438 Am. J. Trop. Med. Hyg., 59(3), 1998, pp. 438–444 Copyright  1998 by The American Society of Tropical Medicine and Hygiene HIGH PREVALENCE OF HANTAVIRUS INFECTION IN INDIAN COMMUNITIES OF THE PARAGUAYAN AND ARGENTINEAN GRAN CHACO JORGE F. FERRER, COLLEEN B. JONSSON, EDUARDO ESTEBAN, DAVID GALLIGAN, MIGUEL A. BASOMBRIO, MONICA PERALTA-RAMOS, MAUSUMI BHARADWAJ, NORAH TORREZ-MARTINEZ, JOHN CALLAHAN, AMALIA SEGOVIA, AND BRIAN HJELLE New Bolton Center, University of Pennsylvania, Kennett Square, Pennsylvania; College of Arts and Sciences, New Mexico State University, Las Cruces, New Mexico; Area de Virologia, Universidad Nacional del Centro de la Provincia de Buenos Aires, Tandil, Argentina; Laboratorio de Patologia Experimental, Facultad de Ciencias de la Salud, Universidad Nacional de Salta, Salta, Argentina; Health Science Center, University of New Mexico, Albuquerque, New Mexico Abstract. Serologic evidence of past infection with a Sin Nombre-like hantavirus(es) was demonstrated in 78 (40.4%) of 193 Indians living in western Paraguay and in 38 (17.1%) of 222 Indians inhabiting the Salta province of northern Argentina. In both populations seroprevalence increased with age, with the most striking increase occurring at 18 years of age in the Paraguayan population and at 35 years of age in the Salta population. The peak prevalences in both populations (66.6% and 44.0%, respectively) were seen in Indians  53 years old. Although no sex difference was observed in the Paraguayan Indians, in the Salta population seroprevalence was greater in males than in females. Familiar clustering of the infection was observed. The data indicate that the Indian populations of the Gran Chaco are frequently exposed to and survive infection with a Sin Nombre-like virus(es). Possible explanations of this novel epidemiology are discussed. Hantaviruses constitute a genus of trisegmented, negative- sense RNA viruses of the Bunyaviridae family. 1 Each han- tavirus is maintained in nature in single rodent reservoir spe- cies in which it causes a generalized, chronic, asymptomatic infection with virus shedding in urine, feces, and saliva. Transmission of hantaviruses to humans is believed to occur through the inhalation of aerosols contaminated with infec- tious rodent urine or feces or following the bite of an in- fected rodent. 2,3 A relatively large number of antigenically and genetically distinct hantaviruses have been detected in a wide array of rodent species throughout the world. At least four strains, termed Hantaan (HTN), Seoul (SEO) Puumala (PUU), and Dobrava/Belgrade (DOB), are causative agents of diseases characterized by hemorrhagic manifestations and renal in- volvement. These diseases, collectively named hemorrhagic fever with renal syndrome (HFRS), are endemic in several regions of Asia and Europe, but cases of HFRS have also occurred with varied frequency in other parts of the world, including South America. 2–5 The most severe cases of HFRS are caused by the HTN and DOB viruses, whereas the milder forms of the disease are associated with the PUU and SEO viruses. 3,6 A novel hantavirus strain, designated Sin Nombre (SN) virus, was etiologically linked with an outbreak of a severe and often fatal respiratory disease termed hantavirus pul- monary syndrome (HPS) that occurred in 1993 in the south- western United States. 7,8 Other HPS-associated hantaviruses indigenous to North America, including the Black Creek Ca- nal (BCC), Bayou (BAY) and New York viruses, have been identified. The reservoir species for all these viruses are ro- dents of the family Muridae, subfamily Sigmodontinae. 3,8 As of August 1997, more than 185 cases of HPS with a mor- tality rate of approximately 45% have occurred in North America. Between March 1995 and July 1997, 108 cases of HPS with a mortality rate of 48% were reported in Argentina (Weissenbacher MC, unpublished data). Approximately one- third of these cases occurred in and about El Bolson, a lo- cality in the province of Rio Negro in southwestern Argen- tina. The remaining cases appeared in clusters in other parts of the country, including the northern province of Salta, the islands of the Parana River and Patagonia. 9 The genome of an hantavirus named Andes was enzymatically amplified from tissues of one of the El Bolson cases and found to be molecularly related to but distinct from SN and other Sig- modontine rodent-borne hantaviruses. 10 Prior to the El Bol- son outbreak, several studies reported serologic evidence of hantavirus infection in humans and rodents of Argentina, and clinical cases compatible with HPS and HFRS were diag- nosed retrospectively in several parts of the country. 4,11–13 A genetically novel hantavirus (Rio Mamore [RM] virus), which is closely related to the Andes virus, was identified in Olygoryzomys microtis from Bolivia, but its role in human disease has not been determined. 14,15 More recently, several new hantavirus sequences were detected in rodents and HPS patients in central Argentina, which may be related to RM virus. 16 The reservoir of one set of sequences, from the Le- chiguanas Islands, is O. flavescens, a close relative of O. microtis. In western Paraguay, a new SN-like virus termed Laguna Negra has been isolated from Calomys laucha and its genetic sequences have been matched with those obtained from tis- sue of HPS patients identified in the area. 17,18 With the purpose of studying the epidemiology of emerg- ing infectious diseases in isolated and relatively closed com- munities, a research team headed by one of us (JFF) col- lected serum samples as well as biographic and demographic data from a large number of Indians from several ethnic groups inhabiting Argentina and Paraguay. This paper re- ports our findings on the presence and distribution of han- tavirus antibodies in these populations. MATERIALS AND METHODS Study populations. The Indians studied inhabit the fol- lowing areas (Figure 1). 1) The northwestern part of the province of Salta in north-