What Veterinarians Need to Know about New World Screwworm (NWS): June 2026

What is New World Screwworm?

New World Screwworm (NWS) is a parasitic fly whose larva (maggots) burrow into the living tissue of warm-blooded animals. Female flies will lay eggs in wounds and mucous membranes, such as in the nose, eyes, ears, anus, umbilicus or genitalia. Myiasis causes serious disease that can be fatal within one week if left untreated.

Why is NWS in the news?

NWS is a reportable foreign animal disease that has recently been detected in Texas, near the US-Mexico border. Preventing further spread into the United States is critical for protecting the livestock industry, economy, and food supply.

Before northward spread to the United States, outbreaks of NWS were first detected in Central America and Mexico and have caused the United States Department of Agriculture (USDA) to implement an aggressive NWS outbreak prevention program, including limiting the movement of livestock across the US southern border and releasing sterile flies.

Has there been NWS in the United States before?

NWS was eradicated from the United States in 1966 using sterile male flies. This technique remains one of the most successful and important tools to preventing NWS from spreading further. NWS is still common in parts of South America, Cuba, and the Dominican Republic and has been spreading northward into Central America, Mexico and the United States. In 2017, there was a small outbreak of NWS in the Florida Keys that was eliminated using sterile flies.

What species can get NWS?

NWS can infest livestock, pets, wildlife, birds (occasionally), and humans (rarely). NWS is most often detected in cattle, swine, horses, sheep, goats, and dogs.

How do animals get NWS?

Female NWS flies lay eggs on the animal, which then hatch into larvae. The larvae eat into the flesh of the animal, causing wounds which may attract more flies and may become infected.

What are the risk factors of NWS infestation in dogs and cats?

NWS flies are attracted to wounds. Animals that have recently suffered an injury, had a surgical procedure, been bitten by a tick, or given birth are most vulnerable. Additionally, pets who spend significant time outside or interact with livestock, breeds with skin folds or long hair and pets with medical conditions such as allergies are at higher risk.

What are the clinical signs of NWS?

NWS infection is most often associated with the presence of a pre-existing wound, but may also be found in mucous membranes, such as around the eyes, nose, gums, and genitalia. NWS eggs and maggots may be visible in the affected area. Affected areas may have a bad smell, bloody discharge, or pus and may rapidly enlarge. Pets may show signs of pain, including depression, irritability, not eating, or isolating away from other animals or people

See the USDA NWS Photo Gallery for photographs of animal infestations (Warning: the gallery contains graphic images of New World screwworm infestations in animals).

How does NWS affect international pet travel?

Pets traveling to countries affected by NWS are at risk of infestation. Owners should be advised of the risk, and USDA APHIS and CDC regulations must be reviewed before travel to ensure owners can adhere to the export/import requirements. Pre- and post-travel veterinary examination should be recommended, and medications to administer before returning to the United States should be considered. If risk factors are severe or management is not feasible, owners should be discouraged from traveling with pets to NWS-affected countries. Pets suspected of infestation or those failing to adhere to regulations may be denied entry into the United States. (As per USDA Guidelines)

What should you do if you suspect NWS in a patient?

NWS is a reportable foreign animal disease. If you suspect NWS myiasis, you MUST contact your APHIS VS Area Veterinarian in Charge (AVIC) for your state and your State Animal Health Officials (SAHO) immediately. Do not wait for a confirmed diagnosis before reporting. Only if approved by the AVIC or SAHO should maggot samples be collected and submitted to the National Veterinary Services Laboratories. Refer to the Standard Operating Procedure for Possible Detections of New World Screwworm in Animals created by the USDA for detailed instructions on sample collection, quarantine, premises decontamination, and post-release monitoring.

Can NWS be treated?

Yes. NWS is treatable but may require extensive and painful wound care, maggot and egg removal, systemic antimicrobials, and analgesia. Wounds left untreated may develop serious infections. Treatments must be guided by the AVIC or SAHO-do not treat suspected cases of NWS independently.

Has the FDA approved any medications for the treatment of NWS in dogs and cats?

Currently, no animal drug has received full FDA approval for NWS myiasis. However, the FDA has authorized several drugs through two regulatory pathways: Conditional Approval and Emergency Use Authorization.

For a current list of authorized products, please visit: New World Screwworm: Information for Veterinarians | FDA

When using these products, please keep in mind:

• Veterinarians should use only the FDA-authorized products as indicated and discuss risks, benefits and alternatives with clients when prescribing emergency use authorization products.
• Appropriate recordkeeping and client communication are required when using these therapies.
• Prudent use is critical to help reduce the risk of antiparasitic resistance.

Is NWS zoonotic?

In rare cases, there have been cases of NWS in people. Always wear gloves, disposable fluid-resistant gown, and shoe covers when handling or treating pets with suspected NWS myiasis and wash hands well after removing PPE. People should seek care with a healthcare provider if they have a wound or injury or are concerned that they may be infected with NWS.

Additional questions about New World Screwworm?

For additional details and the most up-to-date information about NWS, visit the USDA and CDC websites.