RABIES VIRUS (RABV)
LEVELS: Likely to occur: Effective control measures not fully understood; Public exposure risk: Non-foodborne transmission pathways, beyond occupational exposure, that can lead to human infection; Unlikely to be effective: One or more pathways of farm-to-farm transmission exist that cannot be controlled by on-farm biosecurity; Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s); Moderate: Manageable losses related to endemic (population) or chronic (individual) occurrence; Negligible: Little or no market disruption when disease occurs on one or more farms; Minimal risk: Agent inherently unlikely to develop clinically important resistance to antibacterial or antiviral treatments; Minimal risk: Antibacterial or antiviral treatments rarely occur, or are typically limited to short-course individual animal therapy; No availability: Effective treatments not currently available in the US (or have not been developed); No availability: Effective vaccines not currently available in the US (or have not been developed); Not feasible: Eradication extremely unlikely
OVERVIEW
Rabies is caused by members of the genus Lyssavirus in the family Rhabdoviridae, with Rabies virus (RABV) being the most significant species globally. RABV has a wide geographic distribution and broad host range, existing worldwide except for certain islands and countries with strong geographical barriers. Natural reservoirs include several bat species and terrestrial wildlife mammals (foxes, skunks, mongooses, raccoons) with spillover to domesticated species including dogs, cats, and farm animals. In pigs, RABV infection is extremely infrequent compared to cattle, probably reflecting the physical separation between wildlife and swine in modern confinement operations. The United States averages about one case per year in domestic swine. Cases in pigs have been associated with exposure to dogs, bats (Brazil and China), and wildlife (raccoons, foxes) in Canada and the United States. Rabies presents as acute fatal encephalomyelitis, classically divided into furious and dumb (paralytic) forms. Clinical signs in pigs include behavioral changes, progressive neurological dysfunction, paralysis, and death. The incubation period is highly variable, ranging from 17 to 132 days depending on bite location and viral dose. Once clinical signs develop, RABV infection is usually fatal, though rare recovery has been documented in experimentally infected pigs. Diagnosis requires laboratory testing of brain tissue using direct fluorescent antibody (DFA) testing. There are no licensed rabies vaccines for swine, and prevention relies primarily on controlling disease in wildlife reservoirs and protecting swine from wildlife contact through physical barriers.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Likely to occur: Effective control measures not fully understood
Rabies is recognized as "an important zoonotic disease because of its high mortality rate in humans." While the primary transmission route is through bite wounds rather than food consumption, the presence of virus in tissues of infected animals creates theoretical foodborne risk. The chapter notes that "transmission occurs via the bite of an infected animal that has virus in its saliva" and that virus spreads centrifugally to "non-nervous tissue, including epidermis, cornea, epithelium of the mouth, nasal mucosa, intestine, lacrimal glands, pancreas, muscle fibers, myocardium, lungs, kidneys, adrenal medulla, and salivary glands." The widespread tissue distribution in infected animals means that handling, butchering, or consuming undercooked meat from rabid animals could theoretically pose risk, though documented foodborne transmission is not the typical route. The documented zoonotic capacity of rabies and its presence in multiple tissues justifies this level assignment.
NON-FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Public exposure risk: Non-foodborne transmission pathways, beyond occupational exposure, that can lead to human infection
Rabies is one of the most important zoonotic diseases globally due to its near 100% case fatality rate in humans once clinical signs develop. The chapter explicitly states that "rabies is an important zoonotic disease because of its high mortality rate in humans." However, regarding swine specifically as a source: "there is little documented evidence of swine as a source of rabies for humans." The chapter cites that "of 521 people exposed to presumed rabid swine in the subcontinent of India, Pakistan, and Bangladesh from 1908 to 1972, none developed rabies. Notably, each exposed person received rabies prophylaxis." Nevertheless, the chapter advises that "studies have not been conducted to determine the amount and duration of virus shedding in swine, but it should be assumed that virus is present when determining post-exposure treatment options in humans." This precautionary approach reflects the documented zoonotic capacity of RABV even if swine-to-human transmission is rarely documented.
EFFECTIVENESS OF ON-FARM BIOSECURITY IN PREVENTING FARM-TO-FARM TRANSMISSION
Level: Unlikely to be effective: One or more pathways of farm-to-farm transmission exist that cannot be controlled by on-farm biosecurity
RABV transmission bypasses standard biosecurity because it depends on wildlife contact rather than typical agricultural transmission routes: (1) Wildlife reservoir transmission: "the predominant RABV reservoirs are foxes in Europe; dogs and foxes in Asia; dogs, jackals, and mongooses in Africa; dogs and vampire bats in South and Central America; and foxes, raccoons, skunks, and bats in North America"; (2) Unpredictable wildlife incursions: "epidemiologic investigations may identify events that support the possible exposure of pigs to rabies-infected wildlife such as the presence of a skunk in an outside pen"; (3) Large outbreaks possible: "in China, 20 of 56 pigs died after being bitten by a rabid dog"; (4) Direct bite transmission: "transmission occurs via the bite of an infected animal that has virus in its saliva"—this requires direct wildlife contact that standard biosecurity (focused on fomites, vehicles, personnel) cannot fully prevent. While confinement housing provides significant protection ("the spillover of rabies into swine is low and probably reflects the separation between wildlife and swine that occurs with confinement operations"), outdoor or semi-outdoor production systems remain vulnerable to wildlife incursions.
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s)
Clinical recognition of rabies in swine is challenging because "published reports on the clinical signs of rabies in swine are limited and, as in other species, are not consistent." Clinical signs overlap with other neurological diseases: "RABV infection must be differentiated from other viral agents that may produce neurologic signs, e.g. porcine teschovirus, encephalomyocarditis virus, pseudorabies virus, West Nile virus, Japanese encephalitis virus, porcine reproductive and respiratory syndrome virus, porcine sapelovirus, and eastern equine encephalomyelitis virus infection." However, once rabies is suspected, laboratory confirmation is well-established: (1) Direct fluorescent antibody (DFA): "the preferred diagnostic test in animals, including swine, because of its speed and accuracy"—detects viral antigen in brain stem and cerebellum; (2) RT-PCR: "a good alternative for the detection of RABV, particularly for testing samples with low viral concentration"; pan-lyssavirus assays can detect RABV and 13 other lyssaviruses; (3) Virus isolation: in cell culture or mouse inoculation, though not routinely used; (4) Histopathology: Negri bodies in neurons are "almost pathognomonic for the diagnosis of rabies" though their absence has been described in infected pigs. Antemortem testing via corneal impressions and tactile hair biopsies is possible but postmortem brain examination is standard.
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Moderate: Manageable losses related to endemic (population) or chronic (individual) occurrence
Rabies causes significant but infrequent losses in US swine production: (1) Very low incidence: "the United States averages about one case per year in domestic swine"—the spillover rate is low due to confinement housing; (2) High case fatality: "once clinical signs develop, RABV infection is usually fatal"—affected animals are lost; (3) Outbreak potential: when exposure occurs, significant losses can result—"in China, 20 of 56 pigs died after being bitten by a rabid dog" (36% mortality in the exposed group); (4) Variable incubation: the highly variable incubation period (17-132 days documented) creates uncertainty about which animals were exposed and may develop disease. The economic impact at the national level is minimal due to the extreme rarity of cases in swine. Individual farm impact when an outbreak occurs can be significant but remains manageable rather than catastrophic given the infrequent occurrence.
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Negligible: Little or no market disruption when disease occurs on one or more farms
Rabies detection in swine does not trigger significant trade restrictions or market consequences: (1) Not a trade-limiting disease: rabies is endemic in wildlife populations throughout most of the world; presence in livestock does not change a country's or region's rabies status; (2) No movement restrictions on swine industry: unlike diseases such as FMD or CSF, rabies cases in swine do not result in industry-wide quarantines or export bans; (3) Localized response: investigation focuses on identifying the source of exposure (wildlife, dogs) and potentially exposed animals rather than broad regulatory action; (4) Public health focus: the primary concern is human exposure assessment and post-exposure prophylaxis decisions rather than trade implications. The main consequence of detection is the need to assess human and animal contacts for post-exposure treatment decisions.
PATHOGEN'S ABILITY TO DEVELOP AND SPREAD RESISTANCE
Level: Minimal risk: Agent inherently unlikely to develop clinically important resistance to antibacterial or antiviral treatments
RABV is a viral pathogen (negative-sense single-stranded RNA lyssavirus) that does not carry, acquire, or transmit antimicrobial resistance genes. The virus poses no AMR concerns. The chapter discusses viral diversity in terms of phylogroups and the observation that "current licensed RABV vaccines are able to protect against lyssaviruses within phylogroup I but failed to protect against lyssaviruses associated with phylogroups II and III," but this represents inherent viral diversity rather than antimicrobial resistance.
AMR DEVELOPMENT DRIVEN BY DISEASE MANAGEMENT
Level: Minimal risk: Antibacterial or antiviral treatments rarely occur, or are typically limited to short-course individual animal therapy
No antiviral treatments exist for rabies. The disease progresses rapidly to death once clinical signs appear, so antimicrobial treatment of affected animals is not practiced. Exposed but asymptomatic animals may be quarantined and observed, but antimicrobial treatment is not part of rabies management. Given the extreme rarity of rabies cases in US swine (approximately one case per year nationally), any antimicrobial use associated with this disease would have negligible impact on AMR development.
AVAILABILITY OF EFFECTIVE TREATMENT OPTIONS
Level: No availability: Effective treatments not currently available in the US (or have not been developed)
No specific treatments exist for rabies infection. The chapter states that "once clinical signs develop, RABV infection is usually fatal." Management options are extremely limited: animals can only be quarantined and observed if exposure is suspected but clinical signs have not developed. Post-exposure prophylaxis (vaccination after bite exposure) is used in humans and valuable animals of other species but is not routinely practiced in swine. Interestingly, the chapter notes that rare recovery from rabies has been documented in experimentally infected pigs: "four of six pigs developed clinical signs, including progressive paralysis... the signs subsided in 1-2 weeks with the pigs developing high antibody titers to RABV." However, this represents spontaneous recovery rather than treatment, and cannot be relied upon clinically.
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: No availability: Effective vaccines not currently available in the US (or have not been developed)
The chapter explicitly states: "Because of the expense of conducting duration-of-immunity efficacy tests and the limited market, there are no licensed rabies vaccines for use in swine." While vaccines exist for other species (dogs, cats, horses, cattle), no products are licensed for swine in the US. The chapter notes that "off-label use of inactivated vaccines may be justified in expensive breeding stock in rabies-endemic areas, but vaccine efficacy has not been demonstrated." Prevention relies on other measures: "the best way to prevent rabies is to control the disease in wildlife reservoirs and use physical barriers to protect swine from wildlife." This reflects both the limited market for swine rabies vaccines and the effectiveness of confinement housing in preventing exposure.
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Not feasible: Eradication extremely unlikely
Rabies eradication from the US is not feasible because: (1) Established wildlife reservoirs: "foxes, raccoons, skunks, and bats in North America" serve as endemic reservoirs; multiple distinct wildlife rabies cycles exist geographically; (2) Bat reservoirs: bat rabies cannot be eliminated and provides ongoing source of spillover to terrestrial animals; (3) Multiple species involved: the complexity of multiple reservoir species in different geographic regions makes coordinated elimination impossible; (4) Continuous threat: even areas that achieve control of terrestrial rabies cycles remain at risk from bat rabies; (5) No vaccines for swine: without licensed vaccines, protecting susceptible swine populations is limited to physical barriers. While significant progress has been made in controlling specific terrestrial rabies cycles (such as raccoon rabies through oral vaccination programs), complete eradication of rabies from North American wildlife is not achievable with current tools. For swine, the practical approach is containment through confinement housing and controlling disease in wildlife and domestic dog/cat populations rather than attempting elimination.