Original Contributions Clinicopathologic Study and Laboratory Diagnosis of 23 Cases With West Nile Virus Encephalomyelitis JEANNETTE GUARNER, MD, WUN-JU SHIEH, MD, PHD, STEVE HUNTER, MD, CHRISTOPHER D. PADDOCK, MD, TIMOTHY MORKEN, MT, GRANT L. CAMPBELL, MD, PHD, ANTHONY A. MARFIN, MD, MPH, AND SHERIF R. ZAKI, MD, PHD The differences in pathologic findings of fatal cases of West Nile virus (WNV) encephalitis in the context of underlying conditions and illness duration are not well known. During 2002, we studied central nervous system (CNS) tissue samples from 23 patients who had serologic and immunohistochemical (IHC) evidence of a recent WNV infection. Fifteen patients had underlying medical conditions (5 ma- lignancies, 3 renal transplants, 3 with diabetes or on dialysis, 2 with AIDS, and 2 receiving steroids). WNV serology was positive for 18 patients, negative for 2, and not available for 3. Perivascular lympho- cytic infiltrates, microglial nodules, and loss of neurons were pre- dominantly observed in the brainstem and anterior horns in the spinal cord. IHC using antibodies against flaviviruses and WNV showed viral antigens in 12 (52%) of 23 patients. Viral antigens were found inside neurons and neuronal processes predominantly in the brainstem and anterior horns. In general, the antigens were focal and sparse; however, in 4 severely immunosuppressed patients, extensive viral antigens were seen throughout the CNS. Positive IHC staining was observed in tissues of 7 of 8 patients who died within 1 week after illness onset, compared with 4 of 14 with more than 2 weeks’ illness duration. WNV causes an encephalomyelitis by primarily affecting brainstem and spinal cord. Differences in the amount of viral antigen may be related to underlying medical conditions and length of sur- vival. IHC can be an important diagnostic method, particularly during the 1st week of illness, when antigen levels are high. HUM PATHOL 35: 983-990. © 2004 Elsevier Inc. All rights reserved. Key words: West Nile virus, encephalitis, pathology, immunohis- tochemistry. Abbreviations: WNV, West Nile virus; CNS, central nervous sys- tem; CDC, Centers for Disease Control and Prevention; IHC, immu- nohistochemical; Ig, immunoglobulin; EIA, enzyme immunoassay; PRNT, plaque-reduction neutralizing; PK, proteinase K; HIV, human immunodeficiency virus; COPD, chronic obstructive pulmonary disease. West Nile virus (WNV), a member of the Flavi- viridae family, is known to cause a febrile disease accompanied by a spectrum of neurologic complica- tions that range from isolated headache to life-threat- ening encephalitis. 1 In young individuals, the disease is usually benign, whereas elderly patients may present with severe central nervous system (CNS) disease. During the summer of 1999, investigation of a cluster of 8 elderly patients with encephalitis in New York City led to the recognition for the first time of WNV in the American continent. 2 In 2002, nearly 3000 cases of WNV neuroinvasive disease and 300 fatalities with laboratory evidence of WNV infection were reported to the Centers for Disease Control and Prevention (CDC), 3 (http://www.cdc.gov/ncidod/ dvbid/westnile/surv&controlCaseCount02.htm). Al- though transfusion- and transplant-related WNV in- fections were documented for the first time in 2002, most WNV infections are acquired through mosquito bites. 4-6 Study of epidemiologic trends worldwide during the 1990s suggest that human dis- ease may be increasing in severity and that there has been an increased frequency of outbreaks among humans. 7 Previous pathological studies of 4 fatal WNV en- cephalitis cases that occurred in the United States during 1999 showed a spectrum of neuropathologic features that included encephalitis and meningoen- cephalitis. 8,9 In addition, single case reports have con- firmed these findings. 10-12 The present report describes the neuropathologic and immunohistochemical (IHC) findings for 23 cases of WNV encephalomyelitis and correlates these findings with a variety of clinical fea- tures, including illness duration and underlying medi- cal condition. In addition, we assess the role of IHC in the diagnosis and use this technique to better under- stand the pathogenesis of WNV infection. From the Infectious Diseases Pathology Activity, Division of Viral Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta, GA; Department of Pathology, Emory University Hospital, Atlanta, GA; and Division of Vector-Borne Infectious Diseases, Cen- ters for Disease Control and Prevention, Fort Collins, CO. Accepted for publication March 30, 2004. Address correspondence and reprint requests to Jeannette Guarner, MD, Centers for Disease Control and Prevention, Mailstop G32, 1600 Clifton Rd, NE, Atlanta, GA 30333. Timothy Morken is now at Lab Vison/NeoMarkers, Fremont, CA. 0046-8177/$—see front matter © 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.humpath.2004.04.008 983