Public Health Response to a Potentially Rabid Bear Cub -- Iowa, 1999

On August 27, 1999, a 5-6 month-old black bear cub in a petting zoo in Clermont, Iowa, died after developing acute central nervous system signs; the initial direct fluorescent-antibody (DFA) test results available on August 28 indicated the bear had rabies. On August 29, in response to the positive laboratory report, the Iowa Department of Public Health (IDPH) initiated a campaign to identify and inform persons potentially exposed to the bear's saliva. Within 72 hours, IDPH staff verified contact and exposure information for approximately 350 persons. Subsequent testing found no evidence of rabies virus in brain or spinal cord tissues. This report describes the public health response to this potential rabies outbreak and reviews testing procedures and protocols for rabies.

On August 27, the bear developed acute neurologic signs, progressing from mild tremors and anisocoria to coma and death within 4 hours. The attending veterinarian submitted the bear to Iowa State University's Veterinary Diagnostic Laboratory (ISU VDL) for a full postmortem examination. On August 28, ISU VDL notified the veterinarian that the bear had tested positive for rabies*. The veterinarian immediately alerted IDPH. After consultation with CDC, IDPH established a conservative estimate of the period of potential rabies exposure to humans as 28 days before the bear's death. IDPH contacted media statewide to help publicize the potential exposures of the zoo visitors.

The local county health department and the area hospital established a rabies exposure assessment and treatment clinic in the emergency department. Based on information from a voluntary sign-in log for visitors, IDPH used a variety of tools (i.e., media campaign, Internet locator sites, directory assistance, and law enforcement) to reach persons from 10 states (Arizona, California, Florida, Illinois, Iowa, Minnesota, New Mexico, New York, Ohio, and Wisconsin) and Australia; 200 visitors were identified. On August 29, IDPH personnel began contacting the 200 visitors. In addition, efforts were made to contact 150 potentially exposed persons who attended an August 14 "barnwarming" at which the bear was present. On September 3, a dispatch was published in MMWR (1) to notify other health departments of efforts to locate zoo visitors. By September 1, an estimated 99% of potentially exposed persons had been contacted.

On August 30, IDPH, the Iowa State Veterinarian's Office, and the U.S. Department of Agriculture visited the petting zoo to assess exposure factors and implement quarantine measures. On August 31, the ISU VDL reported a positive reverse transcriptase polymerase chain reaction (RT-PCR) for rabies** and submitted brain tissues to CDC to identify the potential wildlife reservoir species associated with the virus. During the ISU VDL necropsy, no alternative cause of death was identified; however, pathologic studies were limited by the advanced state of postmortem autolysis. On the evening of September 1, IDPH was notified by CDC that the DFA of the tissues submitted for virus typing were negative for rabies virus. On September 2, brain and spinal cord tissues were submitted to University Hygienic Laboratory (UHL) and CDC. On September 3, DFA testing at UHL was reported as negative; DFA, RT-PCR, and nested PCR tests at CDC on brain and spinal cord tissues also were reported negative.

On September 3, the available information included the bear's clinical presentation of acute death atypical for but consistent with rabies; the initial positive DFA test and the positive PCR test at ISU VDL; the negative tests conducted by CDC on the bear's brain and spinal cord; the negative DFA test conducted by UHL on the bear's brain; a documented case of a rabid bear with a DFA-negative test on brain tissue (2); the paucity of literature on rabies and rabies testing in bears, and follow-up of humans after exposure to animals with negative laboratory results; and the lack of a reasonable alternative explanation for the bear's neurologic illness and death. IDPH also was aware that the risk for death from symptomatic rabies was 100% and the risk for receiving vaccine was minimal. Consultation with national clinical infectious disease specialists and other medical experts, including epidemiologists, resulted in the conclusion that the vaccine series be continued. IDPH then issued a press release stating that the negative tests made it less likely the bear died from rabies (3). By the end of September, an estimated 150 persons had completed the rabies vaccination series. On approximately October 18, ISU VDL reported mouse inoculation studies negative for rabies.

Reported by: SC Gleason, DO, R Currier, DVM, P Quinlisk, MD, State Epidemiologist, Iowa Dept of Public Health. Viral and Rickettsial Zooneses Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and EIS officers, CDC.

Editorial Note:

The false-positive test result for rabies in a bear in Iowa affords an opportunity to review testing procedures and protocols for rabies virus infection, the public health record in the United States resulting from these procedures and protocols, and recommendations for handling inconsistent test results.

The DFA test for detection of rabies virus antigen in brain tissue is used as the primary diagnostic test in all public health laboratories in the United States. The test has a sensitivity approaching 100% (4,5). Rabies diagnosis and administration of prophylaxis to potential human exposures are based on the observation that, in all mammals, rabies virus reaches the salivary glands and is excreted in saliva only after replication in the central nervous system. Absence of rabies virus antigen in the brain of an animal by DFA (i.e., a negative diagnostic test result) essentially precludes the presence of virus in saliva, the risk for rabies transmission, and the need for postexposure prophylaxis. Clinical signs leading to a suspicion of rabies occur only after substantial virus replication. At that time, most tests for rabies reveal considerable amounts of viral antigen in all areas of the brain.

DFA test results in which staining of antigen is weak or that reveals sparse or focal inclusions often are caused by nonspecific antibody binding or less-than-optimum test conditions. Cross-contamination of negative samples at necropsy with material from strong positive samples tested earlier also can cause sporadic staining in a negative sample. DFA tests that are not clearly positive or negative should be repeated by remaking slides from reserved brain tissue and repeating the test, using reagents from two different commercial sources and using additional specificity controls. If test results remain equivocal, alternative confirmatory tests, such as virus isolation (through cell culture or mouse inoculation) or PCR assays, should be performed (5). Additional amplification, such as a nested RT-PCR assay, is unnecessary and inappropriate for routine diagnostic applications. Postexposure prophylaxis can be initiated during the diagnostic testing process and discontinued if negative results are obtained.

In 1997, approximately 100,000 animal brains were tested for rabies virus antigen by DFA; of these, 8509 (8.5%) were positive (6). The absolute number of persons potentially exposed to an animal with suspected rabies and who did not receive prophylaxis because of a negative diagnostic test result is unknown. Nevertheless, since the initiation of current rabies testing procedures in 1958, there is no evidence that a false negative laboratory test has ever led to rabies in a person subsequently left untreated.

Each laboratory that provides rabies diagnostic services should plan routine evaluation of its DFA test procedures and should participate in national rabies virus proficiency testing. Negative test results obtained by appropriate and systematic examination of specimens can be interpreted reliably by public health practitioners so that no postexposure prophylaxis is required or postexposure prophylaxis that was initiated pending laboratory evaluation can be curtailed (7). To assist state and local health departments, national and international reference laboratories, such as the World Health Organization Collaborating Center for Reference and Research on Rabies at CDC, are available to clarify and interpret rabies test results.

References

1. CDC. Multiple human exposures to a rabid bear cub at a petting zoo and barnwarming--Iowa, August 1999. MMWR 1999;48:761.
2. Taylor M, Elkin B, Maier N, Brady M. Observation of a polar bear with rabies. J Wildlife Dis 1991;27:337-9.
3. Iowa Department of Public Health. Initial rabies tests unconfirmed; Other studies underway [press release]. Des Moines: Iowa Department of Public Health; September 3, 1999.
4. Velleca WM, Forrester FT. Laboratory methods for detecting rabies. Washington, DC: US Department of Health and Human Services, Public Health Service, CDC, 1981.
5. Smith JS. Rabies virus. In: Murray PR, Baron, EJ, Pfaller MA, Tenover FC, Yolken RH, eds. Manual of clinical microbiology. 7th ed. Washington, DC: American Society for Microbiology, 1999:1099-106.
6. Krebs JW, Smith JS, Rupprecht CE, Childs JE. Rabies surveillance in the United States during 1997. J Am Vet Med Assoc 1998;213:1713-28.
7. CDC. Human rabies prevention--United States, 1999: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1999;48(no. RR-1).

* This was subsequently described as a weak DFA positive test. A repeat DFA test was again described as weakly positive and ISU VDL set up reverse transcriptase polymerase chain reaction (RT-PCR) testing.

** This test was subsequently determined to be a positive nested PCR obtained following a negative primary RT-PCR. Sequencing of the amplified product from the nested PCR did not reveal a rabies gene product.

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