Recently, the U.S. Centers for Disease Control and Prevention (CDC) confirmed that a dairy farmer from a dairy farm in Texas was confirmed to be infected with the highly pathogenic avian influenza virus H5N1. The patient is the world's first known case of avian influenza infection linked to diseased dairy cows and, if further confirmed, could also be the first case of human infection by other mammals. CDC officials believe that the risk of human infection with the H5N1 virus is still low, but as of April 4, 2024, cows infected with the avian influenza virus have been found in 11 cattle farms in five states in the United States, and the risk of avian influenza infection still needs to be carefully monitored.
The current detection of the H5N1 virus clade in dairy cows is consistent with the clade that is endemic in birds in the United States and causes a large number of bird deaths, and these cows may have been infected by contact with wild birds infected with the H5N1 virus. Visual China|Figure
The H5N1 avian influenza virus is a highly pathogenic RNA virus that mainly infects birds and can also infect mammals, with a high fatality rate of nearly 100% to birds. Since it was first identified in the 90s of the 20th century, H5N1 avian influenza has been one of the major threats to the poultry industry. The ongoing avian flu outbreak that began in 2003 has killed a large number of poultry or wild birds worldwide, with about 150 million birds killed or culled. In recent years, the global incidence of H5N1 avian influenza has increased, with the worst outbreak since the discovery of H5N1 avian influenza in the United States in 2022-2023, with nearly 60 million birds killed or killed, and more than 50 million birds killed due to avian influenza in Europe.
At the same time, the number of cases of avian influenza outbreaks infecting mammals is increasing. Since 2022, at least 48 species of mammals have been infected with avian influenza in 26 countries around the world, including wild animals such as mink, seals, sea lions, foxes, otters, bears, cougars and skunks, as well as domestic animals such as cattle, sheep and cats, raising concerns about the increased ability of the H5N1 virus to infect humans.
On March 25, 2024, cases of infection with the H5N1 avian influenza virus in dairy cows were reported in New Mexico, Texas, and Kansas in the United States, which is also the first detection of the H5N1 avian influenza virus in dairy cows in the world. Dairy farms in Michigan and Idawa have since reported cases of dairy cow infection. According to Science, the H5N1 virus clade currently detected in dairy cows is consistent with the clade that is circulating in birds in the United States and causes a large number of bird deaths, both of which are clade 2.3.4.4b. These cows may have been infected by contact with wild birds infected with the H5N1 virus. As of April 4, 2024, H5N1 is circulating in 11 dairy herds in 5 states in the United States, including 7 dairy farms in Texas, which strongly suggests cattle-to-cattle transmission.
On April 1, 2024, the CDC confirmed that a dairy farmer from a dairy farm in Texas tested positive for H5N1 virus and that the patient had been in contact with cows infected with the H5N1 virus. Fortunately, the patient presented with only mild symptoms, mainly conjunctivitis, and is currently isolated and receiving antiviral medication. According to the viral genetic test, the virus clade infected by this patient is still clade 2.3.4.4b. The researchers found only a point mutation in the genetic sequence of the virus infected with the patient, which was associated with the increased adaptation of the virus to the mammalian host, which was also found in previous cases of mammalian and human infection, and did not enhance the transmissibility of the virus. Therefore, CDC considers that the current human health risk of the H5N1 avian influenza virus remains low and the public should not be worried. In addition, the USDA does not require the culling of infected herds on farms with infected cattle, and in the event of an outbreak on a poultry farm, it is generally required to cull all poultry on the farm. USDA officials also emphasize that pasteurization kills viruses in milk produced by infected cows.
This is the second human case of H5N1 infection in the continental United States since 2022. In April 2022, a confirmed case of infection with the H5N1 virus was reported in Colorado, USA, who contracted the disease due to contact with and participated in the culling of infected poultry, and his symptoms were relatively mild, but he felt tired for a few days and recovered quickly. At that time, the U.S. Centers for Disease Control and Prevention had identified the H5N1 virus in poultry in 29 states and wild birds in 34 states, and after tracking the health of more than 2,500 people who had been exposed to H5N1-infected poultry, only one human case of infection had been identified. To be on the safe side, the U.S. Department of Disease Control and Prevention also quarantined the patient and treated him with antiviral therapy.
According to the current data, the infection rate of the H5N1 virus in humans is not high, but the mortality rate is more than 50%. As of April 2024, the World Health Organization has reported a total of 889 confirmed human cases since 2003, resulting in 463 deaths. In most years, human cases of avian influenza have been sporadic and only in 2006 and 2015 did the number of global infections exceed 100. In the past year, there have been just over a dozen cases of avian influenza in humans worldwide, 10 of which occurred in Cambodia and five deaths. Genetic testing of the virus showed that the H5N1 virus clade 2.2.3.2.1 c was infected with patients in Cambodia, which has been circulating in wild birds and poultry in Cambodia for many years, and has some genetic variation with the virus clade 2.3.4.4b circulating in birds in other countries such as the United States.
Studies have shown that the low rate of human infection with the H5N1 avian influenza virus may be related to the presence of some key factors against the avian influenza virus. According to a paper published in the journal Nature by researchers from the Centre for Virus Research at the University of Glasgow in the United Kingdom, a protein expressed in the human trachea - BTN3A3 (eosinophilic acid protein subfamily 3 member A3) is an effective inhibitor against avian influenza virus, and its antiviral activity evolved in primates, mainly by inhibiting avian influenza virus RNA replication in the early stages of the virus life cycle.
However, after all, the H5N1 avian influenza virus infection and death rate are more than half, and it should not be taken lightly. As a result, the CDC also reminds that people who are in close or prolonged unprotected contact with infected birds or other animals, including livestock, or who have been in contact with an environment contaminated by infected birds or other animals, are at greater risk. Currently, CDC is working closely with federal departments such as the Department of Agriculture, the FDA, and relevant state health departments to continue to monitor workers who may have been exposed to infected or potentially infected birds/animals and to test people with symptoms.
Southern Weekly Contributing Writer Tang Bo
Editor-in-charge: Zhu Liyuan