How cows transmit avian flu to humans

Recent work reported from the USA and Netherlands has clarified some of the concerns about the spread of the bird flu H5N1 strain of influenza, from cows to humans¹. We reported recently about the surge of cases of bird flu in cows in several USA states and the concurrent infection of in contact workers.

New reports suggest that the risk of spread to humans from cows is less likely than previously thought. Workers from the University of Tennessee have reported that the udders of cows contain a high number of bird flu receptors¹. Influenza viruses bind to sialic acid receptors and there are a wide variety of those present throughout nature. The detection of avian flu receptors, in bovine udders, raised the spectre of a widespread reservoir in which the avian virus could evolve into a strain which would more readily infect mammalian tissues.

Over the last few years there have been numerous reports of a H5N1 avian viral strain infecting humans, with the resulting mortality rates exceeding 50%² . The prospect of infected cattle providing an environment for the evolution of a new mammalian specific strain, has become a serious concern to scientists.

However, Carrasco and colleagues, from the University of Utrecht in the Netherlands³, have used more sophisticated analytic techniques to demonstrate that, while avian type receptors are widespread in cattle udders, no human receptors are present. Furthermore they have demonstrated that there are no bird or human receptors in the upper respiratory tract of cows, making it less likely that airborne aerosols of a new viral strain could come into contact with humans.

This new work firmly focuses the attention back onto milk from infected cows as the primary source of human infection. While this does not totally remove the risk of human exposure, it does mean that cow to cow infection is likely to be mainly from milk, particularly milking equipment⁴ (And, of course, mastitis control in cows has had a strong focus on milking machine issues for many years.). It is less likely that a large number of humans will become infected from milk, particularly as pasteurisation destroys the virus. However the window of evolutionary opportunity to the avian virus is not totally closed; each individual that contracts the virus from milk sources provides an opportunity for mutation to a more potent strain. While the risk may be reduced by these reports, it is not totally removed.