Screwworms are moving north—are we prepared?

Screwworms may not be top of mind for most veterinarians today, but that could soon change. Once eradicated from the United States, these parasites have been steadily moving northward through Central America and have now reached Mexico. In this interview with dvm360, Christopher Lee, DVM, MPH, DACVPM, explains why the potential return of screwworms is a serious concern, how they differ from other maggot infestations, and what veterinarians need to know to be prepared if the spread continues in the US.

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Transcript

Christopher Lee, DVM, MPH, DACVPM:
All right. Well, thank you for having me. My name is Christopher Lee. I'm a preventive medicine specialist. I've worked across the board in veterinary medicine, from scrubbing kennels as a young kid to being an associate, a medical director, and an owner. I've done emergency medicine, day practice, and I still do a lot of ER work.

Recently, I was honored to be part of the editorial advisory board for dvm360, and I am also one of the NAVLE item writers. So, for those who have recently passed the boards—I do apologize—because a lot of those questions were mine!

I'm looking forward to talking about screwworms. It's something we haven't thought about for quite a while. While the government does worry about it, it has always seemed theoretical—probably not an immediate concern. However, it is something that could impact all of us, and the implications are terrifying.

To put it into perspective: if we look back at white-tailed deer in the southeastern United States and across the country, in 1950, we had fewer than half a million deer, and they were under threat. There were other factors involved, but screwworms alone were threatening extinction in certain areas.

By 1966, screwworms were eradicated from the U.S., and within 10 years, the deer population increased tenfold. That is the kind of impact we are talking about.

I recently saw a video and a picture while talking to a friend. We were looking at a deer that had been invaded by screwworms. The parasites had burrowed through the skin, through the muscle, and through the bone—into the brain. And the poor animal was still alive.

This is what we could be facing.

Screwworms belong to the genus and species Cochliomyia hominivorax, which is the New World, or Western Hemisphere, screwworm. It has always been present in South America, but over the last few years, we've seen a steady northward movement.

It has spread through all of Central America and has now reached Mexico. The question of whether it will arrive in the United States is not "if," but "when." If that happens, we will see myiasis cases on a regular basis.

What makes the New World screwworm so concerning is that it causes primary myiasis. To clarify, myiasis occurs when a human or animal has live maggots feeding on them. Right now, in the US, we only see secondary myiasis, which means maggots are feeding on dead or decaying tissue.

For example, if you're a large or small animal veterinarian and you see an animal with maggots, you always ask, What led to that? As an ER vet, I see this regularly. It’s dramatic when people see maggots in a wound, but as a clinician, I recognize that secondary myiasis can actually be beneficial—maggots clean out dead tissue and help prevent gangrene.

However, primary myiasis is a different story. Cochliomyia hominivorax is an obligate parasite, meaning it requires living tissue to survive. That’s what makes it so frightening—you don’t need an existing wound or infection for an animal or human to be infested.

If screwworms establish themselves in the U.S. again, we will be facing a severe and urgent problem in veterinary and public health.