Clinical rabies is caused by a virus from the genus Lyssavirus. Within the Lyssavirus genus, a number of other viruses have been identified that infect mammalian hosts (animal and human) causing fatal encephalitis. Rabies virus is the Lyssavirus associated with rabies in bats and terrestrial mammals around the world. The other Lyssaviruses have been identified in bats in Europe, Africa, Asia, and Australia.
Rabies is transmitted from animal to animal through transfer of virus-contaminated saliva by bites or mucous-membrane exposures. In the United States, rabies virus subtypes have become associated with the mammalian species in which the subtype is generally found. In Indiana, the North Central Skunk virus and numerous bat subtypes of rabies virus have been identified.
Since 1990, bats have been the predominate species diagnosed with rabies at the Indiana State Department of Health (ISDH) Laboratory (the only Indiana lab that does rabies testing). Bats continued that trend in 2002, as 61 of the 63 animals that tested positive were bats. The horse diagnosed with rabies in 2002 was infected with a bat strain of the rabies virus. Figure 1 shows animal rabies cases by species in Indiana for 1998-2002.
In 2002, the United States experienced three human rabies deaths. The last human rabies death in Indiana occurred in 1959.
During the five-year period 1998-2002, 3 percent of the skunks (2 skunks) submitted to the ISDH Laboratory were found to be rabies positive, contrasted with the five-year period 1978-1982 when over 30 percent of the skunks submitted to the ISDH Laboratory were rabies positive. Since 1990, all skunks with rabies originated in four southern Indiana counties (Washington, Crawford, Harrison, and Floyd). Positive skunks were identified in these counties in 1996 (1), 1997 (4), 1998 (1), and 2002 (1). The rabid skunks in 1998 and 2002 were both from Washington County. The four-year gap between cases in the same county suggests that skunk rabies continues to be transmitted but at low enough levels that ill skunks are not being observed by humans and submitted to the ISDH Laboratory.
During 1998-2002, 6.2 percent of bats submitted to the ISDH Laboratory were determined to be rabid. In 2002, there was a large increase in the number of rabid bats compared to the previous years. The increased number of positive bats in 2002 was the result of increased submissions for testing, as the percentage positive was 6.4 percent, only slightly higher than that for the five-year time period.