So far, five cases have been confirmed: three calves and a goat in Texas, and a dog in New Mexico, according to the US Department of Agriculture.
The screwworm is a fly larva that eats living tissue. The flies lay eggs in open wounds on livestock and pets, and when the eggs hatch the larvae eat the tissue. Without treatment, the animals can die as the larvae eat their way into the brain.
In rare cases, humans can also be affected.
To limit the spread, authorities have established two-mile quarantine zones around each confirmed case. Canada has also temporarily halted all imports of live cattle and horses from Texas.
The larva attacks only live animals and does not affect meat products.
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