Ag Minute: New World Screwworm - A Threat to U.S. Beef Supply
- Sangamon County News
- Sep 19
- 4 min read
New world screwworm is a three word term that no beef producer wants to hear. The term evokes fear, concern and flashbacks to outbreaks like the one in Texas in 1976. For over a half-century, the United States has led a charge in eradicating the pest, which has mostly been successful, aside from recurrences in Texas in the 1970s and an outbreak amongst Key Deer in the Florida Keys in 2016-2017. Recent outbreaks in South America and Mexico have placed U.S. authorities on high alert. In fact, a human case of new world screwworm was confirmed in Maryland on August 4, 2025 in an individual who had traveled to El Salvador. The harm that screwworms cause both physically and economically is immense which has led to large investments to ensure their eradication.
Cochliomyia hominivorax, more commonly known as new world screwworm, is a parasitic fly that is native to the Western Hemisphere. Warm blooded animals with open wounds are the victims of the pest, with flies laying eggs in those wounds where larvae to grow. The larvae (maggots) feed on animal flesh in open wounds, leading to immense pain, a rotting smell, and commonly death if left untreated. Treatment involves the use of larvicides to kill larvae in open wounds.
Considered endemic in nations like Cuba, the Dominican Republic, Jamaica, and much of South America, fortunately new world screwworm has not had an impact on the United States in a long time. There is documented evidence of the existence of new world screwworm in the United States since 1842, with efforts being taken to reduce their impact starting in 1935 according to the USDA’s Animal and Plant Health Inspection Service (APHIS). In 1951, scientists in Florida developed a sterilization method for male flies, using radiation. An associated eradication method called the “sterile insect technique” (SIT) followed. SIT involves the mass release of sterile male flies. Once a female new world screwworm mates with a sterile male, it will not mate again, so SIT ensures no new offspring are produced. SIT proved to be an effective way to manage new world screwworm beginning in the Southeast, then the Southwest. By 1966 eradication was considered complete in the U.S. and an eradication border had been established with Mexico. Unfortunately, this border did not last very long and resulted in Texas being forced to spend heavily on both treatment of infected animals and eradication in the 1970s that culminated in the creation of the Joint Mexico-U.S. Screwworm Eradication Commission to drive the border even further south to Mexico’s southern border. Since that time, the SIT method has been very effective and largely kept the pest population south of the Darien Gap in Panama until 2023, when it was observed that new world screwworm had begun moving north again.
Economic gains from the successful eradication of new world screwworm have been massive, with a 1996 report estimating that the U.S. economy benefited from the eradication efforts by $2.8 billion per year. With today’s national cattle herd being at the lowest point since the 1950s, one can only imagine that an infestation of the pest would cause massive economic losses and supply chain struggles for beef in the United States. An August 2025 letter from livestock and dairy producer associations to USDA Secretary Brooke Rollins cited estimates that a contemporary outbreak in Texas could result in $1.8 billion in annual losses. The USDA has not been idle on the new world screwworm issue. In June 2025, the USDA announced a five-pronged approach to ensure the safety of American borders from screwworm. The plan includes augmenting the production of sterile flies in several ways. A sterile fly production facility in Panama working in partnership with the USDA is now committed to producing 100 million sterile flies weekly as opposed to the 20 million that were being produced prior. A fruit fly facility would be retrofitted in Metapa, Mexico to support sterile screwworm fly production. Significantly a new domestic sterile fly production facility is being constructed at Moore Air Base in Edinburg, Texas, with a goal of producing 300 million sterile flies per week.
Not only does new world screwworm pose a threat to the American beef supply, it could also have profound effects on other warm-blooded species, including pigs, sheep, horses and game animals. Given the devastating effects of the pest that were demonstrated prior to eradication, both the agriculture industry and the government have decided that it is worth spending significant amounts of money and effort to alleviate. While fears are mounting as the fly spreads northward, there is cause for optimism because it is clear that the USDA and broader industry in the U.S. are committed to preventing this devastating pest from ever returning.
For information on the facts of new world screwworm, follow the link bellow to Texas A&M Extension services factsheet;
For information relating to the timeline of events and the economic impact of new world screwworm, follow the link to APHIS’s site;
For information relating to the USDA’s commitment and 5 pronged approach to keeping new world screwworm out of the U.S., follow the link below to a USDA press release;
