WASHINGTON — The Centers for Disease Control and Prevention should have results later this month that provide more insight into how a Missouri resident, who hadn’t had any contact with infected animals or food, contracted a case of highly pathogenic avian influenza.

Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases at the CDC, said on a call with reporters Friday the agency is working through its investigation of that bird flu case, while providing several more details.

“As we previously reported, CDC would be able to perform partial sequencing of the avian influenza H5 virus from the case in Missouri, despite a nearly undetectable level of viral RNA in the patient sample,” Daskalakis said.

That process is complex and time-consuming, in part because the patient had rather small amounts of the virus in their system when the test was taken.

Another contributing factor, he said, is “that the virus has two potentially important mutations, meaning two amino acid differences, in comparison with the viruses previously characterized during this event that could affect antigenicity.”

Daskalakis explained that antigenicity is when someone is able to produce “a specific immune response, such as creation of specific antibodies.”

Both the mutations and small sample size have presented challenges for the CDC, but the agency expects to announce results of the test later this month after completing the complicated lab process, he said.

Two cases in California

The Missouri case is the only bird flu diagnosis in the United States this year where the person hadn’t had direct contact with infected poultry or dairy cattle.

The remainder of the 16 people diagnosed with H5N1 during this calendar year had direct contact with farm animals, with nine of those cases linked to poultry and six related to dairy cows.

One of those cases was diagnosed in Texas, two in Michigan, two in California just this week and 10 in Colorado.

Public health officials on the call emphasized that the risk to the general public remains low and that several studies undertaken by the Food and Drug Administration show pasteurized dairy products as well as other foods remain safe to eat.

Since February, the CDC has tested more than 50,000 samples that would have “detected Influenza A, H5 or other novel influenza viruses,” Daskalakis said.

The Missouri case was the first case of bird flu detected through that influenza surveillance system, he said.

Public health officials at the state and federal level have been trying to determine how the Missouri patient, who officials are not identifying for their privacy, contracted the virus through a series of “intense interviews,” Daskalakis said.

That is how they learned someone living in the same house had been symptomatic with various gastrointestinal issues at the same time the patient had been ill.

That simultaneous onset of symptoms implied “a common exposure, rather than human-to-human transmission,” Daskalakis said, before reinforcing that the second person never tested positive for the virus and isn’t considered a case of bird flu.

“At the time of the interview, the household contact had also completely recovered and had not been tested for influenza while they were sick,” he said. “To be clear, there is only one case of H5N1 influenza detected in Missouri.”

Because the person living in the same house as the Missouri patient had been symptom-free for more than 10 days when they were interviewed by public health officials, Daskalakis said there was “no utility in testing the contact for acute influenza.”

Instead, officials in Missouri took blood samples from the two people so the CDC could test for “antibodies against H5 to assess for possible infection with this virus,” he said.

A separate investigation was taken at the hospital where the Missouri patient had been diagnosed to see if any health care workers had contracted H5N1.

Out of 118 health care workers who interacted with the patient in some way, 18 had higher-risk interactions before the patient was diagnosed and began using what Daskalakis referred to as “droplet precautions.”

Six of those health care workers later developed respiratory symptoms, though only one of them had symptoms by the time the public health investigation had begun retroactively, he said.

That one person’s PCR test for acute influenza came back negative and the other five health care workers, who had recovered, did not require a PCR test, he said.

“Since exposures could only be assessed retrospectively, Missouri has also obtained blood specimens from these individuals for antibody or serology testing at CDC to search for any evidence to support the unlikely possibility that their symptoms were related to H5 infection resulting from their interaction with the patient,” Daskalakis said. “Despite the low risk, this testing is important to complete the public health investigation of this case.”

The CDC began working on that serology testing in mid-September when it received the samples from Missouri, though the complicated process likely won’t conclude until later in October.

“For serology testing to be conclusive, it needs to be done using a virus that is genetically identical to the one obtained from the human case from Missouri or there is a risk of a false negative test,” Daskalakis said. “Since this H5 virus was not recoverable, we could not grow it because there was not enough for the Missouri specimen.”

The CDC, he explained, has to “create the right virus for the test using reverse genetics to match the one from Missouri, so that we can use it in these serology tests.”

“We realize people, including all of us at CDC, are anxious to see results from this testing,” he said. “CDC is moving at a very accelerated pace while conducting rigorous science to assure the validity of these results.”

Poultry, dairy cases

In addition to human cases, bird flu continues to infect poultry flocks and dairy herds within the United States.

While the poultry industry has had years of experience supplying its workers with personal protective equipment and culling affected farms, the dairy industry has had to figure out how to address the virus this year.

Eric Deeble, deputy under secretary for marketing and regulatory programs at USDA, said on the call Friday that Colorado’s mandatory testing program of bulk milk tanks, which began in July, offered a hopeful case study for ridding farms throughout the country of H5N1.

“Initially, this revealed a significant local prevalence, about 72% of dairies, centered in Weld County,” Deeble said.

But following months of hard work by farmers and public health officials, Colorado has just one dairy herd that’s currently affected by H5N1 out of 86 dairy herds within the state, he said.

“Mandatory surveillance in the state allows for continuous monitoring of herds and helps detect any instances of non-negative results early on, ensuring timely intervention,” Deeble said. “This decrease in Colorado cases, even in the absence of a vaccine, gives us further confidence that H5N1 can be eliminated in the national herd, even in places where we have seen an initial rapid increase in cases.”

Data from the USDA show that during the past month, three dairy herds in Idaho and 53 in California have tested positive for H5N1.