USDA photo by Sohath Yusseff-Vanegas

Not actually a worm but a fly, the New World screwworm gets its name from the way its larvae burrow, or screw, into living flesh.

Dr. Will McCauley remembers a parasitology unit in veterinary school when his professors at Texas A&M University described the New World screwworm fly, an insect that once wreaked havoc in Texas but has long been eradicated in the United States.

"It was here in the '60s and '70s, but not anymore," McCauley recalled the instructors saying. "You'll probably never see it."

But it's looking like that won't be the case. Federal officials are now preparing for the return of the catastrophic pest. Dr. Timothy Schell, acting director of the U.S. Food and Drug Administration's Center for Veterinary Medicine (CVM), told a gathering of the Association of American Feed Control Officials last month that CVM was preparing for screwworm to arrive in the U.S. as early as September.

The larvae of the parasitic blowfly feed on the live flesh of warm-blooded animals, and it is their attraction to live tissue that sets them apart from other flies. An outbreak presents a risk to livestock, such as cattle and sheep; wildlife, such as deer and rabbits; domestic animals, such as cats and dogs; and humans. The Latin name of the pest, Cochliomyia hominivorax, translates to "man eater."

Besides introducing painful, potentially lethal wounds in their hosts, a screwworm outbreak can be financially damaging — a 1976 incursion in Texas cost the state's economy $1.8 billion in today's dollars, according to a U.S. Department of Agriculture fact sheet on historical economic impact.

Officially eradicated from the U.S. in 1966, there have been only sporadic outbreaks since then, such as in Texas in 1976 and the Florida Keys in 2016.

The current concern over screwworm is due to continuous northward movement of the parasite. It began with a surge of cases in Panama in 2023. Cases spread through Central America before being confirmed on a cow in Mexico in November 2024. Subsequent cases in Mexico have indicated that there is a high likelihood of screwworm entering the U.S. From July 3, 2023, when the outbreak was declared, authorities have reported over 40,000 cases in Central America and Mexico. The majority of cases have been bovine; canine cases are a distant second.

Officials in the federal government and in southern states now anticipate the return of the screwworm to the U.S. Already, there has been confirmation of a human case — a Maryland resident who had traveled to El Salvador. The patient was treated, and there has been no known spread.

Screwworm myiasis, also known as flystrike, is the infestation that results when an adult fly lays eggs in an opening in the skin or in the mucous membranes of a host. The eggs hatch as larvae and feed on the host's flesh. An infestation is treatable if caught early. Effective treatment requires removing all larvae and eggs from the wound. Untreated, screwworm myiasis can be fatal to the host in seven to 14 days.

In advanced states of screwworm myiasis, the larvae-infested wound is obvious to the naked eye, but detecting the presence of screwworm in time to save the host and limit spread requires vigilant monitoring. The initial wound site may be small and in an obscured location, such as the genitalia or umbilicus.

Animals with screwworm may display less obvious signs of infestation, such as head shaking. They may become lethargic, act as though they're in pain and isolate themselves. Additionally, screwworm myiasis produces a putrid smell.

Experts are saying that controlling the spread of screwworm will take a cohesive effort among those who directly handle animals — including wildlife officials, ranchers and veterinarians.

"Veterinarians are one of the first lines of defense," in the view of the Texas Animal Health Commission, according to an email from a spokesperson. The commission has been collaborating with the Texas Veterinary Medical Association (TVMA) to prepare veterinarians in the border state. McCauley, who is on the board of directors of the TVMA, said one of the best things veterinarians can do is be up to speed on screwworm and ready to respond when it arrives — which he expects it to do in the not too distant future.

In Texas, McCauley recommends attending a screwworm informational meeting at the Southwest Veterinary Symposium later this month, or at TVMA's annual meeting in February. For veterinarians in all states, the USDA, FDA and the Centers for Disease Control and Prevention have all put out informational resources, including a webinar recording on YouTube. 

"Veterinarians are probably going to be the ones to really spot this and definitively identify it once it gets here," McCauley said.

Geographic barrier breached

The New World screwworm fly can complete its life cycle in as few as 21 days. After mating, a female fly seeks a host for its eggs, looking for either a wound — something as small as a tick bite will do — or a mucous membrane, such as around the nostrils or eyes. The female may lay 200 to 300 eggs at a time, and up to 3,000 eggs over its short lifespan.

Larvae hatch from the eggs within 24 hours and begin feeding. At this point, the wound site on the host animal may appear small, possibly obscuring a much larger infestation below the skin. After a week or so, the larvae fall to the ground, where they bury themselves to pupate. In approximately another week, they emerge as mature flies.

Screwworms thrive in warm, humid environments. When screwworm was present in the U.S., its range was primarily in the southern states. After being eradicated in the U.S., its northernmost range was pushed continuously further south over the next three decades. By the early 2000s, screwworms were contained below the Darién Gap. Located on the border of Panama and Colombia, the Darién Gap is one of the narrowest points in Central America. 

Efforts against screwworm in North and Central America are led in part by the Panama-United States Commission for the Eradication and Prevention of Screwworm, formed in 1994 through a partnership between the USDA and Panama's Ministry of Agriculture Development. Today, one of its main tasks is the breeding of sterile male screwworms for use in countering outbreaks and maintaining the geographic barrier.

Female screwworm flies mate only once in their lives, whereas male screwworm flies mate multiple times. By introducing sterile males to an outbreak population, the females will lay eggs that will not hatch. 

animal-nws-infestation

USDA photo

New World screwworm infestations are fatal if left untreated.

The Panamanian facility can produce 100 million insects per week. That may sound like a lot, but a female fly may have thousands of offspring. To keep up with the population's exponential growth potential, the U.S. plans to build its own breeding facility in Texas, with the aim of producing up to 300 million sterile male flies per week, according to an announcement last month by U.S. Secretary of Agriculture Brooke Rollins. The timeline for completion is unclear.

Veterinarians prepare to detect, report

Though the majority of cases since the outbreak began have been bovine, McCauley of the TVMA said that small animal veterinarians also must be very alert.

"Since the screwworm larvae can eat and grow in any warm-blooded animal, veterinarians really must be on the lookout for potential larvae in the wounds of not just livestock, but also pets and wildlife in their area," he said.

"It very well could be a small animal practitioner who's the first one to say, 'Oh, this looks like screwworm,' and potentially identifies the outbreak in the southern United States."

Veterinarians will be responsible for treating animals with a screwworm infestation, but there are no animal drugs approved by the FDA for this purpose.

Therefore, veterinarians will need to rely on off-label use — also known as extra-label use — of drugs, a practice that is legal and common.

"If this thing arrives tomorrow ... one of the most important tools for veterinarians will be extra-label use of animal drugs," Dr. Tristan Colonius, chief veterinary officer for the FDA's Center of Veterinary Medicine, said during a webinar for veterinarians last month. To assist, the FDA has created a list of drugs that may be helpful.

Colonius recommended veterinarians check the online list frequently, as he expects it to be updated as the agency works to expedite the availability of more options. The U.S. Department of Health and Human Services said on Aug. 19 that the FDA may issue Emergency Use Authorizations for animal drugs to treat or prevent screwworm infestations.

Suspected cases of screwworm must be reported to the relevant state veterinarian, who can provide guidance on next steps, including how to safely collect and submit samples to the state. 

McCauley, who has been disseminating information about detecting and reporting screwworm to TVMA members, said it's important for veterinarians to be prepared. "This is going to be an issue for years to come," he said.