What is New World Screwworm and could it spread to humans?

Metro

02-06-2025

A flesh-eating parasite known as New World Screwworm is moving closer and closer to the US border in Texas.
The devastating parasite eats cattle and other animals alive and has travelled north from Central America to Mexico.
In response to the spread within 700 miles of the Texas border, Washington halted cattle imports from Mexico in May.
But what is New World Screwworm and can it spread to humans?
Screwworms are parasitic flies whose females lay eggs in wounds on any warm-blooded animal.
Once they do this, the eggs hatch and hundreds of screwworm larvae burrow through living flesh using their sharp mouths.
This helps them to feed on and make the wound bigger, which eventually kills the host if left untreated.
It means that when screwworms infect a cow, something like a tiny scrape, a recent brand or a healing ear tag can quickly become a gaping wound.
Screwworms were eradicated from the US in the 1960s after researchers released massive numbers of sterilised male screwworm flies that mate with wild female screwworms to produce infertile eggs.
New World Screwworm typically impacts livestock but it can also affect humans.
Dr Sonja Swiger, entomologist at Texas A&M University, told Metro: 'While it is rare for humans to get an infestation of NWS, it is not impossible.
'The flies are known to lay their eggs on any warm-blooded animal, but prefer livestock and wildlife over birds and humans, with dogs being another known target.
'One main reason humans are on the rare side is that most will keep their wounds clean or follow hygiene practices that might exclude them.'
Dr Timothy Goldsmith, a veterinary medicine professor at the University of Minnesota, said homeless people are especially vulnerable to infestation.
This is because they sleep outside and have less access to hygiene products and medical care.
Dr Swiger recommended that if people are travelling to areas dealing with an outbreak or endemic for NWS, they should protect themselves and animals they may bring with them from attacks.
This includes doing checks on animals before entry/re-entry into countries that are not endemic for NWS.
'The fly can only travel 10-20 km on its own, so animal movement or humans moving with infestations is of concern,' she said.
Countries where NWS is endemic include Cuba, Haiti, the Dominican Republic, and countries in South America, with cases spreading north to Costa Rica, Nicaragua, Honduras, Guatemala, Belize, El Salvador, and Mexico.
Dr Swiger said that it is not yet clear what is behind the drive north, though animal movement is a major concern.
A factory designed to breed and sterilise screwworms in Panama is releasing 100 million sterile flies every week, but the flies have already passed through the narrowest stretches of land in Panama and Mexico.
Screwworms cannot fly more than 12 miles on their own, but they can cover large distances while burrowed inside their hosts.
The spread to the US would cause problems with livestock population and economic concerns. More Trending
'The largest impact would be to the livestock and wildlife industries if NWS were to return to the United States,' Dr Swiger said.
'That is currently being estimated in the multi-billions of dollars. With an estimated impact of up to $2 billion for livestock and $9 billion for wildlife.'
It was announced by the US Department of Agriculture on Tuesday that it would invest $21million (£15.6million) to convert a fruit fly factory in Mexico to produce sterile screwworms.
It said the border would likely reopen to cattle imports by the end of the year.
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