A Missouri case marked the 14th person to contract highly pathogenic avian influenza, or H5N1, this year amid ongoing outbreaks among poultry and dairy cattle. The Veterinary Diagnostic Laboratory at Iowa State University tests samples from animals for viruses such as avian influenza. (Photo courtesy of Veterinary Diagnostic Laboratory)
WASHINGTON — Public health officials are still trying to determine how a Missouri resident contracted bird flu without having any contact with infected animals, but said Thursday there is no evidence of human-to-human transmission taking place in small enclaves or in a more widespread manner.
The Missouri case marked the 14th person to contract highly pathogenic avian influenza, or H5N1, this year amid ongoing outbreaks among poultry and dairy cattle, though it marks the first time someone without contact to those animals was diagnosed with the virus.
Nirav Shah, principal deputy director at the Centers for Disease Control and Prevention, said on a call with reporters the “evidence points to this being a one-off case, and those do happen with novel influenza.”
“Thus far, we have not seen any evidence of unusual levels of influenza activity in the area where this individual resides,” Shah said during the hour-long briefing. “There have been no increases in the volume of visits to emergency departments for influenza and no increase in laboratory detections of influenza cases in Missouri more broadly.”
The state’s public health laboratory, he said, is subtyping all positive cases of Influenza A, following the CDC recommending ongoing detailed surveillance this spring.
The practice is how doctors and public health officials confirmed this case and would likely be how they diagnose any uptick in cases in the future in the “overwhelming” number states that are taking the extra step of subtyping, he said.
There are several types of influenza virus that are classified by the letters A, B, C and D. The viruses within the influenza A category are further categorized or subtyped based on the proteins hemagglutinin (H) and neuraminidase (N). There are at least 130 combinations of subtypes within Influenza A, according to the CDC.
“So here’s the bottom line, our influenza surveillance system is designed to find needles in haystacks,” Shah said. “And as this case and others show, it is working. And here, in this case, we found such a needle, but we don’t know how it got there.”
The Missouri patient, who has significant underlying medical conditions, was admitted to a hospital on Aug. 22 after presenting to health care providers with “acute symptoms of chest pain, nausea, vomiting, diarrhea and weakness,” according to the CDC. The patient is not being identified out of privacy concerns.
The Missouri Department of Health and Senior Services announced the diagnosis on Sept. 4, as did the CDC in a separate statement.
Missouri DHSS noted the patient had recovered and been sent home from the hospital, while the CDC pointed out that the state has reported cases of H5N1 in commercial and backyard poultry flocks this year.
The patient hadn’t been in contact with any livestock or poultry and didn’t indicate in a detailed questionnaire that they had consumed any food products, like raw milk, that could have potentially transmitted the virus.
Shah said on the call that public health officials are technically classifying this as a case of H5 and not H5N1 as they work to sequence the virus more fully, though that might not be possible.
The CDC has begun classifying the virus’ genetic sequence, but since the patient’s viral RNA levels were “extremely low” the agency may not be “able to generate a full flu genome, including the neuraminidase or the N part of the virus,” he said.
“We’re throwing everything we’ve got at this, but ultimately a full sequence may not be technically feasible because of the low concentration of viral RNA,” Shah said. “The data that we do have and that have been generated thus far show an H5 virus that is closely related to the (H5N1) virus circulating among dairy cows.”
The CDC, he said, is “continuing to look for evidence of genetic changes that would suggest, for example, an increased potential for spread.” None have yet been found.
Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases at CDC, said the risk to the general public from bird flu remains low.
“We assess risk continuously with every case and with every sort of change, and we continue to look at it as low,” Daskalakis said. “If there are changes, we would reassess that risk in real time.”
While the CDC investigation is ongoing, Shah said the further out from the case public health officials get without seeing any new diagnosis, the less likely they become.
“We are beyond the typical 10-day window for transmission,” Shah said. “And so with each passing day, the likelihood of this being something that’s happening deep underwater goes down.”