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A Missouri resident who shared a home with a patient hospitalized with bird flu in August was also infected with the virus, federal officials reported on Thursday.
But symptomatic health care workers who cared for the hospitalized patient were not infected, testing showed. The news eased worries among researchers that the virus, H5N1, had gained the ability to spread more efficiently among people.
Still, the number of human cases is rising in the United States. California said this week that it had confirmed 15 human cases of bird flu. Washington State has reported two poultry workers who are infected and five others presumed to be positive.
There are 31 confirmed cases in the country, but experts have said the figure is likely to be an undercount. “Additional cases may be found as investigations continue,” Dr. Nirav Shah, the principal deputy director of the Centers for Disease Control and Prevention, said at a news briefing on Thursday.
“The identification of these additional cases of H5 in people with exposures to infected animals does not change C.D.C. risk assessment for the general public, which continues to be low,” he said.
The poultry workers in Washington State were infected with a version of the virus that is distinct from the one circulating in dairy cattle, he added.
Except for the two people in Missouri, all infections have been linked to exposure to infected poultry or cattle. None of the infected are thought to have passed the virus to anyone else, although uncertainties about the Missouri infections persist.
Investigators do not know how the Missouri patient and the household member became infected. They had no exposure to infected animals and had not consumed raw milk products that may have carried the virus, officials said.
Jennifer Nuzzo, the director of the Pandemic Center at Brown University, said, “We need to know how the two people in Missouri who had no known contact with animals contracted the virus.”
“I’d feel better if we know these two people likely got sick from a freak encounter with wildlife than a food product that may be distributed to other people,” she added.
Federal officials said they had ruled out person-to-person infection in Missouri partly because the two people developed symptoms at the same time, suggesting a shared source of infection.
But other experts have said that two people may develop symptoms simultaneously even if they were exposed to the virus at different times.
If it were the case that one person had infected the other, “that’s pretty huge,” said Dr. Gregory Gray, an epidemiologist at the University of Texas Medical Branch.
“Some limited human-to human transmission is in the pathway to full-on high human-to-human transmission, and so it is concerning,” he said.
The hospitalized patient in Missouri had been tested for influenza as part of routine surveillance and turned out to be positive for H5N1, the bird flu virus.
The C.D.C. later said that one household member and six health care workers who had been in contact with that patient had developed symptoms associated with flu.
Only one of those individuals had been tested for the virus at the time of illness. That health care worker was negative for flu.
To determine whether the others had been infected, C.D.C. scientists customized a test to detect antibodies to the version of the virus taken from the hospitalized patient. None of the health care workers carried antibodies, suggesting that they had not been infected, the agency said at the news briefing.
Results from such tests are not always definitive. They may miss evidence of infection if someone produces too few antibodies. The health care workers did not experience severe symptoms.
“There may not have been a robust enough infection to induce a very strong immune response,” said Seema Lakdawala, a virologist at Emory University.
Technically, test results for the household contact did not meet criteria set by the World Health Organization for it to be called a “case.” Yet the individual carried antibodies to the virus, suggesting an infection, C.D.C. officials said.
Other factors, including the low levels of virus found in the hospitalized patient, also suggested that the person did not pass the virus along to anyone else, federal officials said.
“There is no evidence that I think meets the bar to suggest that it’s spreading human to human,” said Ryan Langlois, a viral immunologist at the University of Minnesota.
Both the infected people in Missouri had low levels of virus when tested, and had gastrointestinal problems, but not the respiratory symptoms more usually associated with flu infections.
“It doesn’t appear to have any of the traits that I would be worried about for a virus that is fit for respiratory spread,” Dr. Langlois said. “Of course, that can change with a just a couple of mutations.”
Genetic analysis of the virus taken from three infected Californians showed some mutations, but none that would make it easier for the virus to spread among people, according to the C.D.C.
Still, every new human infection gives the virus opportunities to evolve and gain the needed mutations.
“It is clear that there’s a lot of human infection going on,” said Scott Hensley, a viral immunologist at the University of Pennsylvania. “What makes me worried is that there’s just a whole heck of a lot of shots on goal.”
Experts are particularly worried as the flu season draws nearer. Flu viruses can acquire new abilities by swapping genes with one another.
An individual infected with both bird flu and a seasonal flu virus might provide the perfect opportunity for H5N1 to gain the ability to spread as easily among people as seasonal flu does.
California is the nation’s biggest dairy producer, but the state did not detect its first infected herds until the end of August. As of Oct. 24, the virus had been confirmed on 137 dairy farms in the state, the hardest hit in the nation.
The 15 human cases were identified in dairy workers who had direct exposure to infected cows. Their symptoms were mild, and none were hospitalized, according to state officials.
But the number of infected cattle herds suggests that the virus is widespread, and the state health department acknowledged that more human cases could be identified.
“California is incredibly concerning,” Dr. Hensley said.
“It’s unclear if the number of cases that we continue to see come in each week — is that due to just good surveillance and good reporting, or is something different going on in California right now?” he added. “We just don’t know.”
Ideally farmworkers would be tested for antibodies using so-called serology tests to detect people who may have been infected without developing symptoms, and to learn more about the course of infection.
“There’s probably so many undocumented exposures,” Dr. Hensley said. “Through serology, we may be able to learn more about that.”
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