New World Screwworm Parasite
The United States recently confirmed its first human case of New World screwworm infestation. The case involved a traveller from El Salvador and was identified in early August 2025. Authorities say the risk to public health remains low. This event marks the potential reemergence of a parasite once eradicated from the region.
What Is New World Screwworm?
- New World screwworms are blue-grey blowflies native to South America and the Caribbean.
- Female flies lay eggs on open wounds or natural body openings of warm-blooded animals and rarely humans.
- Each female can lay up to 3,000 eggs during her lifespan of 10 to 30 days.
- The eggs hatch into larvae, also called maggots, which burrow into living flesh, feeding on tissue.
- The larvae later drop to the soil and mature into adult flies.
- The name “screwworm” comes from the larvae’s screw-like burrowing action.
Symptoms and Effects
Infestation causes painful wounds that often worsen over time. Symptoms include non-healing sores, bleeding, movement sensations in the wound, and foul odours. If untreated, larvae can invade sensitive tissues such as the brain. This can lead to severe infections like sepsis and has a high mortality risk. The infestation also attracts more flies, increasing the severity and spread of the condition.
Historical Eradication in the United States
The US eradicated New World screwworms in 1966 using the sterile insect technique. This method involved breeding billions of sterile male flies and releasing them to mate with females. Mating with sterile males prevented reproduction, breaking the parasite’s lifecycle. This technique also helped eliminate screwworms in Mexico and Central America by the early 2000s. In 2017, it was used again to control a small outbreak in Florida.
Reasons for Recent Reemergence
New cases have appeared in Panama, Costa Rica, Nicaragua, and Honduras recently. Experts suspect that the parasite’s return is linked to the movement of infested livestock across borders. Another concern is that the sterile insect technique may be less effective against the current screwworm strains. These factors combined pose challenges for controlling the parasite’s spread in the Americas.
Public Health and Control Measures
While the current risk in the US is low, surveillance and prompt diagnosis are critical. Awareness among travellers and healthcare providers helps early detection. Continued use and improvement of sterile insect release programmes remain vital. Cross-border cooperation is also necessary to manage livestock movement and reduce infestation risks.
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