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. 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. 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.
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. 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. 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.