The USDA says that since the discovery of the infected cow, they have identified 10 different feed suppliers that delivered animal feed to the farm.

CNN: USDA quarantines 2 farms in mad cow investigation

Determining if the cow became sick from feed is an area where investigators are focusing close attention.

CNN: USDA quarantines 2 farms in mad cow investigation

The fat content of raw milk varies with cow breed, feed, and stage of lactation among other factors.

FORBES: Why Milk Isn't As "Natural" As You Think

"If the cow were exposed to the typical strain of BSE via animal feed -- and the government says that's not the case here -- that would have represented a significant failure, " she said.

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In this case, the Agricultural Department reported that the cow had a rare form of BSE not likely carried by contaminated feed.

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The fatal brain-wasting disease is believed to be spread through contaminated cattle feed and cannot be passed from cow to cow.

CNN: Canada nears end of mad cow probe