Centers for Disease Control and Prevention officials told reporters Thursday that there is no evidence that there has been any case of human-to-human spread of H5N1 bird flu in the mysterious situation reported last month in which several people in Missouri suffered illnesses unrelated to known animal contact.
In September, Missouri public health officials and the CDC began investigating the case of H5N1 in a human with no known animal exposure. Out of an abundance of caution, blood samples were taken from five healthcare workers and two household contacts of the infected person who had all displayed symptoms that could have been associated with bird flu.
Dr. Demetre Daskalakis, head of the CDC Center for Immunization and Respiratory Diseases, told reporters Thursday that the series of complicated blood testing studies in Missouri revealed no evidence of human-to-human transmission of the virus.
Of the multiple different types of blood testing from the healthcare workers’ samples, none came back positive for antibodies of the virus, indicating that none had been exposed to or infected by the virus because they did not produce an immune response.
Testing of the samples from the household contacts indicated that they had an antibody immune response to the H5N1 virus in one specific type of blood test but tested negative in two other tests necessary to confirm a retrospective case.
In other words, these people cannot count as confirmed human H5N1 cases, but they were likely exposed to the virus at some point, given the presence of bird flu antibodies in their system.
Daskalakis told reporters that a “constellation of data” points to a single source of animal exposure, despite the fact that retrospective blood testing could not reveal the origin.
“These results, taken as a whole — serology, epidemiology, and a positive PCR test in this single case — are all suggestive of possible infection caused by a common exposure, not person-to-person transmission, and that that infection may or may not be responsible for the symptoms of the two individuals,” Daskalakis said.
Dr. Nirva Shaw, the CDC principal deputy director, told reporters that human-to-human transmission can be ruled out because both household contacts of the known positive case developed similar gastrointestinal symptoms exactly at the same time.
The Department of Health and Human Services and the Agriculture Department have been working together to manage the outbreak of bird flu among dairy cattle populations since April.
Since the spring, the number of cattle herds and poultry flocks infected with the virus has significantly increased across the country. Public health experts in government and academia have been concerned that the rapid spread of bird flu among mammal species could be a starting point for a worldwide pandemic.
USDA and CDC researchers told reporters that the overall risk of the virus changing to spread more easily between humans remains low despite the growing number of human infections.
“To date, human-to-human spread of H5 bird flu has not been identified in the United States,” CDC officials said in a medical press release on the Missouri situation. “CDC believes the immediate risk to the general public from H5N1 bird flu remains low, but people with exposure to infected animals are at higher risk of infection.”
Except for the one confirmed patient in Missouri, all cases have come from agricultural workers exposed to infected cattle herds or poultry flocks.
CDC officials told reporters that more information on the total number of cases of human bird flu infections would be released Thursday afternoon, as the testing of samples collected in Washington state continues.
As of noon on Thursday, there have been 31 cases of H5N1 infection: 15 in California, 10 in Colorado, two in Michigan, two in Washington, one in Texas, and one in Missouri.
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Although the virus is deadly among both birds and various mammal species, symptoms of infected humans in connection to the current outbreak have remained comparatively mild.
The most common symptom of bird flu, according to the CDC, is conjunctivitis, or pink eye infection, and mild to moderate upper respiratory symptoms.