Screwworm Update Shared At TWA Annual Convention

From Dick Gill...

Chris:

The latest screwworm update from Livestock Weekly is below. The two facts that struck me are:

The screwworm found in Veracruz was in a native animal, which means that there is a resident population of screwworms there. It was not in an animal that had been brought from somewhere else.

No screwworms have been found in Texas at this point, despite rumors that screwworms are already here.

There are other interesting facts in the article as well.

NOTE: this article was originally published to Livestock Weekly on August 6, 2025. It was written by Colleen Schreiber.

SAN ANTONIO – The potential reinfestation of screwworms in the U.S. is top of mind these days; and that’s true for not just livestock groups but wildlife groups as well.

At the recent Texas Wildlife Association’s (TWA) annual gathering screwworm updates were offered during several committee meetings, and in some of the educational sessions as well.

Dr. Thomas R. “TR” Lansford III, Texas Animal Health Commission (TAHC) assistant state veterinarian, offered an in-depth update during the Wildlife Conservation committee meeting. He shared the basics and then delved into more specifics. He told participants that the most recent case found in Veracruz put the fly now only 370 miles from Texas’ southern border with Mexico.

Another important fact is that it was a native calf.

“The fact that it was born in Veracruz is a concern because that means there is a fly population established there,” said Lansford.

USDA-APHIS Veterinary Services is working on response plans and playbooks.

“As a state, we’ve been really pushing USDA to get us this plan,” said Lansford.
TAHC has already developed a state plan, but it too must be approved by the Feds and that approval has yet to come.

“There are trade implications and with the size of Texas if the fly is in South Texas, we don’t want the whole state shut down,” he explained.

“We want to safely maintain continuity of operations for livestock as well as wildlife operations.”

Management of the pest entails treatment, surveillance and movement controls. As most know by now, the primary treatment for eradicating screwworms is sterile flies, but there isn’t currently enough sterile fly production. Additionally, there are no systemic treatments approved for screwworms in the U.S. at this time.

“It’s being worked on,” Lansford assured participants. “Some of the drug companies are working with FDA to get some of the products used in other parts of the world approved for use in the United States.”

There are some approved topicals, namely permethrin products, available over the counter for both livestock and wildlife. He added that TAHC is working closely with TPWD to offer some guidance and some best management practices for how best to use these to deal with screwworms in wildlife.

Lansford acknowledged that there has been some consternation about movement restrictions, particularly in the livestock industry. As some point out, there were no movement restrictions in the 1970s during the first eradication effort. However, the assistant state vet also reminded that there was a large sterile fly facility in South Texas at that time. That’s obviously no longer the case, at least not currently

“The fly will move naturally on its own and the wildlife and other hosts will move it as well, but we’ve got to do everything we can to not move it long distances on trucks,” said Lansford. “Therefore, there needs to be movement controls, inspections and treatment protocols.”

Surveillance is the third leg of the stool.

“We're going to need everybody's help to report cases,” he stressed.

Surveillance will be particularly critical so that the sterile flies can be used in the right places most effectively.

He also told the group that there is no cost to submitting samples for verification. Veterinarians, TAHC; TPWD biologists are trained in sample collection and submission protocol.

Since the first screwworm case was reported in Mexico, the USDA has had some issues dealing with Mexico, namely getting the sterile flies released in a timely manner. Lansford also pointed out that Mexico has not closed its southern border to cattle coming from countries where the screwworm fly is endemic. In fact, some 500,000 head of cattle are reportedly still coming from Central America.

“That’s a big risk factor,” said Lansford.

Additionally, since December government officials have been requesting information from Mexico as to what they’re doing to surveil wildlife.

“Despite repeated requests, we have no reports on wildlife surveillance,” Lansford said. “What I’m hearing through other contacts is that it has wreaked havoc on the wildlife populations, but again Mexico has not officially said one way or the other. We're not aware that there is any surveillance.”

Lansford also addressed the use of treated feed supplements for wildlife, namely ivermectin treated corn, which have been a hot topic. He told the group that it’s not generally a bad idea, but there are several issues that need to be resolved before use.

When talking about feedthrough products many refer to the Cattle Fever Tick Eradication Program which uses molasses tubs treated with crystalline ivermectin

“It is a highly restricted use product,” Lansford stressed. “It requires a lot of oversight.”

For example, as he explained the tubs must be weighed on certified scales every week. That along with the number of cattle in each pasture is needed to calculate dosage. All of that must be turned into FDA annually to keep their permit.
He also pointed out that there has been no research done for oral or feedthrough application uses for NWS.

“The only way to do that is with screwworms so that means doing the research at the COPEG facility in Panama.”

Additionally, there is withdrawal requirements and drug residue issues as well as concern that systemic use of ivermectin and doramectin for screwworms may speed up the resistance of these products labeled for other ectoparasite use. As he pointed out, Doramectin is one of the few things that provides a long treatment interval for the control of cattle fever ticks.

“It’s a critical tool in that it means producers do not have to pen and spray cattle every two weeks for fever ticks,” he explained.

Still, he reiterated that research groups are looking at it along with USDA, but there is a process before FDA might sign off on use of these drugs for screwworms.
Lansford closed his presentation by asking participants to be vigilant and to report any suspect case to an accredited veterinarian, the Texas Animal Health Commission or a Texas Parks and Wildlife biologist.

“With more public awareness the number of suspicious case calls have gone up, but that’s what we want because we don’t want to miss it.”

“We are not in a crisis. We do not have New World screwworms despite rumors that they’re already here, but it is time to prepare,” he concluded.

Kory Gann, TPWD’s big game program director, also offered brief update during the big game committee meeting. He told the group that a screwworm infestation following a mild winter could potentially cause 50 percent mortality in adult deer and 80 percent fawn mortality. That’s particularly worrisome given there are 753,000 whitetail deer hunters in Texas, which generates about $4.6 billion in direct revenue back to the state. Additionally, Gann noted that hunting enterprises are extremely important to rural economies and to land values.

Since the first case of screwworms were reported in Mexico, there has been a lot of talk about the 2016 Key Deer outbreak, in which 14 percent of the Key deer population was lost, 135 animals out of an estimated 1000 individuals. It was mostly bucks that were lost given it was during the rut.

One of the tools used to get the outbreak under control was inspection stations for live animal movement.

“It was pretty easy to do because there is only one road in and out of the Keys,” Gann explained. “It would be a whole different ordeal in South Texas.”

Additionally, 188 million sterile flies were released, and given the Key deer are endangered and are not hunted for human consumption, Doramectin was allowed as a special use treatment, though Gann acknowledged there is not any good data on its efficacy.

“They were able to dart the animals, inject them with Doramectin and clean the wounds of larvae,” said Gann. “There were able to do a lot of things that may not necessarily be available to us.”

Eradication in the Key deer was announced in March 2017.

Should the screwworm arrive on Texas soil, TPWD is tasked with doing surveillance on TPWD properties which entails working with state parks and state natural areas, as well as Wildlife Management Areas. Thus far some 70 TPWD biologists, game wardens and state park staff have been trained by TAHC personnel on identifying, reporting suspicious cases, as well as management protocols.

“Our three divisions will interact with the public differently,” said Gann. “Our Wildlife division staff, our field staff, they're going to work with landowners. Our state park staff is going to have a lot of interaction with the public, and they're going to be able to do their surveillance on state parks, and our game wardens will be interacting with hunters, landowners and the public.”

Suspect cases will be funneled through TPWD field biologists, Gann said.
“We have a biologist assigned to every county so because of their relationships, because they are on the ground in those counties, we wanted them to be our main source of reporting,” he explained. “From there, they will forward on to our wildlife health team and a member of that team will facilitate the collection and shipment to the appropriate laboratory.”

Upon confirmation of NWS, several response components will be implemented under the Texas Animal Health Commission Response Plan, which is awaiting approval by USDA. The plan will include the implementation of the sterile insect technique and live animal movement controls, Gann said. Surveillance, outreach and education are also part of the plan as well as proper disposal of infested animals from the landscape.

Gann reiterated that while surveillance is critical there is no plan to shut down hunting season should a screwworm infestation occur.

“We will need to monitor those populations through regulatory surveys, property specific surveys like our MLDP landowner deer surveys, and if we do start seeing populations decline then we will need to take action to modify season days and bag limits,” he told the group.

On June 25, TPWD and TAHC were directed by Governor Abbott to establish the Texas New World Screwworm Response Team. It is designed to be a central hub for coordination, information sharing and stakeholder engagement among state and federal partners, Gann said.

Additionally, the Screwworm Coalition of Texas, made up of 13 Texas landowner, livestock, equine and wildlife groups, recently launched a new website designed to deliver reliable information about the New World screwworm.