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The World Health Organization (WHO) expressed serious concerns on Monday regarding outbreaks of avian flu detected on dairy cow farms in the United States. Maria Van Kerkhove, the agency's director for epidemics and pandemics, emphasized the troubling aspect of the virus infecting new animal species, thereby increasing the risk to human exposure.
To date, 36 farms across nine U.S. states have reported cases of avian flu in their herds. These outbreaks came to light in March following investigations that commenced in January when several farms noted a decline in milk production. The affected animals exhibit mild or almost undetectable symptoms. While investigations are ongoing, the suspected source of infection is wild birds that have come into contact with livestock.
The mode of transmission among cows remains unclear. Richard Webby, head of the WHO Collaborating Center for Studies on the Ecology of Influenza in Animals, stated that transmission does not seem to occur through typical respiratory routes. Instead, there is a hypothesis that some element involved in milking the animals may be facilitating the spread of the virus.
The movement of livestock between farms likely explains why the virus has been detected in numerous farms. Webby added that the actual number of farms with cases is probably higher than the officially detected 36. Only one worker has been infected so far, a man from Texas who experienced mild conjunctivitis-type symptoms, but not respiratory issues. It is suspected that the virus entered his eyes when he scratched them with his infected hand after touching an infected cow.
A meeting with experts and journalists on Monday revealed that raw milk from affected animals has a high viral load, but pasteurization eliminates pathogens capable of infecting. Both the WHO and U.S. authorities advise against consuming raw milk or products made from it, a practice that, while a minority, is widespread in the country.
Data also indicates that up to 20% of tested milk in affected states contains viral particles, though these are non-infectious. Investigations thus far have not found viruses capable of infecting other animal products such as meat and eggs in food sales circuits. The WHO currently considers the risk to the general population as "low," and "low to moderate" for farm workers and others in contact with livestock.
Infected cats and raccoons have also been found in farm environments. A risk report published on April 23 by the WHO, along with the Food and Agriculture Organization of the United Nations (FAO) and the World Organization for Animal Health, warned that avian influenza A(H5N1) viruses, especially clade 2.3.4.4b, are diversifying genetically and spreading geographically, increasing the potential for viral reassortment and new genotypes with varied clinical signs. This includes the detection of a new form of influenza A(H5N1) virus in poultry in Cambodia, Laos, and Vietnam.
The report also notes limited evidence of transmission between mammals despite the rise in mammalian infections. While direct evidence is lacking, the large mortality of marine mammals and infections in fur animal farms in Finland and mink in Spain suggest potential mammal-to-mammal spread.