It’s been about two months since we last updated you, and the H5N1 situation in the United States has continued to develop.

We’ve seen further spread of the virus in cattle, with outbreaks now confirmed in 9 different states, with further outbreaks in alpacas and poultry farms. There have also now been three associated cases in people, though all with mild symptoms. One mutation was also identified in the virus that facilitates mammalian adaptation. Low levels of the virus have been found in one sample of beef (though this was a test from a sick animal and not from beef that had entered the market) and in multiple samples of raw milk (though pasteurization would likely destroy the virus and no human infections have been reported through consumption of milk). However, it is recommended to avoid consumption of raw milk products as recent research has shown that mice can be infected from consuming raw milk from H5N1-infected cattle. Unfortunately, we are now seeing that the testing system in the US is geared specifically toward birds and that the regulations to enforce testing in other species are not in place. As a result of this, our information about the situation is severely lacking, as is our ability to control the continued spread of the virus. Because of this, we will likely see continued spread of the virus within cattle in the US, with further transmission events to other animals and additional people. The good news is that the virus seems to be continuing to only cause mild symptoms, both in people and in cattle. However, we know that viruses continue to evolve and that our understanding of the current outbreak is limited at present. With the currently inadequate surveillance and outbreak management there is a real risk of continued spread with unknown outcomes. As such we are paying close attention to the situation and will continue to update you when and if the situation changes.

One side point to note is that there was a recent human case of fatal avian H5N2 influenza in Mexico. This is not the same type of avian flu that is causing the outbreak in cattle in the US and does not appear to be connected at all to the US outbreak. There are multiple types of avian flu and most can cause severe or fatal illness in people and other mammals, though they do not seem to spread well outside of birds. We have seen these limited severe outbreaks for over 20 years now and they tend to behave very differently from what we are seeing with H5N1 avian flu in the United States right now. The reason for this difference remains unknown.