Federal agricultural inspectors found a case of New World screwworm larvae — maggots that burrow into the flesh of live animals and sometimes people — in a 3-week-old calf in south Texas, near the US-Mexico border. Officials have been anticipating the arrival of the screwworm in the United States and say they are ready to contain it.
New World Screwworm, also known as Cochliomyia hominivoraxis very different from normal maggots that feed on decaying organic matter such as garbage, rotting food or dead animals, said Tom Talbot, a veterinarian and member of the California Cattlemen’s Assn.
That’s because the worm’s larva “attacks living flesh,” Talbot said.
On Thursday, the US Department of Agriculture confirmed the discovery of the New World worm in a cattle ranch in Zavala County, Texas, more than 60 kilometers from the northern border with Mexico.
As of Friday morning, no additional cases of infected animals have been reported.
Screwworm is endemic in South America and parts of the Caribbean, according to the Centers for Disease Control and Prevention. However, the insect fly has been slowly moving from Central America to Mexico since 2023.
The USDA says it is actively monitoring the fly’s movement. Last month, the USDA became aware of more than 200 cases of screwworm infestation in the border states of Nuevo León and Tamaulipas, according to the Secretary of Agriculture and Rural Development in Mexico. There are currently more than 2,000 active cases throughout Mexico.
It is believed that the New World screwworm will enter the US by 2025, “however, thanks to the hard work of the entire Trump administration and our industry, regional, and local partners, we were able to buy time for this moment,” said Dudley Hoskins, USDA’s undersecretary for Marketing and Regulatory Programs, in a statement.
The potential economic impact of New World screwworm on the cattle industry due to import restrictions, reduced production and animal losses is significant, said Sally DeNotta, director of the University of Florida’s Equine Performance Laboratory.
Last year, 175 major agricultural organizations signed a letter calling for more federal funding for worm control measures, citing USDA estimates that New World screwworm outbreaks in the US could cost producers $4.3 billion a year and cause economic losses of more than $10.6 billion in the southern United States.
“Although the fly does not survive in temperatures below freezing, infected animals can carry the virus northward and spread the infection during the summer months, and Southern California’s hot climate can support screwworms year-round,” DeNotta said.
Talbot said that from the state to the local level, everyone in the livestock community has been talking about the arrival of the worm and how to fight it.
“What I expect is that there will be a small number of cases [New World screwworm] in California,” he said.
This is because there are many stations along the Southern California border, he said, that collect information, monitor any cases of parasitic flies and catch them.
Talbot says he hopes that effective action on behalf of the federal government will reduce the reach of the screwworm and therefore impact the beef supply locally or nationally.
How worm infections are spread
Female screwworm flies are attracted to the smell of wounds — which can be as small as tick bites — and body openings such as the nose, eyes, ears and mouth where they can lay eggs, according to the CDC.
A female screwworm fly can lay 200 to 300 eggs at a time and may lay up to 3,000 eggs during her 10 to 30 day life span.
When the eggs hatch into maggots, the maggots feed on living tissue, causing a sore that becomes worse, often painful and foul-smelling, according to the CDC.
The screwworm has come to the United States before
An outbreak of hookworm in the southwestern United States in 1965 prompted Mexican and US livestock producers to sign a declaration to establish a joint hookworm eradication program in states on both sides of the Mexico-US border, according to the National Agricultural Library.
By 1966, the United States had eradicated screwworms, but livestock remained at risk of reintroduction by worms from Mexico.
Eradication was possible by using the sterile insect technique, which uses gamma rays to kill the screwworm pupae and create sterile male flies.
The US Department of Agriculture’s Animal and Plant Health Inspection Service maintains a sterilization facility for screwworm pupae in North America and is currently building a new facility in south Texas.
When produced and released in large numbers, sterile male flies mate with female flies, which then lay unfertilized eggs, according to the USDA.
“Since female screwworms only mate once, the population is dwindling and eventually extirpated,” according to USDA officials.
Last year, the Trump administration cut thousands of grants and programs from the US Agency for International Development, including animal disease monitoring projects funded by the United Nations Food and Agriculture Organization, reports Argi-Pulse Communications. Among the reduced programs were some dedicated to monitoring and containing the New World screwworm in Central America.
Today, hookworm infestations are not a common occurrence in the US, but incidents have occurred in travelers returning from areas where the flies are present, according to the CDC.
Can infected animals be treated?
Infected wounds are cleaned and debrided to remove any screwworm larvae, after which the animal is treated with an approved insecticide, DeNotta said.
Last month, the US Food and Drug Administration issued an emergency use authorization for several pesticides known to be effective against the worm.
There are approved system options and topics for a variety of animal species, including cattle, horses, small ruminants, cats and dogs, DeNotta said.
“Several days of treatment are required, and antibiotics and analgesics may be given to treat secondary infection and control pain,” he said.
If left untreated, the tissue destruction caused by carnivorous worms can be extensive and severe, often leading to the eventual deterioration and death of the host, DeNotta said.
“Heavy animals may lose weight, grow poorly and reduce production due to pain and discomfort,” she said.
Screwworm can infect humans
Human infection is rare, DeNotta said, but it can happen.
People are at risk of getting infected with screwworms if they travel to areas where the flies are present, such as South America and the Caribbean, according to the CDC.
CDC officials said your risk of hookworm infection increases when:
- Spend more time outside during the day, especially if you are sleeping or unable to block flies.
- Have open wounds. Small breaks in the skin, including from a scratch, insect bite or recent surgery, may attract screwworm flies.
- Have a medical condition that causes bleeding or open sores, such as skin or sinus cancer, or from treatments that can cause skin wounds.
- Live, work or spend more time in or around livestock or other warm-blooded animals in areas where screwworm flies are present.
Symptoms people experience when infected with screwworm
The following are the symptoms of screwworm according to the CDC:
- Hearing maggots moving or seeing maggots inside skin wounds, sores or body openings.
- Painful skin sores or sores that get worse within a few days.
- Bad smell from the area where it was attacked.
- Bleeding from open wounds.
Bacteria can also infect wounds where screwworms are present and can cause an infection that can lead to symptoms such as fever or chills.
To treat ringworm, DeNotta said, people receive the same combination of wound dressings and pesticides used on animals.
