UNIVERSITY PARK, Pa. — Over the past two weeks, there have been 12 reports out of Texas and New Mexico of new cases of the New World screwworm — a parasitic fly once declared eradicated in the U.S.
Female screwworms lay their eggs in the wounds of mammals, which then serve as a food source for larvae. If left untreated, infections can be fatal. Screwworms can infest any warm-blooded animal, including wildlife, livestock and horses, and domestic animals like dogs, but rarely humans.
In this Q&A, Erika Machtinger, associate professor of entomology in Penn State's College of Agricultural Sciences, discussed the state of the current outbreak — and what risks the insect poses to Pennsylvania and beyond. Penn State Extension is also providing updates and resources about the outbreak.
Q: What is the New World screwworm and what danger does it pose?
Machtinger: New World screwworm is not actually a worm; it is the larval stage of the fly species Cochliomyia hominivorax. Unlike typical maggots that feed on dead tissue, screwworm larvae feed on living tissue. Female flies lay eggs in wounds, surgical sites, navels of newborn animals, antler velvet injuries, tick bites or other openings. After hatching, the larvae burrow into the flesh like a screw, causing rapidly enlarging wounds, severe pain, secondary infections and potentially death, if untreated.
The greatest concern is for animal health and livestock production, as infestations can spread rapidly if not detected and treated promptly.
Q: What’s the state of the current outbreak and how is the U.S. Department of Agriculture (USDA) responding?
Machtinger: New World screwworm was eradicated from the United States in the 1960s, and since then, permanent populations have been contained south of the biological barrier in Panama. Although there have been occasional reinfestations over the years, the parasite has recently spread north through Central America and Mexico and has now re-entered the southern United States. USDA and state animal health officials have responded aggressively through surveillance, quarantines, movement controls and the release of sterile screwworm flies.
The sterile insect technique is the same strategy that successfully eradicated screwworm from the United States decades ago. Sterile male flies are released into affected areas, where they mate with wild females that generally only mate once in their lifetime, resulting in no offspring and gradual population suppression. USDA has expanded production and release efforts while increasing surveillance and response activities in affected areas.
Q: Does screwworm pose a threat to Pennsylvania?
Machtinger: At present, the threat to Pennsylvania remains low. There are no confirmed cases in Pennsylvania, and current detections are concentrated in the southwestern United States.
The risk is low for two main reasons. First, Pennsylvania is far from the current outbreak area, so natural spread to the Northeast is unlikely in the near term. Second, New World screwworm is a warm-climate fly. Its ability to survive, reproduce and establish populations is limited by temperature, especially cold winter conditions. Pennsylvania’s seasonal climate makes long-term establishment much less likely than in warmer southern regions.
The greatest risk to Pennsylvania would come from the movement of infested animals from affected areas rather than natural spread. Animal owners should know where livestock, horses, pets and other animals are coming from, especially if they were recently transported from the South or from areas under animal health restrictions. The Pennsylvania Department of Agriculture has also recently issued a quarantine order, limiting animal movements and urging tightened biosecurity measures to protect livestock and pets.
Even though the current risk is low, early detection matters. Owners should monitor animals for unusual wounds, wounds that worsen rapidly, foul odor, visible maggots, or signs of pain and irritation, and contact a veterinarian or animal health official if they see anything suspicious.
Q: What does the average citizen need to know?
Machtinger: For most Pennsylvanians, there is no reason for alarm. New World screwworm is primarily an animal health issue, not a public health emergency. The food supply remains safe. Meat and milk sold through regulated food systems undergo inspection and food safety safeguards, and New World screwworm does not make properly inspected meat or dairy products unsafe to consume.
The most important thing the average citizen can do is stay informed through trusted sources, such as USDA Animal and Plant Health Inspection Service, the Pennsylvania Department of Agriculture, veterinarians and university Extension programs. Be cautious of rumors or misinformation on social media, especially claims that are not supported by animal health authorities.
Q: What do animal owners need to know?
Machtinger: Animal owners should pay closer attention, especially when purchasing, transporting or receiving livestock, horses, pets or other animals from affected regions. Know where animals are coming from and stay aware of any current animal movement restrictions or quarantine requirements.
Inspect animals regularly for wounds, especially wounds that are worsening, have a foul odor, contain maggots or are not healing normally. Maintain good fly control and animal husbandry practices and contact a veterinarian promptly if anything looks suspicious. Early detection and rapid reporting are the best tools for preventing spread.