Rabies virus (RABV) is a member of the genus Lyssavirus within the family Rhabdoviridae. At least 16 distinct Lyssavirus spp have been formally recognized, 10 of which have been isolated from multiple bats species, and all appear to cause the disease known as rabies.1 Rabies virus infection results in a fatal encephalitic illness in mammals. Although any mammal can be infected with RABV, there are specific reservoir species that are associated with enzootic transmission. In the continental United States and US Caribbean territories (Puerto Rico and US Virgin Islands), RABV reservoir species include striped skunks (Mephitis mephitis), raccoons (Procyon lotor), Arctic foxes (Vulpes lagopus), gray foxes (Urocyon cinereoargenteus), small Indian mongooses (Herpestes auropunctatus), and a diverse assemblage of bats (order Chiroptera). Rabies virus variants are transmitted primarily within a single reservoir species, although cross-species transmission can occur.2
Among humans with rabies acquired in the United States, bat RABV variants account for most deaths3; thus, rabies in bats is a major public health concern. The RABVs associated with silver-haired bats (Lasionycteris noctivagans), Brazilian free-tailed bats (Tadarida brasiliensis), and tricolored bats (Perimyotis subflavus) are most commonly associated with human RABV infections.2,4 However, silver-haired and tricolored bats are not often found in human habitats, and there is limited knowledge of the RABV variants circulating in these inconspicuous bat species populations.2,5,6 In contrast, big brown bats (Eptesicus fuscus), little brown bats (Myotis lucifugus), and Brazilian free-tailed bats are conspicuous species that roost frequently in human-made structures, increasing the likelihood of contact with humans and domestic animals.7–9 As a result, these bat species are more commonly tested for rabies in public health laboratories.
The circulation of RABV in bats is unique to the western hemisphere.10 However, the epizootiology of bat rabies as a whole is similar to the perpetuation of RABV variants among carnivore species.11 There are approximately 60 bat species indigenous to the continental United States,12–14 Puerto Rico, and the US Virgin Islands, of which at least 31 have been reported to be infected with RABV.2,5,15,16 There are more than 20 RABV variant phylogenetic lineages described for bat species in the United States, and most are associated with individual species.2,17 Bat RABV variants may cause infection in other susceptible bat hosts, most often causing dead-end rabies spillover infections as observed for carnivore RABV variants. Multiple RABV variants may be associated with a given bat species,17–19 and transmission dynamics may be shaped by the frequency of interspecies contacts resulting from geographic range overlap, foraging, and roosting behaviors.17 The genetic similarity among bat species, as well as their geographic overlap, may be a major influence on RABV host-shift events among bats.11,20 Bat-associated RABV host shifts to terrestrial mammals have been implicated in epizootics among gray fox and striped skunk populations in the southwestern United States20,21 and are responsible for at least 1 historic shift of RABV into carnivores in the United States.20 Compared with carnivore reservoir species, less is known about RABV circulation in bat species in the United States.22 Given the impacts that bat RABV has on public health and the role of bat RABV in host-shift events, understanding the transmission dynamics of RABV variants within and among bat species is crucial.
The introduction into the United States of RABV variants associated with nonindigenous bat species, such as the common vampire bat (Desmodus rotundus) and the hairy-legged vampire bat (Diphylla ecaudata), is also a risk. The circulation of RABV among common vampire bats causes considerable economic and livestock losses throughout the geographic range of this bat species.23–25 Surveillance along the United States-Mexico border is essential to detect the potential introduction of nonindigenous bat species.26 Furthermore, in the event that nonindigenous bat species do establish colonies within the United States, local identification of novel RABV variants may serve as a sentinel signal for novel host species introduction. A timely public and animal health response to novel RABV variant introductions would minimize the risk of exposure of humans and animals and protect livestock from an incursion of rabies with potentially severe economic impacts.
Animal rabies is a notifiable event in the United States.2,5 Currently, national rabies testing is primarily performed by state public health and veterinary diagnostic laboratories. Each year, state health departments and the USDA APHIS Wildlife Services’ National Rabies Management Program submit data to the national RSS, which is maintained by the Poxvirus and Rabies Branch at the CDC. In some states, additional laboratory analysis to determine the RABV variant, referred to as virus characterization, can be performed. The CDC serves as the national reference laboratory in the United States.
The purpose of the study of this report was to assess the frequency with which bat species are identified when specimens are submitted for rabies testing, determine which rabid bat specimens merit prioritization for virus characterization, and estimate the additional testing load for public health laboratories if all prioritized specimens underwent RABV characterization.
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the CDC or the USDA.
Rabies surveillance system
Specimen of epizootiological importance
Microsoft Access 2013, Microsoft Corp, Redmond, Wash.
STATA 13.1, StataCorp LLC, College Station, Tex.
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Recommendations for storage of specimens obtained from rabid bats.
Suggested duration of storage
Any RABV-positive specimen that undergoes virus characterization should be retained for long-term storage.
Suggested preservation methods
Fresh, well-preserved cross sections of brainstem containing 3+ or 4+ intensity of fluorescein isothiocyanate–labeled antibody against RABV should be stored long term. The ideal amount of tissue to freeze and store is the amount of cross-sectioned brainstem or whole bat brain that can be fitted in half of the capacity of a 2-mL cryotube (with an O-ring cap assembly). Samples submitted in tin or large containers (volume, > 2 mL) should be aliquoted in 2-mL cryotubes after an adequate 3+ or 4+ section (as determined by direct fluorescent antibody testing) was selected.