The United States has only seen one case of Mad Cow disease which was in 2005 when a cow tested positive for the disease. Officials later linked the isolated case to Canada because the cow was born on a farm in Alberta. 
The United States has seen two cases of Mad Cow disease which were in 2005 when a cow tested positive for the disease. Officials later linked one of the cases to Canada because the cow was born on a farm in Alberta. The later test was done in England where samples tested had conflicting results in 2004.
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only seen one case
seen two cases
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was
were
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the isolated case
one of the cases
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The later test was done in England where samples tested had conflicting results in 2004.