U. S. Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health, National Institute of Allergy and Infectious Diseases, Rocky Mountain Laboratory, Hamilton, Montana
A technique was developed based on the release of CO2 for the collection of active stages of Dermacentor andersoni (Stiles). Unengorged ticks, with the possible exception of the larvae, appeared to be guided by a CO2 gradient to its source. Ticks were recovered near the CO2 source (dry ice) with the aid of a white flannel cloth. This was accomplished by placing the cloth beneath the dry ice and allowing the ticks to crawl onto its surface, or by dragging the cloth near the dry ice site after attraction of the ticks. This system appeared to be more sensitive than the conventional flagging method for adults, and it proved useful in areas that could not be flagged properly. Observations indicated that CO2 acted as a guiding stimulus for host attainment as well as a probable mechanism which caused adult ticks to concentrate along game trails.
Colorado tick fever virus isolations from 200 unengorged adults and 350 unengorged nymphs in the study area indicated estimated infection rates of 9.7% and 0.6%, respectively. No isolates were determined for 650 unengorged larvae collected from scattered points throughout this area. This evidence supports the thesis of a maintenance of the virus by a cycle of infection between nymphs and engorging larval ticks and their small mammal hosts rather than of transovarial transmission.