Return of the Screwworm Is Bad News for Endangered Deer

12:10 minutes

Illustration of screwworm blowfly (Cochliomyia hominivorax) laying eggs in an open wound. Credit: USDA
Illustration of screwworm blowfly (Cochliomyia hominivorax) laying eggs in an open wound. Credit: USDA

A long-vanquished foe of North American agriculture has come back from the dead: the New World screwworm, a blowfly that lays its eggs in open wounds and whose larvae then eat their way—in corkscrewing spirals—through living flesh.

In 1960, screwworm cost the Southwestern agricultural industry as much as $100 million. A decades-long eradication effort that pioneered the use of sterilized male flies to slow reproduction finally succeeded in 1982. A multinational effort has been keeping them out ever since with border checks and a regular drop of sterile males in the Darien Gap in Panama.

But screwworm has been spotted again this fall, on a string of islands at the foot of Florida: Big Pine Key, Sugarloaf Key, and half a dozen others. The state has declared an agricultural state of emergency and established quarantines to prevent the parasite’s spread to the mainland. But one species, Florida’s tiny, endangered Key deer, has been decimated by the infestation. As of Thursday, 102 of the estimated 1,000-deer herd had been euthanized because of untreatable screwworm infections—10 percent of the population, most of them male.

Can the Key deer survive? How did screwworm make its way back to Florida, anyway? Ira talks to Phillip Kaufman, a veterinary entomologist at the University of Florida, and Chris Eggleston, acting manager at the National Key Deer Refuge, about the screwworm’s return and efforts to protect deer and other mammals in the Keys.

Segment Guests

Phillip Kaufman

Phillip Kaufman is an Associate Professor of Veterinary Entomology at the University of Florida in Gainesville, Florida.

Chris Eggleston

Chris Eggleston is acting manager for the National Key Deer Refuge in Big Pine Key, Florida.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow.

It sounds like a Halloween horror story– a flesh-eating parasite long thought dead returns to infect beloved endangered wildlife. But it is real. The screwworm is back, the parasitic fly larva named for the long, corkscrewing path it chews through living tissue. Sounds really awful.

And 100 years ago, it was a pest that cost the livestock industry millions of dollars per year. But then scientists in the 1940s began an effort to eradicate them with a now common tool, millions and millions of sterile male flies. And it worked. The United States hasn’t been an established population of screwworms since 1982.

Haven’t seen that. But now it’s been spotted on half a dozen of Florida’s Keys. Bad news for the endangered Key deer. Since late September, a crisis has been unfolding for the little deer. More than 100 have been euthanized because of untreatable screwworm infections. And well, there are only about 1,000 deer left in the entire population.

So can the Key deer survive? How did the screwworm get back there in the first place? And can we banish it again? That’s what we’re going to be talking about.

Let me introduce my guests. Dr. Phillip Kaufman is an associate professor of veterinary entomology at the University of Florida in Gainesville, Florida. Chris Eggleston is acting manager at the National Key Deer Refuge on Big Pine Key. An all-hands-on-deck effort to save the deer is underway. Welcome, Dr. Kaufman, and welcome, Chris, to Science Friday.

CHRIS EGGLESTON: Thank you very much.


IRA FLATOW: You’re welcome. Dr. Kaufman, this sounds like a horrifying little parasite.

PHILLIP KAUFMAN: It is It is. And it’s something we haven’t seen in this country, as you said, in over 30 years. The fly is very aggressive when it does establish an infestation. And it is something we need to take seriously and are taking seriously.

IRA FLATOW: How does it does it kill the deer?

PHILLIP KAUFMAN: Well, so an infestation will start wherever there is any open wound is the most likely place. And so the female fly will sense that. She will find that host, lay her eggs on it. Those eggs hatch. And the larvaes begin burrowing into the animal.

What makes this fly different than all the other blow flies we have is that it specifically goes after the living tissue and burrows deeper and deeper. And as it burrows, it creates more of a wound. That wound draws in more flies. And when those infestations get large enough or extensive enough and they get to a sensitive area where maybe they get into the organs or open wounds that can’t be healed, the animals will die. And it may take seven to 14 days. But untreated, this is a very deadly condition.

IRA FLATOW: Chris, you’re on the ground in the Keys. What do you see? Do you see them being ravaged by the screwworm?

CHRIS EGGLESTON: Yeah, well, it’s been a really tragic situation down here. This started back in late summer, and started to get progressively worse where we started to notice deer with infections, which aren’t entirely uncommon. It’s more and more of an urban area. So they run into dog situations or perhaps vehicle strikes or getting stuck in fences. And so we see wounded deer and respond to the situation.

But they started to show up with worse and worse infections. And the infections were pretty– well, frankly, kind of horrifying. We hadn’t seen wounds that had fly larvae in them previously. When we started to really identify, wow, this is a problem that we haven’t experienced before, we took some samples, sent them in. And then it came back as positive for being a screwworm.

And from there, the mortality rate really climbed high. And there’s been deer walking around here that they’re still trying to keep on, but they’ve got large wounds on them that are being eaten. They’re full of fly larvae. And they’re really eating the flesh.

And it’s really uncomfortable for the deer. And it’s really a tragic situation for the community down here as well, who are all really deeply involved with the Key deer down here.

IRA FLATOW: Sorry to hear that. Dr. Kaufman, what’s the remedy? I mean, you got rid of them once. Can you get rid of the screwworm again?

PHILLIP KAUFMAN: We can. And it’s actively being pursued at this point. So there’s a number of federal and state agencies that are working collaboratively. The primary method for eradication is using what’s called the sterile insect technique. And this method is how we eradicated them in the first place.

We have production facilities down in Panama, where we produce these flies by the millions. The flies are irradiated when they’re in the pupal stage, which sterilizes the males. Through that process, those males are then brought up and released into the environment. If they mate with a female– and this is where the process is most interesting– that female only mates once. And so if she mates with a sterilized male, she will not produce any viable offspring, and essentially, ends her genetic line.

This is used in combination with other techniques such as trapping. That’s used for monitoring as well as removing the flies from the environment, as well as people on the ground checking animals for infestations and assisting those animals to remove those maggots as well.

IRA FLATOW: Can you get this to work fast enough to save the remaining 1,000 deer?

PHILLIP KAUFMAN: Well, there are other steps that are being taken as well. But this process is going to take about six months before they’re going to be probably very sure that they’ve eradicated it. Now, the numbers are going to progressively drop. But there are other things being done to assist the deer, such as trying to treat them with medications that are going to kill any fly larvae that might establish on them before they get too large. But it is a battle. And it’s something that is being taken very seriously. And a lot of people are working to try and save this deer.

IRA FLATOW: Chris, let’s talk about the methods. Can you give medicine or something to the deer to feed them?


IRA FLATOW: Can you attract– will they come over and take it?

CHRIS EGGLESTON: Yeah, it’s interesting. The Key deer here, they’re known for being fairly docile and used to people. And so there is a group out here, a subset of the population, that will come right up to you and eat out of your hand. And that’s something that we’ve always discouraged in the past. But right now, it’s kind of working in our favor in this emergency situation.

So what we’re doing is we’re luring them in and using fruits and vegetables. And then when they get up close enough, we’re feeding them a piece of bread with a duramectin. And that’s an anti-parasitic, part of the avermectin drug family. And it will move through the body. And it will work prophylactically. And it will also treat some infections, very early stage. So we’re trying to get out there and get everything treated and get them on a seven-day rotation. And that’s our plan right now.

But we’ve got a whole lot of other things, a lot of other plans, that folks are working on. And right now, this is the best one we’ve got for us. But things are constantly changing. So we may move on to something else tomorrow.

IRA FLATOW: Like what? What would you do besides that?

CHRIS EGGLESTON: Well, I mean, I don’t know for sure. I’ve got the experts working on it right now. But it may be that we have to move to a topical solution if we can’t get enough to eat. One of the things we’ve run into is, there’s that subset that will eat out of your hand. There’s another group that will stay out about 10 or 12 feet. And we can still throw baits to them, the duramectin-laced baits.

And then there’s another group that won’t come anywhere near us. And they’re out in the woods. And so it’s a real challenge trying to treat those guys, especially when we’re trying to do it on a seven-day rotation. So for those, we may have to try a different technique. We may have to put out maybe feed blocks that have this drug in it, or maybe we figure out a way to get this on them topically and keep those ones safe.

And so right now, what we did is we started some Keys down at Sugarloaf in Cudjoe Key, where there are some smaller populations of deer that haven’t shown positive for screwworm at this point. So we started feeding them this drug first. And then, as we got all the ones that we could get down there, we started to move up to Big Pine and No Name Key.

And one of the interesting parts of this is as we feed the deer, we’re trying to mark them. So we put a little bit of nontoxic paint on them. So on Tuesday, it might have been yellow. So we know that we treated those on Tuesday. And the next day, we might treat with blue. So we know Wednesday is blue. So we can kind of track which deer have been treated and which haven’t and when they’ll need to get their next dose.

But because there are so many deer out there that may not come to us and be easy to get squirted with paint, it doesn’t necessarily mean that a deer without paint hasn’t been treated. So that’s something that we’re grappling with right now.

IRA FLATOW: Dr. Kaufman, the Florida Department of Agriculture has declared a state of emergency. There’s a quarantine for the Keys. And people are advised to have their pets checked if they’re leaving that area. Is that a threat to people’s pets? Is that a big threat?

PHILLIP KAUFMAN: It is a threat, although it’s much less than to the wildlife. And that’s really because people interact with their pets and will see that they have a wound and perhaps even notice there might be maggots in it. To date, there have been very few– I’m aware of one dog that that was confirmed as positive and a couple of others that were possibly infested earlier. But everything else that has been found, as I understand it, has been a wild animal. But it is critical that we keep the infestation from spreading and keep it, in particular, off of the Peninsular Florida, where we have a much more diverse wildlife, as well as our domestic livestock.

And that’s why it’s important that people present their animals to be inspected. This is not an instance where it’s a disease. It is not. These maggots are removed. The animals treated and made better. And the animal continues.

The animals that you’re hearing that are being euthanized are animals that the infection has gotten so bad that the animal can’t be rescued. And that’s really not generally going to be the case for anyone that has a pet that’s infested. You would see that well ahead of time.

IRA FLATOW: Well, considering about Zika virus is such a problem in the Miami area, you don’t want this coming up, as you say, to the peninsula and infecting other wildlife outside of the Keys. I want to thank both of you for taking time to be with us today. And good luck in your efforts.

CHRIS EGGLESTON: Thanks so much.

IRA FLATOW: You’re welcome. Dr. Phillip Kaufman, Associate Professor of Veterinary Entomology, University of Florida, Gainesville. Chris Eggleston, acting manager of the National Key Deer Refuge on big Pine Key.

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