A recent call among veterinarians and entomologists regarding the New World screwworm (NWS) delved deep into this potential agricultural threat.

After a May shutdown of cattle movement, USDA announced a risk-based port reopening plan for July. It was short-lived when authorities discovered NWS was moving northward from Central America. Livestock movement through southern ports of entry is once again prohibited.

NWS is a serious foreign animal disease pest, and the recent increase in cases have brought heightened concern. Cattle producers nationwide should understand NWS and remain vigilant for its potential entry to the U.S.

Entomologist Dr. Sonja Swiger, Texas A&M professor and Extension entomologist, said the larvae of this fly infest healthy tissue of live animals, and although NWS prefers warm-bodied animals such as livestock and wildlife, it also lays eggs on humans, pets and sometimes birds.

NWS populations often occur in unevenly distributed clusters, which makes it more difficult to monitor over large areas.

“The historical range for this fly was into southern Canada and throughout North America,” said Swiger. “They prefer areas where the temperature ranges from 64º to 91º F, with peak activity around 86º. They don’t do well in cold temperatures, but are known to range farther north in summer … They do better in areas with heavy rainfall, but that isn’t required.”

The NWS is a large blowfly species with metallic blue-green bodies and orange eyes. It has a characteristic three stripes on its body.

Its lifecycle from egg to adult is about four weeks. Adults lay eggs in groups of 200 to 300 on livestock, with a preference for moist areas such as wounds or exposed mucous membranes. As larvae hatch, they feed on tissue, resulting in severe damage. Multiple flies often lay eggs in one location.

Larvae are tapered on one end and have distinctive mouthparts for feeding. “The screwworm name is from their screw-like appearance,” said Swiger. “Their bodies are covered with spines and they feed with their heads down.”

The clinical signs of NWS in livestock and wild game include animals appearing depressed and off feed or separating themselves from the herd. Wounds may appear very small on the surface but will likely have deep pockets under the hide with hundreds of maggots inside. As wounds expand and tissue necrosis spreads, more flies are attracted. Secondary infections often cause more fatalities than the initial damage from maggots.

Animals can become systemically ill from secondary infections and sepsis caused by expanding wounds. NWS often targets naval mucosa on newborns.

One of the biggest challenges for those surveilling NWS is movement patterns. Flies will travel six to 12 miles to find a host. The bigger concern is the movement of infested livestock or wildlife, which can spread the NWS hundreds of miles if not stopped by inspections.

The Best Management Tools

Dr. Phillip Kaufman, professor and head of the Department of Entomology at Texas A&M, said although there are several management tools, the sterile insect technique (SIT) is a critical aspect of managing this pest.

Managing NWS via IPM is different from typical IPM because the goal is eradication. “IPM drops the pest population to where it isn’t a problem for the commodity,” said Kaufman. “In this case, eradication is the goal.”

Eliminating this threat involves all livestock producers. For this program to work successfully, individuals must be willing to report every instance of NWS encounters.

“If people don’t report it, the fly will successfully develop, produce many offspring and it will become more challenging to solve the issue,” said Kaufman.

Trapping adult NWS is an option because they are attracted to traps using the scent “swormlure,” which contains the same volatile organic compounds produced by decomposing flesh.

Trapping helps identify adult locations, but officials also monitor for larvae presence. Finding the insect in an area triggers inspections or limits on animal movement.

While the SIT is highly successful, it isn’t a stand-alone process – it’s one component of a comprehensive pest management program. For the SIT program, flies are produced at a plant in Panama.

“Normal production for this plant is 20 million sterile flies per week,” said Kaufman. “They’ve cranked up production to over 100 million sterile flies per week due to the seriousness of this outbreak.”

SIT involves mass-producing insects and exposing pupae to an irradiation treatment that renders them sterile.

Sterilized pupae are released into areas with known fly populations. “Females only mate only once in their life, so if she mates with a sterile male, she can’t produce offspring,” Kaufman explained. “[Sterile] flies are released regularly in mass numbers and may be aerially released in areas where the NWS is widespread.”

Government Response & Treating Affected Animals

Dr. TR Lansford, DVM, Texas Animal Health Commission, is determined to make sure all cattle producers understand the serious nature of NWS.

There’s a lot to gain if the U.S. remains NWS-free. “Because we’re free of NWS, we’re realizing about $1 billion in direct benefit to livestock producers in the U.S. and about $3.7 billion to the overall economy of the U.S.,” said Lansford. “In the ‘30s, when NWS was in Texas, the state lost about 180,000 cattle in one year. The potential cost if the NWS is in the U.S. again is about $540 million to livestock producers.”

In early June, Mexico reported 1,828 NWS cases in animals, primarily livestock. The most alarming case was 127 miles past the Mexican buffer zone established to contain the spread of NWS. This was about 600 miles from the U.S. and was found on a native animal – not an animal that recently moved in.

Sterile fly production in Panama has been moved to Mexico and officials are working to establish an effective buffer zone.

To deal with the current outbreak situation in Mexico and Central America, close to 200 million sterile flies/week will be needed. “The [Panama] facility is maxed out at 110 million flies/week,” Lansford said. USDA recently announced Mexico had agreed to convert a sterile fruit fly facility into production for sterile NWS, supported by USDA funding.

A big management gap has been additional sterile fly capacity, so USDA is currently establishing a production facility and a sterile fly dispersal facility outside Mission, Texas.

If NWS enters the U.S., cattlemen will see increased production costs. “The animal death aspect is factored into that, and this pest can create productivity issues,” Lansford said. “Infected lesions cause significant animal stress and damage, which results in chronic conditions that make the animal less marketable and less productive.”

Other costs include increased veterinary and prevention expenses, especially related to keeping animals free of open wounds of any size. Producers should also consider the extra labor required to conduct increased animal inspections.

The price tag for eradicating NWS is high. Historically, it cost tens of millions; today’s eradication costs would be into the billions. A major expense is expanding sterile fly production capacity – between $300 million and $600 million, depending on location and resources. Research dollars will be spent on developing more effective treatments and to offset infestations.

“Treatment with parasiticides can be successful when detected early,” said Lansford. “Detection is important because it eliminates the maggot in the environment and helps control fly populations. Treatment also limits damage to animals and relieves the discomfort and pain associated with lesions.”

Keeping livestock facilities in good order helps prevent wounds. Wounds should be cleaned and covered to prevent fly infestation. If NWS enters the U.S., it’s critical to avoid accidentally moving flies or maggots from one place to another through infested animals or vehicles in which flies become trapped.

The primary treatment labeled for NWS includes permethrin topical pesticides. Products are available for killing larvae discovered when cleaning wounds. These products can also be used on the animal itself to kill larvae. The challenge in using topical treatment is that wounds can be quite deep, so it’s important to clean wounds well and apply treatment so live maggots in the wound are killed.

There are no systemic larvicides currently approved and labeled for use in the U.S. The treatment for NWS infection such as macrocyclic lactones, ivermectin and related compounds are extra label and can only be used by a licensed vet.

If NWS enters the U.S., producers will need to consider changing production practices such as calving to occur during a time of year when NWS is less active. Procedures such as ear tagging and castration that create open wounds should be done when flies are less active.

Ongoing preparedness efforts over the past several months include strike teams developed by USDA Veterinary Services that have been training and setting up for rapid response in the event of a U.S. case.

“We do not have NWS in Texas, but it’s in Mexico and we’re concerned about northern progression,” said Lansford. “It’s important to understand how fast this pest can move and how fast an infestation can spread, especially when you factor in livestock movement, which was a big contributor to how fast it spread through Central America and into Mexico.”

Dr. Tom Hairgrove, veterinarian and professor of animal science at Texas A&M, explained how producers can prepare for NWS.

“Be vigilant but don’t panic,” said Hairgrove. “If you think you have a NWS issue, such as sudden loss of calves or wildlife, contact your veterinarian immediately.” Treatment guidance will likely include parasiticides, larvicides and treatments for secondary infection.

Hairgrove urges cattlemen to create a management program according to risk. “Be vigilant and report any abnormal events, like more deer dying,” he said. “Build a relationship with neighbors. Monitor and treat for ticks and other external parasites. This is important because certain ticks damage tissue, creating the ideal environment for NWS.”

The type of operation influences the management plan – are replacement animals purchased, or is the herd closed? “Monitor hunting and other recreational activities on your property,” said Hairgrove. “Dogs should be checked often. Keep your squeeze chute in good shape – we see a lot of injuries due to damaged squeeze chutes, holding pens and alleys.”