Spread of avian influenza virus on US dairy farms worries WHO | Society

The World Health Organization (WHO) this Monday expressed concern about outbreaks of bird flu detected in recent weeks on dairy cow farms in the United States. “It is concerning that the virus is infecting new species and thereby increasing the number of people exposed to the pathogen,” said Maria Van Kerkhove, the agency’s director of epidemics and pandemics.

To date, a total of 36 farms in nine US states have identified cases of avian influenza in their flocks. The outbreaks were discovered in March after an investigation began in January when several farms noticed a drop in milk production. The symptoms exhibited by animals are very mild or almost unnoticeable. Although the investigation is still ongoing, they believe the source of the infection may have been wild birds in contact with livestock.

How the virus is transmitted between cows remains a mystery. “It does not appear that transmission occurs (directly between animals) through common forms of respiratory infections. Rather, it appears that some element used in the milking of animals contributes to the spread of the virus, although at the moment these are hypotheses under study,” said Richard Webby, director of the WHO Avian Pathology Research Centre. The movement of livestock between farms could explain why the virus was found on dozens of farms. “It is very likely that the number of farms with cases is higher than officially identified (36),” the expert added.

So far, only one worker has been infected in connection with these outbreaks. This is a man from Texas who developed mild conjunctivitis-type symptoms, but not respiratory symptoms. It is believed that the virus got into his eyes when he scratched them with his infected hand after touching an infected cow.

Information presented at a meeting with experts and journalists this Monday shows that the milk of sick animals has a high viral load, although pathogens capable of infection disappear during the pasteurization process. The WHO, like US authorities, recommends against consuming milk or products made from it that have not been subjected to this thermal process, a rare but widespread practice in the country.

Available data also shows that up to 20% of tested milk sold in affected states contains viral particles, although they are not infectious. Research to date has not identified viruses capable of contaminating other animal products, such as meat and eggs, in the food supply chain.

For now, WHO continues to consider the risk these episodes pose to the general population to be “low” and “low to moderate” for farm workers and other people exposed to livestock. Cats and raccoons infected with the virus have also been found on farms.

No obvious risk to people

Despite this, those responsible for the organization do not hide their concern about the danger posed by the presence of the virus in livestock, since although there is currently no data indicating a greater risk to humans, the fact is that the pathogen is circulating and spreading. mammals that are close to humans increase the likelihood that they will develop mutations that will allow them to adapt.

The virus causing the outbreaks is clade (variant) 2.3.4.4b, which emerged in 2020 and quickly spread across much of the world via migratory birds, causing the deaths of hundreds of millions of birds since then. During those four years, the viruses also infected other mammal species, prompting increased concern among experts about whether the mutations that make this possible could bring the pathogen closer to adapting to humans. However, to date, this clade has had little impact on humans. WHO has only 13 reported infections in humans, almost all of them mild or very mild.

In a risk report published April 23 jointly with the Food and Agriculture Organization of the United Nations (FAO) and the World Organization for Animal Health (WHO), WHO warned that “Avian influenza A(H5N1), especially 2.3.4.4b, continues to diversify genetically and spread geographically,” which, together with the transition to mammals, gives the pathogen “greater opportunities for viral reassortment, creating new genotypes.” An example of this was “the discovery of a new form of influenza A(H5N1) virus in poultry in Cambodia, Laos and Vietnam” that contains “clade 2.3.2.1c surface proteins circulating locally but internal clade 2.3.4.4b genes.”

The document recalls that “there are limited reports of transmission between mammals to date, despite an increase in the number of infections,” and states that “although direct evidence is lacking, high mortality of marine mammals has caused infection on numerous animal farms in Finland and mink in Spain in these cases correspond to mammal-to-mammal transmission.”