Parasite Reappears in the Americas
The New World screwworm, a parasite that feeds on the living tissue of warm-blooded animals, has re-emerged in Central America and Mexico. Once eradicated from North and Central America, the pest is again spreading among cattle and wildlife, prompting livestock and trade concerns. Health officials in the United States and Canada have also confirmed two rare human cases linked to travel in the region.
What Is the New World Screwworm?
The parasite is the larval stage of the fly Cochliomyia hominivorax. Female flies lay eggs in open wounds or mucous membranes, and the larvae then burrow into and feed on healthy tissue. This type of infestation, known as myiasis, can cause painful lesions, infections, and in animals, potentially death if untreated.
Rare Human Cases
On August 4, the U.S. Centers for Disease Control and Prevention (CDC) confirmed the country’s first travel-associated case. A Maryland resident contracted the parasite after visiting El Salvador and has since recovered. In Canada, an elderly man developed an infestation after sustaining leg injuries while hiking in Costa Rica. Doctors removed dozens of larvae in Costa Rica and later in Toronto before confirming the case. He was treated and discharged in stable condition. Human infections remain rare. A fatal case was documented in Costa Rica in 2024, the country’s first in decades.
Treatment and Precautions
Treatment involves removing the larvae from affected tissue. Depending on the depth of the infestation, this may require surgery. Travellers to affected regions are advised to clean and cover wounds, wear protective clothing, use insect repellent, and seek medical care if unusual skin lesions appear after returning home.
Livestock and Trade Impact
The greatest concern is for livestock. Infestations in cattle can lead to serious production losses. Some countries restrict animal imports from affected regions to reduce the risk of spread. The screwworm was eradicated from the U.S. in the 1960s through the release of sterile male flies, a method later extended to Mexico and Central America. Authorities are again implementing this technique in southern Texas and northern Mexico to contain the outbreak. Texas officials estimate that an uncontrolled outbreak could cost the state at least $1.8 billion in livestock losses, labor, and treatments.
Monitoring and Control
The Canadian Food Inspection Agency states the parasite is not present in Canada and cannot survive winter conditions. U.S. and Canadian authorities continue to monitor the situation closely as eradication measures are expanded in affected regions.
