New Delhi, July 25 -- With Mexican cattle again barred this month from entry to the United States over fears of spreading a flesh-eating parasite, ranchers and veterinarians in Mexico hundreds of miles from the border are fighting what has U.S. agricultural authorities so on edge.

In the southern state of Chiapas, which borders Guatemala, the New World screwworm fly's rapid spread appears to have caught most ranchers off guard, despite memories of previous outbreaks in the 1980s and 1990s.

Mexico is building a plant with U.S. support in Chiapas to produce sterile flies, which have proven effective at stopping the spread, but it won't be ready until next year. Meanwhile, the price of medicines used to treat livestock infected with the sc...