USDA Taps Dogs to Sniff Out Screwworm

Fighting Screwworm

For about seven days, New World screwworm larvae feed inside the wound of a living animal. Then they drop to the ground and disappear into the soil.

USDA is now asking dogs to find them in both places.

The agency’s Animal and Plant Health Inspection Service plans to train canine teams to detect screwworm infestations in animals at ports of entry and along the U.S.–Mexico border. A separate, two-year, $404,000 USDA-funded study will test whether dogs can locate a related blowfly buried in soil—a first step toward canine surveillance of screwworm during its underground stage.

One search begins with a wounded animal. The other begins after the larvae have left that animal behind.

If both efforts succeed, dogs could help inspectors locate the parasite at two points when it is difficult to find by sight: while larvae are feeding inside living tissue, and after they have entered the ground to continue their development.

USDA has tried this before.

Nearly 40 years ago, a German wirehaired pointer named Cazador learned to perform both searches.

A Parasite That Hides Twice

New World screwworm, Cochliomyia hominivorax, is a parasitic fly whose larvae feed on living tissue.

A female fly lays her eggs along an open wound or near a body opening. The wound can be a surgical site, a newborn animal’s healing navel, or something as small as a tick bite or superficial scratch.

After the eggs hatch, the larvae burrow into the wound and feed with sharp mouth hooks. The injury becomes deeper and larger as they consume healthy tissue. Without treatment, an infestation can cause extensive damage, severe suffering, and death.

Cattle, horses, sheep, goats, pets, wildlife, birds, and people can all become hosts.

After feeding for about a week, mature larvae leave the wound, fall to the ground, and burrow into the upper layers of the soil. There, they form pupae. Depending on temperature and humidity, adult flies can emerge seven to 54 days later and begin the cycle again.

The move underground creates a difficult break in surveillance. An infested animal may be treated, moved, or no longer present, while the next generation remains hidden beneath the ground.

A dog that can locate that buried stage could point investigators toward a place where the parasite completed its feeding cycle and entered the environment before adult flies emerge.

But first, researchers have to establish what the dog can actually find.

Finding Screwworm on a Living Animal

APHIS says its canine teams will be trained to detect infestations in animals at ports of entry and along the U.S.–Mexico border. The agency has not publicly identified the odor source it will use during training.

That is a central question for the program.

An infested wound may contain larvae, blood, damaged tissue, bacteria, drainage, medication, and the individual odor of the animal. The scent picture may change as the larvae develop, the wound worsens, or treatment begins.

The dogs cannot simply respond to the presence of a wound. They must recognize odor associated with screwworm while ignoring unrelated injuries, infections, treatments, and normal animal odors.

The assignment may also require them to work across different host species, wound locations, and stages of infestation. Odor collected from one type of animal or one stage of larval development may not represent every case a team could encounter in the field.

USDA has not yet released its training progression, validation standards, number of teams, breeds, or anticipated deployment date. Those details will determine whether the program produces dogs that recognize an odor under controlled conditions or teams that can search reliably around live animals.

A canine alert would not diagnose screwworm. Inspectors or veterinarians would still need to examine the animal, collect larvae, and confirm their identity.

The dog’s job would be to identify the animal that deserves that closer look.

Searching Beneath the Soil

The soil-stage study takes on a different problem.

Because work with New World screwworm is tightly regulated, researchers will begin with a related blowfly from the same family as a proxy. Dogs will be tested in laboratory odor work and field searches designed to measure accuracy, time to detection, and the range at which they can locate a buried target.

Using a proxy allows researchers to begin studying the search without handling New World screwworm itself. It also creates an important limit on what the initial results can prove.

A dog that locates the proxy species has demonstrated that it can find that buried insect odor under the tested conditions. It has not yet demonstrated that it can detect New World screwworm.

That distinction will be easy to lose if early successes are reduced to photographs of dogs finding hidden samples. The useful results will be found in the data: how often the dogs locate the target, how often they miss it, how they perform when no target is present, and how accurately handlers interpret their behavior.

Field trials will also show whether dogs that recognize the odor in a laboratory can follow it through real soil. Roots, insects, microorganisms, animal material, and other biological odors will surround the target. Burial conditions and the amount of odor reaching the surface may change during the search.

Finding a buried sample once makes a good demonstration. Building a surveillance method requires repeatable performance.

The Dog Who Did It First

In 1988, USDA scientist John B. Welch began training Severn Run’s Cazador, an American Kennel Club-registered German wirehaired pointer, for the agency’s Screwworm Research Unit.

Welch wanted to know whether a dog could detect animals carrying active infestations and locate screwworm pupae after the larvae had left their hosts and entered the ground.

After five months of training, Cazador could find infested animals. Three months later, he was locating pupae in soil.

He worked primarily in Costa Rica, Mexico, Panama, and Texas, traveling with Welch and demonstrating a search most people had never imagined assigning to a dog.

The USDA National Agricultural Library now preserves photographs, correspondence, working gear, and other records from Cazador’s career. Its collection identifies him as the first dog trained to detect the presence of screwworm.

Cazador established that the odor was available to a dog. The surviving public record does not answer the questions expected of a modern operational program: how many blind trials he completed, how often he alerted incorrectly, how often he missed a target, how burial depth affected his performance, or whether other teams reproduced his results.

His achievement remains significant. It also explains why the current work cannot end with proving that another dog can do what Cazador did.

The goal must be a system that can be tested, repeated, and trusted.

From a Dog’s Response to Reliable Information

USDA already has the infrastructure to develop agricultural detector-dog teams.

The National Detector Dog Training Center in Newnan, Georgia, prepares dogs and handlers to inspect passenger baggage, cargo, parcels, and vehicles for prohibited agricultural products and harmful pests. Other USDA-trained teams locate invasive species in the environment.

Screwworm presents a different search.

Fruit, meat, or plant material concealed in luggage is a comparatively stable target. An infestation exists within a living biological process. The host moves. The wound changes. Larvae develop. Treatments add new odors. Other injuries and infections may be present.

The buried stage presents another variable: odor must travel through soil before the dog can locate its source.

Any field program will need blind testing, blank searches, realistic distractions, clear alert criteria, and independent confirmation. Handlers should not know where the target is—or whether one is present—when a team’s reliability is being measured.

False alerts will be as important as successful finds. A dog that responds to ordinary wounds, infections, or unrelated insects could consume time and resources without improving surveillance. A dog that misses lightly infested animals or deeper buried targets could create false confidence.

The standard is not whether a dog can be trained to show interest in the odor.

The standard is whether a trained team can provide information accurate enough for inspectors, veterinarians, and response officials to act on.

Why USDA Is Moving Now

The United States eradicated established New World screwworm populations in 1966 through a campaign centered on the sterile insect technique. A localized outbreak in the Florida Keys was eliminated in 2017.

The sterile insect technique works by releasing large numbers of sterile male flies. Female New World screwworm flies generally mate once. When a female mates with a sterile male, her eggs do not hatch, reducing reproduction across the population.

After the parasite began spreading north through Central America in 2023, Mexico confirmed a case in November 2024. USDA confirmed the first U.S. animal case in the current outbreak on June 3, 2026, in a calf in Zavala County, Texas. Additional cases were later confirmed in Texas and New Mexico.

USDA’s response includes animal inspections, surveillance, treatment, movement controls, sterile-fly releases, and new research. Through its New World Screwworm Grand Challenge, the agency selected 40 projects for approximately $105 million in funding.

The projects include traps, lures, treatments, diagnostic tools, surveillance technology, sterile-fly production methods, forecasting systems, and other approaches. Canine detection is one small part of that investment, but it addresses a problem few other tools are built to solve: locating odor directly where the parasite is feeding or developing.

The dogs would not replace visual inspections, laboratory identification, fly traps, or sterile-insect releases. They would give response teams another way to decide which animal or patch of ground deserves immediate attention.

Cazador answered the first question nearly 40 years ago: a dog could find screwworm on an animal and beneath the soil.

The work now underway must answer the harder one—whether that performance can be reproduced, measured, and trusted when an alert sends inspectors toward a living animal or a patch of ground.

That is the standard USDA’s dogs will have to meet.

 

Sources Consulted

U.S. Department of Agriculture, Animal and Plant Health Inspection Service, “Detector Dogs.”

U.S. Department of Agriculture, Animal and Plant Health Inspection Service, “New World Screwworm Grand Challenge Award Selections” and “USDA Invests in Projects to Strengthen New World Screwworm Preparedness and Response.”

USDA-funded project description, “Detection Dogs as a Surveillance Tool for Locating Soil-Stage New World Screwworm Using Proxy Calliphoridae Species.”

USDA National Agricultural Library, “Severn Run’s Cazador, Screwworm Detection Dog.”

Centers for Disease Control and Prevention, “Clinical Overview of New World Screwworm.”

U.S. Department of Agriculture, Animal and Plant Health Inspection Service, “New World Screwworm,” “USDA Confirms Presence of New World Screwworm in the United States,” and “Confirmed Detections of New World Screwworm.”