The reemergence of the New World screwworm, which poses significant health risks for livestock, has prompted the Department of Agriculture to halt cattle shipments from Mexico to the United States.

The move marks at least the second time this year that cattle imports from Mexico have been suspended over the parasitic fly.

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Secretary of Agriculture Brooke Rollins made the announcement late Wednesday, issuing a warning about the “deadly pest.”

Here are details on the New World screwworm and whether it poses a danger to humans.

What is the New World screwworm?

The New World screwworm remains endemic in parts of the Caribbean and South America. The insect was previously eradicated from the U.S. in 1966, although there’s potential for its reintroduction in the nation through Central America and Mexico.

A female screwworm typically lays eggs in the wounds or mucus membranes of a warm-blooded animal, such as a horse or cow. Once the eggs hatch, the fly larvae feed on the host’s living tissue with their sharp mouths.

The insect’s name refers to how the species burrows, or screws, into healthy tissue.

If left untreated, the host could eventually die from myiasis.

Health risks for humans

While the disease primarily infects livestock, New World screwworm myiasis can affect humans, albeit rarely.

The infestations in humans often cause painful wounds and bleeding from open sores, among other symptoms that the Centers for Disease Control and Prevention cite. A foul smell from the wound may also occur.

People who travel to South America and the Caribbean, where these insects are located, have a greater risk of becoming infected if they have open wounds, the CDC says. Others who have compromised immune systems or are otherwise malnourished also have a higher risk of infection. Mexico reported the first case of a human infection in April, and the patient was treated and remained in stable condition. As of May 30, there were 8 cases in Mexico.

Early detection of New World screwworm is crucial for the host’s survival. Treatment includes prompt removal of the larvae from the human’s wound or orifice. There is no medication to treat the disease.

Closures of cattle shipments

The U.S. last halted cattle, horse, and bison imports from Mexico in May. The decision was made due to the rapid spread of screwworm cases in northern Mexico, about 700 miles from the southern border.

The imports briefly resumed this week, only to be shut down again. The initial reopening phase began with a port of entry near Douglas, Arizona.

AGRICULTURE DEPARTMENT HALTS LIVESTOCK TRADE AT SOUTHERN PORTS DUE TO PARASITIC FLY

The latest detection of screwworm cases happened about 370 miles south of the border.

If the New World screwworm were to start another outbreak in the U.S. like in the 1950s and 1960s, the parasite could fatally affect livestock, pets, wildlife, and potentially humans. An outbreak could also lead to costly economic losses for the U.S. beef industry and contribute to higher beef prices.