Cattle ranchers may be facing a new pest in town. The New World Screwworm (NWS) is sneaking up on the United States and could affect thousands of herds nationwide. Originating from Central America, screwworms travel virtually on everything and spread like wildfire.   

What is the New World Screwworm?  

Almost out of a scene from a horror movie, the screwworm has a horrific way of harming herds. This fly lays eggs in open wounds of cattle, and maggots feed on cattle tissue, causing open wounds that can lead to infections and death.   

This isn’t the first time we’ve seen these pesky suckers. In fact, we have a history with the screwworm.  In the 1950’s, Raymond Busland and Edward F. Knipling developed a technique of sterilizing male flies and releasing them in the wild to reduce the number of eggs and larvae.  

Ranchers worked closely with scientists, vets, and government agencies to identify infestations, treat them, and release sterilized male flies in their region. Over time, the fly was eradicated from the U.S., then Mexico, then all of the neighboring countries in Central America, down to Panama. The United States and Panama have united to maintain a barrier zone of sterile male flies from reinfesting. Unfortunately, that barrier is beginning is becoming weaker. The most recent outbreak in the United States was in 2016, on a key deer in Monroe County, Florida.  This outbreak was contained quickly and taken care of by 2017.  

How do Screwworms Travel?  

Screwworms like to hitchhike off of people, livestock, wild animals, vehicle, and cargo. They will hitch a ride however they can, and the best way to stop the outbreak is to be aware.  Monitor your cattle for any open wounds, especially if they are infected. Clean and treat the open wounds, and restrict the movement of live animals.  

Any Plan of Action?    

According to the USDA, Secretary Rollins has suspended live animal imports through the ports of entry into the United States. Now, there is a $8.5 million sterile NWS fly dispersal facility in South Texas, and a five-pronged plan to eradicate the worm. This is planned to be opened by the end of 2025. 

What Can Ranchers do to Battle the New World Screwworm Worm?  

The screwworm flies have orange eyes, metallic blue bodies, and three dark stripes on their backs. If you find any on your cattle, the first step is to contact your local veterinarian, and to make sure that state animal health officials are contacted.  

It is also important to evaluate your water. Like most insects, these nasty bugs like to hang around the watering hole. If your water sources aren’t clean, screwworms can multiply heavily.   

If you have a few cattle infected with screwworms, your best bet is to quarantine them from the rest of the cattle. Section off your pasture into different zones, depending on the severity of screwworm infection. Check all animals for open wounds and infections. You can treat cattle infected with screwworms with screwworm aerosol insecticide spray or blue screwworm spray. 
 

Provide individual water troughs/tanks for each zone. This way, the disease is sectioned off from the rest of the herd, and it will give your cows time to recover. Watch water levels in each area and adjust accordingly.  

Ensure Your Water is Clean  

Check all of your water troughs and tanks for algae and sediment. Cattle need clean water in order to fight infections and prevent diseases from spreading.   

To monitor your water, Ranchbot is here for all of your water monitoring needs. To learn more, contact us today!   

Sources: USDA, TAMU, CBS News, FDACS 

UPDATE 7/10/2025

The U.S. southern border shuts down livestock trade due to the northward spread of the New World Screwworm. View the map below to view recent detections: