Rabies is a viral disease normally transmitted by the bite of an animal. The virus affects the brain and is usually fatal. All warm-blooded animals are susceptible to rabies, but certain carnivorous mammals and bats are most likely to be sources of infection.
The major host species for animal rabies in the United States has changed during the last 40 years with a marked decline of reported cases in dogs, cats, and foxes. At the same time, reported cases in skunks, raccoons, and bats have increased, and the geographical distribution of rabies in these species has expanded. In Colorado and Wyoming, similar changes in the host species have occurred, except that rabies in raccoons has remained rare.
In Colorado, rabid bats have been reported most frequently from the densely populated counties, particularly around the Denver metropolitan area, although they have been reported from 27 counties and all sections of the state. Typically rabid bats are reported every year in Colorado with Boulder, Denver, El Paso and Larimer Counties having the highest number of rabies-positive bats each year. Other than rabies in bats each year, rabies in other species had been uncommon until 2007. In 2007, a rabies positive coyote and skunk were reported from Prowers County, and subsequent positive skunks from Washington, Mesa and Las Animas Counties also were detected. All were diagnosed with “South Central skunk strain” rabies, except the skunk from Mesa County, which could be strain typed due to the condition of the tissue. In 2008, 18 of 89 skunks positive with skunk strain rabies were submitted from Arapahoe, Baca, Powers, Cheyenne and Kiowa Counties. An additional rabies-positive skunk was found in Denver County that had bat strain rabies. In addition, a skunk strain rabies positive raccoon (first positive raccoon in Colorado since 1963) and a skunk strain rabies positive house cat (first positive cat in Colorado since 1985) were found in Cheyenne County.
The numbers of rabies-positive animals increased in 2009 with 103 animals representing five species being reported. Skunks were the most numerous (37) and were found in ten Counties east of I-25. Those counties were: Kiowa, Kit Carson, Lincoln and Prowers Counties (1 each); Otero County (2); Elbert, Arapahoe and Yuma Counties (5 each); El Paso and Morgan Counties (8 each). In addition rabid red fox were found in Prowers and El Paso Counties (1 each) and one mountain lion from El Paso County. El Paso County also reported a horse that tested positive for rabies. Rabies in bats was reported at 62 from 13 counties, similar to numbers reported in previous years.
To date (5/20/2010) the number of rabies cases in Colorado in 2010 is significantly higher than found during the same time period for 2009. A total of 43 rabies positive animals have been diagnosed, including 38 skunks, one horse, one muskrat, one mule deer, one red fox and one cat.
Rats, mice, squirrels and other rodents have been examined over the years, but have rarely been found to be rabid.
Rabies Virus and Transmission
Rabies virus in the saliva of the infected animal enters the tissue of the victim when bitten by the rabid animal. If the victim is susceptible, the virus travels slowly up nerve fibers to the part of the brain that controls the area bitten. The virus multiplies and spreads to other parts of the brain, eventually causing a variety of signs in the infected animal. At the same time, the virus spreads from the brain to the salivary glands and is released in the saliva to be introduced by bite into another animal, thus perpetuating the virus in nature.
The virus can also be transmitted by contamination of cuts and abrasions of the skin with infected saliva or virus-infected tissues. Oral transmission by eating infected animals or tissues has also been demonstrated in certain animal species. Transmission by aerosol where animals and humans acquired rabies infections by breathing the air in bat caves has only been reported in Texas. No arthropod vector is involved in the transmission of rabies virus. Not all infected animals will have rabies virus in their saliva as some die before the virus leaves the brain. It is important to note that the virus will rarely be present in the salivary glands or saliva without first occurring in the brain. With the possible exception of several species of bats, the virus is present in the saliva of the infected animal for no more than a few days before clinical signs appear. Rabies is not transmitted by the spray from skunks.