A new study across South America finds New World screwworm flies (Cochliomyia hominivorax) thrive in warm, moist regions and may persist through wildlife reservoirs. Researchers raise concerns about surveillance gaps and the need for coordinated cross-border action to protect animal and human health. (Photo by Paul Langlois, USDA APHIS ITP, Bugwood.org)

By Melissa Mayer

Melissa Mayer

Traumatic myiasis is about as gross as it gets.

A gravid New World screwworm fly (Cochliomyia hominivorax) lumbers through the air, looking for the perfect place to deposit her heavy burden of eggs—maybe an open wound or an exposed mucus membrane. Eventually, they’ll hatch out tiny maggots, each sporting a treacherous spiral of backwards-facing spines. These will anchor the maggots as they use razor-sharp mouth hooks to burrow and tear into tender living tissue.

Historically, New World screwworm has been considered a livestock issue, affecting domesticated animals like cattle, sheep, and goats. But a wide range of warm-blooded animals can be hosts—including wildlife, household pets, and even humans.

That’s one takeaway from a recent study, published in August in the Journal of Medical Entomology, that applied a One Health approach to modeling New World screwworm distribution across South America. The team comprised disease experts, data analysts, entomologists, and veterinarians from institutions in Chile, Paraguay, and Venezuela.

“We all knew the fly affects livestock, but as we went through the literature, we came across cases in wildlife, such as maned wolves, giant otters, ocelots, lesser grisons, and wild boars,” says lead author Marlon Mauricio Ardila, a Ph.D. student at Chile’s Universidad de Concepción. “And that’s when you grasp the real problem. Those wild populations are completely outside the reach of any veterinary control, so they act as a reservoir that can keep the cycle going—even when you think you’ve already brought the fly under control in livestock.”

Finding and Filling Surveillance Gaps

Marlon Mauricio Ardila, is a Ph.D. student at Chile’s Universidad de Concepción and lead author on a new study that applied a One Health approach to modeling New World screwworm (Cochliomyia hominivorax) distribution across South America. (Photo courtesy of Marlon Mauricio Ardila)

New World screwworm once affected a large swath of the Americas, from the southern United States through central Argentina. A program utilizing the sterile insect technique began in the 1950s and continued through the turn of the century to eradicate the fly in the U.S., Mexico, and central America. But it has returned to Central America in the past couple of years, reaching the U.S. this summer. All the while, populations remained strong in South America. But co-author Leidi Herrera of the Universidad Nacional de Asunción, Paraguay, and the Universidad Central de Venezuela says population data was limited.

“My research into metaxenic diseases and zoonoses … revealed significant gaps in surveillance in our region,” she says. “Although screwworm myiasis has a severe impact on health and the economy in Latin America, studies in South America were scarce.”

Existing models mainly looked at Argentina and Chile plus some nation-level analysis in some other South American countries. Now, the team wanted to model the distribution of New World screwworm for the entire region, with a strong focus on finding environmental conditions that might lead the flies to persist or disperse.

To do this, they used modeling tools along with records of New World screwworm myiasis cases in humans and other warm-blooded animals across nine geographic areas. The data included cases of more than 9,000 New World screwworm larvae, which broke out to about 38% involving human myiasis and nearly 62% involving non-human animals or unspecified myiasis.

A new study across South America finds New World screwworm flies (Cochliomyia hominivorax) thrive in warm, moist regions and may persist through wildlife reservoirs. Researchers raise concerns about surveillance gaps and the need for coordinated cross-border action to protect animal and human health. (Image courtesy of Marlon Mauricio Ardila)

A Call to Surveil Together

This ecological modeling found that temperature and rainfall heavily influence the distribution of New World screwworm in South America—especially winter thermal stress. It turns out  screwworm larvae and pupae don’t do well when the temperature dips to its lowest seasonal extreme. They also struggle when the temperature during the day is unstable, which is known to affect how well insects do normal things like fly, mate, and lay eggs.

The models also predict that some places—like the humid lowlands of eastern Paraguay, northeastern Brazil, and parts of the Amazon basin—have exactly what New World screwworm need to thrive: sufficient moisture so their pupae don’t dry out and plenty of opportunities for the flies to find vulnerable (and ostensibly tasty) wounds.

The team used epidemiological records to confirm and extend the models.

The ‘Cross-Border’ Problem

Overall, the team say that this problem is one that combines issues of the environment, animal health, and public health—and therefore requires an integrated approach to surveilling and responding to the threat.

“This is a starting point, not the end,” says Ardila. “For now, the paper calls for strengthening surveillance, training producers, and coordinating control across countries—because this is a cross-border problem.”

It also leaves many questions still open: confirming how wildlife act as a reservoir, figuring out the genetic barrier that prevents the flies from taking up areas that seem perfect from a climate perspective, and looking at how other flies that cause myiasis overlap New World screwworm. Herrera says they’re seeking partnerships and funding to design integrated monitoring tools and control strategies.

One thing is certain: It’s work that needs to be done together.

Melissa Mayer is a science writer and the human behind Washington State University’s science cat, Dr. Universe. Email: [email protected].