What 3rd case of bird flu with unknown source of infection could mean in fight against disease

(SAN FRANCISCO) -- A child in San Francisco was recently confirmed to be the third human case of bird flu in the United States in which it's unclear how the person got infected.

Cases have been spreading across the country since April 2024 with 67 confirmed as of Thursday, according to the Centers for Disease Control and Prevention (CDC).

Most human cases have occurred after coming into contact with infected cattle, infected poultry farms or other culling operations.

The CDC and other public health officials say there is currently no evidence of human-to-human transmission and the risk to the general public is low.

Doctors tell ABC News they agree but, with few cases that have an unknown -- or unclear -- source of infection, there may be evidence of some cases slipping through the cracks.

"There are reassuring factors here, which is the child appears to have had mild disease recovered…and kind of mild symptoms," Dr. Tony Moody, a professor of pediatrics and infectious diseases specialist at Duke University, told ABC News.

Moody added, "That's reassuring on the one hand, but it's also concerning, because we don't know, does this represent the only case, or is it one of 10,000 cases that just haven't made their way into the health care system?"

Health officials in San Francisco first reported the bird flu case in the child earlier this month before it was confirmed by the CDC.

The child experienced symptoms of fever and eye irritation, and has since fully recovered, officials said. Investigators said they're looking into how the child was exposed to the virus.

A CDC spokesperson confirmed this is the second child infected with bird flu in the country, the first case being in late November in California, also with unknown exposure.

The agency noted this is the third time that an exposure source has not been identified for a bird flu case with most other cases directly linked to exposure by infected livestock.

Moody said it's hard what to make of the case because, while the CDC has bumped up surveillance, there are still gaps.

"It's not universal surveillance. We're not able to capture all of the cases that we might like to catch," Moody said. "And so, it's kind of hard to know what to do with isolated data points like this, when you get a report of, yes, this is a confirmed case. But it's also like, what is the actual denominator here? How many cases are there really out there? And it's kind of hard to tell."

"So, I'm not sure that the identification of this case tells us a whole lot, other than, yep, it's circulating," Moody added.

Dr. Meghan Davis, an associate professor of environmental health and engineering at Johns Hopkins Bloomberg School of Public Health, told ABC News that because many of the cases have been mild, people with similarly mild symptoms may not be getting tested for bird flu.

For example, someone with pink eye, also known as conjunctivitis, may not associate it with bird flu, even with recent exposure to cattle.

"I'm certain that we're missing some cases, because not everybody is going to even go to a health care provider if they're sick and get swabbed," she said. "There may be people who have more mild symptoms, and it doesn't graduate to the level of 'I need to go to urgent care' or 'I need to go to the hospital.'"

Both Moody and Davis said more surveillance needs to be conducted to catch cases that fly under the radar. Davis points out that the CDC is already doing this, announcing Thursday it is calling for a shortened timeline for subtyping all tests that are positive with influenza A to identify non-seasonal influenza.

The CDC said it is reminding clinicians and laboratories to test for influenza in patients with suspected cases and to expedite subtyping to determine if they have bird flu rather than seasonal flu.

"The reason this is important is that what you do for someone who has seasonal flu may be a little bit different than what you do when you're dealing with a virus that's novel and you don't know entirely what to expect clinically, and you don't know entirely what to expect in terms of its potential to continue to spread," Davis said.

Moody added that it's reassuring the recent pediatric case in California did not occur within a cluster of cases, such as an entire family becoming infected.

He explained it would be much more jarring to have a cluster of cases with unconfirmed infection compared to an isolated case.

"When we see a report of a cluster of cases, that's when my blood pressure is going to go up," Moody said. "Given everything else we know, I think let's keep our worry proportional for now."

ABC News' Youri Benadjaoud contributed to this report.

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