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 the flu, the Centers for Disease Control and Prevention reported Friday.
The CDC revealed the previously classified information in FluViewits weekly report on flu activity. During an hour-long press conference involving CDC officials on Thursday, no mention was made of this possible additional case.
STAT reached out to the CDC and the Missouri Department of Health and Senior Services for comment on Friday, but had not received statements from either at the time of publication.
On Thursday, the CDC’s chief deputy director, Nirav Shah, suggested that it was beginning to appear that the Missouri case was a single infection of an unexplained illness caused by a non-human flu strain. Although rare and troubling – all human infections with influenza viruses that do not circulate among humans raise the specter of pandemic influenza – such events occasionally occur with swine flu viruses.
“Right now, the 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 further spread from the infected individual. He did not mention that a close contact may have been infected at the same time.
“So far we have not seen any evidence of person-to-person transmission. None of this person’s close contacts have any evidence of onward transmission. None of the people this person came into contact with have developed signs and symptoms. So we haven’t seen any evidence of it at this point,” he said.
The FluView report provided no information about the timing of the second person’s illness, or whether an investigation into his or her movements could explain how the H5N1 virus made its way to at least one and possibly two people in a state that has reported no outbreaks in dairy cows.
Since the second person was not tested while he or she was ill, it is not clear whether that person was also infected with bird flu, or had an unrelated respiratory illness. An answer could come from blood tests looking for antibodies against the H5N1 flu.
During the press conference on Thursday, Shah said the agency hopes close contacts of the confirmed case will agree to provide blood samples to see if there may be undetected transmission from or around the individual. It’s a bit too early to conduct such tests, Shah said, because antibody development takes some time after an infection.
H5N1 viruses, originating from wild birds, have long been high on the list of possible pandemic viruses. Humans have no immunity against this family of flu viruses. The virus has circulated on and off since the late 1990s, causing massive and deadly poultry outbreaks in many parts of the world and infecting about 900 people. About half of the people known to be infected with H5N1 have died.
The H5N1 bird flu has been spreading among dairy cows in parts of the United States since late last year or early this year. The unprecedented outbreak – until now the virus was not thought to pose a risk to cows – has so far spread to 203 herds in 14 states. And this year, 14 people have been infected with the virus, including the case in Missouri. The American cases before 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 that it had succeeded in subtyping the virus’s neuraminidase protein — the N in its name — and that it is indeed H5N1. The virus is genetically similar to the virus circulating among dairy cattle.
The Missouri individual, who had multiple health problems, was hospitalized on August 22 for symptoms that did not immediately appear to be related to the flu: chest pain, nausea, vomiting, diarrhea and weakness. While in the hospital, the person was tested for flu and given antiviral medications after testing positive. The person recovered and was discharged.
So far, public health researchers have been unable to explain how the infection occurred; the individual had no known exposure to animals or poultry, wild birds or wildlife, and did not consume raw dairy products.
This story has been updated to reflect that the CDC has confirmed the subtype of the virus in the infected person in Missouri, N1.