#19,199
Since Monday, the number of New World Screwworm (NWS) cases in the U.S. has more than doubled, with cases reported in 5 different counties (Texas=4, New Mexico=1). Although most have been reported in cattle, the screwworm parasite has also been detected in 2 goats and 1 dog.
While these numbers are still small, the problem is the massive spread of the parasite in Mexico (see map below), which provides numerous opportunities for it to enter the U.S.
Not surprisingly, a number of southern tier states have raised their threat levels, and/or have strengthened their surveillance and prevention efforts, including:
While the NWS has been eradicated before in the United States by releasing sterile NWS flies, the current supply isn't sufficient to combat a multi-state outbreak. The USDA is reportedly able to disperse between 4 and 8 million sterile screwworm flies per week into the known infested counties of Texas.
New production facilities are being constructed, including a massive sterile fly production facility at Moore Air Base in South Texas, which is expected to produce 100 million sterile flies per week by November 2027 (increasing to 300 million a week by the end of 2028).
Meanwhile, much of the fly production from the COPEG facility in Panama is being used to suppress the spread in Northern Mexico, to try to prevent more incursions into the United States.
Although there is a good chance of eradicating this pest once again in the United States, it isn't going to happen overnight. The USDA's own risk analysis from 2025 reads:
In a worst-case scenario, the initial detection of NWS myiasis would not occur until 3 weeks or more after the initial introduction of NWS larvae into a United States location favorable for pupation and ongoing maintenance of the NWS life cycle.
In this scenario, a viable population of NWS flies would already exist in the United States at the time of myiasis detection in an animal or person. Releases of sterile NWS flies would be required for at least 9 to 12 weeks in order to control and eventually eradicate that population. The sterile fly releases would need to begin as soon as possible. During the 2016–2017 NWS outbreak in Florida, samples were not sent to the NVSL for identification until 84 days (4 life cycles) after presumptive positive identification by local veterinarians.
Complicating matters is the timing; with the initial outbreak detected in early June - we've 4 or 5 months of very hot weather ahead - conditions which are highly conducive to the spread of the screwworm.
While the eradication of the NWS 60 years ago in the U.S. was a tremendous achievement, this is a somber reminder that such victories are often fleeting, and that nature always bats last.
