Oh Good, Screwworms Are Back - by polimath

Marginally Compelling

SubscribeSign in

Oh Good, Screwworms Are Back<br>You say you want to know more about the horror bug that we eradicated only to discover it is making a comeback? You are in luck.

polimath<br>Jul 17, 2025

24

Share

There’s always a crisis or what seems to be a crisis or people trying to make something a crisis. Sometimes the crisis is real, sometimes is a manipulation, sometimes it’s a genuine danger that could become a crisis. It is often hard to sort these things out and my coping strategy for dealing with some new crisis is to take that moment to learn about things in detail.<br>For example: have you heard that the Newworld Screwworm is threatening to infect our livestock supply? This sounds bad. Fortunately, I was able to research this and discover that it is very bad and there is a whole fascinating story about this horrific creature, how we eradicated it, and how Covid, open borders, and a stunning and frankly inexcusable amount of institutional incompetence has managed to take a solved problem and allow it become a fresh danger.<br>What Fresh Insect Hell Is This?

Unlike most flies that lay eggs in dead and rotting organic matter, the Newworld Screwworm lays eggs in the open wounds and mucus membranes of living animals. The maggots then eat the animal (or human!) alive. Its scientific name is cochleomyia hominivorax which literally means “man-eating snail”.<br>These little monsters have plagued livestock and humans in tropical and subtropical regions of the Americas for centuries. This includes the southern US, the Caribbean, and most of Central and South America. The devastation of these terrible creatures amounted to billions of dollars of yearly damage on top of the fact that they are extremely gross and cause massive human and animal suffering.<br>So why am I not already familiar with this thing? I’ve lived most of my life in a region of the US that is the historic breeding ground for the screwworm, why have I not heard anything about it?<br>That’s because, similar to the eradication of small pox, we used science and massive international institutional coordination to destroy this nasty little pest.<br>Human Ingenuity Gives The Middle Finger To Nature

In the 1930’s, Raymond Bushland and Edward Knipling were studying the screwworm in Texas where it was devastating livestock herds. These two scientists proposed and developing the “sterile insect technique” (SIT), which involves breeding the insects, sterilizing them with radiation, and releasing them into the wild. Because the female screwworm fly mates only once, if she mates with a sterile male fly any eggs she lays will not produce maggots.<br>This has the immediate benefit of stopping the maggots from killing livestock but also has the long term benefit of eradicating these little assholes. Of course this would mean intentionally breeding sterile flies on an industrial scale and releasing millions of them into the wild. So that is what they decided to do.<br>In 1954, this strategy was tested on a small island of Curaçao. The screwworm was eliminated from the island in the space of seven weeks.<br>Over the next 30-40 years, the there was a major push for screwworm eradication in North America. It was driven out of the US in the 60’s. With enormous international cooperation, they were pushed out of Mexico and Belize in the 80’s and eradication was pushed down to Panama by the 1990’s.

By a happy accident of geography, Panama was an excellent choke-point for the screwworm eradication. We could effectively maintain a screwworm border in Panama with a minimal effort because the geographic area to sterilize was physically small and politically stable. This also meant that screwworm control could be maintained through limited screwworm production facilities based in Panama and managed by COPEG, a joint commission between Panama and the US. COPEG is an institution specifically founded to maintain control over the screwworm barrier in Panama.<br>It wasn’t plausible to push screwworm elimination past Panama for a number of reasons that include political instability and the fact that Brazil is an enormous and terrifying place.<br>Ok, Why Are You Even Writing This, Oh No, Something Went Wrong Didn’t It?

But then something went wrong.<br>Apparently in 2022, the screwworm barrier was breached. I say “apparently” because there seems to be wide agreement that 2022 is when this happened but no one can point to an event or any form of data about when this happened. The year 2022 seems to be a backward extrapolation from the fact that in 2023 there were 6,500 screwworm cases in Panama. Since then, cases have spread up through Central America and into Mexico.

How did this happen? That’s an interesting and mysterious story. The official line is that there were supply chain disruptions associated with the Covid pandemic that limited the Panama production of sterile flies necessary for screwworm containment.<br>I’m skeptical that this is...