanguineus, A. americanum, and A. cajennense are worthy of consideration as possible additional agents of transmission to man. As regards A. americanum, evidence is cited which suggests that it may now be acting in this capacity.

6. It is indicated that spotted fever virus is adapted to survival through successive stages of and to transmission by a variety of genera and species of Ixodidae, and that in all sections of the United States there are ticks that are actual or potential carriers of the virus either in nature only or to man as well.

¹ Contribution from the Laboratories of the U. S. Public Health Service and Montana State Board of Entomology, Hamilton, Montana.