A parasitic pest once eradicated from the United States has made an alarming comeback. A calf near the Texas Mexico border has been confirmed as the first US livestock case of the New World screwworm in six decades, triggering immediate quarantine measures and a coordinated response from federal and state agriculture authorities.
What Is the Screwworm?
The New World screwworm is the larvae of a parasitic blowfly that feeds on living flesh rather than decaying tissue. Once it infests an open wound in a warm blooded animal, the larvae burrow deeper, consuming muscle and tissue and causing severe injury or death if untreated. Cattle, sheep, horses, and even humans are all vulnerable. A single wound can harbor hundreds of larvae, and females can lay hundreds of eggs at a time, causing infestations to escalate rapidly.
How the US Eradicated It Before
Before eradication, the screwworm caused an estimated $20 billion in annual damages to the U.S. livestock industry in today’s dollars The USDA eliminated it in the 1950s and 60s using the Sterile Insect Technique mass rearing and sterilizing male screwworms using radiation, then releasing them into infested areas. When sterile males mated with wild females, eggs were infertile, collapsing the population over successive generations. The continental US was declared screwworm free in 1966.
How This Case Was Found
The infected calf was discovered in Hidalgo County, directly bordering Mexico, where the screwworm is still present but managed through a buffer zone program. Authorities have not disclosed how the calf became infected, though the border location raises concerns about cross border transmission. State and federal agencies immediately quarantined the affected premises and launched surveillance of surrounding livestock.
What This Means
Authorities plan to deploy the Sterile Insect Technique again in coordination with Mexican counterparts, as fly movement does not respect borders. Wildlife populations including deer are also at risk, complicating surveillance efforts. The case highlights the ongoing threat of agricultural pests crossing US borders and the need for continuous biosecurity monitoring and serves as a warning for ranchers to report any suspicious animal wounds immediately.
