USDA develops resource to help veterinary professionals recognize, respond to screwworm

Veterinarians will play a critical role in mobilizing a response should New World screwworm (NWS, Cochliomyia hominivorax) make its way into the United States.

The parasitic pest, known for its larvae that eats the living tissue of warm-blooded animals, has been moving quickly toward the U.S.-Mexico border after breaching a part of Panama that had been a biological barrier to the NWS fly.

To prevent an incursion, U.S. Department of Agriculture’s (USDA) National Veterinary Accreditation Program has developed the training module, “Module 41: New World Screwworm: A 21st Century Perspective.” It goes over signs of NWS, its life cycle, control measures, and reporting methods. The presentation, released in early April, also covers the potential animal, human, and financial impacts associated with an incursion of NWS.

The USDA recommends that every veterinarian and veterinary technician, regardless of practice type, review this module to learn how to recognize and respond to NWS.

“In terms of prevention, you are our first line of defense as you very well may be the first one to see an NWS larvae that’s entered the country,” the module narrator said. “In terms of response, you will play a key role in surveillance should an outbreak occur.  You’ll also play a key role in providing the public essential key information.”

What to look for

NWS could enter the U.S. via an infected animal brought into the country illegally or legally, or through screwworm flies if an outbreak establishes in northern Mexico, according to the training module.

“The math is alarming. The arrival of one female NWS could result in staggering numbers of infestations very quickly. One female can lay 200 to 300 eggs at a time and may lay up to 3,000 eggs during her 10- to 30-day lifespan,” the presentation moderator said. “Because of the very short lifecycle from egg to egg-producing adult, populations of NWS can grow quickly without early recognition and reporting.”

Veterinary professionals are asked to look for the following:

• NWS infestation is often associated with a pre-existing wound, which can be as small as a tick bite. This often occurs near mucous membranes, such as nostrils, eyes, ears, mouth, and genitalia. Drainage, pus, discharge of blood and serum, enlargement, and distinctive odor are common.
• Larvae are visible in the wound by the third day. By that point, hundreds or thousands of larvae could already be present, each 2 millimeters to 1.5 centimeters long.
• In addition to characteristic lesions and the presence of larvae, other symptoms seen in NWS infestations include irritated behavior, head shaking, and the smell of decay. Animals that are isolating, rubbing against trees or other structures, and or standing in water are also indicators of an NWS infestation.

If there is a suspected case of NWS, USDA-accredited veterinarians are required to notify both their state animal health official and a USDA veterinary officer. The after-hours reporting line is (866) 536-7593.

All veterinarians and veterinary technicians are encouraged to ensure that animals entering or reentering the country are inspected for screwworm and have proper documentation; remember that NWS larvae could arrive in any state and can be found deep within wounds; and, if directed to submit samples, collect maggots from all areas of the wound, including the deepest parts of the wound.

USDA’s Animal and Plant Health Inspection Service (APHIS) is also asking all producers and pet owners along the U.S. Southern border to watch their animals for signs of screwworm and immediately report cases to their local veterinarian.

NWS’ northward spread

After moving north past Panama’s Darien Gap—a 60-mile stretch of rainforest between Panama and South America—in 2022, NWS detections in the country exploded from an average of 25 cases per year to more than 6,500 cases in 2023, according to USDA data.

Since then, NWS has continued northward and has been detected in Costa Rica, Nicaragua, Honduras, Guatemala, and Mexico, according to USDA information. Nicaragua’s health ministry has confirmed at least 30 cases of NWS in people, two of whom required lifesaving hospital treatment.

In November last year, Mexico notified APHIS of a positive detection in Mexico. As a result, APHIS suspended Mexican cattle and bison imports. Imports have since resumed as of February under a new protocol that includes pre-clearance inspections and treatment for screwworm. This protocol ensures that imported cattle are free of NWS before entering the U.S.

APHIS continues to partner with other USDA agencies, the U.S. State Department, the Food and Agriculture Organization of the United Nations, and affected countries to respond to the outbreak.

“If NWS makes its way to the U.S., our means of control will be effective, but are costly and take time,” according to the training module.

A version of this story appears in the June 2025 print issue of JAVMA