Health officials announced on Thursday that an investigation into a recent H5N1 bird flu case in Missouri found no evidence of the virus spreading from one person to another.

The Centers for Disease Control and Prevention (CDC) said in an Oct. 24 press call that while the patient and a household contact showed possible exposure to the virus, there is no indication of human-to-human transmission.

A case of H5N1 avian influenza was detected in Missouri last month through routine surveillance, officials said. The patient had no known exposure to infected animals or animal products, raising concerns about potential human transmission.

Dr. Demetre Daskalakis of the CDC explained in the call that an extensive epidemiological investigation was conducted.

He said a household contact who experienced gastrointestinal symptoms at the same time as the patient was identified, along with 112 health care workers who had interacted with the patient, six of whom reported respiratory symptoms.

Serological testing was performed on the patient, the household contact, and the symptomatic health care workers to detect antibodies against H5N1 avian influenza.

“The five healthcare workers were definitively seronegative against H5 in all assays, which means that they were not infected with H5,“ Daskalakis said. ”This finding ... also rules out person-to-person spread between the index case and any of the five health care workers with which they interacted at the hospital, and clarifies that the symptoms that they experienced could not be attributed to H5N1.”

Regarding the patient and the household contact, he said both showed some evidence of antibodies to H5N1 but did not meet the full criteria to be considered positive cases.

“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 investigation concluded that while the patient and household contact may have been exposed to or infected with H5N1, there is no evidence the virus spread between them or to health care workers. Officials emphasized that no animal or animal product exposure explaining the possible infections has been identified.

In related developments, the CDC reported human cases of H5N1 in California and Washington state among individuals with direct exposure to infected animals.

In California, 15 human cases have been confirmed among dairy farm workers known to be positive for H5N1, officials said in the call.

All individuals reported mild symptoms, primarily conjunctivitis (eye redness), and none were hospitalized. Genetic sequencing showed no changes in the virus that would increase its ability to infect humans or spread from one person to another.

In Washington state, two human cases were confirmed among workers involved in depopulating an infected poultry flock of approximately 800,000 chickens.

Both cases exhibited mild symptoms, including conjunctivitis and mild respiratory issues. The virus detected is genetically different from the one affecting cattle, suggesting a separate introduction, possibly from migratory birds.

Dr. Nirav Shah, the Principal Deputy Director of the CDC, stated that despite the new cases, the risk to the general public remains low.

“The identification of these additional cases of H5 in people with exposure to infected animals does not change CDC’s risk assessment for the general public, which continues to be low,” Shah said.

Health officials said they were closely monitoring the situation and working with state authorities to implement biosecurity measures, testing, and preventive strategies. Efforts are focused on controlling the spread of H5N1 among animal populations and preventing potential human exposure.