New World Screwworm
While USDA eradicated New World screwworm from the U.S. in 1966, there’s a constant risk of re-introduction. The parasitic fly larvae eat the living tissue of warm-blooded animals, such as cattle, causing serious, often deadly damage. The National Cattlemen’s Beef Association has warned it’s not a matter of if — but when — the pest makes its way back into the U.S.
USDA Guidance
New World Screwworm: What Every Cattle Producer Needs to Do Now
NWS is inching closer to the U.S. Based on the USDA-APHIS NWS response playbook, here is what ranchers and livestock owners should know.
- Minimize wounds before they happen. When possible, castrate, dehorn, brand and tag outside of peak fly season. Any open wound is a potential infestation site.
- Immediately treat all wounds with approved insecticides. Newborn navels, fresh brands, tagging nicks and foot lesions are the highest-risk sites. Apply treatment the moment any wound occurs.
- Tighten your calving season. Compressed calving windows reduce the number of vulnerable calves on the ground at any one time, so there are fewer animals to monitor and a narrower risk window.
- Update your parasite control program now. Sit down with your veterinarian to identify which products are currently available and labeled under Emergency Use Authorization.
- Build your response plan now. Know which veterinarian you'll call, how herd health checks might need to evolve, which products you'll use and what your movement plan is if a quarantine zone is established near you.
- "Eyes on animals" is your most powerful tool. USDA officials are clear: fly traps help, but animal inspections are more sensitive. Nothing replaces a producer who knows their herd and checks it regularly.
- Know the signs — sight, smell and behavior. Look for unusual irritation or head-shaking; wounds that swell overnight or ooze cloudy fluid; and the unmistakable smell of decay. If you peel back skin edges and see rice-grain-sized maggots with dark spines — act immediately.
- Know what the fly looks like. Adult NWS flies have orange eyes, a metallic blue-green body and three dark stripes along their backs — the center stripe is shorter than the outer two. Larvae burrow into living tissue and spiral deeper like a screw driving into wood.
- Prioritize high-risk sites every check. Navels on newborns, vulvas on fresh cows, sheaths on bulls and recent wound or surgical sites. These are NWS entry points — inspect them first, every time.
- Use technology to add "eyeballs" when labor is short. Game cameras, virtual fencing and behavior tracking tags can help flag problems sooner. NWS is an infestation of individual animals — finding one early means the rest of the herd might be clear.
- Watch wildlife, too. Deer, wild pigs and other warm-blooded wildlife are all susceptible. In past outbreaks, wildlife were a significant driver of spread. Hunters, wildlife managers and taxidermists should all be watching for signs of myiasis.
NWS is a federally reportable foreign animal disease. If you suspect an infestation, you are legally required to immediately report. Officials would rather investigate 1,000 false alarms than miss a single real case.
- Call your veterinarian or USDA APHIS immediately. Contact your USDA area veterinarian in charge or your state animal health official. Do not wait. Every generation of unreported flies dramatically expands the population and risk to your neighbors.
- Hold the animal — don't move it. Premises with a suspected infestation will be placed on a hold order while under investigation. Restricting movement until inspection is completed is required and the right call for the industry.
- A quarantine is not a shutdown. A structured process of inspection, treatment and health certification allows animal movement to continue. You will need an animal health certificate (iCVI for interstate, state-issued for intrastate) to move animals out of an infested zone — but movement is not halted.
- NWS is not a food safety issue. Animals presented to slaughter are processed under FSIS regulations. There are no movement requirements for animal products. This is an animal welfare and trade concern, not a meat safety concern.
- Think about your neighbors. Failure to report results in multiple generations of additional flies, expanding the risk for every operation around you. Reporting fast protects the whole community, not just your ranch.
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