New World screwworm is an emerging threat to U.S. cattle. CALES is coordinating Cooperative Extension, the College of Veterinary Medicine and the Arizona Veterinary Diagnostic Laboratory with Arizona cattle growers and the Arizona Department of Agriculture to turn university expertise into hands-on preparedness — training producers to recognize, sample and report suspected cases before the threat reaches their herds.

New World screwworm has reached the United States, and Arizona cattle growers are getting ready. 

For a rancher, preparedness starts with knowing what to watch for: a wound that doesn’t heal, an animal that separates from the herd, a subtle change in behavior. It also means knowing what to do when something looks wrong.

That’s the work underway at the University of Arizona.

Cooperative Extension, the College of Veterinary Medicine and the Arizona Veterinary Diagnostic Laboratory are working with cattle growers and industry organizations to bring veterinary, diagnostic and field expertise together — and take it directly to producers.
About 150 cattle growers recently gathered at the U of A’s Campus Agricultural Center for the first of what will become a series of hands-on workshops across Arizona. 

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The goal was straightforward: give cattle growers the knowledge and practice to recognize a suspected infestation, collect a sample and know what to do next.

The workshop, offered simultaneously in English and Spanish, was organized through a partnership among the Southern Arizona Cattlemen’s Protective Association (SACPA), Arizona Department of Agriculture and Cooperative Extension. The bilingual presentations help broaden the reach of the university’s knowledge transfer on a topic that is impacting both sides of the U.S.-Mexico border. 

“The more information that we're provided, to guide us to develop operational plans, to evaluate or inspect our facilities, the better we are going to be when we do have cases,” said Juan Lopez, president of SACPA.

Ranchers know how damaging the fly can be.

Females lay eggs in open wounds on animals, often targeting cattle, then the larvae burrow into the living tissue, causing a condition called myiasis. It can kill cattle if not treated.

State Veterinarian Ryan Wolker opened the workshop with a history of the screwworm across its range, a look at the current progression of the fly through Mexico and into Texas and a brief look at the potential impact.

The screwworm costs the livestock industry in Brazil, where the fly is endemic, $340 million annually and $50 million in neighboring Uruguay. Vigilance is key, Wolker told attendees.

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“The best tool that anybody in this room has at their disposal is observation, putting eyes on your animals, looking for signs of infestation,” he said.

The workshop came about after SACPA approached Ashley Wright, a University of Arizona Cooperative Extension area associate livestock agent in Pima and Santa Cruz counties (https://extension.arizona.edu/person/ashley-wright), for help educating the ranching community. The working relationship between SACPA and Extension was a natural foundation for the project, Wright said.

Organizers plan to work with producers in other areas to offer similar workshops, she said.

“We’re definitely going to do it again. It’s a great model that we can start repeating in other parts of the state,” Wright said.

The trust producers have for Extension and the state Department of Agriculture helps foster a “coalition approach” to preparing for the screwworm, said Amy Ganguli, a U of A range ecology professor and Extension specialist.

“It’s a federally coordinated issue, and Arizona Department of Agriculture is the lead for the state. We’re all supporting those efforts, but we work in partnership. It’s truly a coalition,” Ganguli said.

Recognizing New world screwworm

The screwworm is the parasitic larva of the fly Cochliomyia hominivorax. This parasite was endemic in the U.S. but cooperative international efforts saw it eradicated in 1966, though outbreaks have occurred since. In the current U.S. outbreak, the screwworm has only been detected in southwest Texas and eastern New Mexico. In Mexico, the parasite was found in southern Sonora and 18 miles south of the border near El Paso, according to the U.S. Department of Agriculture.

Veterinarian Christina Brownlee, an assistant professor in the College of Veterinary Medicine, and entomologist Lucy Li of the Department of Entomology gave workshop attendees a primer on recognizing signs of the screwworm.

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The parasite can be found in a broad range of animals, but cattle are the most common, with sheep close behind, in the Texas occurrence, Brownlee said.

“We’re ahead of this right now, and the reason we’re ahead of it is reporting,” she said.

Because the flies spend 90 percent of their time resting in vegetation, ranchers should focus on identifying the larvae or eggs in wounds.

“You are not going to identify this by the fly. She lays her eggs and gets out of there,” Brownlee said.

The flies will lay eggs in any areas where injuries normally occur on cattle, including lesions or wounds around the eyes, mucous membranes, injection sites, branding sites and umbilical cords on newborns to name a few, Brownlee said.

“Many day to day activities on ranches cause wounds, including castrations, branding, disbudding, ear tagging, etcetera, she said.

Ranchers should check pens, chutes and gates for sharp edges that could injure an animal. Newborn calves are a common target and major concern. If the larvae enter through the umbilical cord, the resulting bacterial infection is usually fatal, Brownlee said.

The females lay eggs on the surface, but they hatch quickly, especially in warm weather, burrow deep inside the wounds and are not always easy to spot. Brownlee urged ranch workers to look for behavioral signs in livestock.

“They’re going to be uncomfortable. They might isolate themselves. They might be off feed. You might have calves that are not nursing. Lame animals might have an infestation in their feet. It’s going to be subtle at first,” she said.

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The screwworm gets its name because the segmented, quarter-inch larvae with rings of dark spikes resemble screws. They burrow head-first into open wounds and mature through three stages, then fall to the ground and pupate in soil, emerging as flies, Li said.

The red-eyed flies are metallic blue-green with three dark stripes running front to back. Neither the flies nor larvae are a foodborne illness risk, Li said.

“They are not contagious, and they do not spread directly from livestock to people or from people to people. The only way for them to spread is for females to lay eggs in or around wounds on an animal,” she said.

Brownlee stressed the importance of not moving affected animals, because the pupae drop off to mature in soil. To prevent this, ranchers should cover the ground with a tarp while cleaning wounds and collect any debris that falls out of wounds and properly dispose of the contents, most recommendations are burning or freezing, she said.

The flies are sensitive to temperature, and past outbreaks in Arizona have generally been on the state’s southernmost rangeland, where the insect can survive the winter, Li said.

“They can finish their life cycle from eggs to adults very fast. They hatch within 12 to 24 hours after eggs are laid, and in temperate areas, they can be adults in 21 days,” Li said.

In warmer parts of Arizona that can be as short at 18 days. Once the eggs hatch and the larvae begin feeding, they attract more flies. The species can go through 14 generations in a year, and they will fly up to 10 miles looking for a host to lay eggs. Stopping that exponential growth early is important, she said. 

“Over 1 million flies can originate from one infested cow,” Li said.

The life cycle of the species offers a chance to halt the parasite in its tracks. Because females generally only mate once, releasing sterile males significantly reduces the population. Repeated release of male flies eventually stops the spread, she said.

“That’s how we eradicate them,” Li said. 

Brownlee suggested ranchers or other stakeholders check out the USDA's NWS Response Playbook for more specific details about how outbreaks are handled. The 29-page guide from USDA's Animal and Plant Health Inspection Service breaks the response into eight key activities, covering everything from establishing infested and surveillance zones to fly control and safe movement of livestock.

How to get your ranch ready for NWS

Wright urged attendees to start preparing by having a plan.

“Not just a plan in your head, a written plan,” she said. “Have the contact numbers for the Department of Agriculture, the disease reporting emails. Have those either in your phone, written down on a card in the glove box of the truck or wherever you find it convenient for you and the people who work for you.”

In the past year, Cooperative Extension has distributed thousands of sampling kits that include instructions, ethanol, test tubes and forceps for collecting suspected screwworms. The kits are available at Extension offices. Cattle operations should have the kits available and make sure everyone who needs to knows how to use them, Wright said.

Even just having tarps available is important. When affected livestock are treated with medication, the larvae detach and drop to the ground. 

“Some of those larvae will be far enough along that if you drop them on the ground, they will become adult flies,” Wright said.

She urged ranchers to walk their properties looking for things that can injure livestock, like pieces of metal sticking out from fences, chutes or gates. Vigilance is key, in the environment and on the livestock, she said.

What to do if you find suspected NW screwworms

Emily Janovyak, assistant director of the U of A Veterinary Diagnostic Laboratory, led ranchers through the process of collecting suspected screwworms and the ensuing investigation.

“There’s a surprising amount of complexity, and what we try to do in the lab is make it as simple as possible for the person who is submitting the sample,” Janovyak said.

Although the screwworm is a parasite, not a disease, any likely detection must be officially investigated as a USDA Foreign Animal Disease following strict guidelines to keep American agriculture safe and disease free. The guidelines require identification at the National Veterinary Services Laboratory in Ames, Iowa. After the Texas outbreak, the national lab set up a satellite lab in Texas to speed up the process during the crisis, she said.

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Ranchers should report suspected screwworms in livestock to the disease reporting email AZDA at diseasereporting@azda.gov, Janovyak said.

Once a rancher reports a screwworm – to AZDA for domestic animals or the Arizona Game and Fish for wildlife – the agency will decide if an investigation is warranted, which triggers expedited, free testing, Janovyak said.

"If they say no, we can still get it tested for you, but it’s on your dime, and it’s just a regular parasite investigation,” she told the ranchers.

She urged the audience to use sampling kits created and distributed by the Arizona Livestock Incident Reporting Team. If no ALIRT collection kit is available, put the sample in an unbreakable container filled with 70 percent ethanol, which is available at drug stores. Do not preserve samples in formalin, a common ranch disinfectant containing formaldehyde, because it will discolor the larvae, Janovyak said. 

Samples can be mailed or shipped via UPS to the U of A lab or dropped off there. Initial shipping is not paid. To save cost and expedite delivery, specimens can be dropped off at Extension offices. To avoid delays, ranchers should complete an ALIRT online survey when they collect the samples, she said.

The larvae mature through three stages, each bigger than the last. The smaller maggots are tiny and make confirming New World screwworm difficult.

“The gold sample, the ones that we really want, are the slightly bigger, fatter ones, the third (stage) maggots, because it’s easier to see all the structures in them,” Janovyak said, adding that adult flies can also be submitted for identification.

Keeping the larvae intact is important, because identification is done visually. Be careful not to smash them during collection, she said.

“None of us in this room wants to go to all the trouble of sending this off, having an entomologist look at it, then get back a report that says ‘inconclusive,’ so handle them very carefully,” Janovyak said.

Getting accurate GPS coordinates where any sample is collected is essential, because that’s how the USDA will know where to target sterile fly releases. Coordinates can be captured on cell phones, even outside a service area. The online survey, a link to which is in sample collection kits, allows the lab to start paperwork, she said.

“If you fill out that survey, your paperwork is done. Everything else happens on my end,” she said.

Team effort is preparing producers

People in the Arizona livestock industry have been meeting to discuss New World screwworm since at least a year before it appeared in Texas in June. Their reaction has been keen interest, not fear, said Lopez of SACPA.

“I wouldn’t say they’re scared, but they do recognize, based on the history, what the impacts are, so they’re preparing themselves,” he said.
Collaboration is important in the fight against the screwworm, including cross-border collaboration and information sharing. No one can fight this alone. That’s why Lopez invited producers from Sonora to the workshop, he said.

“The fly doesn’t care where you are. It doesn’t have a passport. It doesn’t see a border. It will go anywhere. We have to fight this from both sides,” he said.

“If Arizona gets it, potentially Sonora can get it and vice versa. We’re not enemies. We’re a team of producers, and where we’re related is that we want to keep our ag industry going, our lifestyle going, producing good, healthy cattle.”

To learn more, see the Cooperative Extension New World screwworm resource page.