Ravenous, flesh-eating screwworm flies have busted through containment barriers and have now reemerged in the US. On Monday and Tuesday, the US Department of Agriculture reported three new cases, bringing the tally to five. One of the cases is in a dog, though it’s unclear where it became infected; the dog lives in New Mexico, had its infection reported in Texas, and may have recently traveled to Mexico, where the flies are also spreading.
The screwworm fly, technically known as _Cochliomyia hominivorax_ (Coquerel), is a parasitic blowfly that can lay up to 3,000 eggs. The females mate only once in their 10–30-day lifespan and deposit hundreds of eggs when they find an opening on the body. The eggs hatch within a day, and the resulting eponymous screw-shaped larvae quickly begin ruthlessly boring into and feasting on their victim’s living flesh.
While livestock are the easiest and costliest prey for screwworms, humans are also at risk. Human cases are far less frequent than those in livestock, but when they do occur, they are just as severe. Infestations in humans cause rapidly enlarging, painful wounds that can progress to deeper tissues, with risks of secondary infection, sepsis, and mortality.
The fly’s larvae can destroy muscle, cartilage, and bone if they aren’t caught in time. They can even break through a human skull. Understanding the threat requires examining the parasite’s lifecycle. The screwworm fly is attracted to the smell of wounds, mucous membranes, and orifices of warm-blooded animals.
The US saw a concerted effort to eradicate the screwworm population decades ago using Sterile Fly Technique. This involved breeding millions of male flies in specialized facilities, sterilizing them with gamma radiation, then dropping them from the air like bombs. The technique works by exploiting the fact that females mate only once; if they do so with a sterile male, there will be no offspring, and the population will collapse.
However, around 2022, the barrier was breached, and the flies have been eating their way back up. With the northward movement, reports of human cases of screwworm infections, called myiasis, have trailed them in Central America. They offer a glimpse of the risk that people in the US now face as the flies invade.
In early 2024, researchers in Costa Rica reported what is thought to be the first identified human myiasis case in the country since the reemergence. The case was in a 71-year-old man from a small rural community close to the border with Panama.
The man sought care on January 12, 2024, for wounds on his feet, specifically between his toes, which had developed over the prior four months. Doctors noticed a deep lesion between the first two toes on his right foot and pulled out approximately 160 screwworm larvae. The man was treated with antibiotics and creams and had healed by the time of his six-week follow-up.
The US Department of Agriculture has estimated that if the flies stage a comeback rivaling isolated outbreaks of the past, they could cost Texas producers $732 million per year and the Texas economy $1.8 billion.
Source: Original article