A parasitic fly called the New World Screwworm has recently been found in US animals for the first time in decades. Although the current risk to people in the US is very low, it’s worth knowing the signs, especially if you have pets and live in an affected area.
What is New World Screwworm (NWS)?
New World Screwworm (Cochliomyia hominivorax) is a parasitic fly whose larvae (maggots) burrow into, AKA infest, and feed on the living tissue of animals with warm blood, including livestock, pets, and people. This is unlike most other types of fly larvae that feed on dead tissue. It can cause severe wounds and, if it’s not treated, can cause death.
(Source: CDC)
Although people can be affected too, this is very rare. Human cases typically involve larvae burrowing down into an existing wound. The main concern right now is livestock. If New World Screwworm gains a foothold in the US, it could cause shortages of animal products like meat, milk, and other products that use animal components. And this could result in economic damage to rural areas.
How did we get here?
The US eliminated New World Screwworm in the 1960s using a technique called the Sterile Insect Technique (SIT), which involves releasing huge numbers of lab-sterilized male flies to interrupt reproduction. This technique was used all the way south to Panama and a biological barrier created using these sterile insects in Panama kept the flies in check for decades. But unfortunately in 2023, Panama and Costa Rica identified a new outbreak. Since then, every country in Central America and Mexico where screwworm had previously been controlled has reported cases and there have been more than 185,000 animal cases and over 2,200 human cases reported in the region [archived link].
🤓Nerd alert: Although many sources speak of the eradication of New World Screwworm, in the traditional epi-sense this isn’t strictly true. Let’s break it down:
Eradication means a disease has been wiped out globally, so not just in one part of the world or one country. There are only two diseases that have truly been eradicated: smallpox (in humans) and rinderpest (in animals).
Elimination means that a disease has been wiped out in a specific geographical region, like the US.
So in the case of New World Screwworm, it was eliminated from the US through Central America as far as Panama, but it remains endemic (meaning it circulates at a steady, ongoing level) across much of South America and parts of the Caribbean..
What’s happening in the US right now?
On June 3, 2026, the U.S. Department of Agriculture (USDA) confirmed the first animal case in the United States since a small outbreak in deer in Florida in 2016. Additional cases in Texas and New Mexico have since been confirmed in a dog, some sheep and goats [archived link]. The USDA has a dashboard tracking cases, which you can access here and our friends at GenomicEpi.com have also created a great tool to keep track.
So far there have been no reports of human cases from this outbreak. The risk to people remains low and is concentrated in areas where screwworm flies are actively circulating. However, some humans are more at risk than others [archived link]. Unhoused people with limited access to healthcare are especially vulnerable, as well as agricultural workers and people with open wounds, for example due to diabetes or other skin conditions. If you travel to Central America or Mexico, you should make sure any wounds on your body are covered and watch for signs of an infestation. If you live in the Southwest and have pets that go outside, you should regularly check them for wounds and keep them indoors if they are injured.
⚖️ Health Equity Alert
Farm workers, a workforce that is majority immigrant in many US regions, can be at higher risk for several reasons: Close contact with livestock raises their risk of exposure to screwworm flies. Once affected, additional barriers can delay or prevent people from seeking treatment. These can include the fear of immigration consequences or job loss if seeking health care. Limited access to healthcare can also be due to language barriers, long working hours beyond clinic opening times and living in rural locations with long distances to healthcare facilities. Unhoused people who sleep outdoors are also at higher risk for many health problems, including New World Screwworm and complications arising from infections.
So, should I be worried and is there anything I can do?
🔶 If you live in the US, but DON’T have livestock or pets, and you live in an area that is NOT currently affected, the risk of getting an infestation for you or your animals right now is very low. The bigger concern in the near term is about animal health and the impact on farming.
🔶 If you DO live in an area where NWS has been reported, and in particular if you have contact with or own cattle or pets, keep an eye out for any suspicious-looking maggot-infested wounds in your animals or in yourself.
This includes unusual wound activity, like swelling, an unpleasant smell or discharge from wounds in animals or in yourself. Wounds may also look different because they contain maggots or eggs, and you may see little openings where parts of the maggots poke out.
(Source: USDA National Agricultural Library)
Those wounds can smell really bad, and an affected animal may not want to eat, may be distressed, or isolating itself from others. So, if you are in doubt, get them checked out.
😅 There is one bit of good news though: The FDA recently gave emergency use authorizations for drugs that can treat dogs and cats infested with New World Screwworm [archived link].
🔶 If you’re traveling to Mexico,Central America or South America, it’s worth knowing the signs of an infestation that we mention above, and if you have existing wounds, keep them clean and covered, especially if you have contact with livestock and pets. It is also recommended to wear long-sleeved tops and pants and to use an EPA-approved insect repellent.
Bottom line: New World Screwworm is worth taking seriously. This is not because most of us face a personal health risk, but because it’s a developing situation and the stakes for animals, agricultural communities, and vulnerable people in affected areas are real. If you have livestock or pets in affected areas, stay alert. For everyone else, stay curious. We’ll keep watching this one and update you as things develop.
Stay well, stay curious.
Love,
Those Nerdy Girls
Useful links:
Centers for Disease Control and Prevention (CDC): NWS – Clinical Overview [archived link]
NWS – Outbreak Summary [archived link]
Animal and Plant Health Inspection Service (APHIS): Confirmed detection of NWS
Report suspected cases of NWS [archived link]
Genomic Epi – NWS tracker
The Guardian – Farm workers at high risk amid US NSW outbreak
