Countries across the Americas have spent billions trying to control the New World screwworm since the flesh-eating parasite devastated cattle in the US in the 1950s.
But a recent surge in infections in Central America suggests that containment efforts could be failing, imperilling the region鈥檚 food supply and tens of billions of dollars鈥� worth of cattle.
What is screwworm?
The New World screwworm fly (Cochliomyia hominivorax) is an insect endemic to South America and the Caribbean.
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The female fly lays hundreds of eggs in the open wounds of warm-blooded animals. The larvae hatch into maggots within 24 hours, then burrow into the wound to feast on the living flesh, sometimes eating the host alive.
Theon eradicating the fly after it devastated the country鈥檚 cattle in the 1950s, and estimates its absence saves the nation鈥檚 livestock industry around .
Is it dangerous to humans?
Screwworm can colonise all warm-blooded animals 鈥� including humans and their pets 鈥� through open wounds or body cavities with mucous membranes, such as the nasal passage. The infection is painful and can be deadly in people as there is no medication to treat it. Quick removal is key.
Infections of people are fortunately rare compared with cattle, dogs and sheep. Costa Rica has recorded 39 cases in humans since July 2023, comprising 0.34 per cent of all cases. of screwworm infections in June 2024, the first fatalities since the 1990s.
As well as farm animals, humans and pets, there is concern for wildlife.聽Cases in sloths, howler monkeys and tapirs, but it is difficult to know the extent or impact of these infections given they are harder to track.
Where has it been occurring recently?
Central America is facing its worst outbreak of screwworm in decades, having wiped out the pest in the 1990s and 2000s.
Panama usually sees tens of cases each year, but, as of 23 November, it had recorded more than
There have also been 8674 recorded cases in Costa Rica, 5978 in Nicaragua, 71 in Honduras and 18 in Guatemala. Prior to this, those countries since the 1990s.
The disease has spread north through Central America in a few months and the first case in Mexico was reported in the southern state of Chiapas on 22 November.
Why is it making a comeback?
A leading hypothesis is that the Dari茅n Gap, a 100-kilometre stretch of dense rainforest connecting Colombia with Panama, is no longer functioning as effectively as a natural barrier between South and Central America.
The surge in screwworm cases coincides with a spike in migration heading north through the Dari茅n Gap, with half a million people taking the week-long trek in 2023. The unprecedented flow of migrants could be helping to carry the fly into Central America.
Another factor could be that COPEG, the scientific programme run by the US and Panama to create a barrier against the fly, is failing. Every week, the programme drops millions of flies that have been sterilised with radiation into Central America. Releases are concentrated around the Dari茅n Gap to stop population growth and prevent the small number of flies that make it through the jungle continuing north.
The sterile male flies may have lost their effectiveness, says at North Carolina State University. 鈥淭he thinking is that the strain that was being used was starting to weaken. Over time, it accumulates deleterious mutations and needs to be replaced periodically,鈥� he says. 鈥淭here could be a lot of factors. Maybe they鈥檝e become less attractive to the females that are out there, they don鈥檛 live as long or they don鈥檛 fly as far.鈥�
Once the screwworm got past the COPEG barrier, it probably hitched a ride on Central America鈥檚 booming illegal cattle trade, which is used by cartels to launder drug money, says at the Wildlife Conservation Society. The fly appears to have moved slowly to Costa Rica, then accelerated to Nicaragua and travelled 1100 kilometres from the Nicaragua-Honduras border to Mexico in under three months. 鈥淭he routes perfectly mirrored what we know about illegal cattle ranching and trafficking routes used by cartels,鈥� says Radachowsky.
Research suggests that growing deforestation for cattle and the climate are also aiding the fly鈥檚 spread.
Could it spread to the US and other countries?
After screwworm arrived in Mexico, the US took swift action to block imports of cattle from its neighbour on 25 November. But that may not be enough to stop the fly from getting in.
鈥淚t will probably be the southern part of the United States that will be affected if it spreads there,鈥� says at the University of Costa Rica. 鈥淚f it does spread internally, it will not be easy to control there, either. It is a strong fly that can travel several kilometres a day and infects not only cattle, but also other vertebrates, including wild animals that we cannot control.鈥�
What are authorities doing and what needs to be done to stop it?
Central American countries like Costa Rica have ramped up national programmes monitoring farms for screwworm outbreaks and opened WhatsApp lines where the public can report infections.
COPEG has increased its production of sterile flies from 20 million a week to more than 95 million and has shifted its releases northwards, with 25 weekly dispersal flights in Nicaragua and Guatemala.
鈥淐ontrol measures on farms now will not be enough, as wild areas will be affected,鈥� says Troyo Rodr铆guez. 鈥淪terile flies will be needed.鈥�
