Two cases of likely H5N1 human spread

Lack of infection control measures common theme

Though human transmission of avian influenza A (H5N1) is extremely rare, there are two published investigations that strongly suggest the emerging virus was transmitted to health care workers in 1997, and from a patient to family contacts in 2004.1,2 Interestingly, both cases involve prolonged exposure to very ill patients in the absence of rigorous infection control measures.

"When we do epidemiological investigations, we think there has been some probable, limited person-to-person transmission," said Tim Uyeki, MD, MPH, a medical epidemiologist in the national center for infectious diseases at the Centers for Disease Control and Prevention. "There is some evidence in studies. If you look back to 1997 in Hong Kong, there were two health care workers who were exposed to H5N1 patients who actually had serological evidence. [They] seroconverted from a baseline serum specimen. They did not have any contact with poultry, so it suggests limited patient-to-health care worker transmission."

The two workers who seroconverted did not wear masks while treating a very ill adult patient who eventually died, CDC investigators reported.1 They were following standard precautions, wearing gloves and hand washing. Spread could have occurred via airborne or droplet routes or direct contact and self-inoculation of the nasal mucosa, the CDC reported. The investigation revealed that actually eight workers exposed to case patients had presence of H5N1 antibodies. Of the two workers who seroconverted, one remained asymptomatic and the other reported a respiratory illness two days after exposure to a case patient. Bathing and changing the patients’ bed linens were activities associated with being seropositive. The activities essentially were considered a surrogate marker for increased intensity of exposure to the infected patient.

A more recent case that actually led to death in the patient contact was reported last year in the New England Journal of Medicine.2 The case, which occurred in Thailand in 2004, represents some of the strongest evidence yet that person-to-person transmission of H5N1 can occur, Uyeki recently said in Chicago at the annual meeting of the Society for Healthcare Epidemiology of America (SHEA).

The index patient was an 11-year-old girl who became ill and was hospitalized three to four days after being exposed to dying household chickens. Her mother — who came from a distant city to care for her in the hospital and had no recognized exposure to poultry — died of pneumonia after providing 16-18 hours of unprotected nursing care. An aunt also provided unprotected nursing care. She had fever five days after the mother first had fever, followed by pneumonia seven days later, investigators reported.

"[The girl] lived with her aunt," Uyeki said. "Her mother did not live with her in this village. The mother lived in the summer in Bangkok. The mother came to the hospital and spent about 24 hours at the bedside with the daughter. The mother subsequently developed H5N1 and she died. Virus was isolated from her. The aunt actually became a case as well and she survived. The girl died. [The case] suggests that the girl transmitted H5N1 to the mother and possibly the aunt."

By the time the cluster was recognized, the index patient and her mother had died and had, respectively, been cremated and embalmed. Serum from the index patient and the aunt was tested for antibodies to H5N1. Autopsy tissue from the mother and nasopharyngeal and throat swabs from the aunt were positive for H5N1. No additional chains of transmission were identified, and sequencing of the viral genes identified no change in the receptor-binding site of hemagglutinin or other key features of the virus. The sequences of all eight viral gene segments clustered closely with other H5N1 sequences from recent avian isolates in Thailand.


  1. Bridges CB, Katz JM, Wing HS, et al. Risk of influenza A (H5N1) infection among health care workers exposed to patients with influenza A (H5N1), Hong Kong. J Infect Dis 2000; 181:344-348.
  2. Ungchusak K, Auewarakul P, Dowell SF, et al. Probable person-to-person transmission of avian influenza A (H5N1) N Engl J Med 2005; 352:333-340.