U of A researchers build model to predict screwworm spread in Arizona

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University of Arizona researchers are spearheading an effort to map and predict how New World screwworm might affect wildlife across the state.

Much of the attention surrounding screwworm has focused on livestock, but researchers are also looking at what the fly could mean for wildlife, which can be harder to monitor and treat. Though no wildlife has been impacted by a recent emergence of screwworm in Texas and New Mexico, the fly larvae, which burrow into living flesh and can kill host animals, have been found in wildlife across the fly’s range in the Americas.

The initial work will include mapping “areas of suitability” for the fly, or potential hotspots where it is likely to take hold, said Leigh Combrink, an assistant professor of wildlife disease and conservation in the School of Natural Resources and the Environment.

Researchers are adapting an existing model developed at the university to predict the distribution and abundance of mosquitoes that carry West Nile virus. The model will help researchers predict where screwworm could spread in Arizona, including areas where it could affect livestock and wildlife. It incorporates factors such as temperature, rainfall, and the presence of buildings and other structures, Combrink said. 

“Our model basically asked, ‘What does a mosquito need?’ We will take the same approach with New World Screwworm, as many of those same factors are going to affect the flies, as well,” she said.

Wildlife vector research part of broader grant

Combrink is leading wildlife vector research under a $3.7 million USDA grant recently received by U of A to bolster Arizona’s preparedness and response to the fly’s re-emergence.

Researchers will examine where the screwworm is found in Texas and New Mexico, and using those cases, will identify  similar areas in Arizona as potential outbreak hotspots, Combrink said.

Data collected since screwworm began spreading north from Panama in 2023 can help researchers predict how the fly could spread in the U.S. The model the task force is developing will be specific to Arizona but adaptable, Combrink said.

“Our plan is to make our framework available so that people can make it specific to other states,” Combrink said.

The model will include data on rainfall patterns, cattle movements, vegetation cover and other factors. Other states will be able to plug in local data and adapt the model to identify areas where the fly might emerge in their area.

New World screwworm can impact many species

A literature review this year by the USDA found 35 wildlife species in which the screwworm has been documented throughout the fly’s range, including several deer species, wild boar, rabbits, ocelots, pumas, sloths and one howler monkey. It has been identified in only a handful of birds.

The New World screwworm fly (Cochliomyia hominivorax) is endemic to South America and parts of the Caribbean. The fly gets its name from its larval stage, known as screwworms. The flies lay eggs in the wounds of warm-blooded animals, and the larvae feed on living tissue, causing wounds that can kill host animals if left untreated.

The fly plagued ranchers across the U.S. through much of the 20th century. After the release of millions of sterile male flies, which interrupt the breeding cycle, the fly was declared eradicated in the U.S. in 1966. In June, it showed up in Texas, first in cattle, then a dog, then sheep and goats.

The USDA Animal and Plant Health Inspection Service has reported no cases in wildlife, and no flies have been trapped.

The last major emergence of the fly in the U.S. was in wild deer. In the summer of 2016, an endangered deer hit by a car in the Florida Keys was found to have severe wounds caused by the fly larvae.

In the ensuing months, scores more infested Key deer and a handful of infested domestic animals were found, including one dog, according to a report from the U.S. Department of Agriculture.

More than 130 deer were euthanized — a huge blow to the subspecies, with a pre-emergence population estimated at about 1,000. More than 50 cases — all negative — were investigated in 20 other states and Puerto Rico, including five in Arizona. No commercial livestock were infected, USDA reported.

The USDA stopped the re-emergence by releasing sterile screwworm pupae in Florida.

Wildlife surveillance will be key to tracking the fly

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Courtney Duchardt is an assistant professor and Cooperative Extension specialist in the School of Natural Resources and the Environment. Her research focuses on wildlife in rangelands.

“There are only a couple of observations of birds having it. We don't know if it's more rare for them to have it or more rare for us to detect it in birds,” Duchardt said.

Birds are a concern because they migrate long distances and can fly across the U.S.-Mexico border, where the U.S. border wall stops larger mammals. Smaller mammals, like jackrabbits and cottontails, can also cross the border, she said.

“Having that wide array of species that it could impact can really be a challenge,” Duchardt said.

Combrink suspects that although any free-rainging warm-blooded animal could be a target, the most-commonly infested wildlife may be deer, partly because they share overlapping ranges with cattle and share water sources, she said.

Any researcher who works with wildlife in Arizona has to get a Scientific Activity License from Arizona Game & Fish, and everyone on that list has been asked to keep their eyes open, Combrink said.

Although researchers in South America have tried using game cameras to identify wildlife carrying the larvae, it’s not ideal at this stage. Cameras might be able to identify injuries on animals, but they generally can’t identify the cause of those injuries, Combrink said.

Spotting and dealing with screwworm in wildlife is harder than in livestock, in part because livestock are monitored more closely. Also, with smaller wildlife species, like rabbits, rodents and birds, the screwworm may be harder to spot, she said.

Coordinating efforts to identify and track the screwworm in wildlife is difficult because there are many organizations and types of people involved, including hunters, hikers and wildlife rehabilitation workers, she said.

“Everyone recognizes this threat and realizes there’s no point gatekeeping information. Everyone’s willing to share what they know and work together. The sense of camaraderie among all the wildlife people has been really great,” Combrink said.

Communication—a key role for the university—will be critical, Combrink said. 

“We do have a lot of people monitoring wildlife all over the place, but if everybody keeps everything in isolation, we're never going to be able to combat this,” she said.

Combrink sees progress in responding to the fly’s re-emergence, and collaboration and communication will increase knowledge and give Arizona the tools to face the screwworm head-on, she said.

“In three years' time, we're going to be leaps and bounds ahead of where we were at the beginning of this year, just from everyone working to try and solve this problem, so I'm very hopeful,” she said.