CHICAGO (Reuters) -The U.S. Department of Agriculture has again halted imports of Mexican cattle into the United States due to the spread of the damaging livestock pest New World Screwworm in Mexico.

Screwworms are parasitic flies whose females lay eggs in wounds on warm-blooded animals, usually livestock and wild animals. Once the eggs hatch, hundreds of screwworm larvae use their sharp mouths to burrow through living flesh, eventually killing their host if left untreated.

The USDA said in a statement late on Wednesday that it ordered the closure of livestock trade through southern ports of entry effective immediately following the detection of screwworm about 370 miles south of the border in Ixhuatlan de Madero, Veracruz.

The decision was a quick reversal after the USDA said last week it would resume cattle imports from Mexico on Monday at a port of entry in Douglas, Arizona, as part of a phased reopening of the border. 

Washington suspended cattle imports from Mexico in May as New World Screwworm was detected in farms in Oaxaca and Veracruz, Mexico, about 700 miles from the U.S. border.

"We must see additional progress combatting NWS in Veracruz and other nearby Mexican states in order to reopen livestock ports along the Southern border," USDA Secretary Brooke Rollins said in a statement.