[Emerging Infectious Diseases]  
[Volume 4 No. 4 /October-December 1998]

Dispatches

An Outbreak of Hantavirus Pulmonary Syndrome, Chile, 1997

Jorge Toro,* Jeanette D. Vega,  Ali S. Khan, James N. Mills, Paula
Padula, William Terry, Zaida Yadn, Rosa Valderrama, Barbara A. Ellis,
Carlos Pavletic,* Rodrigo Cerda, Sherif Zaki, Shieh Wun-Ju, Richard
Meyer, Mauricio Tapia,# Carlos Mansilla,# Michel Baro,** Jose A.
Vergara,** Marisol Concha,* Gladys Calderon, Delia Enria, C.J. Peters,
and Thomas G. Ksiazek
*Ministry of Health, Santiago, Chile; Pan American Health Organization,
Santiago, Chile; Centers for Disease Control and Prevention, Atlanta,
Georgia, USA; Instituto Nacional de Enfermedades Infecciosas ANLIS "Carlos
G. Malbran," Buenos Aires, Argentina; Aysen Region XI Health Service,
Coyhaique, Chile; #Coyhaique Regional Hospital, Coyhaique, Aysen, Chile;
**Llanchipal Health Services, Puerto Montt, Chile; Instituto Nacional de
Enfermedades Virales Humanas, Pergamino, Argentina; and Catholic
University of Chile, Santiago, Chile.

---------------------------------------------------------------------------
      An outbreak of 25 cases of Andes virus-associated hantavirus
      pulmonary syndrome (HPS) was recognized in southern Chile from
      July 1997 through January 1998. In addition to the HPS
      patients, three persons with mild hantaviral disease and one
      person with asymptomatic acute infection were identified.
      Epidemiologic studies suggested person-to-person transmission
      in two of three family clusters. Ecologic studies showed very
      high densities of several species of sigmodontine rodents in
      the area.

Hantavirus pulmonary syndrome (HPS), first recognized in 1993 after a
cluster of acute respiratory distress syndrome deaths in the southwestern
United States, is now considered a pan-American zoonosis (1,2). HPS is
characterized by fever, myalgia, gastrointestinal symptoms, and headache,
with subsequent characteristic cardiopulmonary dysfunction and a 40% to 60%
case-fatality rate (3). Although the etiologic agent, Sin Nombre virus
(SNV), causes clinical symptoms that appear different from those that
Eurasian hantaviruses cause in hemorrhagic fever with renal syndrome, SNV
shares many features with these Old World hantaviruses (4,5), including an
association with a single primary rodent host (Peromyscus maniculatus [deer
mouse] in the case of SNV), which acts as the natural reservoir (6).

After SNV was identified, numerous other New World hantaviruses were
rapidly identified throughout the Americas by reverse
transcription-polymerase chain reaction (RT-PCR) amplification of viral RNA
from captured antibody-positive rodents and, occasionally, from infected
patients (2). HPS was first identified in Chile in 1995 in the Cochamo, Los
Lagos region. Genetic sequencing of RT-PCR products from the autopsy
tissues of another patient presumptively infected in this region in 1996
confirmed infection with Andes virus (7), which had previously been
identified in southern Argentina as a cause of HPS, with Oligoryzomys
longicaudatus postulated as the reservoir host (8). Sporadic cases were
reported in Chile until an outbreak that included two family clusters was
recognized in the Coyhaique Health District, Aysen region, in August 1997;
a third cluster was reported in January 1998. This outbreak prompted a
joint investigation by the National Administration of Laboratories and
Institutes of Health (Argentina), Pan American Health Organization, Centers
for Disease Control and Prevention (CDC, USA), in collaboration with the
Ministry of Health of Chile, to define the epidemiology and ecology of the
syndrome in Chile.

The Investigation

Epidemiologic Surveillance and Case Identification

National and local surveillance for HPS was reinforced with explicit case
definitions for HPS, asymptomatic hantavirus infection, and mild hantaviral
disease. HPS was defined as an acute febrile illness (temperature >38.3C)
characterized by unexplained acute respiratory distress syndrome or
bilateral interstitial pulmonary infiltrates, with respiratory compromise
requiring supplemental oxygen, or an unexplained illness resulting in
death, with autopsy results showing noncardiogenic pulmonary edema without
an identifiable specific cause of death. In addition to a compatible
clinical illness, the following laboratory evidence of infection was
required: 1) hantavirus-specific immunoglobulin M (IgM) or a fourfold rise
in IgG titer, or 2) positive RT-PCR results for hantavirus RNA, or 3)
positive immunohistochemical results for hantavirus antigen.

Asymptomatic hantavirus infection was defined as laboratory evidence of
acute hantavirus infection (presence of IgM antibodies) in persons with no
documented concurrent illness. Mild hantaviral disease was defined as
IgM-positive results and a febrile illness without objective pulmonary
dysfunction (i.e., hypoxemia and radiographic abnormalities). All other
persons with isolated detectable hantavirus-specific IgG antibodies were
classified as "other seropositives."

All available clinical charts and case reports from the Aysen region were
reviewed in conjunction with interviews of family members from the two
described clusters and survivors. A third cluster was identified in January
1998 and was also reviewed.

Ecologic Studies

Rodents were trapped at two principal sites near the residence sites of the
first two reported family clusters, as well as in several areas of less
disturbed native vegetation in Lago Atravesado. Traps were placed at each
of the two case-households, as well as at two neighboring controls for each
case-household (one within 500 m and the other 500 m to 1,000 m). Each
evening, 150 to 200 live-capture traps were placed within and around
residences, outbuildings, gardens, and woodpiles, along fence lines, and
within remnant patches of native vegetation. Traps were checked in the
early morning, and samples of blood and organ tissues were collected
according to standardized protocols (9). For comparison, we used similar
trapping techniques at additional sites near Valdivia (700 km north of the
outbreak area) and the capital city of Santiago (1,400 km north). Rodent
taxonomy is as described by Musser and Carleton, 1993 (10).

Laboratory Testing

Human serum specimens were tested for IgG antibodies reactive with SNV and
Andes antigens by an enzyme-linked immunosorbent assay (ELISA) (11). Rodent
whole-blood specimens were similarly tested for IgG antibodies to SNV. An
IgM-capture ELISA was performed on human sera by using inactivated Laguna
Negra virus at CDC and an IgM-capture ELISA with recombinant Andes
nucleocapsid antigen at the National Institute for Infectious Disease,
Buenos Aires, Argentina (P. Padula, unpub. data). Immunohistochemical
analysis for hantaviral antigens was performed by using a cross-reactive
monoclonal antibody directed against conserved hantaviral nucleocapsid
epitopes (12). Specimens from HPS patients were also examined for viral
genetic materials (M and S segment) by nested and heminested RT-PCR from
either autopsied tissues or blood clot samples (13,14). Genetic sequencing
of RT-PCR products derived from patient samples was also performed.

The Outbreak

						 From October 1995 when HPS was first
                                     recognized in Chile through June 1997,
 						 seven additional cases were identified
                                     (Figure 1). Between July 1, 1997, and
[Figure 1.]Temporal distribution of  January 22, 1998, 25 HPS patients and
 hantavirus pulmonary syndrome       three family clusters were identified
 cases, Chile, 1995-1998.            in Chile. By RT-PCR we confirmed that
                                     16 of these patients had been infected
                                     with Andes virus. Except for six
cases, the recent outbreak was centered in Regions X (Llanchipal) and XI
(Aysen) (combined population = 1.1 million) (Figure 2). The areas where
these patients lived are sparsely populated, with numerous lakes, in a
Nothofagus (southern beech) forest ecosystem. From October 4, 1995, to
January 22, 1998, a total of 33 cases were identified in Chile, with a
case-fatality rate of 54%. The mean age was 31.4 years (range: 1 year 11
months to 60 years); five (15.2%) of the patients were children under 17
years of age; and 76% of the patients were male. No cases were reported
among health-care workers.

No significant difference in mean age, case-fatality rate, or proportion of
male patients was evident between the July to January outbreak and previous
cases; however, all the pediatric cases occurred during the July to January
outbreak.

Family Clusters

The first family cluster (Cluster 1) of  
HPS cases was reported from Cisne Medio,
Lago Verde community, Aysen region. The  
39-year-old male head of the family
became ill with an acute febrile illness [Figure 2.]Geographic distribution
on July 15 and died in transit to the    of hantavirus pulmonary syndrome
regional hospital on July 21; HPS was    cases, Chile, 1995-1998.
subsequently confirmed by
immunohistochemistry and RT-PCR (M and S
segment), with a genetic sequence consistent with Andes virus. The family
moved to the village of Villa Amengual, 7 km away, on July 22 and remained
there except for a brief visit to Cisne Medio on July 27 to retrieve
personal belongings. The wife of the index patient became ill with HPS on
August 2 and died on August 8. Her 2-year-old and 12-year-old sons became
ill 1 week and 16 days, respectively, after she became ill. HPS was
confirmed in both children by serologic testing; both children survived.
The brother-in-law of the index patient, who continued to reside
intermittently in the original household, became ill 33 days after his
sister became ill and died (Figure 3). Thus, the immediate family members
became ill 12, 19, and 28 days after leaving the family homestead, and the
intervals between the onset of the index and later cases were 18, 25, and
34 days.

In contrast, the second family cluster (Cluster 2) included all four
members of a household in Lago Atravesado, Coyhaique community, Aysen
region, who became ill within 5 days of each other; one of them, a
2-year-old child with no respiratory symptoms, was classified as having a
mild hantaviral infection. The third family cluster (Cluster 3) included a
husband, who worked in a rural area, and his wife, who remained in the
family home in urban Coyhaique, a city of 60,000 inhabitants. The husband
became ill with symptoms suggestive of HPS 12 days after returning to his
family home. He was hospitalized and died on December 19. His wife became
ill 22 days after the onset of the husband's symptoms. She had not traveled
outside the town of Coyhaique during the previous 12 months and reported no
exposure to rodents or their excreta. The only known exposures for the wife
were washing her husband's clothing and caring for him while he was ill.
She survived and was discharged from the hospital on January 28.
						  
						  Genetic sequencing of viral RNA from
                                      cases of immediate family members
 						  from Clusters 1 and 2 demonstrated
                                      3.6% divergence in a 167-nucleotide
 [Figure 3.]Time-line listing for     G2 fragment between the clusters.
 hantavirus pulmonary syndrome cases  However, the genetic sequence was
 in Cluster 1, Chile, 1997.           identical within each of the two
                                      clusters. Genetic sequencing of
                                      viral RNA from an additional patient
from Cisne Medio not related to the family members in Cluster 1, and the
brother-in-law in Cluster 1, demonstrated a distinct G2 sequence, with
1%-4% sequence divergence between them (P. Padula, unpub. data).

Clinical and Histopathologic Features

In general, the clinical description of HPS cases in Aysen was similar to
that of the cases in North America. However, included in the Aysen cases
were three children with petechiae; one of them died rapidly with
hemorrhagic pulmonary secretions and bleeding from puncture sites. Six of
the seven patients on whom urinanalysis was performed had microscopic
hematuria and casts; three had proteinuria of >100 mg/dl. In addition, one
child in the Llanchipal region had concurrent acute rubella infection. One
of the most recent cases occurred in Maule, Region VII, in a 23-week
pregnant woman who had typical HPS symptoms and fetal death. Subsequently,
disseminated intravascular coagulopathy developed and the woman died of
multisystem failure.

The histopathologic features in the lung of HPS case-patients included an
interstitial mononuclear infiltrate and intraalveolar edema.
Immunohistochemical staining showed hantaviral antigens in the
microvascular endothelial cells of the lung and other tissuestypical
SNV-associated HPS cases. Prominent immunostaining of pulmonary macrophages
was also noted in some cases. A unique feature in the series was fine
granular immunostaining in the hepatic cells in some cases, a pattern not
observed in a large series of North American HPS cases previously examined
(Figure 4) (12).

Laboratory Test Results

Serologic testing was conducted for all patients and contacts of patients
with confirmed HPS cases in Aysen. Among 53 contacts of 14 patients, two
(3.8%) had serologic evidence of an acute infection (one had no illness,
and another [described previously in cluster 2] had a mild febrile illness
without pulmonary disease that did not meet the HPS case definition), and
one was IgG positive. In addition, two hospitalized patients had mild
hantaviral disease (M. Tapia, unpub. data). Test results of specimens (SNV
and Andes antigens were used) were 100% concordant. Nested and heminested
RT-PCR assays were conducted for 20 (71.4%) of 28 HPS cases; all 20 were
positive. Sixteen (57.1%) were sequenced and characterized as Andes virus.

Ecologic Studies

A total of 253 rodents were captured during 3 nights of trapping (574 trap
nights) in the vicinity of Coyhaique. Overall trap success (captures per
100 trap nights) at case and control home sites was 37% at Cisne Medio and
50% at Lago Atravesado. Trap success at Cisne Medio within and adjacent to
the case-household was 66%, compared with only 18% at control homes; this
difference may reflect the use of rodenticides in and around the control
households. Trap success at distances greater than 100 m from the
households was more similar between case and controls (63% versus 43%). The
most frequently captured rodent species was O. longicaudatus, which
comprised 47% of the captured rodents in the area; 13 (12.7%) of 102 tested
were hantavirus-antibodypositive. Akodon olivaceus comprised a further 33%
of the captures (6 [7.5%] of 80 were antibody-positive), and 16% were
Akodon longipilis (1 [2.7%] of 36 antibody-positive). Eight of 10 rodents
captured inside the case-home at Cisne Medio were O. longicaudatus, and one
was hantavirus-antibodypositive. High trap success in a forested area near
Lago Atravesado (22 captures in 40 trap nights, or 55%) indicated that high
rodent population densities were not restricted to peridomestic areas.

						  Trap success was moderate near
                                      Valdivia (52 captures from 660 trap
 						  nights, or 8% trap success) and very
                                      low near Santiago (8 captures from
 [Figure 4.] A) Low-power             453 trap nights, or <2%). The species
 photomicrograph of lung showing      composition was also very different
 interstitial pneumonitis and         from that encountered in the
 intraalveolar edema. B) Andes virus  Coyhaique area: despite trapping in
 antigenpositive intraalveolar       rural areas, 31% of rodents captured
 macrophages. C) Fine granular        near Valdivia and Santiago were the
 immunostaining of hantaviral         murine species, Rattus rattus, R.
 antigens in endothelial cells of     norvegicus, and Mus musculus. No
 pulmonary microvasculature. (A,      hantavirus-antibodypositive rodents
 Hematoxylin and eosin; B,C,          were found from Valdivia or Santiago.
 napthol-fast red with hematoxylin
 counterstain; original               Conclusions
 magnifications A x 50, B and C x
 250.)                                This outbreak of HPS and the presence
                                      of endemic disease in Chile support
                                      the assumption that HPS will continue
to occur throughout the Americas (range of the sigmodontine rodents that
host the genetic group of HPS-causing hantaviruses). In Latin America,
endemic HPS and epidemics have been reported in Argentina, Brazil, and
Paraguay (15-19). Sporadic cases and SNV-like viral sequences have been
reported in these countries and in Bolivia and Uruguay (P. Padula, pers.
comm.,7,20). Retrospective studies have verified that HPS cases occurred in
the United States as early as 1959 (21). The factors responsible for the
outbreaks in the Americas have not been well defined and may not be the
same in all regions. However, periodic climatic and ecologic events cause
dramatic local increases in rodent reservoir populations, known as
"irruptions" or "ratadas" (22) and may lead to an increase in the number of
rodent-human contacts.

Small-mammal trap success in the Coyhaique area was 5 to 30 times higher
than in rural areas 700 km to 1,400 km further north. These extremely high
trap success values indicate a rodent irruption in southern Chile whose
causes may be related to the flowering of a species of bamboo and a
relatively benign preceding winter (22). Regardless of causes, however, the
extremely high rodent densities and evidence of hantavirus infection in the
two most common species strongly implicate contact with rodents as the
primary mode of disease transmission to humans. The rodent species with the
highest antibody prevalence, O. longicaudatus, is the reservoir for Andes
virus in Argentina and Chile (8,23). No hantavirus has been associated with
A. olivaceus. RT-PCR conducted on tissue samples from seropositive animals
yielded amplifiable viral RNA from seven of seven O. longicaudatus, two of
four A. olivaceus, and one of one A. longipilis. Sequencing showed that all
PCR products represented Andes virus, and sequences were nearly identical
among all three species (S. Morzunov, unpub. data). The near identity
between sequences from all three species and the fact that two of four
samples from A. olivaceus were PCR negative suggest that hantavirus
infection in A. olivaceus and A. longipilis represents spillover of virus
from the primary reservoir, O. longicaudatus. The absence of viral RNA in
two of the four samples from A. olivaceus also suggests that the infection
is of shorter duration in this presumably nonhost species.

A number of features may eventually differentiate HPS caused by Andes virus
from that caused by other New World viruses. Several mild and asymptomatic
hantavirus infections have been reported in Argentina and Chile, whereas in
North America the case-to-infection ratio is approximately 1:1 (24,25). In
this investigation, four (11%) of the patients with serologic evidence of
acute infection did not have HPS. More mild and asymptomatic disease in
some South American hantavirus infections, in addition to infection with
nonpathogenic hantaviruses, may be among the factors explaining antibody
prevalence as high as 40% among indigenous persons of Argentina (C.
Johnson, unpub. data). Similar case-to-infection ratios may explain the
high prevalence (35% to 57%) of hantavirus antibodies in some regions of
Paraguay (16,26).

Disease in Chile has also been characterized by an increased propensity
toward bleeding, with petechiae in 50% of the pediatric cases and increased
renal involvement with microscopic hematuria and cellular casts in the
three patients whose urinanalysis results were available. Flushing of the
head and upper thorax is indicative of vascular dysregulation and is a
well-described sign in hemorrhagic fever with renal syndrome. It has also
been described in Andes virus-infected patients from the adjacent areas of
Argentina (17,27), but not in Chile.

The relatively large proportion of children with HPS reported in Chile is
unexplained. Infection in children is relatively rare in the United States,
where only 8 (4.5%) of 179 reported cases were in children 16 years old or
younger. Moreover, this dearth of pediatric cases has also been reported
for hemorrhagic fever with renal syndrome (28). It is unclear if this
greater incidence of pediatric cases is Andes virus- or outbreak-specific.
However, Argentina has also seen a larger proportion of pediatric HPS cases
than the United States, caused by three different genotypes: Andes, Oran,
and Lechiguanas.

Immunostaining showed hantaviral antigens in the microvascular endothelial
cells of the lung and other tissues with prominent staining of pulmonary
macrophages in some cases. A unique feature not previously described in
SNV-associated HPS cases was fine, granular immunostaining in the hepatic
cells of three patients. The specificity of this hepatic staining is being
investigated. In Argentina, person-to-person transmission has been
demonstrated in at least one epidemic of Andes virus infection (17).
Although the mechanism of that transmission is unknown, the increased
staining of intraalveolar pulmonary macrophages suggests the presence of
virus within the alveolar space with the potential for small-particle
aerosolization. A comparison of disease variants that may be
hantavirus-strainspecific will be required to address this hypothesis.

No discrepancies were observed in serologic results from testing with SNV
or Andes antigens, which underlines the current recommendation that
diagnosis be based on serologic testing, especially given the difficulties
in handling and transporting fresh tissues for RT-PCR and the possibility
of cross-contamination. Formalin-fixed tissues from case-patients who died
should be used for immunohistochemical tests, particularly if no serum or
blood was obtained. However, even in fatal cases, an attempt should be made
to obtain heart blood for serologic tests.

Person-to-person transmission cannot be excluded in the first or third
family cluster. We did not trap rodents within the town limits at Villa
Amengual or in the city of Coyhaique. However, although large numbers of
rodents were seen in rural areas, rodent infestation in Villa Amengual or
Coyhaique had not been reported. Further, since O. longicaudatus is a
sylvatic species not generally associated with urban habitats, subsequent
exposure to infected O. longicaudatus was likely not responsible for the
wife's illness in family cluster 3. However, as in Cluster 2, Andes and
other associated hantaviruses can cause clustering attributable to a single
pointsource contact with infected rodents.

Person-to-person transmission of Andes virus-associated HPS has been
documented (17,29), although this type of transmission has not been
observed with SNV-associated HPS (24,30). The lack of cases among
health-care workers suggests that current protective methods are adequate;
however, standard precautions should be reinforced. Until the propensity of
Andes virus to be transmitted from person to person is clarified, patients
with suspect cases should be isolated in separate rooms, contact and
droplet precautions should be used, and respiratory precautions should be
considered.

Acknowledgments

      We thank Drs. Alex Figueroa, Roberto Belmar, Hugo Salinas, and J.
Montecinos for their support to complete these studies, and John O'Connor
for editorial review of this manuscript. Dr. Roberto Murua provided
valuable support of rodent trapping efforts, and Dr. Milton Gallardo
confirmed the rodent species identifications.

      Dr. Toro is an epidemiologist with the Chilean Ministry of Health,
professor of public health at the Universidad de Chile in Santiago, and
director of the Chilean Society of Epidemiology.

      Address for correspondence: Ali S. Khan, CDC, Mail Stop A26, 1600
Clifton Road, Atlanta, GA, 30333, USA; fax: 404-639-1509, e-mail:
ask0@cdc.gov.

References

  1. Duchin JS, Koster FT, Peters CJ, Simpson GL, Tempest B, Zaki SR, et
     al. Hantavirus pulmonary syndrome: a clinical description of 17
     persons with a newly recognized disease. N Engl J Med 1994;330:949-55.
  2. Khan AS, Ksiazek TG, Peters CJ. Hantavirus pulmonary syndrome. Lancet
     1996;347:739-41.
  3. Khan AS, Khabbaz RF, Armstrong LR, Holman RC, Bauer SP, Graber J, et
     al. Hantavirus pulmonary syndrome: the first 100 U.S. cases. J Infect
     Dis 1996;173:1297-303.
  4. Ksiazek TG, Peters CJ, Rollin PE, Zaki S, Nichol S, Spiropoulou C, et
     al. Identification of a new North American hantavirus that causes
     acute pulmonary insufficiency. Am J Trop Med Hyg 1995;52:117-23.
  5. Nichol S, Spiropoulou C, Morzunov S, Rollin P, Ksiazek T, Feldmann H,
     et al. Genetic identification of a hantavirus associated with an
     outbreak of acute respiratory illness. Science 1993;262:914-7.
  6. Childs JE, Ksiazek TG, Spiropoulou CF, Krebs JW, Morzunov S, Maupin
     GO, et al. Serologic and genetic characterization of Peromyscus
     maniculatus as the primary rodent reservoir for a new hantavirus in
     the southwestern United States. J Infect Dis 1994;169:1271-80.
  7. Espinoza R, Vial P, Noriega LM, Johnson A, Nichol ST, Rollin PE, et
     al. Hantavirus pulmonary syndrome in a Chilean patient with recent
     travel in Bolivia. Emerg Infect Dis 1998;4:93-4.
  8. Levis SC, Morzunov SP, Rowe JE, Enria D, Pini N, Calderon G, et al.
     Genetic diversity and epidemiology of hantaviruses in Argentina. J
     Infect Dis 1998;177:529-38.
  9. Mills JN, Childs JE, Ksiazek TG, Peters CJ, Velleca WM. Methods for
     trapping and sampling small mammals for virologic testing. Atlanta
     (GA): U.S. Department of Health and Human Services; 1995. p. 61.
 10. Musser GG, Carleton MD. Family Muridae. In: Wilson DE, Reeder DM,
     editors. Mammal species of the world, a taxonomic and geographic
     reference. Washington: Smithsonian Institute; 1993. p. 501-755.
 11. Feldmann H, Sanchez A, Morzunov S, Spiropoulou CF, Rollin PE, Ksiazek
     TG, et al. Utilization of autopsy RNA for the synthesis of the
     nucleocapsid antigen of a newly recognized virus associated with
     hantavirus pulmonary syndrome. Virus Res 1993;30:351-67.
 12. Zaki SR, Greer PW, Coffield LM, Goldsmith CS, Nolte KB, Foucar K, et
     al. Hantavirus pulmonary syndrome: pathogenesis of an emerging
     infectious disease. Am J Pathol 1995;146:552-79.
 13. Lopez N, Padula P, Rossi C, Lazaro ME, Franze-Fernandez MT. Genetic
     identification of a new hantavirus causing severe pulmonary syndrome
     in Argentina. Virology 1996;220:223-6.
 14. Hjelle B, Jenison S, Torrez-Martinez N, Yamada T, Nolte K, Zumwalt R,
     et al. A novel hantavirus associated with an outbreak of fatal
     respiratory disease in the southwestern United States: evolutionary
     relationships to known hantaviruses. J Virol 1994;68:592-6.
 15. Da Silva MV, Vasconcelos MJ, Hidalgo NTR, Veiga APR, Canzian M,
     Marotto PCF, et al. Hantavirus pulmonary syndrome: report of the first
     three cases in So Paulo, Brazil. Rev Inst Med Trop Sao Paulo
     1997;39:231-4.
 16. Joachini L, Thompson M, Rojas de Arias A, Iwasaki E, Torrez-Martinez
     N, Von Bredow C, et al. Prevalence of hantavirus antibodies in select
     populations of Eastern and Western Paraguay. In: Abstracts of the
     Fourth International Conference on HFRS and Hantaviruses; 1998 Mar
     5-7; Atlanta, Georgia.
 17. Wells RM, Sosa ES, Yadon ZE, Enria D, Padula P, Pini N, et al. An
     unusual hantavirus outbreak in southern Argentina: person-to-person
     transmission? Emerg Infect Dis 1997;3:171-4.
 18. Levis SC, Briggiler AM, Cacase M, Peters CJ, Ksiazek TG, Cortes J, et
     al. Emergence of hantavirus pulmonary syndrome in Argentina
     [abstract]. Am J Trop Med Hyg 1995;54 Suppl:441.
 19. Williams RJ, Bryan RT, Mills JN, Palma RE, Vera I, Velasquez F, et al.
     An outbreak of hantavirus pulmonary syndrome in Western Paraguay. Am J
     Trop Med Hyg 1997;57:274-82.
 20. Hjelle B, Torrez-Martinez N, Koster FT. Hantavirus pulmonary
     syndrome-related virus from Bolivia [letter]. Lancet 1996;347:57.
 21. Frampton JW, Lanser S, Nichols CR, Ettestad PJ. Sin Nombre virus
     infection in 1959 [letter]. Lancet 1995;346:781-2.
 22. Murua R, Gonzales LE, Gonzales M, Jofre YC. Efectos del florecimiento
     del arbusto Chusquea quila Kunth (Poaceae) sobre la demografa de
     poblaciones de roedores de los bosques templados frios del sur
     Chileno. Boletn de la Sociedad de Biologa, Concepcin, Chile
     1996;67:37-42.
 23. Lpez N, Padula P, Rossi C, Miguel S, Edelstein A, Ramirez E, et al.
     Genetic characterization and phylogeny of Andes virus and variants
     from Argentina and Chile. Virus Res 1997;50:77-84.
 24. Wells RM, Young J, Williams RJ, Armstrong LR, Busico K, Khan AS, et
     al. Hantavirus transmission in the United States. Emerg Infect Dis
     1997;3:361-5.
 25. Levis S, Rowe JE, Morzunov S, Enria DA, St Jeor S. New hantaviruses
     causing hantavirus pulmonary syndrome in Central Argentina [letter].
     Lancet 1997;349:998-9.
 26. Ferrer JF, Jonsson CB, Esteban E, Calligan D, Basombrio MA,
     Peralta-Ramos M, et al. High prevalence of hantavirus antibodies in
     Indian communities of the Paraguayan and Argentinean Gran Chaco. Am J
     Trop Med Hyg 1998;59:438-44.
 27. Parisi MDN, Enria DA, Pini NC, Sabattini MS. Retrospective detection
     of clinical infections caused by hantavirus in Argentina. Medicina (B
     Aires)1996;56:1-13.
 28. Khan AS, Ksiazek TG, Peters CJ. Viral hemorrhagic fevers among
     children. Seminars in Pediatric Infectious Diseases 1997;8:64-73.
 29. Padula PJ, Edelstein A, Miguel SDL, Lpez NM, Rossi CM, Rabinovich RD.
     Hantavirus pulmonary syndrome (HPS) outbreak in Argentina: molecular
     evidences for person-to-person transmission of Andes virus. Virology
     1998;241:323-30.
 30. Vitek CR, Breiman RF, Ksiazek TG, Rollin PE, McLaughlin JC, Umland ET,
     et al. Evidence against person-to-person transmission of hantavirus to
     health care workers. Clin Infect Dis 1996;22:824-6.

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

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(ft) for footnote; (sup) for superscript; (sub) for subscript; 
>/= for greater than or equal to.

URL: ftp://ftp.cdc.gov/pub/EID/vol4no4/ascii/toro.txt