WASHINGTON — On Oct. 24, the Atlanta-based Centers for Disease Control and Prevention (CDC) announced the total number of confirmed H5N1 human cases has risen to 31.

The uptick comes after the agency was able to begin testing some of the presumptive cases that occurred in poultry workers in Washington earlier this week. As the CDC continues to test employees exposed to the virus at an infected commercial egg laying operation in Franklin County, Wash., the number of positive cases in humans may continue to increase.

Also contributing to the growing number of cases is a total of 15 human cases confirmed in California since the beginning of October. These were all in workers exposed to infected dairy cows across 13 dairy farms in the state. All symptoms have been reported as mild, with no hospitalizations occurring.

CDC continues to declare the risk of highly pathogenic avian influenza (HPAI) to the general public is low. This assessment is supported by evidence that no person-to-person spread of the disease has occurred.

A deep dive into the case that popped up in Missouri last month without known exposure to infected cattle or poultry has led the CDC to remain confident in its assessment.

The Missouri case was the first — and to this day, the only — time a person has tested positive for H5N1 without reporting exposure to animals. The uncertainty of the source of exposure opened the door to the possibility of person-to-person transmission.

However, after subsequent testing and research, the CDC has ruled out person-to-person transmission.

In a conference call on Oct. 24 with federal health and agriculture officials, Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases at the CDC, detailed the process the agency undertook to come to this conclusion.

The Missouri patient had underlying medical conditions and tested positive for influenza A while hospitalized. During the patient’s stay, they came into contact with 112 healthcare workers. Six were found to have experienced respiratory symptoms. When the CDC tested the healthcare workers for the disease, the agency could serologically rule out they had not contracted H5N1.

Meanwhile, the CDC also looked into people who had household contact with the patient. This avenue of testing proved to be less clear-cut than that of the healthcare workers.

The CDC discovered one person to have a neutralizing antibody to H5N1 similar to that of the patient, suggesting they were both infected with the bird flu. However, the agency will not officially add the household contact to the number of confirmed H5N1 cases. That status could only be achieved if the contact showed signs of contracting the disease through at least two of the three assays run by the CDC, and the subject only showed signs on one test.

The patient and the contact were noted to have displayed symptoms for H5N1 around the same time, which the CDC views as epidemiologic evidence one did not contract the disease from the other but from a third-party source.

“I think what we do know is that the evidence points toward a single source of infection,” Daskalakis explained. “The force of the larger evidence also suggests that likely exposure was an animal or animal product that we have not identified in our epidemiologic survey.”