A retrospective search for lyssavirus in humans in the Northern Territory

Background: Following the 1996 discovery of a rabies-like lyssavirus in Aus tralian flying foxes, it was unclear whether this was a new epizootic or an unrecognised, previously existing disease. Objective: To review cases of unexplained encephalitis in the Northern Terr itory (NT) to test available clinical specimens for lyssavirus and survey t he use of diagnostic tests by clinicians. Methods: The NT hospital morbidity database was searched from January 1992 to September 1996 for all Royal Darwin Hospital (RDH) cases with an ICD-9 c ode encompassing encephalitis or viral meningitis. Final diagnoses were det ermined by hospital record review. For cases of unexplained encephalitis, w e assessed the use of diagnostic tests and located clinical specimens for t esting for lyssavirus-specific inclusion bodies via immunohistochemistry, i mmuo-flourescence and reverse-transcriptase polymerase chain reaction (RT-P CR). Results; Encephalitis occurred in 34/154 (22%) cases located by the search; 53% (18/34) of encephalitis cases were unexplained. Of these, 24% had no s erology performed and 47% had no blood cultures taken. Four (22%) died and two had autopsies. These were the only two cases with clinical specimens av ailable for testing. They were negative for lyssavirus. None of the 71 case s coded as viral meningitis had unexplained encephalitis. Conclusion: There was a considerable proportion of unexplained illness amon g NT cases of encephalitis. Implications: Clinicians should test for lyssavirus in patients with enceph alitic symptoms and a postmortem should be sought where death is unexplaine d. Specimens should be stored to enable testing for emerging infectious dis eases.