The New World screwworm (NWS) is a parasitic fly that no livestock producer or veterinarian wants to see on a farm or ranch. However, with active NWS cases now officially confirmed in South Texas, the threat has become an active reality for U.S. producers. While safeguarding cattle herds is a top priority, establishing a robust perimeter defense requires producers to look beyond their pastures and monitor local wildlife.
Free-ranging wildlife represent an uncontrolled, highly mobile reservoir for the parasite. During the last outbreak of 2016-2017 in the Florida Keys, 136 wildlife cases were confirmed, and 15% of endangered Key Deer died. Along with concerns for wildlife health, wildlife monitoring has become an important tool for following the migration of these flies.
“Wildlife are a cherished natural resource and play vital roles in our ecosystem health,” says Dr. Marcus Blum, AgriLife Extension statewide specialist and assistant professor at Texas A&M. “Texas is home to more than 100 mammal species that are at risk if the fly re-establishes itself here.”
How New World Screwworm Spreads in Wildlife
Most adult screwworm flies fly less than 10 miles searching for a host to lay their eggs in. While this natural migration is certainly affecting the spread of the pest, most of its travel is on infested animals.
“Most screwworm migration does not come from adult flies making heroic flights northward. Instead, it comes from the movement of infested animals — cattle, deer, wildlife — and yes, even in rare cases, people. Wherever the animal goes, the parasite goes with them,” said Dr. Christopher Lee on a recent episode of “Vet Watch”.
One of the biggest challenges with wildlife monitoring is that most free-ranging wildlife avoid human contact. Because of this, NWS infestations can go undetected — meaning large scale mortalities are possible.
Signs of New World Screwworm to Look for in Wildlife
- Visible Larvae and Open Wounds: Look for deep, weeping sores on living wildlife, often with a visible pocket of crawling maggots feeding on the tissue.
- Foul, Decaying Odor: Active infestations emit a distinct, foul smell resembling rotting flesh.
- Abnormal Behavioral Changes: Watch for isolated or lethargic animals exhibiting continuous head shaking, intense scratching, and extreme irritability.
Why White-Tailed Deere and Feral Swine are Potential NWS Carriers
Chief among affected species are the more than 5 million white-tailed deer that call Texas home. White-tailed deer are highly susceptible and often show visible infestations in the head and neck areas. Since the last major NWS infestation in the 1960s, the Texas white-tailed deer population has grown from 500,000 to approximately 5.5 million in 2024. This increase in population also represents an increase in risk for NWS migration, as they are such good hosts for the pest.
Feral swine could also act as NWS carriers as their widespread presence and constant movement means they could spread infestations quickly. For cattle producers, this means that even if your herd is secure and well-maintained, an outbreak in nearby wildlife could easily spread onto your animals.
Wildlife susceptibility to NWS infestations varies by sex and season. This is due to seasonal behavior and environment changes.
NWS activity is at its highest in the summer when temperatures range from 70°F to 90°F, directly coinciding with fawning season. The umbilical region of a newborn fawn, as well as the mucous membrane of the doe genitalia, present highly attractive, vulnerable targets for female flies to deposit eggs.
In breeding season, male wildlife compete for mating dominance, often leading to wounds from fighting. These wound sites are the perfect place for NWS to lay their eggs.
During the autumn rut, male wildlife engage in aggressive fighting for mating dominance, resulting in open tissue wounds that are an ideal place for NWS to lay their eggs. Additionally, the bloody tissue left behind during summer antler growth and velvet shedding creates a perfect, unprotected environment for screwworm strike. For example, the Florida Keys outbreak occurred during rut season, and males had the highest mortality rates in Key deer.
How You Can Help Monitor Wildlife for Signs of New World Screwworm
Everyone who works the land or spends time outdoors can play a part in protecting against NWS. Ranchers, hunters and community members are often the first to spot unusual wounds on deer, feral hogs or other wildlife.
- Trail Cameras: If you’re using trail cameras, consider checking them regularly for any signs of wounded or abnormal appearing wildlife. Position trail cameras near water sources, scrape lines, or low-fence crossings where border wildlife congregate.
- Hunting Stands: A hunting stand can be a great vantage point for identifying infected animals.
- Security Camera Footage: In more residential areas, security camera footage could be useful for monitoring wildlife.
- Vulture Activity: Watch the sky for scavengers. If you see vultures circling, investigate what is attracting them.
As always, do not attempt to approach live wildlife. Maintain a reasonable viewing distance for your own safety. If you suspect NWS in any wildlife you observe, including the observation of maggots, immediately report your sighting to your state animal health official and Animal and Plant Health Inspection Service (APHIS) office.
The battle against NWS isn’t just about livestock — it’s about the ecosystem around them. Wildlife can serve as both an early warning system and a pathway for infestations to spread. By recognizing the importance of wildlife monitoring, producers and veterinarians strengthen the shield that keeps herds and geographical regions safe from this destructive parasite.
Blum and colleagues have released a very informative fact sheet if you’re looking to learn more about wildlife monitoring and management for NWS. The APHIS office has also published a guide for the detection and reporting of NWS.
How to Report Suspicious Wildlife
If you observe wildlife with open wounds, visible maggots, or highly erratic behavior, do not attempt to approach or handle the live animal. Maintain a safe distance and follow these steps immediately:
- Document the Details: Note the exact GPS coordinates or landmarks, the wildlife species, and the specific signs observed. Take photos or video from a distance if possible.
- Notify State Authorities: Immediately contact your state animal health official to report the location.
- Alert Federal Monitoring: Contact your local USDA Animal and Plant Health Inspection Service (APHIS) office.
Early detection protects your neighbors, your herd, and the entire livestock industry.
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