A close contact of the person in Missouri who had an unexplained H5N1 bird flu infection last month was also sick around the same time, but was not tested for influenza, the Centers for Disease Control and Prevention reported on Friday.
The CDC revealed the previously undisclosed information in FluView, its weekly update on influenza activity. In an hour-long press briefing involving CDC officials on Thursday, no mention was made of this possible additional case.
In a statement on Friday, the CDC reiterated that it does not believe there has been spread of H5N1 between the infected individual and any of his or her close contacts.
“There is no epidemiological evidence at this time to support person-to-person transmission of H5N1, though, public health authorities continue to explore how the H5N1-positive individual in Missouri contracted the virus,” the CDC said, adding it would support studies of contacts of the cases to try to determine if any of them also contracted the virus.
A spokesperson for the agency told STAT that a health worker who had contact with the confirmed case in the hospital also developed mild flu-like symptoms, but tested negative for influenza.
On Thursday, Nirav Shah, the CDC’s principal deputy director, suggested it was beginning to look like the Missouri case was a one-off — a case of unexplained disease caused by a non-human flu strain without ongoing transmission. Though rare and unsettling — any human infections with flu viruses that don’t circulate in people raise the specter of pandemic influenza — such events occasionally occur with swine influenza viruses.
“Right now, evidence points to this being a one-off case,” Shah said.
When asked to explain that comment, he suggested there was no evidence of any onward spread from the infected individual. He did not mention the fact that a close contact may have been infected at the same time.
“Thus far we have not seen any evidence of person-to-person transmission. None of this individual’s close contacts have any evidence of onward transmission. None of the individuals that this individual came into contact with have developed any signs and symptoms. So we haven’t seen any evidence of it at this time,” he said.
The FluView report gave no information about the timing of the second person’s illness, or whether an investigation into his or her movements might explain how the H5N1 virus made its way to at least one and possibly two people in a state that has not reported outbreaks in dairy cows.
Lisa Cox, a spokesperson for the state’s Department of Health and Senior Services, said investigators learned that a second individual — a household contact of the confirmed case — had also experienced symptoms only after the individual had recovered. By then it was too late to be able to test by PCR.
“The onset of symptoms for both the case and the case’s household contact occurred on the same day,” Cox told STAT in an email. “The possible opportunities of exposures for this close contact were also evaluated as part of the investigation. Serological testing” — in other words, testing of blood for antibodies to the virus — “is being considered.”
During the Thursday press briefing, Shah said that the agency hopes close contacts of the confirmed case will agree to give blood samples to see if there might have been undetected transmission from or around the individual. It is a bit too soon to conduct such tests, Shah said, because development of antibodies takes a little time after an infection.
H5N1 viruses, which originate in wild birds, have long been high on the list of possible pandemic viruses. Humans do not have immunity to this family of flu viruses. The virus has circulated off and on since the late 1990s, triggering massive and deadly poultry outbreaks in many parts of the world, and infecting about 900 people. About half of the people known to have been infected with H5N1 have died.
H5N1 bird flu has been spreading in dairy cows in parts of the United States since late last year or early this year. The unprecedented outbreak — the virus hadn’t been thought to be a risk to cows before — has spread to 203 herds in 14 states to date. And 14 people have been infected with the virus this year, including the case in Missouri. The U.S. cases prior to the Missouri case were all mild; none of the people needed to be hospitalized.
That was not the case with the H5N1 case in Missouri, which was first reported last week. On Friday, the CDC revealed it had succeeded in subtyping the neuraminidase protein of the virus — the N of its name — and that it is indeed H5N1. The virus is genetically similar to those that have been circulating in dairy cattle.
The Missouri individual, who had multiple health issues, was hospitalized on Aug. 22 for symptoms that did not immediately appear to be flu-related: chest pain, nausea, vomiting, diarrhea, and weakness. While in the hospital the person was tested for flu and was given antiviral medication after testing positive. The person recovered and was discharged.
To date public health investigators have been unable to explain how the infection occurred; the individual had no known exposures to animals or poultry, wild birds or wild animals, and did not consume raw milk products.
This story was updated to include comment from the CDC and the Missouri Department of Health and Senior Services, and to reflect the fact that CDC confirmed the subtype of the virus in the infected person in Missouri, N1.