Health officials in California are sounding the alarm about a parasite that hasn’t been seen in the state for decades but could soon return. The New World screwworm, once a major threat to both humans and livestock, has reappeared in parts of Central America and was recently detected in a U.S. traveler.
This tiny but destructive organism poses risks far beyond its size. Here, we’ll look at what the parasite is, why experts believe it could resurface in California, and what doctors and travelers need to know about prevention.
What is the New World screwworm
The New World screwworm is the larval stage of a fly that targets open wounds. Adult females lay eggs in even the smallest cut, from a razor nick to a scratch. Once the eggs hatch, the larvae burrow into flesh, causing painful sores that can expand quickly if left untreated. Dr. Monica Gandhi, an infectious disease specialist at UC San Francisco, described it bluntly: “It’s so gross […] It can really be very painful”.
While single cases in humans are concerning, the larger risk lies with animals. Cattle, sheep, and other livestock are especially vulnerable. When the parasite spread through the American South in the early 20th century, ranchers lost huge portions of their herds.
The U.S. Department of Agriculture (USDA) eventually launched eradication programs in the 1960s, releasing sterilized flies to wipe out breeding populations. By 1966, the screwworm was declared eliminated from the United States.
But eradication efforts have not been successful everywhere. In recent years, outbreaks have been reported in Mexico and Central America, raising the possibility of reintroduction to the U.S. through cross-border animal movement or infected travelers. A case this summer in Maryland highlighted how easily the parasite can find its way back.
Warnings and precautions for Californians
The California Department of Public Health (CDPH) has issued an alert urging doctors to recognize the telltale signs of screwworm infection. These include non-healing wounds, open sores, and visible larvae. Because the parasite can progress quickly, early diagnosis is critical. Physicians are also being asked to report suspected cases immediately, as public health officials want to prevent the insect from establishing itself again.
Travelers are being advised to take simple precautions. Gandhi notes that bug spray is not the main defense. Instead, covering any wounds is key. Even small cuts should be cleaned and protected when traveling in areas where the parasite is present. This advice is especially important for people working around livestock, as herd infestations can spread rapidly and be devastating for ranchers.
Federal authorities are also on alert. The USDA recently announced new efforts to support Mexico in controlling screwworm populations. The strategy is designed to create a buffer zone that keeps the parasite from crossing north into the United States. Given California’s large agricultural industry and international travel hubs, the state is considered particularly vulnerable if screwworm populations were to return.
The possibility of the New World screwworm resurfacing after nearly 60 years is a reminder that global travel and shifting ecosystems can reintroduce threats once thought eliminated. Covering wounds and staying aware while abroad are the most effective defenses against a parasite that is small, painful, and—as Gandhi put it—”gross”.