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Rabies in Local Bat Populations

Rabies in Colorado is found almost exclusively in bats, and the big brown bat is the species most commonly submitted to public health agencies in Colorado for rabies diagnostic examination. For a detailed summary of other profiles in potential pubic exposure to rabies in Colorado bats see the publication by Pape et al. (1999) [pdf] We found similar patterns in more recent public health records for Larimer County and Fort Collins, although the proportion of bats examined that were rabies positive was somewhat lower.

We also found that there are multiple variants of the rabies virus circulating in Colorado. [pdf] Generally, each species of bat has a unique virus variant (based on sequence differences in the N gene). These variants can appear in another species of bat, but only infrequently. Our findings suggest that interspecies transmission of the rabies virus between bats is limited. Big brown bats harbor more than one variant of rabies virus, but this virus variant is not restricted to differing genetic lineages of bats found in Colorado.

Saliva swabs being taken from big brown bats for testing of rabies viral RNA

We never found evidence of epizootics or die-offs in our bat population due to rabies, and the literature provides little convincing evidence of such events in U.S. bat populations in general. However, we analyzed many hundreds of serum samples from seemingly healthy bats that were captured in Fort Collins, and found evidence of widespread exposure to the virus in bats (seropositive for antibodies) without evidence of productive infection (no rabies). The presence of anti-rabies antibodies was found in bats from almost every colony we sampled, and in every year. These antibodies were not accompanied by any evidence of viral RNA in the bats saliva, as determined by RT-PCR on swabs of bats oral cavities. There was also no evidence of the virus in any tissues of a smaller sample of bats analyzed for this purpose. PIT-tagged bats that were seropositive were known to be alive in the population for several years after first sampling, providing additional evidence that these bats were not infected (if so they would have died much sooner). This information supports the hypothesis that bats can acquire immunity to rabies. This may be through low-dose exposure after bites from rabid bats. The presence of antibodies in juvenile bats suggests the presence of passive immunity transferred to offspring from previously exposed females.

Similar prevalence of antibodies without evidence for rabies in seropositive bats was found in other populations of big brown bats we sampled in Colorado, and in several other species of insectivorous bats, including hoary bats, silver-haired bats, and several species of Myotis [Fort Collins bat species]. This suggests that the acquired immunity hypothesis extends beyond the species and population we studied most intensively.

Obtaining a blood sample from a big brown bat for serology

Our current modeling work suggests that the ability of bats to develop immunity to rabies plays a key role in maintaining the virus at low levels in the population without causing die-offs. The lengthy period of bat hibernation, when the virus also remains inactive, also appears to play an important role. The significance of bat population dynamics to this system is currently under investigation.

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