Jessica Metcalf’s work stinks. Literally. But the Colorado State University professor’s research may be key to protecting livestock and wildlife from a flesh-eating parasite.
In a project recently funded by the U.S. Department of Agriculture, Metcalf is studying foul-smelling substances to figure out what attracts the New World screwworm to its favorite meal – open wounds – so she can develop better lures for trapping this serious threat to the livestock industry. The pest was eradicated in the United States more than 50 years ago but was found in Texas in early June.
When a wound starts to smell, that’s when you know it may be infected and teeming with bacteria. Foul odors are among the chemical byproducts of bacteria, and the same smelly bacterial chemicals that cause revulsion and concern for humans attract the New World screwworm – a type of blowfly.
"We use these bacterial cues to stay away from certain things – to not eat spoiled meat, for example,” Metcalf said. “The insects are using them for a different reason. They're like, 'Yes, bring it, that smells like a great place to lay eggs.'”
The New World screwworm blowfly lays eggs in wounds and body openings. The maggots that emerge then feed on living tissue. It can infest any warm-blooded animal, including people and pets, although there is low risk in the United States. Left untreated, the parasite can be deadly.
The USDA recently announced $105 million in funding for 40 projects, including Metcalf’s, to improve the country’s defenses against the New World screwworm. As of July 23, the parasite has been detected in cattle, sheep, goats and two dogs in Texas and in another dog in New Mexico. Metcalf and her collaborators aim to help keep it from spreading.
Detection is the first step to controlling the New World screwworm. One of the most widely used lures utilizes bacterial-produced smells from rotting meat, so it attracts a closely related blowfly, the secondary screwworm, which prefers a diet of dead flesh. Traps fill with the abundant secondary screwworm, making it difficult to detect the more harmful New World screwworm. Metcalf’s project will develop lures specific to the New World screwworm using bacterial-produced smells from wounds for improved monitoring.
These bacterial-produced smells are known as volatile organic compounds, or organic chemical compounds that vaporize easily.
Metcalf's team will distinguish the difference between the VOCs that attract the New World screwworm versus the secondary screwworm. In other words, what is so appealing to the New World screwworm about live flesh over dead flesh, where it wouldn’t cause harm?
Blowflies also carry their own bacteria, which they use to communicate across the species. These bacteria signal to other blowflies where to find a suitable host, compounding the infestation.
Metcalf’s team will distinguish which bacteria are found on NWS versus the secondary screwworm to help design better lures for traps. Knowing what attracts NWS also potentially could be used to redirect it away from live hosts.
"Because we had been working on the relationship among insects, bacteria and animal decomposition for the last 15 years, and the New World screwworm evolved from a decomposer – an insect that utilizes dead flesh to raise its young – we were well set up to tackle this problem,” Metcalf said.
Metcalf’s work with the bacteria that decompose animals, including humans, has forensic science applications. She and her collaborators found that the microbes present on a body can accurately determine the time of death – useful information for solving crimes.
"We have been gaining a foundational understanding of animal decomposition at our main field site in Huntsville, Texas,” Metcalf said. “The basic science underlies tools we build for forensic science in estimating how long a person's been dead. It turns out we can also leverage decomposition ecology, which relies heavily on blowflies and their bacterial partners, to improve surveillance and control of the New World screwworm.”
Metcalf’s team, including CSU faculty Kayla Borton and Kelly Wrighton, will use sophisticated genomic and chemical data analysis tools through a partnership with the Colorado State Microbiome (CoSMic) Network.
Metcalf and her collaborators will enlist the help of on-the-ground volunteers, including livestock producers, veterinarians and wildlife biologists, to swab animal wounds for lab testing. These stakeholders will receive diagnostic results from the lab tests and will benefit from the solutions developed through the project. Meanwhile, Metcalf’s lab in CSU’s Department of Animal Sciences will compare wound bacteria and smells to dead animal samples.
To build the network of stakeholders, the research team is leveraging statewide partners, including producer associations, state agencies, CSU Extension and CSU’s Agricultural Experiment Station and Veterinary Diagnostic Laboratories. Stakeholders will receive information about the New World screwworm, sampling supplies and training. Project collaborators in Texas, Arizona and New Mexico will do similar outreach and sampling.
Tracey Goldstein, director of CSU’s One Health Institute, will coordinate outreach, training and sampling activities across the network. She said the network needs to be as wide as possible to watch for New World screwworm because if it infests wildlife, it will be especially challenging to control.
"This particular fly doesn't care if you're a person, a deer, a pig or a chicken. It will infest everything, which is a little scary,” Goldstein said. “That's also part of why it's hard to manage it. We really need all eyes and ears out there on the ground to help figure out if and when it gets to Colorado."
Julia Herman, beef cattle specialist veterinarian with the National Cattlemen’s Beef Association, will connect producers across the country with the research. She said producers in infestation zones are required to check every animal for wounds, and wounds must be healed before they can move livestock.
"We can't treat our way out of this problem,” Herman said. “We have to have other ways to manage this fly, and this project provides a way that we can decrease New World screwworm fly populations.”
She added, “While the New World screwworm isn't affecting a lot of animals, it is affecting the cattle industry as a whole, from animal welfare to transportation challenges to extra time and money and resources to find this fly’s larvae and treat the wounds. Hopefully we can take some of that stress off the producer and minimize the time and resource loss with projects like Dr. Metcalf's.”
The current international effort to manage the New World screwworm is largely based on irradiating male blowflies to make them sterile. The USDA and Panama’s Ministry of Agriculture Development operate a facility in Panama that breeds, sterilizes and releases millions of New World screwworm blowflies. Female blowflies mate only once, and when they mate with sterile males, their eggs do not hatch.
This has effectively kept the species out of the U.S. for decades, but in 2023 the flesh-eating insect started to make a comeback in Mexico and Central America. Metcalf’s research will give her team the data to assess whether sterile males lose any of their bacterial attraction during sterilization.
“Another potential outcome of our research is understanding how we could make the sterile flies more fit,” Metcalf said.
CSU Ph.D. student Anthony Grigsby, a forensic entomologist and microbiologist in Metcalf’s lab, said understanding what influences blowfly behavior and their ecological role could be helpful in controlling pests of livestock and humans.
"So much of the New World screwworm control work up to this point has been reliant on methods created over half a century ago,” he said. “We can help modernize the infrastructure underlying Sterile Insect Technique programs along with our understanding of screwworm ecology with this project.”