The continuing outbreak of bird flu in the US has alarmed researchers and

prompted new efforts to track the virus that's already caused the deaths of

tens of millions of birds from Europe to Antarctica. But as William Bringham reports,

as H5N1 continues to jump into mammals, most recently dairy cows,

many scientists are concerned that we're not watching closely enough

as this virus

continues to spread. A warning this story contains scenes of animals in

distress. Evolutionary biologist Michael Warby at the University of Arizona is one of many

scientists around the world trying to untangle bird flu's latest twist.

How and when it spread to dairy cows.

The jump into cattle probably took place between mid-November and mid-January,

and so we're months into this already.

And since then, it's spread like wildfire,

infecting dairy cows in at least 46 herds across nine states.

It seems to be spreading cow to cow in some fashion, but we don't know exactly

how that is. For example, it could just be mechanical transmission as one cow leaves a

milking machine, leaves virus on it, and then the next cow comes in,

or is this going respiratory like flu does with humans?

In late April, the USDA mandated that milking dairy cows being transported across state lines need to be tested for bird flu.

Do you think we have got now enough surveillance out there to know what this virus is doing and where it's moving?

I think we still have a long way to go, honestly.

We are still sort of dealing with a pretty limited number of

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