A household contact of a Missouri patient who contracted bird flu also became ill on the same day, the CDC said in its weekly update on the H5N1 outbreak among dairy cows and poultry across the U.S. The development has raised public health experts' concerns about the possibility of person-to-person transmission.
Neither patient had any known occupational exposure to infected animals. On Sept. 6, the CDC confirmed a Missouri resident developed bird flu, marking the nation's first human case with no known animal exposure. The patient had underlying medical conditions and has since recovered after being hospitalized in late August. The individual was treated with an antiviral and was not severely ill.
It is unclear whether bird flu was the source of the household contact's symptoms, as the individual was not tested and has since recovered, the CDC said in its Sept. 13 update.
"The simultaneous development of symptoms does not support person-to-person spread but suggests a common exposure," the CDC said. A second close contact — a healthcare worker who had cared for the initial patient — also developed mild symptoms but tested negative for bird flu.
On Sept. 12, a day before the CDC shared news of a household contact developing symptoms, the agency held a briefing on the Missouri case and did not make mention of the close contact's symptoms, drawing sharp criticism from health experts.
"There are absolutely no circumstances in which it is acceptable to not have disclosed that information yesterday," Dr. Jennifer Nuzzo, director of the pandemic center at Brown University School of Public Health in Providence, R.I., told The New York Times.
In a statement to news outlets, a CDC spokesperson said the household contact's illness "should have been mentioned in the press briefing, along with the additional context," maintaining that the risk to public health remains low.
Officials with the CDC and Missouri health department are still investigating the case to determine exposure, with officials now asking more detailed questions about whether wild birds or animals had been seen around the patient's home and the type of dairy consumed, including any raw products, according to NBC News.
The H5N1 outbreak has affected more than 200 dairy herds in 14 states since late March. The initial 13 cases all occurred in people who work with infected dairy cows or poultry. Federal officials have reiterated that there is no evidence human-to-human transmission is occurring, though infectious disease experts have repeatedly raised concern that the nation is not conducting enough testing to quickly identify and respond if and when that does occur.
A virus sample from the Missouri patient did not have enough genetic material to obtain a full genetic sequence, the CDC said. However, a preliminary genetic analysis did not show evidence of changes that would make the virus more transmissible to humans, a federal health official who spoke on the condition of anonymity since the investigation is ongoing, previously told The Washington Post.