Screwworms In S. Mexico Has USDA, States Working On Plan

Screwworm eradication is one of the most successful and beneficial agricultural programs. As reported below, the screwworm danger remains.

NOTE: this article was originally published to LivestockWeekly.com on January 8, 2025. It was written by Colleen Schreiber.

Ninety-four-year-old Minnie Lou Bradley was just a kid, perhaps 12 or so, when she had her first real life experience with screwworms.

She had several sows and more piglets that were about half grown when she found that screwworms had gotten into their noses. This was long before there was any such thing as the sterile fly program.

Bradley says that in those days they could buy chloroform over the counter and always had some on hand. She decided that might just be what she needed to kill those flies, so she proceeded to pour it over their feed.

Thirty minutes later she went back to check on her pigs and found that they all appeared to be dead.

Devastated, she ran to the house crying and calling for her dad to tell him that she had killed her pigs.

Explaining what she had done, her dad simply suggested they wait and go back out in an hour or two. She did as was told and when she went back next, all her pigs had awakened.

"I killed those flies, but I sure enough put my pigs to sleep," says Bradley.

She followed that story with another.

"While we had screwworms on our farm, I never realized how bad they could be until I moved out here (just northwest of Childress)," says Bradley. "Back then we could only brand at a certain time of the year. We couldn't do anything that would bring blood."

She recalled the first load of calves they bought after moving to the ranch.

"They came from Bill B. Wallace at Jayton - 300 Hereford steers," says Bradley.

Those calves arrived in October and her father-in-law, Rusty Bradley, decided since it was near the first frost that they would likely be okay to get them branded.

As it went, it started raining and those brands peeled and overnight essentially, they had 300 calves with screwworm.

"I'll never touch another Viena sausage because we'd leave out at daylight with sausage, crackers and water," Bradley says. "We stayed horse back all day long." That was a regular part of their life, riding and doctoring wormies and riding and doctoring some more until one day they heard from Dub Waldrip, who then worked for the Texas A&M Experiment Station, that there was to be an effort to eradicate the screwworm through the production of sterile male flies. All the ranchers were called on to help financially.

Bradley's husband, Bill, was made chairman of their region. He went door to door asking for financial support for the effort.

"You know we eradicated it, and we've had it eradicated for years now," says Bradley.

Then she adds, "If we had to do that today (ride cattle every day, well it wouldn't happen. We don't have the cowboys, and we don't have the horses... I don't know what will happen if it ever gets out of hand again."

That sentiment has been echoed a lot of late.

That's because in late November the Chief Veterinary Officer of Mexico notified USDA Animal and Plant Inspection Service of a positive detection of New World Screwworm (NWS) in Mexico. Specifically, one illegally imported cow from Guatemala and one native Mexican bull calf were presented with NWS, the latter at a checkpoint close to the border with Guatemala in the southern Mexican state of Chiapas.

Given the severity of the threat Dr. Rosemary Sifford, Chief Veterinary Officer of the United States immediately issued an order restricting the importation of animal commodities originating from or transiting Mexico.

Besides livestock, NWS can infest wildlife, pets, occasionally birds, and in rare cases, people. APHIS documentation indicates that screwworm flies are metallic blue, blue-green, or gray, with orange eyes and three dark stripes down their backs. Within a day, eggs hatch into larvae and those larvae or maggots burrow into the flesh of a living animal. Mature larvae then drop to the ground to tunnel and re-emerge as adults.

APHIS further states that in their early stages, screwworms can be difficult to see. The most obvious signs are a change in the wound appearance, deepening and enlarging, and an odor emitted from the wound as well as some bloody discharge. An infestation can kill a untreated animal in one or two weeks.

Although USDA eradicated NWS from the United States in 1966 using the sterile fly technique, there is a constant risk of re-introduction into the United States.

Out of an abundance of caution, in February 1994 the Panama United States Commission for the Eradication and Prevention of Screwworm (COPEG) was created.

In 2006 a new sterile fly production facility was built in Panama. The facility is managed and funded jointly by USDA and Panama's Ministry of Agriculture Development (MIDA) through COPEG. The plant has a production capacity of 100 million flies weekly and it is the only remaining sterile fly production facility in North and South America.

Release of sterile files from the Panama facility began along the Panama/Colombia border in 2009. The release of sterile male screwworm flies in the DarienProvince of eastern Panama, specifically in the Darién Gap, a 60-mile stretch of land that crosses the border between Panama and Colombia, has continued since.

The goal of COPEG has always been to prevent the northward movement of NWS from South America where it is endemic in certain countries as well as Cuba, Haiti and the Dominican Republic. Nonetheless, in the last two years, that barrier has proven ineffective, and the screwworm has spread north throughout Panama and into Costa Rica, Nicaragua, Honduras, Guatemala and now Mexico. In fact, as of early December, COPEG reports that Panama had a total of 22,611 positive cases.

There is also the presence of NWS in Costa Rica with 11,627 cases, Nicaragua 6436, Honduras 105, Guatemala 32 and now Mexico with two confirmed cases.

Consequently in late November, sterile fly drops began in Chiapas, Mexico, along the Guatemalan border.

Sterile fly drops also continue throughout Central America, but the production to meet the needs is insufficient.

USDA has investigated the possibility of reviving old plants in the U.S., but that option proved unworkable They've also investigated the possibility of retrofitting existing fruit fly and boll weevil facilities to produce sterile NWS flies but that too has not proven to be feasible.

Mexico is reportedly reviving an old sterile fly plant that could potentially produce about 40 million flies per week. However, that has not been officially confirmed nor is it known when that plant might get up and running. There are said to be potential distribution facilities in Nicaragua and Honduras, but those do little to solve the production issue itself.

Also to build a new plant in the U.S. could reportedly cost hundreds of millions and would likely take a year or more to build. Plus, they are not cheap to operate as such a plant is said to require a lot of water and electricity to operate at capacity.

In December 2023, APHIS invested $109.8 million to combat new NWS detections in Central America and Mexico to keep the pest from spreading into North America. Another $135 million of dedicated funds were added at the end of 2024. That funding is said to just be a drop in the bucket compared to the amount that is needed to get ahead of the problem.

Meanwhile, APHIS has been working on protocols to reinstate movement of livestock from Mexico. State animal health officials have yet to see a finalized plan.

The Christmas and New Year holidays have obviously slowed USDA's progress.

Texas state veterinarian, Dr. Bud Dinges said that TAHC was first made aware of a potential problem in March of 2024. TAHC was alerted again in August when some in Texas, who regularly work with Guatemalan ranchers, last visited there and learned that the screwworm had migrated to Honduras.

"We first began putting pressure on USDA back in March to take note and step up their efforts," says Dinges.

Dinges is but one of the state animal health officials who have asked for a copy of the surveillance protocols that Mexico has in place. As of January 6, he has learned that the Mexican government has established two barriers in the southern part of Mexico to reduce the risk of introduction and dissemination of the insect through the trade of live cattle. The first barrier consists of four federal inspection points in Chiapas, Tabasco and Campeche.

Trailers will be inspected with canines trained to detect the NWS. Additionally, cattle entering the country will be sprayed with larvicide products and treated with ivermectin or comparable product.

The trailers will be sealed and issued a certificate of treatment. The load is then permitted to the next animal health barrier which covers the roads of the Isthmus of Tehuantepec in Chiapas, Oaxaca and Veracruz.

NWS mitigation information will be distributed to both producers and the public. Also, producers will be supplied with kits to submit larvae samples and will be supplied with treatment regimens for infested animals.

A "risk area" 35 kilometers from an infested premise will be established and all production units will be inspected to verify the absence of the insect.

Also, as of December 18, USDA-APHIS's protocols, for eventually reestablishing trade with Mexico, call for the Mexican government to set up pre-export pens where inspections by accredited veterinarians will be conducted. Cattle are to arrive at these pre-export pens three to five days after the administration of an anti-parasitic, with ivermectin or similar product.

The location of these pre-export pens has yet to be identified by the Mexican government. When identified, USDA-APHIS veterinarians will visit those facilities and approve them as pre-export pens. Memorandums of Agreement are also to be executed with those pre-export facilities.

In the meantime, USDA has trained all Veterinary Medical Officers, Animal Health Technicians and other entry point personnel on New World Screwworm inspection and treatment protocols. Additionally, all USDA's cattle fever tick eradication inspectors and other personnel in that program have been trained on these same protocols.

Likewise, TAHC has trained inspectors and veterinarians in all the regional offices in Laredo, Beeville and Stephenville. These individuals are essentially monitoring animals with any open wounds looking particularly for any maggots or flies in those wounds. If there is a suspicious incident reported, then a USDA foreign animal disease diagnostician or one of TAHC's foreign animal disease trained veterinarians would be called in.

As Dinges noted, TAHC has an 800-phone number for these types of situations monitored 24-7.

The diagnosticians have permethrin spray, Dectomax or Ivermectin products on hand should an animal need to be treated. Also, if needed, samples are taken and expedited shipment to the USDA laboratory in Ames, Iowa is the next step. Additionally, those animals would be quarantined

While the various insecticides may well be effective for treating adult livestock, there are others who voice concern about maggots infecting the naval of newborn animals. This would entail essentially manually treating each animals naval area one at a time.

Texas' state veterinarian is also concerned about the role that wildlife may play in the spread of this foreign animal disease. Containing the movement much less treating infected wildlife is much more difficult. In 2016, more than 130 endangered Key deer, found only in the Florida Keys, succumbed to infestations of NWS. Of that total, 120 were bucks as the outbreak occurred during the rut.

An American Veterinary Medical Association article by Greg Cima in January 2017 reported more than 45 million sterile screwworm flies were released by USDA between October and December 2016.

Additionally, borrowing from work cattle fever tick work in Texas, USDA-Veterinary Services set up Doramectin-coated rollers in front of feeder stations to try and treat the more remote populations.

Also, Florida's state animal health officials launched a monitoring effort and further infestations in pets and a stray dog were identified. That necessitated the need to expand the release of sterilized male flies to the Florida mainland.

Thankfully, five months after the outbreak was confirmed, USDA declared that screwworms had once again been eradicated in Florida. However, one of the big remaining concerns is that USDA never identified the origin of the infestation.

Others have also pointed out that feral hogs were not around in the 1950s and 60s, certainly not in the numbers they are today. While the screwworm would impact that population, they too could further the spread. The same could be said for the migratory avian population, not to mention flies may well be transported in during a hurricane.

There are also concerns with illegal movement of not just livestock and horses but also people and even domestic pets. For now, USDA continues to allow the legal importation of horses though inspection of those animals has been increased significantly, Dinges says.

For example, horses must be sedated to look specifically at the sheath in the prepuce area.

He acknowledges the need for feeder cattle from Mexico particularly given where we are in the cattle cycle. However, the risks of a screwworm infestation, should it get out of hand quickly without sufficient sterile fly production, is a concern that must be carefully evaluated

"Again, our best tactic is to just try to keep them south of the border," Dinges reiterated.

"I really would like to see some efforts by the Mexican government as far as their surveillance goes, before we start importing a great deal of cattle to the United States again."

He echoed Minnie Lou Bradley's comment that this generation does not understand what it was like to deal with screwworm infestations, and that they have no idea, really, of the devastation the disease can cause and the efforts that ranchers went to prior to the 1960s to humanely care for their animals.

He also reminds again that while the parasitic products weren't available back in the 1960s, they are only a treatment and the only way to eradicate the screwworm is with the sterile fly technique.

"If the animal is infested, that's one thing," says Dinges. "We can treat them with the necessary products, but we want to ultimately move the screwworm back down south of the Darien Gap in Guatemala.

Bottom line Dinges opined is that it would be a mistake to depend on the Mexican government alone to hold the line south of the border.

"We need to be prepared on our side to control it."

There was some talk as well that plant in Panama recently put out a new strain of fly and whether that strain may perhaps not be effective. However, that is speculation and not proven.

"It would sure behoove us, I think, to develop a production facility in the United States even if that plant sits idle and is only cranked up on an as needed basis.

It's about National Security and protecting our food supply," Dinges concludes.

More information along with updates are available on USDA's New World Screwworm website. Within are specifics on the importation of cattle, bison and germplasm from Mexico.

Additionally, the temporary suspension of cattle imports will remain in effect until complete implementation. When cattle imports have resumed, an alert will be posted on the website.