What is the New World Screwworm?
The New World screwworm, or Cochliomyia hominivorax, is a type of blowfly. The term “screwworm” comes from the larvae (maggots), which have rows of spines that make them look like a screw as they burrow into flesh. Unlike other maggots that feed on dead tissue, screwworm larvae are obligate parasites, meaning they must feed on living tissue to survive.
The name hominivorax literally translates to “man-eating,” reflecting the parasite’s destructive nature.
The Recent Case in the US
A human case of screwworm infestation was recently confirmed in Maryland. The patient was a traveler who had recently returned from a region in Central America, specifically El Salvador, where there is an ongoing outbreak among livestock. The individual has since recovered.
This case is particularly notable because the New World screwworm was considered to have been eradicated from the United States in the 1960s and 1970s through a large-scale program that involved releasing billions of sterile male flies.
How Does an Infestation Occur?
The life cycle of the screwworm is as follows:
- Egg-laying: A female screwworm fly is attracted to an open wound or a natural body opening (like the nose, eyes, or ears) on a warm-blooded animal or person. She lays a cluster of 100-350 eggs at the edge of the wound.
- Hatching: The eggs hatch into larvae within 12 to 24 hours.
- Feeding: The larvae burrow head-first into the living tissue, where they feed and grow. This process causes significant pain and tissue destruction.
- Pupation: After about 5-7 days of feeding, the mature larvae drop from the wound to the ground, where they burrow into the soil and pupate.
- Adult Emergence: An adult fly emerges from the pupa after about a week and a half, ready to begin the cycle again.
Symptoms and Treatment
Symptoms of a screwworm infestation in humans include:
- A painful, non-healing wound that continues to get worse.
- The sensation of movement under the skin.
- A foul odor coming from the wound.
- Visible maggots in or around the wound.
If left untreated, an infestation can cause severe tissue damage, secondary bacterial infections, and can even be fatal. Treatment involves the careful and complete removal of all larvae, which is often done through surgery. It is crucial to seek immediate medical attention rather than attempting to remove the larvae yourself.
Public Health and Agricultural Concerns
While the risk to the general public in the U.S. is considered very low, the re-emergence of this parasite is a significant concern for the country’s livestock industry. The screwworm poses a serious threat to cattle, horses, and other animals. An outbreak could be devastating to the economy, potentially causing billions of dollars in losses.
Federal agencies like the USDA and the CDC are working to prevent the parasite from gaining a foothold in the U.S. and are coordinating with health officials in affected regions to monitor and contain its spread.
