A USDA “strike force” investigated 15 infected Michigan dairy herds as well as eight turkey flocks in early April. It worked with the state of Michigan as well as individual farmers.

The investigation was launched after local researchers identified a “spillover” event that went from infected cattle to a nearby poultry plant. The state — and farmers — wanted to know how it happened.

What the team found suggests the “control” Topham referred to may be elusive.

From surveys and observations, they found that cats and chickens were free to walk around without containment — potentially migrating between nearby dairies and poultry farms. Some of these animals had become infected; several died.

Asked about their practices regarding isolation of newly introduced cattle, three out of 14 farms said they always isolated, another three said they never isolated, and the remainder didn’t respond.

Then there was the dumping of unpasteurized, contaminated milk into the open waste lagoons on several of the farms. And the feeding of non-pasteurized milk to calves on three farms. Or the potentially contaminated manure that was stored, composted or applied to nearby fields. In one case, a farmer reported they had sold or given away potentially contaminated manure.

Finally there was the issue of humans: On every farm, there were visitors, carcass removal companies, milk suppliers, veterinarians and employees — many of whom traveled between farms.

For instance, of the 14 dairies that reported information about their employees, three had employees that worked at other dairies, one had employees that worked at a poultry farm, and one had an employee who also worked at a swine farm. At four dairies, some of the employees were reported to have their own livestock at home.

As the authors reported, “transmission between farms is likely due to indirect epidemiological links related to normal business operations ... with many of these indirect links shared between premises.”

They noted there was no evidence to suggest waterfowl had introduced the virus to the Michigan herds.

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