The New World screwworm lays its eggs in open wounds and burrows into skin. While human infections are rare, the insect poses an existential threat to cattle farming and dairy production. And it has returned to the United States.
While the U.S. Department of Agriculture officially declared the parasite eradicated from the country in 1966, several notable outbreaks and incursions have occurred since then. One of those is happening now.
On Wednesday, the USDA confirmed a case in South Texas. It could decimate that state’s $15 billion cattle industry, and poses a similar threat to California, where dairy and cattle lead the agricultural economy. Here, UC Riverside entomologists answer some of the questions naturally arising from such a threat.
New World screwworm larvae.
Amy Murillo (AM) is an associate professor of entomology and is principal investigator of a project funded by the California Department of Food and Agriculture to monitor the state for early signs of the fly’s return. She is collaborating on the project with Alec Gerry (AG), a UC Riverside veterinary entomology professor.
Q: In December 2025, the fly was detected within 70 miles of the Texas border. Now that it is definitely in Texas, what tools does the U.S. have to combat its spread?
AG: The USDA and California, as well as other border states, have been preparing for the possible reinvasion of screwworm for many months. The USDA has developed a robust screwworm response plan which is now being implemented in Texas.
AM: The USDA has been releasing sterile screwworm flies throughout the affected region for weeks now. This is still the most effective area-wide New World screwworm control that we have. Adult females only mate once, so if we can flood the area with sterile males then we increase the chance of stopping fly population growth.
Sterile fly release, animal quarantine/inspection, and surveillance are all tools that have been used to slow the spread. In the confirmed case, the maggots were caught early, so inspection worked so those affected animals can be treated and quarantined.
AG: In California we are also pursuing additional tools for control of screwworm including additional wound treatments and insecticides that can receive emergency use authorization for control of screwworm.
While the reported case in Texas is the only case identified in the U.S. thus far, the very rapid spread over the last two years suggests that those who own or work with animals should be particularly vigilant in watching for and reporting potential screwworm larvae (maggots) in animal wounds.
Q: What can you tell us about the time it takes for sterile fly release to become effective?
AG: The sterile males should start controlling the wild fly populations almost immediately following their release, as the adult males begin to mate with wild females. But if your question is when will the sterile flies provide COMPLETE control of the wild New World screwworm population in the released area, that is a bit more complicated. There is no guarantee that in fact we can get complete control or that the flies have not already escaped the quarantine zone where the sterile males are being released.
What is important is that within the known affected area, there will be enhanced surveillance for additional evidence of the parasite, and the sterile male release as well as other measures such as animal wound treatment that is expected to continue until the fly or larvae have not been detected for 21 days, according to the USDA New World screwworm Response Playbook.
Q: What is the likelihood of it reaching California?
AM: It is always possible that an infected animal or person can be moved into the state from an active infestation zone.
Q: In addition to the economic impacts on dairy and beef cattle, what are some of the consequences if screwworm comes here?
AM: The insect can infest any warm-blooded animal, including pets, birds, and humans. There will also be potential negative consequences to wildlife and other at-risk populations.
Q: Is there anything the average individual can do to reduce the likelihood of it reaching this state?
AM: Report any suspicious maggots found in wounds to the CDFA at (916) 900-5002 or the USDA at (916) 854-3950 so that they can be tested. More information is available on either the CDFA or the USDA websites.
Dr. Gerry and I are currently working with California groups to develop and disseminate information about the threat, as well as conducting adult fly surveillance throughout Southern California.
