This Once-Eradicated Larva is Staging a Comeback

Countless diseases and life-threatening pests have been eradicated worldwide. When what was once eradicated makes a comeback, though, there is cause for concern both for social and economic reasons.

A pest that had been eradicated from the United States since 1966 has made a surprising comeback, not far from the U.S.-Mexico border. Despite efforts to rid both countries and even Central America of the devastation experienced in the past, the New World screwworm larva has staged a comeback. This time, however, it is not just the economy that is suffering. For the first time, the larvae are infecting people as well.

Call it a Comeback

The New World screwworm may not be a creature that many have heard of. This is because the flies that lay eggs that create the larvae have been eradicated from the U.S. since the 1960s. Unfortunately, the larvae have not only been found in places where eradication efforts have previously been successful, but also closer to the U.S. border than it has been in decades. Consequently, the alarm has been sounded to get screwworms under control and as expeditiously as possible.

According to Phillip Kaufman, PhD, Professor and Department Head for Entomology at Texas A&M University, the outbreak of screwworms in Mexico is a “continuation incident” from the 2023 Panama outbreak, where over 6,500 screwworm incidents were reported. This is problematic, given the efforts that were put into place to eradicate screwworms from the U.S. and then in Panama in 2004. “Historically, screwworms were identified in the U.S. in the early 1800s,” Kaufman explains. “It essentially was known pretty commonly in the southwest.”

Kaufman says that by 1930, the screwworms survived on the Gulf Coast, and by the next summer, they had officially invaded Florida. It was not until the 1960s that plans were put into motion to eradicate screwworms from the U.S. completely.

The eradication project dropped sterile screwworm flies in areas where screwworms were present. However, according to Kaufman, the project did not stop there. Instead, he says, the flies were dropped a couple of hundred miles into Mexico as well as into the isthmus of Panama. The quarantine line was considered the Panama Canal.

Researchers were able to “figure out how to get ahead of [screwworms] thanks to the eradication program,” Kaufman says. However, the imaginary line that was drawn thanks to the sterile flies was breached in 2023 when Panama had their outbreak. While that was a concern, it was not until November 2024 that Mexico revealed screwworms were present in the state of Chiapas.

When this news was revealed, the Chief Veterinary Officer of the United States, Dr. Rosemary Sifford, announced that the eradication program would be ramping up to protect the U.S. from a screwworm infestation.

“Given the northward movement of NWS, APHIS has in recent months stepped up its efforts in Central America to partner with impacted countries to push this pest out of newly affected areas,” Dr. Sifford says in her announcement via the U.S.D.A. “With this latest find in Mexico, we will further intensify this work to protect American agriculture and reestablish the barrier in Central America.”

As more states in Mexico were reporting screwworm outbreaks, the decision was made by U.S. Secretary of Agriculture Brooke L. Rollins to close the border between Mexico and the U.S. for the import of live cattle, horses, and bison until the screwworms had been proven to be better under control. This suspension will be revisited every 30 days, according to Secretary Rollins, to determine whether the screwworm population is declining in Mexico.

“The United States has ordered the suspension of livestock imports through ports of entry along our southern border after the continued spread of the New World Screwworm (NWS) in Mexico,” Secretary Rollins explains in a press release via the U.S.D.A. “Secretary Berdegué and I have worked closely on the NWS response; however, it is my duty to take all steps within my control to protect the livestock industry in the United States from this devastating pest.

“The protection of our animals and safety of our nation’s food supply is a national security issue of the utmost importance,” Secretary Rollins continues. “Once we see increased surveillance and eradication efforts, and the positive results of those actions, we remain committed to opening the border for livestock trade. This is not about politics or punishment of Mexico, rather it is about food and animal safety.”

This situation has become even more urgent now that not just animals, but people are becoming infected as well.

Signs Of A New World Screwworm Infection

While the cattle industry is most concerned about the potential of a screwworm outbreak in the U.S., it is essential to remember that screwworms can infect any animal with an open wound. This means that while domesticated cattle are susceptible and most prone to infection due to their proximity to one another, any animals that stay outdoors, such as dogs or deer, can also be infected. Even people can be infected, as was most recently seen in a 77-year-old woman in Chiapas, Mexico.

“This case has raised alarms within the health sector, as myiasis is more common in animals, but its occurrence in humans is rare and poses risks to public health and the agricultural sector,” a spokesperson for the Secretariat of Health, or Secretaría de Salud, says to The Pinnacle Gazette.

Consequently, the Mexican government has been working to get the word out about the best health practices and ways to decrease the chance of a parasitic infection caused by screwworms.

Those who are infected by screwworms will have very specific symptoms. Those symptoms include:

• Unexplained lesions that will not heal
• Prolonged sores that are painful
• Bleeding open sores
• Feeling larva movement in oan pen wound
• Foul-smelling lesion

Animals infected with screwworm exhibit distinct symptoms of their own. Those symptoms that animals will present with include:

• Head shaking
• Irritated behavior
• Decay smell
• Signs of fly strike
• Presence of maggots in wounds
• Reduced appetite
• Depression
• Wounds continue to get worse rather than better
• Reduced milk production
• Signs the animal is in discomfort

Because the infection can spread quickly, it is necessary to constantly monitor outdoor animals for not only open wounds but also signs of infection in the wounds. If there are open wounds, they need to be treated appropriately so that they heal as expeditiously as possible, preventing screwworm infection from occurring.

How The Screwworm Is Brought Under Control

Bringing the screwworm outbreak under control is not as simple as spraying an insecticide and killing the flies that lay the eggs. In fact, methods used on regular houseflies and other pests have proven to be mostly ineffective on screwworm flies and screwworms themselves.

The most effective method is complete eradication, which is only achieved by releasing sterile screwworm flies into the environment where screwworms have been positively identified.

According to Kaufman, there is a production facility in Panama that is currently producing 100 million sterile flies per week. Half are female and half are male. It is the sterilization of the males that “knock the population out,” however.

Given the outbreak in Mexico, there are plans to retrofit a plant for flies in Mexico that will have the ability to release 50 million flies per week. Additionally, Kaufman says the USDA is working to create a facility in the US to combat the screwworm outbreak should one occur here.

“The USDA is investing to build a plant in South Texas,” Kaufman tells A-Z Animals. “It is my understanding that the facility will have the capacity to release 300 million flies per week.”

“It is a slow process with slow movement,” Kaufman says. “We need to contain it the best we can until we can get the facilities and methods up to speed.”

The only screwworm fly production facility is currently in Panama, which has been providing the sterile flies to Mexico in hopes of containing the screwworm outbreak before it spreads further north.

Many Often Confuse The Secondary Screwworm For The New World Screwworm

There has been fear that the screwworm has made its way into the US, given the presence of the native secondary screwworm. However, Kaufman asked that it be noted that the secondary and New World screwworm are two very different species.

According to Kaufman, the flies that lay the eggs look similar to one another. The only difference is the secondary screwworm flies have three distinct stripes on the back that the New World screwworms do not. Additionally, the other big difference is where the flies lay their eggs.

“The secondary screwworm fly looks like a blow fly,” Kaufman explains to A-Z Animals. They also act the same. This is because, unlike the New World screwworm being found in live animals, the secondary screwworm will only be found in deceased animals. The flies lay their eggs in decaying animals, will land in trees near the deceased animals, and will even land on people if they happen to be nearby.

Conversely, the New World screwworm flies will not land on people or animals unless there is an open wound. The New World screwworm has very “specific reasons for landing,” Kaufman says.

“Compared to other insects, screwworm flies are not common,” Kaufman states. “They are fairly uncommon to see.”

This is because the New World screwworm fly will not land unless there is a suitable place to lay its eggs, which only occurs when the skin is not intact.

Why The New World Screwworm Population Suddenly Soared

Currently, there are no answers surrounding why the screwworm has been able to make a comeback. While it is clear that the screwworms have been able to move past the containment lines that were previously established, why this has happened is still being debated.

One of the reasons often cited is climate change. A reasonable assumption given that as temperatures continue to rise, the flies that lay the screwworm eggs would have access to more territory to allow the larvae to hatch and flourish. However, according to Kaufman, climate change is not why this latest outbreak has occurred.

“No one is sure what has happened at this point that led to them getting reestablished,” Kaufman explains to A-Z Animals. “They escaped containment and moved north.”

Kaufman goes on to say that having screwworms in the US “isn’t new” as screwworms “have always been here, before we were here.”

The greater likelihood of the screwworm making its way into the US, Kaufman says, is if an infected animal is transported here.

The problem is, those who remember the screwworm outbreaks from a handful of decades ago or more are dwindling in numbers. Ranchers today have never had to deal with screwworms, and the potential economic threat to the industry is frightening.

As researchers work to determine why the screwworm is making a northward move, the best that can be done at this point is to deploy previously successful eradication methods and to remain vigilant. This, according to Kaufman, includes reporting potential screwworm larvae and flies to veterinarians, to state-specific food and agriculture departments, and even to Texas A&M itself.

Those who believe they have seen a screwworm can send pictures of the worm or the fly to [email protected]. To date, the university has received an influx of photos. Fortunately, none have been identified as the screwworm, and hopefully, eradication efforts will work, and none ever will.