The New World screwworm (NWS), caused by the larvae of the Cochliomyia hominivorax fly, affects the flesh of warm-blooded animals and, in some cases, humans. These flies are drawn to open wounds, where they lay their eggs, leading to tissue infestation. While NWS is not commonly found or spread in the United States, it is more prevalent in regions like South America and the Caribbean. Travellers to these areas—especially those who work with livestock, sleep outdoors, or have exposed wounds—are at higher risk of contracting an infestation.
Recently, the New World Screwworm has seen outbreaks in Central America and Mexico, which have spread into global outbreaks after the United States confirmed its one human case, which is linked with travel.
What is New World Screwworm?
The New World Screwworm is a fly larva (maggots) that feeds on living flesh. These parasites can infect any warm-blooded animal, including humans. If untreated, screwworm infestations can be fatal.
What are the Life-Cycle of the New World Screwworm?
Source: cdfa.ca.gov
The life cycle of the New World Screwworm is given below:
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Adult flies copulate.
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A gravid female fly searches for a host with a wound or orifice.
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A gravid female fly lays eggs on a wound or orifice.
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Eggs hatch in 12–24 hours.
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Larval Stage (L-1, L-2, L-3): larvae feed on tissue for 5–7 days.
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L-3 larvae fall from the wound or orifice and bury themselves in the ground to pupate.
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The pupae stage lasts 6–8 days, then transitions into the pupa stage, and the fly emerges.
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The entire cycle can be as fast as 21 days (depending on temperature, moisture, and soil).
What are the Geographic Distribution of the New World Screwworm?
The Geographic Distribution of the New World Screwworm are:
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The Cochliomyia hominivorax (New World screwworm) has been moving north after being pushed past the Darien Gap in Panama.
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Recently, the New World Screwworm has been detected in Oaxaca, Mexico.
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It is mainly found in many areas of South America and some Caribbean Islands.
How does the New World Screwworm take transmission?
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Adult flies live up to 10–14 days.
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Usually fly short distances (6–9 miles / 10–15 km) when hosts and suitable conditions are present.
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Can travel up to 125 miles, though long-distance spread is more likely via movement of infested animals.
How does the New World Screwworm spread among humans?
New World screwworm infestations start when a female fly lays eggs on open wounds or soft tissues of warm-blooded animals. While livestock are the most common hosts, birds and even humans can also be affected.
Female flies are drawn to the odour of wounds or natural openings such as the nose, mouth, eyes, the umbilical cord of newborn animals, or the genital area. Even a tiny injury, like a tick bite, can be enough to attract them.
A single female fly can deposit 200–300 eggs at once, and over her short lifespan of 10 to 30 days, she may lay up to 3,000 eggs.
Once the eggs hatch, the larvae (maggots) burrow into the wound and feed on living tissue. After completing this stage, they drop to the ground, dig into the soil, and later emerge as adult screwworm flies—ready to repeat the cycle.
Conclusion
The New World screwworm poses a serious threat to livestock, wildlife, and humans, especially in tropical and subtropical regions. Its rapid reproductive cycle and ability to infest even minor wounds make prevention and early detection vital. Strengthening surveillance, veterinary care, and traveller awareness is essential to curb outbreaks and reduce risks.
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