Lizeth Olivarez doesn’t know if people will have the stomach to deal with New World screwworm (NWS). She’s a sixth-generation rancher who runs cattle in the U.S. and in Mexico at Las bendiciones Ranch in Realitos, Texas, and Rancho El Cuellareno in Guerrero, Tamaulipas.
Although she hasn’t experienced it herself, she has grown up hearing story after story about NWS from both her grandfather and father who dealt with NWS when the deadly fly struck in the 1960s. With reports of NWS in seven states in Mexico, the U.S. continues to bolster its defenses to keep the pest out.
“The first time around NWS was eradicated by using sterile flies dropped from planes in a box that look like a Big Mac burger box with a target sign on it,” Olivarez says. “The only way to eradicate the NWS is with sterile flies.”
Secretary of Agriculture Brooke Rollins announced a five-pronged plan to combat NWS on June 18 partnering with Mexican counterparts and using this sterile insect technology to stop the spread.
“It’s important to clarify we currently don’t have [NWS] here in the U.S.,” says Russ Daly, Extension veterinarian for South Dakota State University.
He admits it’s a strange name for a fly – New World screwworm – because it makes you think it’s a worm.
“It’s not actually a worm – it’s the larvae of the NWS fly that deposits its eggs in animals,” he explains. “They can get big in size, up to two-thirds of an inch. The first person who described them might have thought they look like little worms. They have ridges that are spiral, and that allows these larvae to burrow down into the animal. That’s how they get their name.”
The NWS fly lays eggs and larvae in open wounds of warm-blooded animals. Daly says NWS’s function is similar (but different) to a maggot. Domestic blow flies lay their eggs on dead tissue. The resulting maggots feed on the dead tissue. The difference with NWS is it lays its eggs on living tissue and will feed on living tissue.
“NWS is not easy to recognize,” Daly adds. “You really need an entomologist to recognize the fly and the larvae. For recognizing a problem in the animal, it might come down to recognizing a larval infestation that is showing up in unusual circumstances.”
It’s Not Just a Cattle Problem
Megan Niederwerder, executive director of the Swine Health Information Center (SHIC), says it’s important to remember this is not just a cattle disease – it impacts other livestock species and humans as well.
“The fly can deposit its larvae in any open wounds as well as surgical sites such as castrations and umbilical sites after a newborn animal is born. It can also deposit its larvae in any mucous membranes,” Niederwerder says. “Keep an eye on non-healing wounds or wounds that have a foul odor or bloody discharge.”
She says the animals often feel discomfort and pain due to this non-healing wound so they may isolate themselves and be off feed.
“It’s really important to stay vigilant as we think about increasing awareness and understanding of what the fly looks like,” Niederwerder says. “We also need to understand what the clinical signs may look like as the introduction and incursions in Mexico have increased the risk for the U.S., too.”
Although the cases in Mexico have been primarily cattle, she notes there have also been cases in pigs, horses, sheep, goats, dogs and even in humans in Mexico.
“We really don’t like to think about that human part of it,” Daly says. “I don’t want to downplay it, but in people, we would certainly know when we have an irritation or when a fly is on us. But we need to think about the people who potentially are debilitated and wouldn’t know they have a fly on them for a long period of time or that something’s wrong.”
Watch Out for the Unusual
Olivarez urges producers to call their veterinarian immediately if they suspect NWS. During the 1960s outbreak in the U.S., she says most ranchers first noticed it around the umbilical cords of newborn calves.
“It affected ranches the most, especially those that had cattle out on the range where you need manpower to monitor for open wounds, cuts or sores of any kind as well as cows with newborns since the fly will attack the umbilical cord,” Olivarez explains.
She says it’s not only important to monitor cattle but also pay attention to wildlife and pets that live on your property since they could indirectly bring NWS.
Daly challenges livestock producers to pay attention to unusual situations. If you have an animal that suddenly has a lot of necrotic tissue that fly larvae (maggots) are trying to clean up, Daly says that’s a sign producers need to take seriously.
“If there isn’t a wound or a very good reason why there would be an infestation of those larvae that should tip off animal owners, caretakers and veterinarians,” Daly says. “There needs to be some sort of break in the skin to attract the NWS fly, but that something can be so minuscule you wouldn’t even notice it.”
He says it could be as small as a bug bite and is often so tiny producers don’t know how the initial entry even happened.
“NWS does its damage by burrowing into living tissue,” Daly explains. “But they can be drawn to the animal through secretions from that minor break in the skin, and then start to invade the living tissue as well. In addition, once the NWS sets up the wound and causes the infestation, normal blow flies then can come in and get in there, too.”
The females lay their eggs very quickly, he adds. It doesn’t take long for the larvae to develop from there. He says the flies like to deposit their eggs on their “victims” in the afternoon.
“They choose this timing because when it cools off at night, it helps the development of the eggs into the larvae,” Daly says. “This all happens within 10 to 12 hours. If you have a fly that gets into a nick on a pig in the afternoon, for example, by next morning, there’s larvae starting to burrow in already.”
The fly has a lifespan of 28 days, Olivarez shared with followers of Texas Farm Bureau on TikTok.
@txfarmbureau Ever heard of the New World screwworm? South Texas rancher Lizeth Cuellar Olivarez breaks down why this tiny pest is a big problem for livestock. #texasfarmbureau #agtok #farmtok #fyp #screwworm ♬ original sound - texasfarmbureau
“During this time, NWS can lay eggs four times and lay 300 eggs each time,” Olivarez says. “If it goes untreated, after three weeks the larvae will fall and convert to a fly that then goes looking for another animal to attack.”
Treatment Tactics Aren’t Pretty
If an animal becomes infested with NWS, treatment is not easy or pleasant. The larvae have to be removed, Daly explains.
“NWS does respond to some insecticides that we would use for show pigs or horses during the summer to keep the flies off,” Daly adds. “But in order to get that animal healed up, the larvae have to be flushed out and removed by a veterinarian. These infestations can lead to secondary infections to worry about. And, if the infestation has gone deep enough, there could be severe muscle damage or damage to other organs that are beyond help.”
Treatment products need to be considered, adds Lisa Becton, assistant director of SHIC. Because of this, wound care is key during NWS infestation.
“If you have animals that have sores, it’s very important to treat that wound,” Becton says. “You may or may not be able to use a sealant, whether that’s iodine or antibacterial. Wound care is very important to help get a wound healing fast, even if you also have to use antimicrobials for an animal to help prevent other infection.”
It is treatable, but NWS is a painful event for animals and humans alike, Becton says. Rapid identification and action are critical.
Stay Alert, But Don’t Panic
Livestock trade and wildlife migrating due to different issues with climate and weather have helped move the infestation further north.
“The flies actually don’t fly that far,” he explains. “They typically fly less than 10 to 12 miles in their life span. So, they aren’t flying over on the wind and getting into these new places. It has more to do with animal movement.”
Fortunately, Daly says NWS is like any other fly in that they need somewhere to overwinter, so they won’t survive freezing temperatures. However, if they moved into this region in June or July, they would have a foothold until the frost.
“When we let our guard down and don’t look for it as much, or don’t use the sterile flies as often, that’s when things fire up again,” Daly says.
He says it’s not time to panic, but it’s important to be observant. The basics of good animal husbandry remain the most important thing for producers to focus on now.
“Sometimes it’s much easier said than done, especially in outdoor raising situations,” Daly says. “From fly control to monitoring animals, the best way to snuff something out like this quickly is good observation and animal care.”
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