In early August, the US Centers for Disease Control and Prevention confirmed a human case of New World screwworm in a Maryland patient who recently returned from El Salvador. As reported by Reuters, a spokesperson for the Department of Health and Human Services claims the public health risk is very low.[1] Nevertheless, the story illustrates crucial vulnerabilities in the current food system and raises questions about our susceptibility to similar parasitic outbreaks in the future.
The USDA’s Animal and Plant Health Inspection Service (APHIS) describes the New World screwworm as “a devastating pest.” It gets its name from the manner by which the fly larvae—maggots—burrow into the wounds of their hosts, “feeding as they go like a screw being driven into wood. . . . [and] tearing at the hosts’ tissue with sharp mouth hooks.”[2]
As the maggots burrow and feed on the living flesh, the wound worsens, attracting other mature female screwworm flies to lay their eggs around the edges of the wound. After about a week of this, the larvae fall from the wound and burrow into the ground, from which an adult fly eventually emerges to reproduce and repeat the cycle. Cochliomyia hominivorax, the Latin name, means “man eater.”
As you might expect, infestation is very painful. And mortality rates are high when it is not promptly treated. One of the biggest fears is that the maggots burrow deep enough to reach the most sensitive areas of the body, such as the brain tissue.[3] Apart from the visible evidence of maggots in festering wounds, what are the other signs of infestation? APHIS warns to watch out for animals with “irritated behavior,” “head shaking,” and, most unpleasantly, “the smell of decay.”[2]
Screwworms were eradicated from the US in the mid-60s. Apart from a relatively small outbreak in the Florida Keys last decade, they have not been a major concern. The strategy used to eradicate them is interesting. Rather than using insecticides, “[scientists at the USDA] decided to use the pest itself as a control agent.”[3] They raised huge numbers of the insects in factories, sterilized them, and released them back into the wild, decimating fly reproduction rates.
However, over the past few years, new cases have been reported in Central America, and there are fears of the pests making their way north again. The primary concern is not from a human health perspective; despite the recent Maryland case, humans are rarely directly affected. Rather, the scare comes mostly from the livestock industry, for cattle are more susceptible to screwworm outbreaks.
According to USDA estimates, a screwworm outbreak in Texas could cost the economy nearly $2 billion.[1] Texas is the biggest producer of cattle in the US and is likelier than other states to have an early outbreak due to its border location and the northward trajectory of screwworms from Central America, but any outbreak in Texas would potentially be only the start of a wider destabilization of the American cattle industry.
According to Max Scott, an expert in genetic pest management at North Carolina State University who recently spoke to an NPR reporter, the latest crop of sterilized flies may be less effective than those used in the past.[3] Additionally, there is only one facility today producing sterile flies, located in Panama. The USDA, responding to concerns from the cattle industry, is hoping to change that within the next few years. A facility planned for construction in Texas could produce 300 million sterile flies weekly, but it could take two to three years to complete.[1] In the meantime, government agencies are increasingly taking precautionary measures to hopefully prevent screwworms from entering the country, including halting imports of live cattle.[4]
An archived post from the USDA’s Agricultural Research Service, published nearly twenty-five years ago, describes how shifting toward meatless diets improves the screwworm eradication program and saves money.[5] But don’t get your hopes up—they’re talking about the diets of screwworms, not humans.
Still, it provokes an interesting thought: what might happen if we focused on changing human diets instead? Obligate parasites, such as screwworms, cannot sustain large populations without a suitable number of hosts. If our society were to shift toward a plant-based diet, the demand for livestock would plummet, as would livestock populations. This would drastically reduce the possibility of screwworm outbreaks.
Although potential hosts would still exist in the wild, parasites thrive when there are more hosts overall and when host density is higher, as is the case in modern animal agriculture systems that cram hundreds if not thousands of animals into confined spaces. Even an incomplete shift toward plant-based diets could substantially limit outbreaks, and current containment strategies, like the sterile insect technique, would be more effective if there were fewer host-parasite interactions.
This reflects a more general yet highly significant point: the eradication of animal agriculture, especially at the industrial scale, would greatly reduce the risk of all zoonotic outbreaks. This includes parasites, like screwworms, as well as bacterial and viral pathogens. We’ve written about this before: the majority of new infections are zoonotic and driven by intensive animal farming characterized by large livestock populations and high animal density. Livestock now make up nearly two-thirds of the world’s mammal biomass, and a huge proportion of that livestock is raised and killed in crammed, unsanitary, stressful conditions, all of which contribute to animals’ weakened immune systems. Their disease susceptibility is alarmingly high, as demonstrated by the fact that the majority of antibiotics in use today are given not to humans but to livestock.
While the public health risk from screwworms might be low, we would be foolish to consider the screwworm situation an isolated episode. Other pathogens, including many that are more deadly and far more contagious than screwworms, can and do transfer from livestock to humans, including influenza (bird or swine origin), SARS, MERS, and COVID-19. With the pandemic still a recent memory, why are we not taking this threat more seriously?
Add to this the environmental destruction caused by diets centered on animal foods, and it is undeniable that by far the greatest threat to a sustainable, healthy earth (including our own health) is not screwworms or anything similar; it’s us.
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