On June 3rd, the U.S. Department of Agriculture announced that an animal infected by the larvae of the New World Screwworm fly had been detected and confirmed in a calf in southern Texas, the first such case found in the United States since 1966. Four additional cases have been detected in the five days since then, three of them in Texas. Two of the newer Texas cases were also calves (like the first), and the third was in a goat. The fourth new case was in a dog residing in New Mexico but diagnosed by a veterinarian in Texas.
The NWS fly attacks warm-blooded animals by laying eggs in open wounds and moist membranes, and then when the eggs hatch into larvae within a day, those larvae burrow into the living flesh of those animals in order to feed themselves. It represents a highly significant threat to beef and dairy cattle, although it has also been known to attack other animals, both mammals and fowl, including humans. There is no known vaccine to prevent infestation, although the U.S. Food and Drug Administration conditionally approved a drug last fall that is intended to help treat NWS infestation in cattle.
The New World Screwworm fly was one of the 16 biosecurity threats to U.S. farmers and ranchers that were profiled in a policy paper commissioned by the Farm Journal Foundation and released in December of last year. At the time of publication, the NWS fly was included in a section describing pests and diseases deemed to be ‘looming threats’ to U.S. agriculture, due to either not yet having been detected within U.S. borders in recent decades (a list that also included African Swine Fever and Foot and Mouth Disease) or that had not yet established a foothold in a major producing area (the fungal disease Asian Soybean Rust, which has been largely limited to modest infestations in the southern United States since it was first detected in November 2004). If that paper were to be written today, the NWS fly would no longer be classified as a ‘looming threat’. That threat is now quite real and immediate.
In a press conference held on June 8 in Kerrville, TX, Secretary of Agriculture Brooke Rollins described the steps that are being taken to combat this outbreak, across several agencies of the federal government and in cooperation with the state government in Texas. She was joined at the press conference by Texas Governor Greg Abbott, and several other officials from across USDA and the relevant Texas state agencies, as well as a representative of the Texas cattle industry.
The primary tool to be utilized for now is to conduct targeted dispersals of sterile male NWS flies, a process which was successful in ending the last NWS outbreak in the United States in the 1960’s. Sterile flies are produced by irradiating the insects so they can no longer successfully engage in reproduction. The sterile male flies mate with female NWS flies which then yield no offspring, greatly reducing the next generation of NWS flies and larvae. Unfortunately, it will take considerable time to increase production levels of sterile flies to the levels needed to blanket-disperse them in the appropriate regions. Right now, the U.S. government’s supply is limited to the 100 million flies provided per week by a joint U.S-. and Panamanian-funded facility in Pacora, Panama, far short of the amount needed to achieve complete eradication. A sterile fruit fly production facility in Mexico is currently being converted to sterile NWS fly production, and the U.S. military is helping to construct a new production facility in Edinburgh, TX. Those facilities are projected to be in operation by early in 2027, so Secretary Rollins spoke about the spring of next year as the target period for when USDA hopes to have the NWS outbreak fully contained.
The other main action that needs to be taken is for cattle owners to assiduously monitor their animals for any signs of NWS infestation on a regular basis and report any cases to designated authorities, a responsibility which both Governor Abbot and Secretary Rollins urged those producers to undertake. According to USDA estimates, there were just over four million head of beef cattle in the state of Texas at the beginning of 2026, so this will not be an easy ask for farmers and ranchers. In addition, considering that two of the five NWS cases identified thus far were not cattle, this request may need to be made to all animal owners in Texas as well as New Mexico, not just cattle owners. There are concerns that some cattle ranchers may be reluctant to report potential NWS cases they detect, for fear of having their operation quarantined.
The FJF biosecurity paper included a number of policy recommendations, and a few of them seem quite relevant for the current NWS outbreak in Texas. They include a recommendation to bolster international cooperation, both in efforts to monitor the spread of NWS flies and implement sterile fly dispersal when appropriate, and also in research efforts to develop new technologies to address this threat. When the U.S. Agency for International Development was dissolved early last year, among the programs that were eliminated were those that fostered such cooperation, including U.S. funding for an FAO project that addressed this very issue. In addition, the paper recommended increasing training for rural veterinarians and extension agents to make sure they would know how to identify a new pest or disease outbreak, such as a NWS larvae infestation, and doing more to educate the general public about the nature of biosecurity threats and how they can help minimize it by paying attention to what they bring back across the border after international travel. If the initial reports that the infected dog in New Mexico spent time recently in Mexico are confirmed, then this would be an example of what not to do.
