Reading through the Equity Livestock Co-op member newsletter that came in the mail today, there was an item about this "pest that was eradicated from the US in the 1960's" but has been in South and Central America. 

The article states that Panama has served as a natural barrier but the fly has recently been found in Mexico "prompting the USDA to stop all imports of live cattle and bison originating in or transiting through Mexico" and "most US producers are not familiar" with it.

This pest sounds pretty nasty:

"This is a fly that lays its eggs on live animals, and when those eggs hatch, the larvae burrow into the flesh of the animal causing massive amounts of damage and often death to the host animal."

Seems like something we really want to keep out of the US.