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Trafficked cattle likely spread screwworm
What breached the barrier
It isn't 100% clear what breached the barrier, and most sources point to several factors at once. The pandemic seems to be partly to blame: livestock inspectors were forced to stay home, vehicles broke down for lack of replacement parts, and power outages at the screwworm plant killed millions of sterilized flies. Starting in 2021, the number of migrants passing through the Darien Gap also skyrocketed.
Cattle trafficking
Ranchers throughout Central America illegally graze cattle on protected forests, often as part of a larger money-laundering operation for narcotics cartels. The cattle are then moved north to livestock markets in Mexico, and hundreds of thousands are estimated to be trafficked each year. The speed at which screwworms have spread suggests the cattle move by truck, carrying infections with them.
Cases climb
The cases climbed fast. Panama had 6,500 in 2023 and 18,000 in 2024, when Costa Rica had 8,600 and Nicaragua 3,300, and screwworm had reached every Central American country and Mexico. In 2025, cases in Mexico rose to over 12,000, and they have since exceeded 30,000.
Why did power outages at the plant matter so much?
Sterile flies are living insects, not a stockpile, so a factory cannot store a surplus and release it later. If the temperature control or ventilation fails, entire batches of developing flies die. A barrier depends on a steady weekly supply of flies, so a gap in production leaves a gap in protection.
How does trucking cattle carry screwworm north?
A truck can cover hundreds of miles in a day, which is far beyond what a fly can manage on its own. Infested animals carry larvae in their wounds, and the larvae drop out to pupate wherever the truck stops. Spread by truck produces new outbreaks that jump past the infested zone, rather than advancing steadily along its edge.
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