Volume 31, Issue 4
  • ISSN: 0002-9637
  • E-ISSN: 1476-1645



A virus very similar or identical to Colorado tick fever (CTF) virus was recovered from the blood clot of one of 104 black-tailed jack rabbits () examined during a survey for various zoonotic agents in mammals and ticks from the University of California, Hopland Field Station, Mendocino County, California, 1974–79. This is the first reported isolation of a CTF-like virus from , and only the second time such a virus has been found in northwestern California. Mendocino County is located far outside the known distributional ranges of the most common mammalian hosts of CTF virus and of , the only proven tick vector for man. The viral isolate is very similar to a CTF-like virus previously recovered from the blood and spleen of a western gray squirrel () from San Luis Obispo County, an area also outside of the previously-known CTF area. Virus was not isolated from 14 additional species of mammals (354 specimens) or from eight species of ticks (4,487 individuals), but CTF-neutralizing antibodies were detected in 28 of 771 (3.6%) sera from seven of 15 mammalian species including significant titers (⩾1:8) in two species and one subspecies not previously reported as natural hosts, i.e., brush mouse (), pinyon mouse (), and Columbian black-tailed deer (). CTF indirect immunofluorescent antibodies also were detected in 26 of 129 (20.2%) sera belonging to four of five mammalian species tested. Neutralizing antibodies were found in sera of deer from other localities in Mendocino County, from a deer mouse from Napa County, and from a brush rabbit from Monterey County as well. These findings suggest that a virus identical or similar to CTF virus is widespread in northwestern-westcentral California, and that surveillance for human cases of CTF or a similar disease should be extended to cover this region.


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