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In the latest episode of the 80,000 Hours podcast, Kevin Esvelt talks about the New World screwworm. He claims that there is a potential for a massive animal welfare win, if we can eliminate this parasite. 

This feels worthy of a discussion, so I'm starting this thread. Is this issue on anyone's radar? Has anyone looked into these claims? They seem potentially important/actionable, if true! 


Relevant section from the podcast:

Kevin Esvelt: [...] the New World screwworm, which has the amazing scientific name of Cochliomyia hominivorax: “the man devourer.” But it doesn’t primarily eat humans; it feeds indiscriminately on warm-blooded things, so mammals and birds. It’s a botfly that lays its eggs in open wounds, anything as small as a tick bite. And it’s called the screwworm because the larvae are screw-shaped and they drill their way into living flesh, devouring it. And as they do, they cultivate bacteria that attract new gravid females that lay more eggs and continue the cycle.

So you have this macabre dance of parasitisation that results in the animal being devoured alive by flesh-eating maggots. And we know that it’s horrendously painful, because people get affected by this, and the standard of treatment is you give them morphine immediately so that surgeons can cut the things out — because it’s just that painful; it’s unbelievably agonising. And by my back-of-the-envelope calculations, there’s about a billion hosts of this every year — so a billion animals are devoured alive by flesh-eating maggots every single year.

We even know that we can eradicate this species from at least many ecosystems and not see any effects, because it used to be present in North America too, and we wiped it out using nuclear technology, oddly enough. Some clever folks noticed if you irradiate the larvae, then they grow up sterile. And if you release enough of them, then the wild ones will mate with a sterile one, and they only mate once, so you can suppress the population to the point of not being there anymore.

So we did this first up through Florida and then across the West, and then down through Texas to the Mexican border. The US Department of Agriculture then inked a deal with the Mexican government to eradicate them from Mexico because the southern border was shorter and therefore cheaper. And then they just went country by country down Central America to Panama. The southern border of Panama is the shortest, so American taxpayer dollars today contribute to the creation and maintenance of a living wall of sterile screwworm flies released in southern Panama that prevents the South American screwworm from reinvading North America — 10 million released every week.

Luisa Rodriguez: Wow.

Kevin Esvelt: But there’s too many of them in South America to wipe out by that means. And so the way forward is obviously gene drive. If the Mercosur countries agree that they want to get rid of the New World screwworm, they can start with something like a daisy drive locally — and Uruguay is working on this — then they can wipe it out from their country. Uruguay loses about 0.1% of their total country’s GDP to the screwworm because they’re so dependent on animal exports. I mean, Uruguay and beef is… To those listeners who eat beef, I’m going to start fights here, but it’s better than beef from Argentina, even. But anyway, they’re all very concerned about their beef, and screwworm is horrific.

It also, of course, preferentially hurts poor farmers who struggle to afford the veterinary treatments for their animals. And of course, they hate to see it, because here you’re watching these animals that you’re caring for literally get devoured by flesh-eating maggots, and it’s agonisingly painful.

But from an animal wellbeing perspective, in addition to the human development, the typical lifetime of an insect species is several million years. So 106 years times 109 hosts per year means an expected 1015 mammals and birds devoured alive by flesh-eating maggots. For comparison, if we continue factory farming for another 100 years, that would be 1013 broiler hens and pigs. So unless it’s 100 times worse to be a factory-farmed broiler hen than it is to be devoured alive by flesh-eating maggots, then when you integrate over the future, it is more important for animal wellbeing that we eradicate the New World screwworm from the wild than it is that we end factory farming tomorrow.

The ethics of CRISPR [02:38:34]

Luisa Rodriguez: What a take. I also love the application of gene drives for animal suffering in particular. I’d heard of many applications for human benefit, but the idea that we could make a dent on some wild animal suffering was just really moving to me. I feel like there are loads of concerns about kind of messing with an ecosystem. In this case, it’s already happening, just through a different method that can’t be scaled up — so it just seems like a really great case of how we’ve got this way to scale it up much bigger, eradicate this horrible insect in more places.

Kevin Esvelt: It might matter to some listeners, they might be concerned about the moral implications of actually driving a species to extinction. Which, of course, is also what we’re proposing for the malaria parasite (but not the mosquitoes) and also for the schistosoma. But for something that is not a major human disease, that’s [not] a microbe, here we’d be proposing eradicating the screwworm itself — the fly, the macroscopic thing from the ecosystem everywhere in the world.

But it’s worth noting that this is actually reversible, because screwworm is one of those comparatively few insects whereby you can freeze the larvae and unfreeze them decades later and they’re perfectly viable. So we don’t have to drive them extinct, we just need to remove them from the wild and then we can keep them on ice. So if for some reason we decide we need them again later, we can reintroduce them. It’s just we’ve got to ensure, if you want the animal welfare benefit…

One of the things that really I find attractive is, when you think about how much suffering humans have inflicted on animals in the course of our species, it almost certainly does not outweigh 1015 mammals and birds devoured alive by flesh-eating maggots. So to the extent that we’re now net negative on the scale, all we have to do is, before civilisation collapses, or we disassemble the Earth or whatever futurists think we’re going to be doing — or even if we lose, even if we fail and civilisation collapses, or even we go extinct — as long as we remove the New World screwworm first, we will be in morally net positive territory when it comes to our impacts on other species’ wellbeing. That’s tremendously inspiring.

Luisa Rodriguez: Yeah, I completely agree.

Kevin Esvelt: But it’s none of my business, because I don’t live in South America. It’s their environment; it’s their call. And so I would urge folks, if you want to reach out and know who to support in South America to fund that project, I’d be happy to connect folks — but moralising about how they have this moral duty to do this for the benefit of all humanity, probably not very helpful. If they decide to do it, it’s going to be for their own reasons, and us hectoring them is not going to be useful to the cause if you care about seeing it happen.






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My understanding is that screwworm eradication in North America has been treated by wild animal welfare researchers as a sort of paradigmatic example of what wild animal welfare interventions could look like, so I think it is on folks' radar. And, as Kevin mentions, it looks like Uruguay is working on this now with hopes of turning it into a regional campaign across South America.

I'm guessing one of the main reasons there hasn't been more uptake in promoting this idea is general uncertainty — both about the knock-on effects of something so large scale, and about whether saving the lives of animals who would have died from screwworm really results in higher net welfare for those animals (in many cases it's probably trading off an excruciating death now for a painful death later with added months or years of life in-between that may themselves be net-negative). So I do think it's a big overstatement for the guest to suggest that eradicating screwworm would be two orders of magnitude better than preventing the next 100 years of factory farming, which basically assumes that the wild animal lives saved directly trade-off (positively) against the (negative) lives of farmed animals.

@saulius might know more about this. One quote from a recent post of his: "To my surprise, most WAW researchers that I talked to agreed that we’re unlikely to find WAW interventions that could be as cost-effective as farmed animal welfare interventions within the next few years."

I remember talking about screwworms with @kcudding and @Holly_Elmore, I don't know how deeply they looked into it but maybe they could comment.

Yes! The biggest issue with eradication in the US and Central America is that it has to be maintained with pretty massive international effort. It uses a non-genetic sterile male technique, which means rearing huge amount of males (which require meat and certain conditions to grow), sterilizing them with x-rays, and releasing them by helicopter over the line Panama/Colombia border where the eradication is maintained. The US needs the cooperation of the rest of the North American countries in providing facilities and granting access to distribute the worms to keep it up, and it needed Mexico's partnership to do the initial eradication, part of which was a public health campaign to get ranchers to treat affected cattle to reduce the spread (as you can see in these amazing comic books the joint commission put out). So far it has proved too difficult to keep going through the jungles of South America to finish the job.

But using gene drives could make it could be easier. There are just all the standard concerns about using gene drives, like it affecting related species. I don't know enough about the relatives of the screw worm to know if it's a good candidate there or not.

Interesting point, that what's at stake here is the delta between an excruciating death now vs a few years of wild animal life and painful death later.

At least some chance of a less terrible death later, no? I'm really not sure what the distribution of causes of death looks like for different types of wild animal hosts

This really piqued my interest when I listened to the Podcast episode, but after some investigation, I was unable to verify his estimated 1 billion screwworm infections per year. I couldn’t find any estimates at all—only a study of its infection rate in a sample of pigs. Still, I think it’s a cause that STRONGLY deserves further research (namely estimates of annual screwworm infections) given its potential benefit.

I also tried and failed to find a good source to support his claim of screwworm prevalence (and would love further research).

This looks like an exciting cause, but I'm not sure why we should have confidence that this screwworm can be completely eliminated in the near future. I know it has been eradicated from central America which is very impressive, but many diseases have been removed from much of the world (malaria, polio, measles, cholera etc.), yet are far from elimination. I have had a very cursory look at literature and I haven't seen a strong argument or pathway as to why eradication might be probable - maybe someone could point me to it. There are a number of reasons this might be very, very difficult

(Note that I'm a global human health guy so basing my reasoning mainly on that, so I might be missing important things here)

1. Humanities track record so far (the best way to predict the future is look at the past). Only smallpox (1980) and rindepest (2011) have been eradicated so far. This is despite billions of dollars being poured into trying to eradicate diseases like polio and guinea worm. Perhaps a parallel to screwworm l is guinea worm, which should in theory be not that hard to eradicate given that humans are the main host, yet guinea worm still infects handfuls of people and animals in multiple countries despite us successfully reducing the global burden by 99.xx percent. We may well eradicate guinea worm soon, but it seems to me an easier task than this screwworm

2. This is a tropical parasite, with North and central America at the top of the range. The distribution map looks not so dissimilar to Malaria. This means that it is likely to be harder to eradicate closer to the equator. We talk about "shrinking the malaria map", which means eradicating malaria from countries where it is less viable, to the south and the north of the equator. That's why southern Europe, North Africa and Southern Africa can clear out their malaria, while most of sub-saharan africa currently can't. The closer you get to the equator, the harder malaria becomes to eradicate. This may be similar


3. The current control method is expensive, sterile flies have to be produced in the hundreds of millions over a number of years. This would have to be performed over a huge land area accross many countries. To be fair this didn't stop the USA and central America from successfully eradicating it there, so I don't think this is a dealbreaker in and of itself

4. Screwworm infects many species as primary hosts, making complete eradication and surveilance much harder - especially among wild species.


5. Political will and conflict. A LOT of countries would need to come on board and there would need to be a LOT of co-operation. Would Venezuela get on board with this right now? A big reason that guinea worm and polio haven't yet been eradicated, is that we lose the ability to vaccinate, treat and monitor in warzones. You would need a lot of co-operation and to have luck with lack of conflict in the region.

I'm not saying we shouldn't invest a lot more money in controlling screwworm, and that we couldn't reduce the population of worms by 95% or even 99% which could be a great investment, but looking seriously at the EV of complete eradication seems premature.

I'm interested in supporting this financially (that sounds like something a rich person would say so I should clarify this would not be a ton of money lol) and possibly in other ways as well (e.g., helping set up a website)

They seem potentially important/actionable, if true! 

Had the same thought as you OP. It had been rattling around in my head for a few days now, so I appreciate you making this post. 

Uruguay is a promising place to start. the IAEA web site says that Chile is the only South American country that doesn't have a Screwworm problem. that takes care of most of Argentina's border. Success for Uruguay would make the job a bit easier for Argentina to follow. it's fun to imagine a bunch more deer and capibaras and bunnies

hopefully international cooperation and funding would be feasible, as each country that undertakes eradication presumably makes subsequent tries a bit easier, especially for neighboring countries

Island-hopping would make sense for thrift, as eradication in Puerto Rico and Curacao doesn't seem to have been as high-maintenance to keep up as eradication in Panama. IAEA web site says Cuba, DR, Haiti, Jamaica, and T&T have screwworm. Islands belonging to South American countries like Colombia's San Andres and Providencia and Ecuador's Galapagos islands may also have them

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