Blood tests conducted by the Centers for Disease Control and Prevention have confirmed that one person in Missouri, with no known exposure to livestock or poultry, contracted H5N1 bird flu in August, and that a household contact of this person may also have been infected. But five healthcare workers who developed flu-like symptoms after caring for the confirmed case were not infected with the virus.
The findings suggest the state has not seen a cluster of bird flu cases, a possibility some experts feared.
The CDC reported Thursday that its examination of blood samples from seven Missourians yielded clear negative results for health care workers but mixed results for the person who shared a home with the confirmed case. The second individual had antibodies to the H5N1 virus, but to be considered a confirmed case under World Health Organization criteria, the individual would also have to be positive based on another type of test – and the person did not meet that bar .
CDC officials speaking at a news conference hosted by the Department of Health and Human Services and the U.S. Department of Agriculture described the household contact as a possible case but said the person would not be added to the list of confirmed cases, which currently numbers 31. in six states so far this year. CDC officials suggested that number will rise, possibly later Thursday.
There won’t be a clearer answer about whether the household contact of the confirmed case was actually infected, said Demetre Daskalakis, director of CDC’s National Center for Immunization and Respiratory Diseases.
“This is the end of what a laboratory can do,” Daskalakis said. “There is no blood test or other research that can give us a clearer definition of the nature of the exposure of these individuals. So from the perspective of where we are with this research, I think we have the conclusion.”
It has been known since early September that one of the individuals, who had been hospitalized for three days the previous month due to other symptoms, had tested positive for H5N1. It later turned out that a household contact of the person was ill at the same time, but was not tested during his illness. The CDC’s blood test attempted to answer the question of whether this person was also infected. The result of the tests was unsatisfactory: the person seems like a possible case, but cannot be considered a case.
The tests failed to pinpoint how the two became infected, and an investigation by Missouri Health Department officials provided no solid clues as to the source of the infections.
In announcing the test results, Daskalakis emphasized that the CDC believes the two people became infected through exposure to a contaminated animal or animal product. The fact that they got sick at the same time makes it unlikely that one infected the other, he said.
Six health care workers who cared for the hospitalized person later reported having had flu-like illnesses — a fact that raised concerns that Missouri may have seen a cluster of cases in which the virus had spread from person to person.
Although there has been limited human-to-human transmission of H5N1 in other countries in the past, this has proven to be rare. Earlier this week, Tim Uyeki, chief physician of the CDC’s influenza division, told a World Health Organization webinar that the last time confirmed person-to-person spread was reported was in Pakistan in 2007.
One of the health care workers in Missouri was tested for the flu when he or she was sick; the test was negative. But the other illnesses only came to light after the CDC confirmed on September 6 that the hospitalized person’s positive flu test was actually caused by the H5N1 virus and the Missouri Department of Health and Senior Services began an investigation. By then, all healthcare workers had recovered, so serology – testing blood for antibodies – was the only way to determine if there had been spread from the patient to healthcare providers. That testing proved challenging to implement, the CDC acknowledged earlier this month.
Although disease researchers have been unable to figure out how the Missouri case became infected, genetic sequences of viral fragments from the person who tested positive showed the virus is related to one that has been spreading among dairy cattle in the country for months. On Thursday, the USDA confirmed 339 infections herds in 14 states.
The ongoing outbreak in cattle, which has sometimes spread to poultry flocks, has led to a rapid accumulation of human cases in this country, where only a single case of H5N1 infection in humans had been recorded before this year. CDC officials said two recent cases from Washington, which the state reported on Sunday, have already been confirmed, while five more possible cases are being tested. All of these individuals had mild symptoms, said Nirav Shah, the agency’s deputy director, although at least a few had respiratory symptoms, which have not been seen in all confirmed cases so far.
The Washington cases involve workers involved in culling infected poultry on a farm with more than 800,000 chickens. Shah said genetic analysis of viruses from the workers showed they were infected with a slightly different version of H5N1 than the one circulating in cows, indicating the poultry outbreak was not related to the cow outbreak.