A new report on the first human bird flu case tied to the outbreak in cows in the United States suggests that the Texas man may be the first detected case of the H5N1 virus transmitting from a mammal to a person.
Nearly 900 people in 23 countries have been infected with the H5N1 bird flu virus since it started spreading from Southeast Asia in late 2003. But previous human cases were all linked to transmission from infected birds, typically domestic poultry.
The report, published Friday by the New England Journal of Medicine, details the unidentified man’s symptoms and his possible route of infection. It was written by scientists from the Centers for Disease Control and Prevention, the Texas Department of State Health Services, and the Texas Tech University Bioterrorism Response Laboratory in Lubbock. The senior author was Tim Uyeki, chief medical officer of the CDC’s influenza division, who has investigated H5N1 outbreaks around the world for more than 20 years.
How the man became infected cannot be proven; while the cattle on the farm where he worked reportedly suffered from a decline in milk and other symptoms seen in herds that have tested positive for H5N1, no animal testing at that farm was undertaken. The authors noted, though, that nearby farms where dairy cows experienced the same symptoms tested positive for the virus.
“Given the infected human was a dairy farm worker with reported exposure to sick, presumably infected cows in Texas and without reported exposure to other mammals or birds, we believe the genetic and epidemiologic data are strong evidence of infection of the human following exposure to presumably HPAI A(H5N1) virus-infected cows,” they wrote in the supplementary materials that accompanied their report.
HPAI A(H5N1) is scientific shorthand for highly pathogenic avian influenza A virus of the H5N1 subtype. The term “highly pathogenic” applies only to how the virus behaves when it infects poultry, though people might easily assume it applies more broadly, given how deadly this virus has proven to be over the decades. Roughly half of the people known to have been infected died. It kills wild birds and dozens of mammalian species, including cats (domestic and large cats), seals, foxes, mink, and many other scavenging carnivores that have had the misfortune of consuming infected wild birds.
In the report the authors describe, the man’s illness was exceedingly mild. His lungs were clear and he had no trouble breathing; he had no fever. His sole symptom appeared to be conjunctivitis, a condition colloquially known as pink eye.
The man and people he lived with were given flu antiviral drugs. He reported that his conjunctivitis cleared up. None of the people he lived with became sick.
Ideally in cases like this, scientists would draw blood from the man and his contacts, as well as from other people working on the farm, to look for antibodies to the H5N1 virus. Presence of antibodies among other farm workers might suggest the virus is passing to people more commonly than has been seen. Antibodies in the blood of the man’s contacts could indicate that he transmitted the infection to them, but that their cases were so mild they didn’t have symptoms.
But this work was not done, the report said, because the man and his contacts would not agree to have blood drawn. “We were also unable to collect acute or convalescent sera to assess seroconversion in the dairy farm worker or household contacts,” they said. Likewise it appears that health authorities were not allowed on the farm to investigate whether more workers might have been infected.
The authors report that nasal swabs taken from the man also showed presence of virus, but at much lower levels than what was found in his conjunctiva, the tissue surrounding the eye.
They hypothesize that the man could have been infected by one of two routes. Either virus in the air in the milking parlor landed in his eyes — reportedly he did not wear eye protection — or he may have had virus on his hands or gloves and transferred it to his eye inadvertently, they suggested.
Analysis of the genetic sequence of the virus retrieved from this man has shown that while it is closely related to the viruses that have been causing the cow outbreaks, it does not fit neatly into the viral family tree that scientists studying the sequences have developed. The authors suggested that it’s possible the virus was from a slightly different offshoot that has died off; alternatively, there could have been more than one spillover event from birds in the Texas panhandle region where these outbreaks were first observed.
To date, 36 herds in nine states have tested positive for the virus, though it is believed that many more have experienced outbreaks but have not been tested. Both the U.S. Department of Agriculture and the CDC have acknowledged that farmers have often refused to cooperate with their efforts to investigate these outbreaks.
The report noted that the virus from the man was closely related, genetically, to the viruses used to produce two batches of H5N1 vaccine that the U.S. government has made and stockpiled as a hedge against a bird flu pandemic. The stockpiled vaccine, of which there are about 10 million doses, “would likely afford immune protection in people if used as vaccines,” the authors concluded.
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