Introduction

The annual rabies surveillance report presents the official statistics for animal rabies in the US, summarizes laboratory and epidemiological data from 54 reporting US jurisdictions, and provides an overview of rabies in Canada and Mexico during 2020. In the US, rabies is enzootic in 4 terrestrial reservoir hosts and at least 20 species of bats. These terrestrial reservoir hosts include raccoons (Procyon lotor), skunks (family Mephitidae), foxes (Vulpes spp and Urocyon cinereoargenteus), and the small Indian mongoose (Herpestes auropunctatus) exclusively in Puerto Rico. Several distinct rabies virus variants (RVVs) have been identified on the basis of viral protein–antibody reactivity and genomic sequence analysis. These RVVs have evolved in the reservoir host species over time and been compartmentalized in specific geographic regions of the US (Figure 1).¹ Dog-mediated RVVs (DMRVVs), the leading global cause of human rabies deaths, were eliminated from the US in the 1970s, yet human exposure to rabies virus remains common in the US owing to interactions with wildlife and unvaccinated domestic animals.² Ongoing surveillance is essential to detect trends in rabies, notably translocation of non-enzootic RVVs and viral host shift events, as these trends can dramatically threaten public health, animal welfare, and wildlife ecosystems.

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Distribution of major rabies virus variants (RVVs) among mesocarnivores in the US, including Puerto Rico, from 2016 through 2020. Darker shading indicates counties with confirmed animal rabies cases in the past 5 years; lighter shading represents counties bordering enzootic counties without animal rabies cases that did not satisfy criteria for adequate surveillance. Small nonenzootic areas with no rabies cases reported in the past 15 years are shaded if they are in the vicinity of known-enzootic counties and do not satisfy criteria for adequate surveillance. ARC FX = Arctic fox RVV. AZ FX = Arizona fox RVV. CA SK = California skunk RVV. E RC = Eastern raccoon RVV. MG = Mongoose RVV. NC SK = North central skunk RVV. SC SK = South central skunk RVV.

Citation: Journal of the American Veterinary Medical Association 260, 10; 10.2460/javma.22.03.0112

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Reporting and Analysis

The US National Rabies Surveillance System (NRSS) is a laboratory-based system consisting of approximately 130 public health, agriculture, and academic laboratories that conduct animal rabies testing. Additionally, 54 state, district, and territorial public health jurisdictions conduct epidemiological investigations. Diagnostic tests accepted by the NRSS include the direct fluorescent antibody test, direct rapid immunohistochemical test, immunohistochemistry, and the LN34 real-time reverse transcription PCR assay. Both genomic and antigenic typing methods are accepted for rabies virus characterization.³ The USDA Wildlife Services monitors select areas of the country to identify the geographic dispersion of rabies and to document the impacts of ongoing wildlife rabies management actions. Human and animal rabies are nationally notifiable conditions in the US, and the Council of State and Territorial Epidemiologists defines data elements, case definitions, and timelines for reporting.^4,5 During 2020, 54 jurisdictions submitted 87,895 animal samples for rabies testing (26.7 animals submitted/100,000 US human population), of which 85,483 (97.3%) had a conclusive (positive or negative) test result.

National animal rabies cases and virus characterization reports from 2020 were summarized and compared with historical temporal (Figure 2) and geographic trends by species. Results from 2020 were considered significantly different from previous results when outside the 95% CI observed during the previous 5-year period. Samples unsatisfactory for testing or with indeterminate test results were excluded from analyses. Because rabies epidemiology in the US is well described, rabid terrestrial animals and bats without RVV typing results were assumed to have the local terrestrial enzootic RVV or bat-type RVV, respectively.⁶ Criteria for counties to be considered free from terrestrial rabies are described in previous reports and based on an absence of case detection over a 5-year period while maintaining adequate testing levels.¹ Summaries of 2020 rabies surveillance for Canada and Mexico were provided by the Canadian Food Inspection Agency Centre of Expertise for Rabies and the Centro Nacional de Programas Preventivos y Control de Enfermedades of the Secretaria de Salud (Mexican Ministry of Health), respectively.

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Cases of rabies among wildlife in the US, by year and species, 1970 through 2020.

Citation: Journal of the American Veterinary Medical Association 260, 10; 10.2460/javma.22.03.0112

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Rabies in Wildlife

During 2020, 4,090 wildlife tested positive for rabies, representing a 5.0% decrease from the 4,305 rabid wildlife reported in 2019 (Table 1). The percentage of rabid wildlife among the total tested (8.9%) was not significantly different from the previous 5-year average (9.3%; 95% CI, 8.7% to 9.9%; Supplementary Table S1).

Rabies in Domestic Animals

During 2020, 389 domestic animals tested positive for rabies, representing a 1.0% increase from the 385 reported in 2019 (Table 1). The percentage of rabid domestic animals among the total tested (1.0%) was significantly higher than the previous 5-year average (0.9%; 95% CI, 0.8% to 0.9%; Supplementary Table S1).

Rabies in Humans

During 2020, antemortem samples from 12 human patients in 9 states and US territories (Virginia, Georgia [n = 2], Texas [2], Minnesota [2], Pennsylvania, Tennessee, Washington, Florida, and Puerto Rico) were submitted to the CDC for diagnostic testing. Postmortem samples from 3 states and US territories (Utah, Washington, and Puerto Rico) were also received. None were found to be positive.

There were 5 confirmed human rabies cases in 2021 (Table 3). Of these, 4 occurred after direct contact with a bat in the US (Minnesota, Illinois, Idaho, and Texas), and 1 occurred after a bite from a dog in the Philippines (New York). Median age was 66 years (range, 7 to 87 years); all were male. All 5 persons died. Postexposure prophylaxis (PEP) was not sought in 4 of the 5 cases; the Minnesota case was classified as a breakthrough infection (ie, rabies illness despite administration of PEP). This patient’s poor antibody response after PEP was attributed to a previously unrecognized immunocompromising condition. The most common reason that patients did not receive PEP was not knowing that bats can transmit rabies.⁹

National Rabies Control Efforts

Primary rabies prevention efforts in the US are led by local and state health departments. Jurisdictions employ preventative measures such as encouraging pet vaccination (to prevent secondary rabies exposure from wildlife reservoirs); providing animal control services to respond to sick, nuisance, and unwanted animals; providing risk assessments and laboratory diagnosis of animals for residents suspected to have a rabies exposure; and assisting with access to rabies PEP for persons confirmed or suspected to have been exposed to rabies. Rabies management in wildlife populations to prevent the spread and eventually eliminate specific RVVs in mesocarnivores is a collaborative effort led by the USDA Wildlife Services, the Texas Department of State Health Services, other state agencies, and the CDC. These landscape-scale programs use oral rabies vaccination (ORV) as the primary rabies control strategy targeting wild carnivore populations. Oral rabies vaccination is used to prevent specific RVVs from gaining a larger geographic footprint, lessen impacts to human and animal health, and reduce the substantial costs associated with rabies prevention and control.

During 2020, the USDA maintained an ORV zone to prevent the spread of the eastern raccoon RVV in 13 states. The zone was located near the US-Canada border in parts of Maine, New Hampshire, New York, and Vermont and then from Lake Erie at the New York-Ohio-Pennsylvania border south through the Appalachia region (Virginia and West Virginia) to the Alabama-Georgia-North Carolina-Tennessee border. An ORV zone in Massachusetts to prevent recurrence of eastern raccoon RVV cases on peninsular Cape Cod was also maintained. Additionally, state and county collaborators conducted ORV programs in local jurisdictional areas of Florida, Maryland, and New Jersey. In total, 8,367,970 baits (vaccinia-rabies glycoprotein recombinant vaccine baits [62%] and adenovirus-rabies glycoprotein recombinant vaccine baits [38%]) were distributed across > 124,000 km². Additionally, 1,180,200 baits (vaccinia-rabies glycoprotein recombinant vaccine baits) were distributed across > 41,000 km² along the US-Mexico border in Texas to prevent the reintroduction of the canine-coyote RVV.

Rabies in Canada and Mexico

Discussion

The NRSS relies on state and local jurisdictions to operate frontline rabies surveillance and management programs and share notifiable data with the CDC, in accordance with Council of State and Territorial Epidemiologists recommendations. This system has operated in the US since its inception in 1944. Although approximately 100,000 animals have been submitted for rabies testing annually in the US, only 87,895 were submitted in 2020. Compared with 2019, there were fewer samples submitted for testing each month of 2020 except for June, and there were > 20% decreases in the number of submissions in April, October, and November, which correlated with peaks observed in COVID-19 case reports.¹³ The 10.2% decline in US testing was consistent with COVID-related rabies testing declines reported from Canada and Mexico. The COVID-19 pandemic has overwhelmed public health systems,¹⁴ and stay-at-home orders may have resulted in reduced human-animal interactions. Despite a notable decrease in submissions, rabies case detection rates decreased by only a small amount, which may indicate that pandemic-associated impacts on testing were more prominent among lower-risk exposures.

The NRSS has consistently evaluated the enzootic terrestrial rabies status of counties since 2008 to infer rabies risks to people and animals. The methods described in this report are applied to every US county annually. In 2020, the number of counties in which terrestrial rabies was enzootic increased by 2% (1,335 vs 1,308), with increases occurring primarily in regions where the arctic fox RVV and California skunk RVV were enzootic. Despite not having active wildlife management programs, the number of enzootic counties in areas where the SCSK RVV and NCSK RVV were enzootic decreased by 4.3% and 7.1%, respectively. These decreases could have been due to natural epidemiological changes resulting from host-virus-ecological factors. Alternatively, limited testing and surveillance in this more sparsely populated region of the US could mask underlying enzootic cycles. The methodology used by the NRSS to determine the presence or absence of enzootic terrestrial rabies has been used for > 10 years; an evaluation of the accuracy of this definition across reservoir territories is warranted to ensure accurate public health guidance is being maintained.

The DMRVV was eliminated from the US after a decades-long campaigns of dog vaccination and population management programs.¹⁵ Maintenance of this DMRVV-free status requires regulations to moderate the importation of dogs from high-risk countries as well as systematic and actionable surveillance systems for dogs in which rabies is diagnosed in the US.^16,17 The NRSS has a goal of variant typing all rabid dogs in the US. In 2020, virus characterization of rabid dogs was significantly improved, but still only 60% of dog cases had the RVV reported to the NRSS. Fortunately, all dogs with virus characterization results had RVVs consistent with local epidemiological expectations, supporting both epidemiological and diagnostic evidence that the US DMRVV-free status was maintained.

Over the past 5 years, Puerto Rico has reported the greatest number of rabid dogs among all reporting jurisdictions, a consequence of their enzootic mongoose RVV and a relatively high proportion of free-roaming dogs. The decrease in rabid dogs in 2020 was most evident in Puerto Rico, which saw the number of rabid dogs decrease from 23 in 2019 to just 4. It is unclear whether this reduction reflects reduced surveillance during the pandemic or a true decline in cross-species transmission of the local mongoose RVV. Since 2018, 2 dogs infected with the mongoose RVV were translocated to the continental US (Maryland and Virginia). The movement of dogs within the US poses a risk for translocating nonenzootic variants. The mongoose RVV resulted from a host shift of the virus from dogs into mongooses, making this variant of greater concern to reestablish itself within susceptible canid populations. Ensuring strong vaccination and surveillance programs in Puerto Rico has direct benefits for the national program.

Rabies virus characterization is becoming a more important tool to understand the epidemiology of rabies and effects of public health and wildlife management actions. However, determining the variant of a rabies-positive sample constitutes an added cost to health departments and may not provide actionable information. The NRSS has developed criteria to determine which samples should be prioritized for virus characterization and defined these as samples of epidemiological importance (SEIs).^18,19 Given that not all samples are of equal value to rabies management, the NRSS now evaluates improvements in variant typing solely on the basis of changes in viral characterization rates for SEIs. In 2020, only 40% of SEIs had a variant result submitted to the NRSS, marking the first time since tracking that this value has decreased. This could be the result of COVID-19 pandemic effects, a reduced state capacity for testing, or a decrease in the number of jurisdictions that included this information in their notifications to the NRSS.

Supplementary Materials

Supplementary materials are posted online at the journal website: avmajournals.avma.org