Links to Other ASM Pages:



• Table of Contents
• ASM News Issues

Experts Debate Whether Andes Hantavirus Spreads Person-to-Person

Rafael Medina, a research associate, working in the field laboratory at Dos Lagunas National Monument near Coyhaique, Chile, in February 2000 during investigations into hantavirus spread in that area.

Can one individual with hantavirus pulmonary syndrome (HPS) infect another? Some investigators analyzing a 1996 outbreak in southwestern Argentina and a separate 1997 outbreak in Chile conclude that the Andes virus species of hantavirus can spread directly from one individual to another. However, others question that interpretation, pointing to studies showing that little or no evidence of person-to-person spread could be found in other hantavirus outbreaks.

Humans typically are infected by hantaviruses through inhalation of aerosolized urine or feces deposited by rodent carriers. However, evidence that the Andes virus might spread directly from person to person came initially from several clusters of cases involving residents of or visitors to three towns near the southwestern border of Argentina. The most telling case involved a doctor in Buenos Aires who treated a resident of one of these towns who traveled to the capital city seeking care. Although the physician had not traveled outside Buenos Aires, she developed HPS. Other cases in Argentina involved family members and friends as well as health care providers who had close contact with infected individuals but apparently were not exposed to rodent-infested settings.

A research team led by Paula Padula, chief of virology at the Carlos Malbran Institute of Microbiology in Buenos Aires, examined the apparent person-to-person "chains" of transmission in Argentina by analyzing viral gene segments obtained from patients within suspected transmission chains. These segments, spanning a total of 1,075 nucleotides, were identical in 16 of 20 cases.

Despite the genomic sequencing evidence, the idea that the Andes virus could spread through personal contact met with skepticism, Padula recalls, and several alternate explanations were offered for these cases. After all, these were all instances in which the source of transmission could not be definitively determined. Moreover, a retrospective study of the 1993 Sin Nombre hantavirus outbreak in the United States designed to look for instances of person-to-person transmission concluded that this mode of transmission most likely had not occurred in that outbreak. The few cases where patients shared connections with other Sin Nombre-infected individuals were most likely the result of common exposure to a rodent-contaminated source.

Similar caution hovers over interpretations of person-to-person spread during the HPS outbreak in Chile. However, two chains of related cases in Chile provide some evidence suggestive of this transmission mode, according to Pablo Vial, head of the infectious diseases program at the Catholic University School of Medicine in Santiago. In one instance, the wife of a man who worked briefly in an affected region of Chile contracted HPS despite never leaving their hometown. In another instance, several members of a family that stayed in a cabin in an area where the outbreak was occurring subsequently developed HPS after returning home to an unaffected area--for some of them, illness developed many weeks after the trip, surpassing the usual viral incubation period. "In [these] two clusters, exposure to a common source of virus can be excluded, so the next most likely explanation for the infection is person-to-person transmission," Vial says. "However, there may be some other source that we did not get to know during interviews with the cases or that we do not know yet."

Other hantavirus experts say that the large amount of case clustering that characterizes the Chilean outbreak may offer even more evidence, albeit indirect, of person-to-person transmission. A joint Chilean-U.S. hantavirus research team recently determined that roughly one-third of HPS patients in Chile were part of familial case clusters. "What makes these Chilean clusters concerning to me is that they suggest that person-to-person transmission may be occurring at some low level whenever there’s an Andes virus outbreak," says Greg Mertz of the University of New Mexico in Albuquerque, who is a lead investigator of this team.

But others caution that the source of infection in these family clusters is too ambiguous to allow any conclusions. The Chilean case clusters have mostly involved family members, with no instances of transmission to health care workers, which commonly occurred among the Argentinean clusters, Vial notes. He says that the family clusters could be the result of common exposure to the same rodent-contaminated sources. "There are many pieces of evidence that show that person-to-person transmission is very rare," he says.

Researchers have few ideas to explain how the Andes virus might spread from one person to another and why this transmission mode has not been noted with any other hantaviruses, including those that have been causing outbreaks in Europe and Asia for many decades. "It seems possible that the virus could be transmitted person-to-person during a viremic stage prior to clinical presentation before the immune response starts limiting its replication," Vial says. "We know, based on a case in which the virus was isolated during the prodrome of the disease, that the virus is present in the blood. However, we do not know yet if it is present in other body fluids."

Christine Stencel
Christine Stencel is a communications manager and science writer at ASM.