IIIA, IIIB, IIIC, and IIID variants share vectors or reser- voir hosts, although IIIA, IIIC, and IIID probably infect Proechimys spp., and Tonate virus (IIIB) may use birds as its reservoir hosts (34). The newly identified subtype IIID strain was isolated from spiny rats (Proechimys spp.), Culex (Melanoconion) spp. mosquitoes and from a patient with fever, chills, and malaise. The symptoms are typical of human VEEV (sub- type I) infection. Using diagnoses based on virus isolation and serology, Watts et al. (4,14) previously reported that VEEV was responsible for at least 3% of febrile illnesses in the city of Iquitos in the Amazon basin of Peru. Subtype ID was thought to be the cause of these human VEEV cases, including those diagnosed only by serologic find- ings. However, our study demonstrated that both VEEV subtype ID and the newly recognized subtype IIID are responsible for human illness in the Peruvian Amazon basin. Whether subtype IIID was responsible for any other of the 183 human cases diagnosed serologically in Peru since 1995 is unclear. More detailed studies to retrospec- tively examine serum samples from these cases are required to evaluate this possibility. Human Disease and Virulence of Enzootic VEEV Strains In Peru, human VEE does not appear to result in neuro- logic manifestations, and fatal human disease has never been reported. In contrast, during recent VEEV epidemics with subtype IC strains in Colombia and Venezuela, an estimated 3,000 cases with neurologic complications and 300 fatal cases (5,35) were reported. Overall, human death rates have generally been estimated at approximately 0.5% during these epidemics, with most of the neurologic dis- ease and fatal cases reported in children. Most of the human VEEV cases we studied in Peru occurred in adults (94.6%), which suggests an occupational exposure or pos- sibly an age-biased recruitment into the NMRCD febrile illness study. The lack of any evidence for neurologic dis- ease in any of the NMRCD cases studied from 1994 to 2003 (D. Watts, unpub. data) suggests a possible difference in virulence compared with epizootic IAB and IC strains. Because we characterized only 183 VEEV cases, and only 10 of these were children, whether a virulence difference exists between enzootic subtype ID and epizootic VEEV strains is impossible to determine with any statistical cer- tainty. Strains of the Panama-Peru genotype of subtype ID are known to have caused fatal human disease (10). Acknowledgments The authors thank Leslie Angulo, Zonia Rios, Marie Lefevre, and Vidal Felices for their invaluable assistance in the execution of this project. Endemic Venezuelan Equine Encephalitis in Northern Peru This work was supported by grant No. AI49725-01 from the NIH, and by Work UNIT NUMBER (WUN) No.847705 8200 25GB B0016. IRB Protocol Number: 31535 “Surveillance and Etiology of Acute Febrile Diseases in Peru.” The study protocol was approved by the Naval Medical Research Center Institutional Review Board (Protocol # 31535) in compliance with all U.S. federal regulations governing the protection of human study participatns. The opinions expressed in this paper are those of the authors and do not reflect the official policy of the Department of Navy, Department of Defense, or the U.S. Government. Ms. Aguilar is a Peruvian citizen and a graduate student in the Microbiology and Immunology Program, University of Texas Medical Branch, Galveston, Texas. Her main research interests are the genetics and pathogenesis of alphaviruses and other arthropod- borne viral diseases and the development of antiviral vaccines. References 1. Kubes V, Rios FA. The causative agent of infectious equine encephalomyelitis in Venezuela. Science 1939;90:20–1. 2. 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