New World Screwworm: Entomologist Q&A Separates Fact From Fear

Since June, New World screwworm (Cochliomyia hominivorax) has been in the news after being detected in Texas and New Mexico. As many entomologists know well, the fly was eradicated in the United States during the 1960s following a concerted effort by the U.S. Department of Agriculture using the sterile insect technique, and it was later eradicated throughout Mexico and Central America. However, it re-emerged across Central America and Mexico in the past few years before finding its way across the border last month.

Because the fly’s larvae feed on living tissue of warm-blooded animals, it poses a serious threat to livestock and wildlife (and, in rare cases, humans). A broad swath of federal and state government agencies, health departments, agriculture departments, wildlife groups, non-governmental organizations, research institutions, and industry partners are collaborating to slow the fly’s northward range expansion and coordinate response efforts.

The Entomological Society of America is doing its part to contribute, including hosting an emergency preparedness exercise on screwworm for public health in April in partnership with the U.S. Centers for Disease Control and Prevention and building a special collection of new and existing research on New World screwworm in the Journal of Medical Entomology and Journal of Economic Entomology, among other resources on New World screwworm.

This week, ESA released the latest episode of its podcast, Stridulation Station, titled “New World Screwworm: Separating Fact from Fear.” The episode features a conversation between Jennifer Gordon, Ph.D., founder and principal entomologist with Bug Lessons Consulting LLC and a past ESA Science Policy Fellow, and Edwin Burgess, Ph.D., assistant professor of entomology at the University of Florida.

Burgess and Gordon discuss New World screwworm basics, such as why it poses a serious threat to livestock and wildlife, how it spreads, and why the risk to humans remains extremely low. They also explore the classic sterile insect technique and tackle common misconceptions about sterilized flies, food safety, surveillance efforts, and the role of the public and various interested parties in responding to the current outbreak.

Below, see an excerpt from the conversation (lightly edited for clarity), and check out the full podcast in the player below or YouTube, Apple Podcasts, or Spotify.

Gordon: So, I’m hearing something called “sterile insect technique.” Can you tell me a little bit about that? How is that involved with New World screwworm?

Burgess: Sterile insect technique is the primary eradication tool that we successfully used in the 1960s to eradicate it from the United States and then, throughout the 80s, 90s, and 2000s, to push it all the way down to South America. And it’s the primary tool we’re going to use, again, to eradicate it from the United States, and the U.S. government is making a great effort and spending a lot of money to facilitate the necessary capacity to do that again.

So, how does it work? And these flies that are sterilized and released, are they dangerous to people?

No, they’re not dangerous to people. The way that sterile insect technique works, what they do is they rear the fly. The fly has four life stages: Their adult stage, the females lay eggs, and then the eggs hatch into larvae, and then the larvae develop and eventually will fall out of the wound, and they find a dry, dark place to go through a process called pupation, which is kind of akin to a moth or a butterfly’s cocoon. And that larvae undergoes metamorphosis inside that little puparium, where they turn into an adult. And then they pop the top off of that puparium, and the adult climbs out of it.

So what they do to sterilize the flies is they rear them in a big facility, they take the puparia, and they irradiate them with a radioactive compound. What that does is the energy that comes off of that radioactive compound breaks up the DNA in all of their reproductive cells. So, eggs are sterile now, and the male sperm is sterile now.

The key driver to this is that females only mate one time in their life. So, when you mass release these sterile flies, and a sterilized male mates with a wild female, the sperm that she’s collected from that male is non-viable—it’s sterile—so she can’t fertilize her eggs, and that makes those larvae unable to develop from that egg. That breaks the life cycle, and that’s the mechanism by which sterile insect technique works.

Gotcha. One thing I have to ask, because, you know, I watch all these YouTube … do I need to be worrying about radioactive flies out in the environment?

No, you don’t. That’s a total myth. You can find stuff, you know, in the media where they’ll call them “nuclear flies” or all kinds of scary names. There’s a really important thing with this, and it’s kind of Radioactivity 101: There is a radioactive compound that emits energy, and the pupae are exposed to that energy, and that’s what breaks up the DNA. The flies do not acquire any of the radioactive compound. They’re just exposed to that energy.

It’s the same thing as, if you were to be exposed to a radioisotope and you were exposed to the energy that it was giving off, it might do damage to you, but you’re not going to be radioactive, because you didn’t acquire any of that radioisotope in your body. You didn’t acquire any of that chemical in your body. You just were exposed to the energy.
So it doesn’t sound like anybody’s going to get some superhuman fly powers like Spider-Man did.

No, unfortunately, no. The flies are totally safe. They just get exposed to the energy from the radioactive materials. They don’t acquire any of the radioactive material themselves.

Technology’s certainly come a long way since the 1960s, and I saw that the USDA recently funded $105 million worth of new research. What other ideas for managing New World screwworm are people proposing right now?
We’re in a technological age unlike what we were in in the 1960s. I mean, the 1960s was before the molecular revolution, where we knew how to look at DNA and all the molecular things that make organisms do what they do. And so we’re well within that era now, where we have those types of tools available to us that we didn’t have back then. There are a number of technologies that are being developed right now for New World screwworm, such as transgenic flies—genetically modifying them so you don’t need all the radioactive materials [to sterilize them].

Some of the projects that are being funded off of that money include developing safer insecticides, and using AI for surveillance. Surveillance is a very important component of managing the spread of New World screwworm. Diagnostics [as well].

Another really critical thing, too, is we’re a generation or two removed from the last group of people that really dealt with this. Because it was not a problem in the United States, we’re under-trained in many areas currently with how to do identification, how to do management, how to communicate what’s going on with this fly. So there’s a lot of money going into training  technicians and people who are on the ground to facilitate all of these control efforts, right now.

If you had a crystal ball, and you were a gambling kind of guy, what do you expect the near term and long term are regarding New World screwworm?

In the near term, until we have all the sterile-fly facilities built to facilitate the full-on eradication efforts, I think you’ll see periodic cases pop up here and there. I know the state of Texas is doing a tremendous amount of work, and they’re doing a tremendous job, monitoring the situation, keeping track of animal movements, making sure that cases are being treated and reported promptly. So, in the near term, because the sterile insect technique is a numbers game, until we get those numbers up [of sterile flies produced], I think you’re going to see sporadic cases popping up here and there. If it gets into another state, it’s going take a while because of how intensely focused everyone is on monitoring this right now.

In the long term, I think we will eradicate it again, and it’s just going to be when we can facilitate enough sterile flies that we can release them over that landmass to fully engage that eradication program again.

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