The lower jaw of the screwworm is strong enough to chew human flesh and can be chewed at a greedy speed. Photo credit: John Kucharski/Wikimedia Commons
The US has confirmed the first human case of infection by a screwworm in the New World (Cochliomyia hominivorax) In decades. The incident includes Maryland residents diagnosed in early August after returning from El Salvador, where the parasites still exist. Health officials said the patient received medical care including removal of the larvae and then recovered.
The screwworms of the New World are parasitic fly born in America. Unlike other fly species where maggots consume dead or decayed material, larvae are considered particularly dangerous because they eat live tissues of warm-blooded animals. Adult female fly lays eggs with open wounds, and once hatched, the larvae’s den deeper into the flesh, actively feeding them, causing extensive tissue damage. Without treatment, invasion can be fatal. This parasite has historically been a serious problem in both humans and livestock. The US eradicated screwworms through sterile insect techniques, including mass release of sterile male fly in the 1960s. When these sterile males were mated with women, no viable offspring was produced, leading to the collapse of local populations. This strategy is considered one of the most successful pest eradication campaigns in history and was later applied in Central America.
Despite this success, the screwworms have persisted in Latin America and parts of the Caribbean. Occasionally, outbreaks have been reported in countries such as Panama and the Dominican Republic. The Maryland incident highlights the continued risk of reintroduction through human or animal movements across borders. Officials emphasize that the infection has been obtained overseas and there is no evidence of local transmission within the United States.
The USDA and state officials responded by strengthening surveillance measures. Texas is preparing a new, large-capacity sterile fly production facility, although it will be the most vulnerable in the event of a cross-border outbreak. Reports show that the centre can produce and release up to 300 million sterile fly per week. These efforts aim to create protective barriers along the southern border and prevent parasites from being reestablished in US livestock populations. Economic interests are important. The invasion of screwworms in cattle, sheep, and other animals can lead to serious losses as it can cause suffering as well as reduce productivity and require culling. The US livestock industry is worth billions of dollars a year, and the reintroduction of parasites will pose a serious threat to farmers and supply chains.
In humans, cases remain rare. When they occur, they are generally associated with migration in areas where parasites are still present. Symptoms may include painful wounds that have failed to heal, visible larvae of affected tissues, and secondary infections. Treatment usually involves physical removal of the larvae, wound care, and administration of medication to prevent reinvasion.
Officials stress that while the emergence of parasites in human patients is surprising, the public health risks in the United States remain very low. Detection and rapid treatment of Maryland cases also demonstrate the effectiveness of medical and surveillance systems. Nevertheless, this case serves as a reminder of the importance of international cooperation in surveillance and control of parasitic diseases. As global travel continues to increase, rare but serious parasites can also appear in countries that were once excluded.
