ASK DR. BRAZIL: What's the deal with New World Screwworm?

Secretary of Agriculture Brooke Rollins recently announced the Southern border of the United States would be closed to all live imports of cattle, bison, and horses as a measure to prevent the spread of New World Screwworm.

"The protection of our animals and safety of our nation's food supply is a national security issue of the utmost importance," Rollins said in her public statement.

In this video, Dr. Rich Brazil explains what New World Screwworm is and why this closure is an important step to protect U.S. herds and food safety.

Dr. Rich Brazil is a large animal veterinarian and ranch management consultant who has been practicing in Northern California for nearly 40 years. He is the founder and creator of Ranch Vision, a revolutionary ranch planning and profit maximization software designed for livestock operations.

What is New World Screwworm?

New World Screwworm is a parasite. These flesh-eating maggots can infest an animal's wound or mucous membranes. Without treatment, the fly larvae can kill a grown cow in a matter of days or weeks. They even pose a threat to humans.

How the USDA successfully eradicated New World Screwworm in the 1970s

For many decades, the U.S. Department of Agriculture (USDA) has worked to eradicate this dangerous pest from U.S. herds.

A 1976 outbreak in Texas impacted over 1.4 million cattle as well as hundreds of thousands of sheep and goats. The USDA estimates that a similar outbreak today would cost the Texas economy about $1.8 billion.

In the 1960s and 70s, the U.S. and Mexico launched a successful effort to completely wipe out the New World Screwworm by releasing hundreds of millions of infertile male flies. The sterile flies caused females to lay defunct eggs, effectively exterminating the parasite completely.

"Barrier zone" north of Panama

In the years since this successful program, the USDA has fought to keep the New World Screwworm out of a significant barrier zone in Central America, with minimal sightings north of Panama in that time.

But now, experts are sounding the alarm after tracking an alarming surge of cases in Central America. There have even been some larvae found in remote farms in Mexico, as close as 700 miles to the U.S. Southern border.

Protecting U.S. animal health and food safety

Back in November 2024 when the first case of New World Screwworm was found in Mexico, the U.S. stopped all live animal trades at the border. Mexico worked to strengthen inspection and treatment protocols, and subsequently reached an agreement with the U.S. to lift the trade ban in February 2025.

Secretary Rollins' announcement on May 11 came after an "unacceptable northward advancement of NWS."

Until further notice the U.S. will restrict all imports of live cattle, bison, and horses across the Mexico border. This ban will be evaluated on a "month-by-month basis, until a significant window of containment is achieved," the USDA states.

The USDA Animal Plant Health Inspection Service (APHIS) is continuing to release sterile flies in Central America and Southern Mexico to curb and hopefully limit the population.