Relatives of avian flu patients have asymptomatic cases

Mar 9, 2005 (CIDRAP News) – Two relatives of avian influenza patients in northern Vietnam have tested positive for the virus without being sick, according to reports from Vietnam today.

The 61-year-old widow of a man who died of avian flu in late February and the 80-year-old grandfather of two patients currently under treatment in a hospital have tested positive, the Associated Press (AP) and other news services reported. Both are apparently healthy.

Both people live in Thai Binh province, where five other avian flu cases have been reported in the past 2 weeks.

The AP said Nguyen Tran Hien, director of the National Institute for Hygiene and Epidemiology in Hanoi, confirmed that the 61-year-old woman had tested positive for the H5N1 virus. The director of a district medical center in Thai Binh, Nguyen Van Thieu, said the woman remained in good health but was isolated at home, the story said.

Her 69-year-old husband fell ill Feb 19 and died Feb 23, Thieu said. He said the family had raised some chickens, but they remained healthy, and the source of the woman's infection was unclear.

The 80-year-old is the grandfather of a 21-year-old man and a 14-year-old girl who have avian flu. The young man has been in critical condition for 2 weeks, while the girl is in good condition though still hospitalized, Agence France-Presse (AFP) reported today.

Testing completed yesterday at the National Institute of Hygiene and Epidemiology showed that the grandfather had the H5N1 virus, the AP reported, quoting Pham Van Diu, director of the province's Preventive Medicine Center. Diu said the man was at home and in good health.

A Reuters report today quoted health officials in the man's village as saying he had drunk raw duck blood during the Lunar New Year festivities in February. But the AFP story said it was unclear whether he had caught the virus from infected poultry or from his grandchildren. None of 12 other relatives tested positive for the virus, the AP reported.

The two new cases are not the first sign that people can contract the H5N1 virus without getting ill or seriously ill. Mild and asymptomatic cases were seen when the virus first jumped to humans in Hong Kong in 1997, Dick Thompson, infectious disease spokesman for the World Health Organization (WHO) in Geneva, told CIDRAP News via e-mail today. (Six of 18 people infected in that outbreak died.)

In addition, a Japanese man was found to have been infected while working at a farm where a poultry outbreak of H5N1 avian flu occurred in February 2003, though he never got sick, according to news reports in December 2004.

The confirmation of asymptomatic cases implies that infections may be more common than previously thought and that the case-fatality rate may be lower. The fatality rate for officially confirmed cases has hovered in the 70% range.

Thompson commented, "The CFR [case-fatality rate] had to be overstated. The cases we were sure of were those which were sick enough to go to a hospital and these extreme cases have very poor outcomes. Surely others were infected and either not getting sick or not getting sick enough to seek treatment at a hospital. Factoring those into the CFR has been impossible. We simply don't know the denominator."

Dr. Arlene King of the Public Health Agency of Canada told CIDRAP News that researchers in Asia have conducted serologic surveys in an effort to find asymptomatic cases, but full results have not been released yet. In a newspaper report yesterday, she said, the WHO's Peter Horby reported that a serologic survey of Asian healthcare workers who had cared for avian flu patients found no evidence of infection among the workers. King is director of the agency's Immunization and Respiratory Infections Division in Ottawa.

King said mild and asymptomatic cases of avian flu could go undetected even if people are tested, depending on test sensitivity. "The sensitivity of tests may be higher in sick patients," she said. "If you're swabbing patients who are less sick, maybe your tests are less sensitive."

If avian flu cases are more common than suspected, it raises the question whether the virus has more opportunities to mix, or "re-assort," with human-adapted flu viruses. Reassortment could lead to a mutated virus that could spread easily from person to person, potentially triggering a flu pandemic in a world population with little or no resistance to the virus.

Thompson said the WHO is assessing the implications of asymptomatic cases for viral reassortment. "We hope to have something soon," he wrote.

King commented, "I think we know so little about the conditions in which that [reassortment] occurs that it's really difficult to speculate on that."

King said it would not surprise her if the two latest cases are found to involve person-to-person transmission, given that some possible cases of human transmission have been seen previously in the past year. "The two settings you're going to see them [cases involving human transmission] in are households and healthcare settings, because that's where infection gets amplified," she said.

Several family clusters of cases have occurred since H5N1 avian flu began spreading in Southeast Asia in late 2003, but human-to-human transmission has been described as probable in only one instance so far. Researchers concluded that the mother and an aunt of an 11-year-old girl probably acquired the virus from her when they cared for her in a hospital in Thailand in September 2004.

Staff writer Amy L. Becker contributed to this report.

See also:

CIDRAP case-count tables
http://www.cidrap.umn.edu/cidrap/content/influenza/avianflu/case-count/avflucount.html

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