What Animal Care and Transport Teams Should Know About New World Screwworm (NWS)

You may have heard of the recent cases of New World Screwworm (NWS) in the U.S., in cows, a dog and other species. Screwworm was previously considered eradicated in the U.S. However, the parasite has been moving north through Central America since 2023, and a few cases of infection have recently been diagnosed in Texas and New Mexico.

The ASPCA is monitoring the situation closely, especially in relation to our work in Southern states. Our current processes, including performing veterinary physical exams, assessing clinical histories and using preventive anti-parasitic medications, are already significant steps towards keeping animals safe. Additionally, protocols that highlight wearing personal protective equipment, covering wounds and reporting injuries also play an important, proactive role in staff safety.

Knowing some basic facts about NWS can help teams working with animals in these areas be informed, assess risk and take appropriate actions.

What is NWS?

A type of fly whose larvae (maggots) burrow into living flesh, which is different than typical maggot infestations that only feed on dead tissue. NWS can cause significant injury and even death and, to date, has primarily been a concern in livestock.

Who is at risk?

NWS can affect all warm-blooded animals, including humans. However, it is most commonly seen in livestock, such as cattle, due to their extensive time outside and exposure to flies. Human infections are rare but possible.

How NWS spread?

It is spread when the female fly lays eggs in a wound or body opening (e.g., nose, eyes, mucous membranes). Any wounds, even one as small as a tick bite, are potential targets for the fly. The NWS eggs hatch into larvae, typically within 12–24 hours, at which point the larvae begin burrowing into flesh and damaging healthy tissues. The infection is not typically spread directly from animal to animal or from animals to people through direct contact.

How is NWS treated?

Any animal infected with NWS needs to be treated by a veterinarian. No medications are labeled to treat NWS; however, the FDA has issued conditional approval and emergency use authorization for some medications that are readily available for other parasites; see this list of drugs with conditional approval and emergency use authorizations. Consult with a veterinarian if you have concerns about an animal’s risk or exposure.

Who needs to be notified if a case is suspected? What happens then?

If a maggot is identified in an animal, or a case of NWS is suspected, a veterinarian should be notified so the animal can be examined immediately. Because of the serious nature of this parasite, it is reportable, and a formal notification system is required involving state and federal officials. Information on reporting is available at the APHIS site.

When a case is diagnosed, APHIS establishes a containment zone around the identified cases to stop the spread of the parasite by restricting animal movement. These zones, which are at least 20 km (12.4 miles), are based on the daily flight distance and lifespan of the fly; however, this range can be up to 125 miles depending on geographic, climate, and other risk factors. This information can be used to establish risk of exposure for animal surveillance and movement activities. 

What should you watch for?

Monitor animals for maggots in wounds or body openings; foul odor or smell of decay; a wound that enlarges quickly or signs of pain ( irritated behavior, depression, not eating and isolating themselves). 

Animal care teams and veterinarians can follow this checklist from APHIS when examining animals at risk for NWS infestation: Best Practices for Evaluation of Animals for NWS.

Examiners should pay special attention to openings and areas where the larva might hide:

• Nostrils, eyes and ears
• In skin folds, axilla (armpit), under tailhead
• Genitalia (e.g., prepuce and vulva) 
• Recent surgical sites
• Umbilicus on newborns 

How should people protect themselves? 

• Wear appropriate personal protective equipment (PPE) when examining animals at risk - at a minimum, use latex or nitrile gloves. 
• Keep any wounds you might have - including cuts and scratches - clean and covered. Coverings should include secure bandages and clothing. 
• Wear close-toed shoes, long sleeves, long pants, hats, socks and items like protective gloves when performing physical tasks to protect skin from injury or abrasions that could contribute to open wounds and elevate risk.
• The CDC recommends wearing an EPA-registered insect repellent to prevent insect bites.
• If you notice anything unusual related to a wound or skin irritation, contact your healthcare provider immediately and notify your supervisor.

Where can you learn more?

• APHIS provides Facts About New World Screwworm.
• The CDC is monitoring the situation in the U.S. and posts regular updates on their website.
• Want a deep dive? Watch this recent National Shelter Rounds presentation.