Deadly Andes hantavirus may spread nosocomially
Person-to-person transmission suggested
An emerging new hantavirus in South America has been strongly linked to person-to-person transmission, and some of the first victims have been health care workers caring for infected patients, the Centers for Disease Control and Prevention reports.
Between Sept. 22 and Dec. 5, 1996, 20 cases of hantavirus pulmonary syndrome (HPS) occurred in southern Argentina, and half of those infected died. A total of five physicians were infected, including three who were directly responsible for the clinical care of an HPS patient. Epidemiologic links between 16 of the cases strongly suggested person-to-person transmission of HPS during the outbreak, the CDC reported.1 The infectious agent was the Andes virus, which was first identified in the lungs of an infected patient in Argentina in 1995.2
Normally spread from asymptomatically infected rodents to humans, the mode of person-to-person transmission in the outbreak could not be determined. In reporting the case, the CDC drew no conclusions, but cited these possibilities: direct contact, droplets, infectious aerosols, or contaminated fomites. The outbreak has raised concerns that hantavirus which has never been associated with human-to-human transmission in studies and observations that include soldiers in the Korean war exposed to the Hantaan virus - may now be finally bridging that gap.
Transmission model could be changing
"This is the kind of episode that is extremely important to follow up on because this could be one of those viruses that is trying to make a change [in transmission mode]," says C.J. Peters, MD, chief of the special pathogens branch at the CDC and one of the principal investigators in the case. "It is clearly a virus that we need to know a lot more about."
No cases have been reported in the United States, and the CDC decided not to advise any additional infection control measures for U.S. patients presenting with hantavirus pulmonary syndrome. Standard precautions are recommended for such patients, but the outbreak in Argentina raises the question of whether health care workers also should don masks to prevent possible respiratory spread. The CDC decided not to make such a recommendation after reviewing data on all U.S. cases associated with the Sin Nombre hantavirus that caused a deadly outbreak in the Southwest "four corners" region in 1993. A study of that outbreak found no disease or hantavirus antibodies in serum specimens taken from 396 health care workers, including 266 who had been exposed to patients with HPS or their body fluids two weeks to six weeks earlier.3
"We scrubbed’ the [U.S.] registry and we haven’t found anything that suggests nosocomial transmission," says Peters. "If anything comes up that [ICPs] are concerned about that looks like it could be secondary transmission either in a family or in the health care setting they should contact us as soon as possible. We would like to look at it and study it in detail."
Overall, 11 (55%) of the 20 patients were male, the mean age was 38 years (range 13-70 years), and the case-fatality rate was 50%. The clinical profile was generally similar to the fever and myalgia reported in HPS cases in the United States. Coughing did not occur earlier or more frequently in Argentinean HPS patients than in U.S. patients, which suggests that coughing was not the critical factor in person-to-person transmission, the CDC reported. The case-to-infection ratio with the virus is very high, so any cases that appear in the United States will likely be symptomatic, Peters adds.
"A very high proportion of [Andes virus] infections are not just symptomatic, but cause recognizable hantavirus pulmonary syndrome," he says.
Partially completed DNA sequencing of the Andes virus indicates it is different from the U.S. hantavirus, which resides benignly in deer mice but continues to cause sporadic infections in humans with about a 50% mortality rate. The Andes hantavirus is carried in the South American colilarga "long-tail" rodent, which has no habitat in the United States.
"These viruses are very rodent-specific and we think it is unlikely that this virus could propagate in the rodents in this country," Peters says.
Rodents not epidemiologically connected
The most frequent mode of hantavirus transmission to humans is inhalation of aerosolized rodent excrement in infested areas. The U.S. outbreak was strongly linked to exposures to rodents, with CDC investigators commonly reporting that 10% to 20% of rodent traps were filled around the area where cases lived. However, few rodents were found in the Argentine outbreak and none of the patients could clearly be linked to rodent exposures.
"There were very few wild rodents down there," he says. "We trapped around the house of the index case and we took one rodent in 400 traps. There was no evidence of rodent excreta. This was a well-to-do area with houses on concrete slabs with good construction and most of them were rodent-proofed. There was very little in the way of evidence of rodent exposure in any of these people."
CDC investigators were initially skeptical that secondary human transmission of hantavirus had occurred in Argentina, but changed their minds after they arrived on the scene and began assisting in the outbreak investigation, Peters notes.
"There was something different going on down there," he says.
A web of linked cases
The outbreak began when the 41-year-old index patient from the town of El Bols became ill with HPS and was transferred to the town of Bariloche to receive intensive care. Three weeks after the index patient became symptomatic, his 70-year-old mother and one of his doctors developed HPS. The infected physician died, and his wife, who was also a physician, contracted HPS 19 days later. She traveled to Buenos Aires for medical care. In a Buenos Aires hospital, an admitting doctor spent one hour taking a clinical history and examining her. The doctor applied pressure to a venipuncture site on the patient’s arm with multiple layers of gauze, but no obvious blood contact occurred.
"She put a gauze sponge over the place where the needle was removed," Peters says. "She didn’t have gloves on but she didn’t obviously soil herself either. There was no exposure incident or needlestick."
The only other contact between the doctor and the patient occurred two days later, when the doctor briefly visited the hospital’s intensive care unit to attend to another patient. Yet 24 days after admitting the patient, the doctor became ill with HPS, even though she had not traveled outside Buenos Aires and reported no contact with rodents.
"I think it is pretty strong case epidemiologically," Peters tells Hospital Infection Control. "For me, that doctor in Buenos Aires clinches it. She was an urban dweller and hadn’t been out of Buenos Aires. The rodents that carry this particular virus don’t even live in the area of Buenos Aires. And for them to get to the hospital downtown or this doctor’s house downtown is just very unlikely."
The outbreak continued, including a 40-year-old doctor from Buenos Aires who developed HPS. He had visited one of the patients in the hospital but was not directly involved in the clinical management of any HPS patients. A fifth physician who developed HPS reported having close contact with several HPS patients, including intubating one and examining two others.
Subsequent cases continued to be epidemiologically linked, including additional cases among those who attended a funeral and rode in a car that included the symptomatically infected housekeeper of one of the victims. One of those in the car who was infected was another person who had not been in the region prior to the funeral, again suggesting that a common source exposure to a rodent population was not likely. In addition, a second group of HPS cases occurred in the town of Bariloche, including four people who had visited or worked in a hospital where many of the patients from El Bols were transferred. That included the hospital’s night receptionist, a visitor of an uninfected patient, and a couple who befriended one of the infected patients while frequenting the hospital to visit their pre-term infant.
"In all, about a fourth of the cases were medical personnel," Peters says. "We went back through our [U.S.] registry, and of the 160 cases we looked at, only one was a health care worker."
That case involved a nurse in Texas in 1993, and the CDC sent out an investigator to confirm suspicion that the first case of nosocomial transmission of hantavirus had occurred in the United States. A review of patient records, however, yielded no indication of any contact with a patient with HPS.
References
1. Wells RM, Estain SS, Yadon ZE, et al and the Hantavirus Pulmonary Syndrome Study Group for Patagonia. Centers For Disease Control and Prevention. An unusual hantavirus outbreak in southern Argentina: Person-to-person transmission? Emerg Infect Dis [serial online] 1997; 3(2). Available: http://www.cdc.gov./ ncidod/EID/eid.htm.
2. Lopez N, Padula P, Rossi C, et al. Genetic identification of a new hantavirus causing severe pulmonary syndrome in Argentina. Virology 1996; 220:223-226.
3. Vitek CR, Breiman RF, Ksiazek TG, et al. Evidence against person-to-person transmission of hantavirus to health care workers. Clin Infect Dis 1996; 22:824-826.
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