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There's no evidence of human-to-human transmission of H5N1 bird flu among patients and healthcare workers in Missouri, the CDC said during a press briefing today.

None of the six healthcare workers who developed respiratory symptoms after coming into contact with an H5N1-infected patient was infected with the virus; one tested negative on PCR, while the other five were confirmed negative on CDC's recent serology testing, the agency said.

Both the index patient and a household contact did test positive on serology for neutralizing antibodies to H5N1, but fell short of meeting the strict serological definition of a case, Demetre Daskalakis, MD, MPH, director of CDC's National Center for Immunization and Respiratory Diseases, said during the briefing. Neither person tested positive on additional immunologic and multiplex tests, one of which would have been required to be confirmed positive, he said.

Nonetheless, the agency considers both to be H5N1 cases, Daskalakis said, even though the household contact is not being counted in official agency totals.

"These similar immunologic results, coupled with the epidemiologic data that these two individuals had identical symptom-onset dates, support a single, common exposure to bird flu, rather than person-to-person transmission within the household," Daskalakis said.

The source of that exposure has not been determined. The predominant symptoms for both people were gastrointestinal, Daskalakis said, and it's highly likely that there was a "common animal or animal product exposure that wasn't identified in our epidemiologic surveys."

Experts have previously told 番茄社区app that these patients could have been exposed to infected birds -- either wild or poultry flocks -- or any other wild or domestic animals that have been infected, including cats. They also could have been exposed to infected cattle herds, or to unpasteurized milk, which has been shown to have very high levels of the virus.

The Missouri patient first presented to the emergency department on August 22 with acute symptoms of chest pain, nausea, vomiting, diarrhea, and weakness. An influenza test was ordered given the patient's significant underlying medical conditions. They were not severely ill and did not have to go to the intensive care unit, were treated with an antiviral, and were ultimately discharged and recovered.

Nirav Shah, MD, JD, principal deputy director of the CDC, gave an update on the case count in this year's outbreak associated with livestock, which reached 31 human cases: 15 in California, 10 in Colorado, two in Michigan, one each in Missouri and Texas, and now two in Washington state.

The Washington state outbreak has an additional five presumptive positive cases waiting for confirmation at CDC, Shah said.

These infections were tied to a poultry depopulation effort involving some 800,000 chickens. Patients have had conjunctivitis, and some have had mild respiratory symptoms, Shah said.

The predominant H5N1 genotype involved in this outbreak has been D1.1, which is different from the B3.13 genotype involved in outbreaks on dairy cattle farms, he added.

All 15 of the confirmed H5N1 cases in California have occurred among dairy farm workers. Most have experienced conjunctivitis, and there have been no hospitalizations, Shah said.

Genetic sequencing of nine viruses from the California outbreak show no changes in the virus' ability to cause infection in humans, or increase the likelihood of human-to-human transmission, he said. Nor is there any evidence of reduced susceptibility to available influenza treatments.

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