RESEARCH Hantavirus Infection in Humans and Rodents, Nor thwester n Argentina Noemi Pini,* Silvana Levis,* Gladys Calderón,* Josefina Ramirez,† Daniel Bravo,‡ Elena Lozano,† Carlos Ripoll,§ Stephen St. Jeor,¶ Thomas G. Ksiazek,# Ruben M. Barquez,**†† and Delia Enria* We initiated a study to elucidate the ecology and epi- demiology of hantavirus infections in northern Argentina. The northwestern hantavirus pulmonary syndrome (HPS)–endemic area of Argentina comprises Salta and Jujuy Provinces. Between 1997 and 2000, 30 HPS cases were diagnosed in Jujuy Province (population 512,329). Most patients had a mild clinical course, and the death rate (13.3%) was low. We performed a serologic and epidemio- logic survey in residents of the area, in conjunction with a serologic study in rodents. The prevalence of hantavirus antibodies in the general human population was 6.5%, one of the highest reported in the literature. No evidence of interhuman transmission was found, and the high preva- lence of hantavirus antibody seemed to be associated with the high infestation of rodents detected in domestic and peridomestic habitats. Hare zoonotic viruses of rodents that produce two antaviruses (family Bunyaviridae, genus Hantavirus) major clinical syndromes in humans: hemorrhagic fever with renal syndrome (HFRS) in Asia and Europe and han- tavirus pulmonary syndrome (HPS) in the Americas. Since HPS was initially characterized in the United States in 1993 and the associated hantavirus (Sin Nombre virus, or SNV) was identified, an increasing number of human cases and SNV-related viruses have been identified in dif- ferent countries of North and South America (1). Three HPS-endemic areas have been recognized in Argentina: northern (Salta and Jujuy Provinces), central (Buenos Aires, Santa Fe, and Entre Rios Provinces), and southern (Rio Negro, Neuquén, and Chubut Provinces). In the North, cases of acute respiratory distress syndrome of unknown etiology have been reported since 1984 at Orán, *Instituto Nacional de Enfermedades Virales Humanas “Dr. Julio I. Maiztegui,” Pergamino, Argentina; †Hospital San Miguel, Yuto, Jujuy, Argentina; ‡Hospital Oscar Orias, Libertador General San Martín, Jujuy, Argentina; §Dirección de Epidemiología, Jujuy, Argentina; ¶University of Nevada, Reno, Nevada, USA; #Centers for Disease Control and Prevention, Atlanta, Georgia, USA; **Universidad Nacional de Tucumán, Tucumán, Argentina; and ††Consejo Nacional de Investigaciones Científicas y Técnicas, Tucumán, Argentina Salta Province. The illness, known in the area as ”Distress of Orán,” had an unexplained etiology until the early 1990s, when these cases were first associated with Leptospira interrogans infections and later with han- taviruses. Of 21 patients tested between 1991 and 1993, eight showed serologic evidence of recent leptospira infec- tion by microscopic agglutination test, and 4 had a positive immunoglobulin (Ig) M enzyme-linked immunosorbent assay (ELISA) using Hantaan virus antigen (2,3). Ultimately, these patients were recognized as having HPS, and a new SNV-related hantavirus, now designated Oran virus, was recognized in the region (4). The first HPS cases in Jujuy Province were confirmed in 1997, and since then, their number has been progressively increasing. Isolated cases were detected in several different localities (San Pedro, La Mendieta, Caimancito, Libertador General San Martín, Fraile Pintado, and San Salvador, the provincial capital), but most originated in the town of Yuto and sur- roundings. A high percentage of confirmed cases had the usual nonspecific prodrome but were not followed by a distress syndrome. The case death rate (4 [13.3%] of 30) was noticeably lower than that reported in other areas of the country and in the literature. Some strains of hantavirus were then hypothesized to produce subclinical disease. Only one hantavirus antibody-prevalence study had been performed among inhabitants of the Gran Chaco of Paraguay and Argentina (Salta Province), and hantavirus antibodies were found in 20% to 40% of participants (5). Some differences in the clinical signs and symptoms of HPS have been recognized in other areas of the Americas compared with those described after infections with SNV; these differences have included possible person-to-person transmission, a different spectrum of clinical illnesses, an elevated incidence of infections in children, and higher antibody prevalence (6). For instance, patients from the area under study had unusually mild clinical symptoms and low death rates, supporting the idea that a less patho- genic hantavirus could be circulating in that area or that host or environmental factors might be responsible for the observed pattern. The objectives of this study were to determine the prevalence of hantavirus antibodies in the 1070 Emerging Infectious Diseases • Vol. 9, No. 9, September 2003