In what could possibly be the first known case in the United States, the H5N1 “bird flu” virus may have now been spread from one person to another, according to the Centres for Disease Control & Prevention, which announced last Friday that someone who lived with a Missouri resident infected with bird flu also became ill on the same day.
The CDC officials said that although there was “no epidemiological evidence at this time to support person-to-person transmission of H5N1”, additional research was needed.
The New York Times reports that the coincidental timing of the illnesses, especially outside flu season, has concerned independent experts, who said neither the initial patient nor the household contact had any known exposure to the virus via animals or raw milk.
Neither patient has been identified, and details are scant. The household contact was not tested, so officials cannot be sure that the individual actually was infected with the bird flu virus.
The household contact had gastrointestinal symptoms, the CDC said, which sometimes accompany flu infections.
In a news briefing on Thursday, the officials said it was unclear how the initially identified patient had become infected, and called the case “a one-off”.
That evening, they said that “all contacts are known and remained asymptomatic during the observation period”.
However, on Friday, CDC officials acknowledged that the household contact’s illness “should have been mentioned in the press briefing the previous day, along with the additional context”. The risk to the public remains low, they added, and both illnesses might have resulted from simultaneous exposure to H5N1 in some unknown way.
But other experts said the appearance of illness cannot be used to determine timing of exposure because people develop symptoms at different rates.
“There is always a range of time between when a person gets infected and when they start feeling sick,” said Deepta Bhattacharya, an immunologist at the University of Arizona.
If someone has additional health problems, as the initially identified patient did, “they might start feeling worse faster than someone healthy”, he said.
The patient with confirmed bird flu was admitted to hospital on 22 August. The close contact developed symptoms on the same day, but the case was missed by routine flu surveillance because the person was not hospitalised, said Missouri health officials.
Even now, however, antibody testing could reveal exposure to H5N1 – and officials said such tests were “being considered”.
Dr Nirav Shah, CDC principal deputy director, said on Thursday that it was too early to detect antibodies in the hospitalised patient, but Bhattacharya disagreed.
“That’s plenty of time to be able to tell,” he said. “It’s been weeks now, so I don’t see how you wouldn’t even try yet.”
Other experts said they were frustrated with the lack of information about the investigation.
In addition to the household contact, a health worker attending to the hospitalised patient became ill but tested negative for the flu, the CDC said on Friday night, adding that its scientists were unable to obtain the full genetic sequence of the virus from the patient because of the sample’s low quality, so they could not be sure of how the virus might be changing.
Since the H5N1 outbreak in dairy cattle was first reported in March, the virus has been detected in 203 herds in 14 states and in 14 people. Experts have warned that without widespread testing, it is likely that additional infections went undetected.
The first 13 human cases all occurred in people who worked directly with infected cows or poultry. The virus has not been detected on any of Missouri’s dairy farms, though commercial poultry flocks and wild birds have been affected.
The Missouri case was detected by routine flu surveillance. The patient, who had other underlying medical conditions, was treated with the antiviral drug Tamiflu and has recovered.
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