Health officials in the United States have confirmed the nation’s first human case of New World screwworm—a flesh-eating parasite notorious for its devastating effects on livestock—in a Maryland patient who recently returned from El Salvador. The confirmation, announced by the Centers for Disease Control and Prevention (CDC) and the Maryland Department of Health on August 4, 2025, marks a rare occurrence that has drawn the attention of federal and state agencies, as well as the agricultural community.
According to the U.S. Department of Health and Human Services (HHS), the case was travel-related, with the patient likely acquiring the parasite during their stay in El Salvador, where an ongoing outbreak of New World screwworm has been impacting both animals and humans. Reuters reported that the individual has since recovered, and Maryland health officials stated on August 25, 2025, that there was no indication of transmission to other individuals or animals. "The risk to public health in the United States from this introduction is very low," HHS spokesperson Andrew G. Nixon told Reuters, emphasizing that the event remains isolated and does not pose a significant threat to Americans, including those in states like Michigan.
The New World screwworm, known scientifically as Cochliomyia hominivorax, is a housefly-sized parasite whose larvae feed on the living tissue of warm-blooded animals. While the parasite primarily targets livestock, humans can also become infested, typically when the female fly lays her eggs in an open wound or mucous membrane—such as the nose, mouth, or eyes—of a host. According to the CDC, the eggs hatch into larvae, or maggots, which burrow into the wound and consume healthy tissue, causing a painful condition known as myiasis. If left untreated, the host may die within one to two weeks.
Symptoms of screwworm infestation include the presence of maggots around open sores, wounds that do not heal or that worsen over time, bleeding, foul-smelling odors from the infestation site, and even the sensation of larvae moving within a wound or in facial orifices. The CDC warns that even wounds as small as a tick bite can attract a female screwworm fly, which is capable of laying 200 to 300 eggs at once.
Historically, New World screwworm infestations have been a major threat to livestock industries across the Americas. The parasite was eradicated from the United States by 1966 through an ambitious program that involved flying planes over affected areas and dropping boxes of sterile flies, a technique designed to disrupt the reproductive cycle of wild screwworms. The U.S. Congress notes that Mexico achieved eradication in the 1970s, and Central American countries followed suit in the early 2000s. However, recent years have seen a resurgence of the parasite in Panama, Costa Rica, Nicaragua, and Honduras, despite previous eradication efforts.
Federal officials remain vigilant as the screwworm spreads northward through southern Mexico, raising concerns about the potential impact on the Texas cattle industry. In response, the U.S. Department of Agriculture (USDA) has taken proactive measures to prevent the parasite’s return. In June 2025, Agriculture Secretary Brooke Rollins announced the construction of an $8.5 million sterile fly dispersal facility at Moore Air Base, just 20 miles from the Mexican border in Edinburg, Texas. The facility, expected to be operational by the end of 2025, will allow the USDA to aerially release sterile male screwworm flies along the border and in northern Mexico. These sterile males will mate with wild females, resulting in no offspring and gradually reducing the population.
"The United States has defeated NWS before and we will do it again," Secretary Rollins declared, as reported by the Michigan Farm Bureau. "We do not take lightly the threat NWS poses to our livestock industry, our economy, and our food supply chain." The agency’s swift action underscores the seriousness with which officials regard even isolated cases of the parasite, given its potential to wreak havoc on agriculture and public health.
To further bolster the nation’s defenses, just last week the HHS issued a declaration that allows the U.S. Food and Drug Administration (FDA) to grant Emergency Use Authorizations for drugs to treat or prevent screwworm infestations in animals. This move ensures that veterinarians and livestock producers have access to the necessary tools should the parasite be detected within U.S. herds.
Despite these precautionary steps, the risk to the general public remains minimal. As of August 2025, there have been no reported human or animal cases of New World screwworm in Michigan or other states beyond the travel-related incident in Maryland. The CDC and HHS continue to stress that screwworm infestations do not regularly occur or spread in the U.S., and that people who travel to endemic regions—especially those spending time around livestock, sleeping outdoors, or with open wounds—are at the greatest risk of exposure.
For those who might be worried, experts point out that the eradication of the parasite in the United States nearly six decades ago stands as a testament to the effectiveness of coordinated public health and agricultural interventions. The sterile fly technique, which proved instrumental in the original eradication campaign, remains the cornerstone of the current USDA strategy. By targeting the reproductive cycle of the screwworm, officials hope to prevent its reestablishment in the U.S. and protect the nation’s vital livestock industry.
The Maryland case, while alarming in its rarity, serves as a reminder of the interconnectedness of global health and agriculture. With outbreaks continuing in parts of Central America, the risk of travel-associated cases will persist. Vigilance at ports of entry, robust surveillance, and rapid response capabilities all play a crucial role in maintaining the U.S. as a screwworm-free zone.
As the USDA’s new facility nears completion and emergency authorizations for treatments are rolled out, federal and state agencies remain on high alert. For now, Americans can rest assured that the threat from New World screwworm is being met with the full force of science, government coordination, and lessons learned from past victories over this formidable parasite.
