The New World screwworm (NWS) is a pest that no producer or veterinarian wants to see on a farm or ranch. However, with the recent active case identified just 70 miles south of the U.S.-Mexico border, it seems only a matter of time before this threat becomes reality.
While NWS is a threat to livestock operations, we must also consider the impact it could have on local wildlife. 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.”
Screwworm Fly Migration
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.
Here are some things you can look for in wildlife you may observe:
- Open sores and tissue damage on living wildlife. Maggots may be visible in wounds
- Foul odor like rotting flesh
- Strange behavior including head shaking, irritability, apparent discomfort, lethargy or isolation from other animals
Wildlife Affected by New World Screwworm
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. Unfortunately, this timing coincides with fawning season. Not only is the umbilical region of newborn animals highly susceptible to infestation, but NWS may also lay eggs on the mucous membrane of the genitalia.
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.
Antler growth, antler shedding and velvet shedding also create opportunities for NWS infestation. For example, the Florida Keys outbreak occurred during rut season, and males had the highest mortality rates in Key deer.
How can you help monitor wildlfe?
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. If you’re using trail cameras, consider checking them regularly for any signs of wounded or abnormal appearing wildlife. A hunting stand can be a great vantage point for identifying infected animals. In more residential areas, security camera footage could be useful for monitoring wildlife. Also pay attention to 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.
Your Next Read: Winter: The Secret to Slowing the Spread of Screwworm
