06/10/2026
As horse owners, we tend to keep a close eye on colic, lameness, and infectious diseases—but there's another threat that deserves our attention: the New World screwworm.
This is a great write-up on this parasite and I wanted to share it with fellow horse owners. Unlike typical maggots that feed on dead tissue, New World screwworm (NWS) larvae feed on living flesh. Even a small wound can quickly become a serious, painful, and potentially life-threatening infestation if left undetected.
The good news is that prevention starts with vigilance. Take a few extra minutes during your daily routine to check your horses for cuts, scrapes, tick bites, surgical sites, or any other wounds. If you notice a wound that seems unusually painful, swollen, draining, foul-smelling, or attracting excessive flies, investigate promptly and contact your veterinarian if you have concerns.
While NWS is not currently widespread in the United States, recent developments serve as a reminder that biosecurity and early detection matter. Horse owners are often the first line of defense. Knowing what to look for and reporting suspicious cases can help protect not only our own animals but the broader equine and livestock community.
An ounce of prevention is worth a pound of cure—especially when it comes to flies with such devastating potential.
🚨 𝐓𝐡𝐞 𝐅𝐥𝐲 𝐄𝐫𝐚𝐝𝐢𝐜𝐚𝐭𝐞𝐝 𝐟𝐫𝐨𝐦 𝐭𝐡𝐞 𝐔.𝐒. 𝟔𝟎 𝐘𝐞𝐚𝐫𝐬 𝐀𝐠𝐨 𝐈𝐬 𝐁𝐚𝐜𝐤 🪰
I am sure if you have been paying attention lately, your feed has been flooded with news about the New World screwworm. So let's take a moment to discuss what it is, why it matters, and where things currently stand.
The New World screwworm is a parasitic fly, native to the Americas, whose larvae infest and consume the flesh of their hosts. Unlike other maggots, screwworms eat healthy, living tissue using sharp mouth hooks that can create extensive tissue damage and result in large pockets of decay.
Animals at risk include any warm-blooded mammal with a wound as small as a thorn prick, which can serve as a site of infestation.
Left untreated, screwworm infestations can cause severe tissue destruction and become fatal within as little as 7 to 14 days. The resulting damage may lead to secondary bacterial infections, sepsis, or vital organ damage, making early detection critical.
🦠 𝐒𝐩𝐫𝐞𝐚𝐝
While the larvae do not spread directly from animal to animal, after about 7 days of feeding, they drop to the ground, burrow into the soil, and pupate. The adult screwworm fly emerges after 7 to 54 days (depending on temperature and humidity) and seeks out a new host.
🌎 𝐖𝐡𝐞𝐫𝐞 𝐓𝐡𝐞𝐲 𝐓𝐡𝐫𝐢𝐯𝐞
New World screwworm thrives in hot and humid climates and prefers temperatures between 77 and 86°F (25–30°C) with 30–70% relative humidity. However, they experience both heat and cold vulnerability that can reduce activity or kill them entirely.
For example, adult fly activity is severely limited below 59°F (15°C) or above 95°F (35°C), and adults do not survive below 20°F (-6.6°C). Additionally, larvae dropping from the host typically die when temperatures fall below 15 to 20°F (-9 to -6°C) and pupal development halts when temperatures exceed 110°F (43°C),
This means year-round populations have historically been restricted to the lower quarter of Texas, southern New Mexico, Arizona, California, and parts of Florida. However, seasonal expansion can occur during spring and summer, with transportation of infected animals representing the greatest risk of introduction into northern regions.
📜 𝐇𝐢𝐬𝐭𝐨𝐫𝐲
Historically, the screwworm was endemic from the southern United States through South America and created a tremendous economic burden for the livestock industry. As a result, controlling it became a national priority.
In the 1930s and 1940s, USDA scientists Edward Knipling and Raymond Bushland developed the Sterile Insect Technique (SIT).
This process proved successful because female screwworms only mate once. Scientists sterilized male flies using radiation and released them into the wild, effectively collapsing the breeding cycle. The technique was first successfully tested in 1954 on the island of Curaçao, located 40 miles (64 km) off the coast of Venezuela. By steadily increasing the release of sterile flies, complete eradication was achieved in just 7 weeks.
The technique was later used to eradicate New World screwworm from the United States by 1966. The SIT barrier continued progressing south through Mexico and Central America until a Panama-based biological barrier was established at the Darién Gap, and Panama was declared screwworm-free in 2006.
The Darién Gap is significant because it is a dense, mountainous jungle between Colombia and Panama. Not only is the forest vast, but its lack of roads creates a natural geographic barrier that limits the movement of wild host animals and helps contain the parasite.
To support this barrier, the binational Panama – United States Commission breeds and releases approximately 15 to 20 million sterile New World screwworm flies each week over the Darién Gap.
🦌 𝟐𝟎𝟏𝟔 𝐅𝐥𝐨𝐫𝐢𝐝𝐚 𝐎𝐮𝐭𝐛𝐫𝐞𝐚𝐤
However, sporadic reintroductions have occurred and resulted in outbreaks within the United States.
In July 2016, an outbreak occurred in the Florida Keys and affected the endangered Key deer population. Officials successfully used SIT to eradicate the parasite, but the process took approximately 7 months and impacted roughly 15% of the Key deer population before the outbreak was contained.
The geographic origin and method of introduction remain unknown. However, because adult screwworm flies cannot travel long distances across open ocean, investigators concluded the outbreak likely resulted from an accidental human-assisted introduction involving an infected animal.
🚨𝐂𝐮𝐫𝐫𝐞𝐧𝐭 𝐎𝐮𝐭𝐛𝐫𝐞𝐚𝐤
While you may just be recently hearing about New World screwworm, the current outbreak actually dates back to 2022. At this time, the New World screwworm breached the biological barrier at the Darién Gap, and cases in Panama surged from approximately 25 annually to more than 6,500.
This increase is believed to be associated with increased animal movement as well as exponential growth in human crossings through the Darién Gap, which increased from approximately 24,000 migrants in 2019 to more than 520,000 in 2023.
Since then, the outbreak has surged north through Central America and Mexico, driven primarily by unregulated cattle movement.
The United States has attempted to reduce the risk of introduction through suspended imports of live animals, implemented periodically since November 2024. Additional measures have included preclearance inspection and treatment protocols, as well as increased funding for screwworm response activities.
Despite these preventative measures, the first cases of New World screwworm were identified in the United States in June 2026.
🏭 𝐖𝐡𝐚𝐭'𝐬 𝐍𝐞𝐱𝐭?
Screwworm control relies heavily on sterile fly production.
To support these efforts, the USDA is opening a sterile fly production facility in Edinburg, Texas. However, the facility is not expected to become operational until November 2027, when it will be capable of producing approximately 100 million sterile flies per week.
The USDA has also invested $21 million to modernize a sterile fly facility in southern Mexico, which is expected to begin operations later this month in June 2026.
✅𝐖𝐡𝐚𝐭 𝐂𝐚𝐧 𝐘𝐨𝐮 𝐃𝐨?
It may feel like a problem far beyond the control of any individual owner, but the reality is that early detection and reporting are our first line of defense. Here are some practical ways you can do your part:
🔹 Preventative care for animals – Treat all wounds and umbilical cords immediately, inspect pens and housing for sharp or potentially harmful objects, and avoid scheduling elective procedures such as dehorning or branding when in or near an infested area.
🔹 Be observant and report suspected cases – Monitor animals daily, carefully inspect wounds, and immediately contact your local State Animal Health Official or USDA APHIS office if you suspect screwworm.
🔹 Travel safely – Inspect pets and livestock traveling internationally or from infested regions, and check vehicles for screwworm flies when leaving an infested area.
𝐂𝐨𝐧𝐜𝐥𝐮𝐬𝐢𝐨𝐧
The New World screwworm is a reminder that disease and parasite control is never truly finished. Decades of scientific innovation and international cooperation successfully pushed this parasite out of the United States, but maintaining that success requires constant vigilance.
While the current situation can feel alarming, it is important to remember that we have defeated screwworm before and we understand how to control it. Early detection, rapid reporting, responsible animal movement, and continued investment in surveillance and sterile fly programs will all play a critical role in limiting its impact.
Whether you own one horse, a small hobby farm, or a large livestock operation, staying informed and paying close attention to wounds may be one of the most important things you can do to help protect your animals and prevent further spread.
Cheers,
Dr. DeBoer
