Rabies virus is a neurotropic virus of the genus Lyssavirus that causes fatal encephalitis in nearly 100% of infected mammals. There are 8 terrestrial RABV variants that circulate in the United States1 in 5 terrestrial wildlife reservoir species, namely striped skunks (Mephitis mephitis), raccoons (Procyon lotor), mongooses (Herpestes javanicus), arctic foxes (Vulpes lagopus), and gray foxes (Urocyon cinereoargenteus). Although rabid domestic animals are identified every year, > 90% of all rabid animals in the United States identified since 1980 have been wildlife species.2
Rabies in an animal is a notifiable event in the United States.3 Each year, state and territorial health departments and the National Rabies Management Program of the USDA APHIS Wildlife Services submit data to the US animal RSS, which is maintained by the CDC's Poxvirus and Rabies Branch. The public health animal RSS in the United States is used to monitor rabies-related events in domestic and wildlife species. Virus characterization by molecular sequencing can determine the variant of the virus in a rabies-infected animal and is a valuable tool for identifying and understanding important events in the epizootiology of rabies in the United States. However, virus characterization is not routinely performed on all RABV-positive specimens. Although the public health animal RSS is robust, there is no clear protocol for identification of specimens that should undergo virus characterization. It would be ideal to characterize all RABV-positive specimens; however, this is neither financially nor logistically feasible. A strategy for selection of RABV-positive specimens for virus characterization that is based on existing animal RSS data would be useful.
Changes in the epizootiology of RABV have the potential to increase the risk of exposure of humans and domestic animals, impact animal control measures, and influence public health policy. Four types of events that have or could substantially alter animal rabies epizootiology in the United States include changes that occur in relation to host-shift events, translocation of RABV variants not previously documented within geographic areas, introduction of novel RABV variants (including importation events) over a given period, and any unusual rabies-related incidents.
Several examples of recent host-shift events highlight their wide-reaching impact. Of the 8 terrestrial wildlife RABV variants in the United States, 5 (California skunk, north-central skunk, mongoose, Texas gray fox, and Arizona gray fox) were the result of a host-shift event from dogs. For example, in the late 1980s, the gray fox (U cinereoargenteus) was recognized as an RABV reservoir species, with a novel RABV variant (Texas gray fox virus variant) consistent with a canine lineage.4,5 The raccoon RABV variant and south-central skunk RABV variant were the result of host-shift events from bats.6 Host-shift events from bats to carnivores can also have important public health implications, especially in areas where terrestrial animal rabies has been eliminated.6,7 In 2001, cross-species transmission of a bat RABV variant into the striped skunk population in Flagstaff, Ariz, occurred, causing a rabies outbreak in striped skunks.7 Bouricki et al8 determined that a 2009 rabies outbreak among gray foxes in California was likely caused by a host shift of the California skunk RABV variant into gray foxes.8 The frequency with which RABV has shifted between hosts6,8,9 emphasizes the importance of ongoing surveillance of RABV distribution among wildlife species and application of virus characterization to identify and understand changes in the epizootiology of rabies in the United States.
Translocation of rabid animals has led to large-scale epizootiological changes with regard to rabies. In the 1970s, a presumed human-mediated translocation of raccoons from their enzootic zone (Florida) to a rabies-free zone in the Mid-Atlantic region was responsible for arguably one of the most important public health events in the United States.10 The newly established raccoon RABV variant spread throughout the east coast of the United States, from Georgia to the border with Canada.10 The raccoon RABV variant now accounts for > 70% of all animal rabies cases in the United States and was the cause of 2 human deaths directly and 2 additional human deaths through transplantation of infected organs.2,11
Since 1995, the USDA has been working with local, state, and federal government partners to conduct ORV of wildlife in targeted areas to prevent the spread of specific terrestrial RABV variants. Enhanced rabies surveillance in these areas is critical, and the data are used to adapt the USDA's rabies management practices and to monitor for RABV translocation events.12,13
The canine RABV variant was eliminated from the United States by the 1970s.5 However, in 1988, a novel domestic dog-coyote RABV variant was introduced from Mexico and became enzootic in coyotes (Canis latrans) along the border between the United States and Mexico.14 In addition to numerous domestic animal and wildlife cases of rabies, the outbreak resulted in 2 human deaths.15,16 In 1994, the dog-coyote RABV variant was detected in Alabama and Florida (likely a result of interstate transport of infected coyotes for hunting purposes) and caused local outbreaks in domestic dogs.4
The United States regained its canine RABV variant-free status in 2007,5 but multiple importations of infected dogs since that time highlight the importance of early identification of such dogs to prevent future outbreaks. Since 2007, the importation of dogs infected with the dog RABV variant has resulted in administration of postexposure prophylaxis to > 50 people, and > 90 animals (unpublished data) have had to be given booster vaccinations and undergo quarantine. Total public health response costs for each importation event have been > $250,000.17–19
The purpose of the evaluation reported here was to identify RABV-positive specimens submitted to public health laboratories that should be considered for routine virus characterization because of their potential to optimize detection of or provide information regarding the 4 key events of epizootiological importance (ie, SEIs). Characterization of specimens not classified as SEIs was also undertaken. In addition, the increased test load and associated costs that laboratories could expect if all those RABV-positive SEIs underwent virus characterization were estimated. We considered that identification and prioritized testing of SEIs would enhance rabies stakeholders’ abilities to better target rabies management efforts.
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the CDC or USDA.
The authors thank Xiaoyue Ma and Lauren Greenberg for technical assistance.
Oral rabies vaccination
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 animals.
Suggested duration of storage
Any RABV-positive specimen that undergoes virus characterization should be retained for long-term storage. Additionally, 10% of RABV-positive specimens from reservoir species (raccoon, skunk, arctic fox, and gray fox) should be similarly stored. Specimens submitted for long-term storage should be geographically and temporally representative throughout the year.
Suggested preservation methods
Fresh, well-preserved cross sections of brainstem that contain a 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 tissue or whole small rodent brain that can be fitted in half the capacity of a 2-mL cryotube (with an O-ring cap assembly). Specimens submitted in tin or large containers (volume, > 2 mL) should be aliquoted in 2-mL cryotubes after an adequate 3+ or 4+ section was selected by direct fluorescent antibody testing. For small rodents, if whole brains are not available, complete carcasses should be submitted in plastic biosafety bags.