Jean S. Smith

Epidemiology of Human Rabies in the United States, 1980 to 1996

One of the oldest recognized zoonotic diseases, rabies continues to plague humankind and causes more than 35 000 deaths annually [1]. These potentially preventable deaths occur primarily in Asia, Africa, and Latin America, where animal control, vaccination programs, and effective human postexposure prophylaxis are not widely available. In contrast, in the United States, deaths in humans caused by rabies totaled 99 in the 1950s, 15 in the 1960s, 23 in the 1970s, 10 in the 1980s, and 22 from 1990 through 1996 [2, 3]. The epidemiology of human rabies is ultimately linked to cycles of rabies virus transmission in animals. With the interruption of dog-to-dog transmission in most regions, the incidence of human rabies in the United States has reached a level that cannot be further reduced without targeting wildlife. An understanding of epidemiologic patterns of rabies virus maintenance in natural populations has emerged in the past 20 years, largely because of advances in immunology and molecular biology. Monoclonal antibody and genetic sequence analyses of rabies virus variants permit detailed descriptions of enzootic maintenance cycles of specific virus variants in the United States [4, 5]. These analyses have led to an understanding of how variants of rabies virus are maintained in natural reservoirs within geographic regions and have provided information on variability of the virus itself. Current epidemiologic patterns of rabies in the United States can be summarized as follows: The annual reports of rabies in wildlife exceed those of rabies in domestic animals [6]; rabies variants in bats are associated with a disproportionate number of infections in humans, although bats constitute only about 10% of all reported rabies cases in animals annually; most other cases of human rabies diagnosed in the United States can be attributed to infections acquired in areas of enzootic canine rabies outside of the United States; most persons with a case of rabies that originated in the United States have no history of an animal bite; and rabies is diagnosed after death in more than one third of the latter group. The last published summary of cases of human rabies in the United States covered the period from 1960 to 1979 [3]. This review discusses the clinical and epidemiologic features of cases of human rabies in the United States from 1980 to 1996. Methods Case Definition This report includes all laboratory-confirmed cases of human rabies in the United States or its territories from 1980 to 1996 [7-31]. All of the cases were reported to the Centers for Disease Control and Prevention (CDC) by health authorities as part of ongoing national surveillance. Variable Definitions Onset of illness was defined as either the first day of reported symptoms attributable to rabies or the date of initial presentation for medical care before confirmation of rabies. Clinical signs attributable to rabies included paresthesia, anxiety, agitation, confusion, disorientation, hydrophobia, aerophobia, hypersalivation, dysphagia, paresis, paralysis, and fluctuating levels of consciousness [32, 33]. The type of transmitting animal and the geographic location of exposure were listed if the case history included a definite animal bite. The reliability of information that linked rabies exposure to a human was assessed by subsequent laboratory typing of the rabies virus variant. All other exposures were defined as unknown. The diagnosis of rabies was considered antemortem if it was tentatively made and samples were obtained specifically for rabies testing before the patients death. Laboratory Tests The diagnosis of rabies was confirmed by using standard tests [34] conducted at the CDC or at a state laboratory. Serology Two tests were used to detect rabies antibody: the rapid fluorescent focus inhibition test and the indirect immunofluorescence assay. The rapid fluorescent focus inhibition test measures neutralizing antibody. An antibody titer of 1:5 or more, as defined by the reciprocal of the serum or cerebrospinal fluid dilution that reduces the challenge virus by 50%, was considered positive. An indirect immunofluorescence assay, using patient serum or cerebrospinal fluid diluted 1:4 or more, detects serum reactive with rabies antigen in infected cell cultures. The presence of antibody in serum was considered diagnostic if no vaccine or antirabies serum was given to the patient. Antibody in the cerebrospinal fluid, regardless of the rabies immunization history, was considered indicative of rabies virus infection. Virus Isolation Suspensions of brain or saliva specimens were added to mouse neuroblastoma cells and cultured for 24 and 48 hours. Culture slides were fixed and examined by direct immunofluorescence assay for antigen. Samples that were initially negative were maintained for an additional 3 to 4 days and retested. The negative result was considered definitive if it occurred both times. Antigen Detection Antigen detection was performed by direct immunofluorescence of assay serial frozen sections of nuchal skin biopsy specimens, touch impressions of corneal epithelial cells, or fresh brain matter. Paraffin-embedded fixed brain matter was sectioned and enzyme-digested before direct immunofluorescence. RNA Detection Standard extraction procedures and reagents were used to obtain nucleic acids from samples of undiluted saliva; from fresh or paraffin-embedded fixed samples of the brain; or, occasionally, from other tissues. Reverse transcription of RNA and complementary DNA amplification were performed by polymerase chain reaction (PCR) with primers derived from the sequence of the N protein gene. The nucleotide sequence of all PCR products was obtained by standard dideoxynucleotide sequencing methods. Rabies virus variants were identified by comparing samples of rabies virus obtained from all known reservoirs for rabies in the United States [5] with samples of rabies virus obtained from dogs in Asia, Africa, and Latin America [35]. Statistical Analysis Data analyses were performed by using EPI INFO 6 (Centers for Disease Control and Prevention, Atlanta, Georgia) or SPSS 6.0 for Windows (SPSS Inc., Chicago, Illinois) [36, 37]. Specific tests are identified in the text. Some variables were dichotomized before statistical comparisons for determination of odds ratios and 95% CIs. All reported P values are for two-tailed tests of significance. Results Demographic Information Thirty-two persons died of rabies in the United States from 1980 through 1996. Patients ranged in age from 4 to 82 years (median, 27 years) and 20 (63%) were male (Table 1). Cases were reported from 20 states; 7 cases (22%) were reported in California and 6 in Texas. Eleven patients were exposed to rabies in eight foreign countries on the basis of variant typing. The onset of illness occurred in all months and had no apparent seasonal pattern. Dates of exposure, based on the history of an animal bite, were obtained for 7 patients (22%) (Table 1). Table 1. Human Rabies in the United States, 1980-1996 Exposure History A definite history of animal exposure was identified in 7 of the 32 patients (22%), and 25 remained unknown or indefinite (Table 1). Of the 7 cases of definite exposure, 6 resulted from a dog bite received in a foreign country and 1 was from a bat bite received in the United States. Although rabies was not diagnosed in any of the animals that inflicted bites, in each case the rabies virus variant identified in the human sample was consistent with that in the animal species implicated as the source of infection (Table 1). Contact with an animal, thereby suggesting the source for infection, was identified in 12 persons (8 with a bat, 2 with a dog, 1 with a cow, and 1 with a cat). This human-animal contact, however, could not be linked to a bite or mucous membrane contact with the saliva of an animal potentially infected with rabies virus. The remaining 13 patients did not report animal contact; thus, a potential source of exposure was not identified. Histories were obtained before death from friends or relatives in 9 cases and from 4 children aged 11 to 13 years. Prophylaxis None of the 32 patients received a complete series of rabies prophylaxis after exposure; patient 7 reported receiving a single injection of an unknown type after a dog bite in Guatemala, and patients 15, 29, and 30 received human rabies immune globulin during the course of their clinical illness. Clinical Presentation For the 7 patients in which a definite animal bite occurred, the median incubation period was 85 days (range, 53 to 150 days). The first signs and symptoms of rabies were often nonspecific, including fever, sore throat, chills, malaise, anorexia, headache, nausea, vomiting, dyspnea, cough, and weakness. Specific symptoms, such as paresthesias at or near the presumed exposure site, were also reported early in the clinical course, and 19 of the 32 patients (59%) had three or more clinical findings suggestive of rabies during the course of their illness (Table 2). The 32 patients were seen by physicians on an outpatient basis a median of one time (range, 0 to 5 times) before hospitalization, and the median length of time from the onset of illness attributable to rabies to hospitalization was 4 days (range, 1 to 10 days). Table 2. Clinical Findings Suggestive of Rabies in 32 Patients* On admission, 21 of the 32 patients (66%) were febrile (oral temperature > 37.8C), including 12 patients with temperatures greater than 39.5C. Of the 11 patients who were afebrile on admission, 5 reported being febrile before admission, 2 became febrile within 2 days of admission, and 4 had no additional temperatures recorded. The antemortem diagnosis of rabies was first considered at the time of hospitalization in 5 patients, within 1 day of hospitalization in 5 patients, and after a median of 6 days of hospitalization (range, 2 to 12 days) in 10 patients. In 12 patients, rabies was diagnosed after death. Th

Emerging Epidemiology of Bat-Associated Cryptic Cases of Rabies in Humans in the United States

In the United States, during the past half-century, the number of humans to die of rabies dramatically decreased to an average of 1-2 per year. Although the number of deaths is low, most deaths occur because individuals are unaware that they had been exposed to and infected with rabies virus, and, therefore, they do not seek effective postexposure treatment. Molecular epidemiological studies have linked most of these cryptic rabies exposures to rabies virus variants associated with insectivorous bats. In particular, virus variants associated with 2 relatively reclusive species, the silver-haired bat (Lasionycteris noctivagans) and the eastern pipistrelle (Pipistrellus subflavus), are the unexpected culprits of most cryptic cases of rabies in humans.

Unexplained Rabies in Three Immigrants in the United States a Virologic Investigation

BACKGROUND Extensive investigation of three patients who died of rabies in the United States failed to reveal any source of exposure to the disease. The three patients had immigrated to the United States from areas in Laos, the Philippines, and Mexico where rabies is endemic. METHODS We studied rabies viruses isolated from the three patients, other patients with a known source of exposure, and animals in the United States, Thailand (as a proxy for Laos), the Philippines, and Mexico. The viruses were characterized by indirect immunofluorescence and neutralization tests according to their reactions to panels of monoclonal antibodies. Transcribed complementary DNA from these isolates was amplified by the polymerase chain reaction; the DNA product was then analyzed by differential digestion with restriction enzymes. RESULTS The viral isolate from each of the three patients was a rabies variant with distinctive antigenic or genetic characteristics. For each of the three isolates, identical variants were found in specimens from rabid animals obtained from or near the country in which the patient lived before immigrating to the United States. None of these variants were found among the isolates collected from rabid animals in the United States. CONCLUSIONS Rabies infection in these three patients did not originate in the United States but resulted from exposures in Laos, the Philippines, and Mexico. Since the three patients had lived in the United States for 4 years, 6 years, and 11 months, our findings suggest that the onset of the clinical manifestations of rabies occurred after long incubation periods.

Bat lyssaviruses (Aravan and Khujand) from Central Asia: Phylogenetic relationships according to N, P and G gene sequences

Bat lyssaviruses Aravan and Khujand were isolated in southern Kyrgyzstan in 1991 and in northern Tajikistan in 2001, respectively. Preliminary studies with anti-nucleocapsid monoclonal antibodies suggested that the viruses were distinct from other lyssavirus serotypes. These data were supported by sequencing of the N gene of Aravan virus. In the present study, we sequenced the entire N, P and G genes of both Aravan and Khujand viruses and compared them with respective sequences of other lyssaviruses available from GenBank. The results suggested that each virus should be considered as a newly recognized genotype according to the current approaches for genotype definition (amount of nucleotide identity of the N gene and bootstrap support of joining to certain phylogenetic groups). Use of different phylogenetic methods and comparison of different parts of the genomes generally suggested that Khujand virus was mainly related to genotype 6, while Aravan virus, on the one hand, was related to Khujand virus, and, on the other hand, demonstrated moderate similarity to genotypes 4, 5 and 6. The potential significance of these new lyssaviruses for veterinary and public health should not be underestimated.

Bat-associated rabies virus in Skunks.

Rabies was undetected in terrestrial wildlife of northern Arizona until 2001, when rabies was diagnosed in 19 rabid skunks in Flagstaff. Laboratory analyses showed causative rabies viruses associated with bats, which indicated cross-species transmission of unprecedented magnitude. Public health infrastructure must be maintained to address emerging zoonotic diseases.

Molecular epidemiology of rabies in the United States

Abstract Changes in demographics, land use, recreation and hunting practices in the last 50 years dramatically increased the public health importance of reservoirs for rabies in wild species in the United States. This article focuses on attributes of host natural history to interpret the molecular phylogenies of the rabies variant transmitted within a particular animal population and the threat to human health presented by this reservoir.

Rabies virus epitopic variation: use in ecologic studies.

Publisher Summary People living in the industrialized societies of Europe and North America are less at risk from dying of rabies than of being struck by lightning.. Before dismissing rabies as a trivial public health problem, however, it must be noted that the low incidence of human rabies is not the consequence of disease eradication; rather it is the result of control programs comprising massive preexposure and postexposure prophylaxis and dog immunization. Recent advances in vaccine technology have suggested new avenues of rabies control that, in addition to preventing human infection, may also actually eradicate the disease from its animal reservoir, thus eliminating the need for human and domestic animal prophylaxis. Oral vaccination of red foxes by field baiting has dramatically reduced rabies in some European countries and may have eliminated it from some areas.

MOLECULAR EPIDEMIOLOGY OF TERRESTRIAL RABIES IN THE FORMER SOVIET UNION

Fifty-five rabies virus isolates originating from different regions of the former Soviet Union (FSU) were compared with isolates originating from Eurasia, Africa, and North America according to complete or partial nucleoprotein (N) gene sequences. The FSU isolates formed five distinct groups. Group A represented viruses originating from the Arctic, which were similar to viruses from Alaska and Canada. Group B consisted of “Arctic-like” viruses, originating from the south of East Siberia and the Far East. Group C consisted of viruses circulating in the steppe and forest-steppe territories from the European part of Russia to Tuva and in Kazakhstan. These three phylogenetic groups were clearly different from the European cluster. Viruses of group D circulate near the western border of Russia. Their phylogenetic position is intermediate between group C and the European cluster. Group E consisted of viruses originating from the northwestern part of Russia and comprised a “northeastern Europe” group described earlier from the Baltic region. According to surveillance data, a specific host can be defined clearly only for group A (arctic fox; Alopex lagopus) and for the Far Eastern part of the group B distribution area (raccoon dog; Nyctereutes procyonoides). For other territories and rabies virus variants, the red fox (Vulpes vulpes) is the main virus reservoir. However, the steppe fox (Vulpes corsac), wolf (Canis lupus), and raccoon dog are also involved in virus circulation, depending on host population density. These molecular data, joined with surveillance information, demonstrate that the current fox rabies epizootic in the territory of the FSU developed independently of central and western Europe. No evidence of positive selection was found in the N genes of the isolates. In the glycoprotein gene, evidence of positive selection was strongly suggested in codons 156, 160, and 183. At these sites, no link between amino acid substitutions and phylogenetic placement or specific host species was detected.

Serologic Evidence of Lyssavirus Infections among Bats, the Philippines

Active surveillance for lyssaviruses was conducted among populations of bats in the Philippines. The presence of past or current Lyssavirus infection was determined by use of direct fluorescent antibody assays on bat brains and virus neutralization assays on bat sera. Although no bats were found to have active infection with a Lyssavirus, 22 had evidence of neutralizing antibody against the Australian bat lyssavirus (ABLV). Seropositivity was statistically associated with one species of bat, Miniopterus schreibersi. Results from the virus neutralization assays are consistent with the presence in the Philippines of a naturally occurring Lyssavirus related to ABLV.

Emerging Pattern of Rabies Deaths and Increased Viral Infectivity

Most human rabies deaths in the United States can be attributed to unrecognized exposures to rabies viruses associated with bats, particularly those associated with two infrequently encountered bat species (Lasionycteris noctivagans and Pipistrellus subflavus). These human rabies cases tend to cluster in the southeastern and northwestern United States. In these regions, most rabies deaths associated with bats in nonhuman terrestrial mammals are also associated with virus variants specific to these two bat species rather than more common bat species; outside of these regions, more common bat rabies viruses contribute to most transmissions. The preponderance of rabies deaths connected with the two uncommon L. noctivagans and P. subflavus bat rabies viruses is best explained by their evolution of increased viral infectivity.