A bat-associated rabies virus variant that spread among northern Arizona's terrestrial wildlife in previous years was not found in those animals in 2010.
And the overall number of rabies cases in the state dropped by nearly two-thirds from 2009 to 2010.
Decreases in carnivore populations and an increased use of vaccination programs are possible contributors to the decline in rabies cases after record-high numbers of laboratory-confirmed cases in 2008 and 2009, with 176 and 280 cases, respectively.
Craig Levy, an epidemiologist and the manager of the vectorborne disease program for the Arizona Department of Health Services, said rabies and canine distemper drove down carnivore populations, and only 102 animals tested positive for rabies infection in 2010. While the adapted virus variant wasn't found in any of Arizona's terrestrial animals that year, he said it could re-emerge as animal populations rise. Effects of the bat-associated variant are the same as for other variants.
A 2006 report in the Centers for Disease Control and Prevention's journal Emerging Infectious Diseases described the discovery in 2001 of a cluster of 19 rabid skunks infected with a bat-associated rabies virus variant, the largest recorded cluster of infection with the bat-associated variant among terrestrial mammals. The virus was isolated from the salivary glands of five affected skunks, and it was found again in 2004.
The report indicates the virus adapted from bats to terrestrial carnivores. Health authorities also found in 2008 that the virus was transmitted among foxes.
David Bergman, Arizona's director of the U.S. Department of Agriculture's Wildlife Services under the Animal and Plant Health Inspection Service, said the USDA started an oral rabies vaccination program in late July 2009 in a 4,100-sq-km area including Flagstaff. About 125,000 vaccine-filled packets were dropped from the air, and about 4,300 were distributed on the ground. Because those oral vaccines do not work on skunks, 110 of them were captured and vaccinated, as were four raccoons.
The USDA had a similar campaign in June 2010 and plans to again in summer 2011.
No rabies-infected terrestrial animals had been found in the area where vaccines were distributed since the 2009 vaccination program began, Bergman said. And results of tests performed on blood samples drawn from captured gray foxes indicate about 64 percent of those foxes had ingested vaccine through the bait packets.
Bergman said greater-than-average snowfall in late 2009 and early 2010 also potentially lowered the population density of Arizona's carnivores and the number of rabies cases.
Dr. Charles E. Rupprecht, chief of the CDC's rabies program, said the bat-associated rabies virus variant is still present in foxes and skunks in northern Arizona. Oregon state officials are also monitoring rabies infections, and CDC officials are analyzing samples from infected animals in southwestern Oregon, where a bat-associated rabies virus variant may have spread among foxes.