New World Screwworm (NWS)
New World Screwworm: A Reemerging Threat
New World screwworm (NWS), or Cochliomyia hominivorax, is a species of parasitic fly that can cause infestation in humans and animals. NWS larvae feed on live tissues of any warm-blooded animal. When larvae from the fly get into a wound or opening in the body, they cause myiasis, or an infestation of fly larvae.
NWS myiasis occurs when a female fly lays eggs near wounds or mucous membranes (eyes, ears, nose, mouth, genitalia) of an animal or person. Female flies can lay hundreds of eggs within minutes. When the eggs hatch, the immature flies (also known as larvae), burrow or “screw” into the opening with sharp mouth hooks and eat the flesh. This can be painful and can even cause death. There is currently no medication labeled to treat an NWS infestation in humans. The larvae must be removed by a health care professional for humans or by a veterinarian for animals.
NWS is generally an agricultural problem. Screwworm myiasis in cattle can lead to the animal’s death and can severely impact ranchers and the agricultural industry. Early detection can save lives and money. If you suspect a pet, livestock, or wild animal has an NWS infestation, get help as soon as possible. For pets or livestock, call your veterinarian and contact the Texas Animal Health Commission (TAHC) at 1-800-550-8242 or through your local TAHC regional office. If you see a live wild animal with larvae, immediately contact Texas Parks and Wildlife (TPWD) through your local wildlife biologist.
NWS and Humans
Though the risk to the public remains low, people can sometimes get an NWS infestation if they visit areas where the fly is present, such as Central or South America, the Caribbean, or Mexico. People who do the following in areas with NWS might be at greater risk for infestation:
- People who sleep outdoors (unhoused, campers)
- People who primarily work outdoors
- Ranchers and those who work with livestock
- People with an open wound
- Those with diabetes or other conditions
People who see or feel larvae in or on their wounds should immediately contact their health care provider. If you think you might have been exposed to C. hominivorax or think you might have an NWS infestation, contact your health care provider.
Information for Health Care Providers
Health care providers play a critical role in stopping NWS. If you suspect a patient might have myiasis caused by an NWS infestation, please contact your local health department to arrange testing. Any larvae collected for identification should be submerged in 70% alcohol (ethanol or isopropanol) in a plastic container with a screw cap that can be tightly sealed. DSHS Regional Zoonosis Control Offices will give providers further guidance on sample collection, submission, and interpretation of results.
Live larvae should not be thrown away in the trash or outside while they are alive. If larvae are not needed for testing, submerge them in 70% alcohol before disposing. This ensures that the larvae die and do not cause any further risk.
Identifying the New World screwworm
The DSHS Laboratory can test to see if a fly or larvae is C. hominivorax. Only health care providers can order these tests and submit specimens to the laboratory. The DSHS Lab does not accept specimens for NWS surveillance from the public. Specimens submitted by the public will be rejected and destroyed. If you believe you have a parasitic infection, contact your health care provider. Laboratory staff cannot provide medical advice.
FREQUENTLY ASKED QUESTIONS
What is New World screwworm?
Despite its name, the New World screwworm is actually a species of fly also known as Cochliomyia hominivorax. NWS larvae feed on live tissues of any warm-blooded animal. When fly larvae get into a wound, they cause myasis, or an infestation of fly larvae.
Can I get NWS from an animal or someone who is infested?
No, you cannot directly catch NWS myasis from an infested animal or person. However, adult flies can lay eggs on and infest different people living in the same space
What are the symptoms of NWS infestation in humans?
NWS infestations are very painful. If you have an NWS infestation, you may see larvae around or in an open wound. They could also be in your nose, eyes, mouth, or other mucous membranes. Tell your health care provider of any recent travel to Central or South America, the Caribbean, or Mexico.
Symptoms can include:
- Feeling larvae moving within a skin wound, sore, ears, nose, eyes, or mouth
- Seeing larvae around or in open sores
- Painful skin wounds or sores
- A foul-smelling odor from the site of the infestation
- Unexplained skin wounds or sores that do not heal within a few days
- Bleeding from open sores
Bacteria can also infect wounds where NWS larvae are present, which may lead to fever or chills. (CDC)
How can I protect myself from NWS?
DSHS urges people to take precautions by following these steps to prevent NWS infestation:
- Keep open wounds clean and covered
- Sleep indoors or in screened shelters
- Wear socks and loose-fitting, long-sleeved shirts and pants
- Use an EPA-registered insect repellent
- Treat clothing and gear with products containing 0.5% permethrin, which is an insecticide and repellant
What should I do if I think I have an NWS infestation?
If you think you might have an NWS infestation, keep your wound covered and see your health care provider as soon as possible. Because NWS larvae can burrow deep into wounds, you can hurt yourself if you try to remove the larvae on your own. Your health care provider will be able to safely remove the larvae. Your provider should contact newworldscrewworm@dshs.texas.gov for more information.
What should I do if I think my pet or livestock has an NWS infestation?
Animals with an NWS infestation typically display:
- Irritation or discomfort
- A smell of death or decay around a wound or orifice
- Open wounds or sores
- Wounds where larvae are visible
If you see or suspect any of these symptoms, contact your veterinarian so the animal can be assessed quickly.
If your pet or livestock is diagnosed with NWS infestation, you or your vet should contact the Texas Animal Health Commission (TAHC). If you see a live wild animal with larvae, report the sighting to Texas Parks and Wildlife (TPWD).
Didn’t the U.S. eradicate the New World screwworm?
NWS was previously eradicated from the U.S. in the 1960s. The fly continues to be endemic in most of South America, parts of the Caribbean, and southern Panama. Since 2023, NWS has been detected throughout Central America. NWS was detected in southern Mexico beginning in November 2024. To learn more about the history of NWS and eradication efforts, visit the U.S. Department of Agriculture (USDA) website.
RESOURCES
DSHS is working with multiple state and federal partners in response to NWS. In Texas, the lead agency is the Texas Animal Health Commission (TAHC).
If you suspect an NWS infestation, please notify the following authorities immediately.
If you live or work with animals:
Livestock: Call your veterinarian and the Texas Animal Health Commission at 1-800-550-8242 or through your local TAHC Regional Office.
Pets: Contact your veterinarian and the Texas Animal Health Commission at 1-800-550-8242 or through your local TAHC Regional Office.
Wildlife: Contact Texas Parks and Wildlife through your local wildlife biologist.
If you suspect you or another person has an NWS infestation:
Contact your health care provider immediately. Your provider should contact your regional public health office.
If you are a health care provider and need to get a sample tested:
The DSHS Laboratory can test samples from health care providers only. Contact your local Zoonosis Control Regional Office for instructions.