Breaking: New World Screwworm Might Have Been Detected in South Texas

A case of New World screwworm (NWS) might have been detected in South Texas near La Pryor, according to USDA. The sample is currently at USDA’s National Veterinary Services Laboratories in Ames, lowa, for confirmatory testing. Personnel have been activated on the ground. Feeder cattle futures markets are reacting to the potential first case in the U.S., falling more than $5 per hundred across contracts.

The U.S. cattle industry has not faced the threat of NWS for more than 60 years. The NWS fly, about the size of a common housefly, has migrated across Central America from Panama through Mexico and now crossed the U.S. border. The harmful pest can travel on humans, vehicles, pets, livestock and even on wildlife species.

Because these infestations can be deceptive, producers should learn to identify the specific signs of New World Screwworm, such as unusual discharge or larvae deep within living tissue. Surveillance, reporting and veterinary partnerships are critical ways to prevent a single case from becoming a national crisis.

NWS is an infestation, not an infection, it is not a systemic disease problem, but an infestation that still demands strong controls.

Screwworm is a Pest, Not a Food Safety Issue

NWS is a pest control challenge, not a threat to the safety of beef. It’s not a disease or food safety concern for consumers.

“This is certainly considered a pest that we are trying to control, not a consumer safety issue at all,” stresses Stephen Diebel, Texas beef producer and Texas & Southwestern Cattle Raisers Association president. “We continue to have the safest, most wholesome protein product on the market today, and it continues to be that.”

Dudley Hoskins, U.S. under secretary of agriculture for marketing and regulatory programs, adds: “The screwworm is not contagious. It does not spread directly from animals to people or from person to person. And the screwworm does not pose a food safety risk. It spreads only when a screwworm fly lays eggs in a wound, not through meat, poultry or dairy products. However, [it] is a serious concern because of the potential disruption it could cause to the U.S. livestock industry if a detection is not quickly identified and treated.”

(Lori Hays)

What is NWS Myiasis?

NWS myiasis is the infestation of NWS larvae or maggots that feed on the living tissues of all warm-blooded animals, including humans, and rarely birds. Adult female flies lay their eggs, often as many as 200 to 300 eggs at a time, at the edges of wounds on animals or at the mucous membranes or body orifices. Within 12 to 24 hours the eggs will hatch, and larvae emerge to feed on living flesh by burrowing into tissue, tearing at the tissue with their hook-like mouthparts, like a screw being driven into wood and hence, their name.

The larvae can be difficult to detect for the first 24 to 48 hours, but as larvae feed on tissue, the wound enlarges and drains a serosanguineous fluid. There is severe inflammation and secondary infection as well as the stench of necrotic tissue.

Screwworm larvae pass through three stages (or instars), and they will reach maturity about five to seven days after the eggs hatch. At maturity, the larvae stop feeding and fall to the ground where they burrow and pupate to become adult flies. Adult flies live for two to three weeks in the field. Females mate only once in their lifetime.

(USDA APHIS)

Use the Playbook to As a Roadmap

The New World Screwworm Playbook, created by USDA-APHIS, provides a science-based roadmap for states, ranchers and veterinarians to combat a NWS outbreak. The goal of the Playbook is to try to balance that constant posture of vigilance, prevention and emergency response coordination.

(Farm Journal)

The Playbook includes the plan for movement restrictions if a premises falls into an infested region. The plan is designed to keep producers in business, keep cattle and products moving, and manage NWS in a way that protects both herds and markets. USDA-APHIS admits there will be movement restrictions. To move animals out of that zone, there will be steps to follow but movement will not be completely shut down.

((Illustration: USDA National Agricultural Library))

Diagnosis, Treatment and Prevention of NWS

Laboratory diagnosis of NWS is usually made by identification of the parasites under the microscope. NWS is a foreign animal disease that is reportable to state animal health authorities and to USDA-APHIS. The U.S. is responsible for reporting NWS to the World Organization for Animal Health and to our trading partners.

Before collecting or sending any samples from animals with a foreign animal disease, the proper authorities should be contacted. Samples should only be sent under secure conditions and to authorized laboratories. NWS can infest humans, so samples should be collected and handled with proper precautions. Larvae should be removed from the wound prior to treatment and placed in 80% ethanol for transport to the lab. Formalin should not be used.

Treatment for NWS myiasis generally includes cleaning and debriding the wounds and applying organophosphate insecticides, which are effective against newly hatched larvae, immature forms and adult flies. Carbamates and pyrethroids are also effective against larvae. Antibiotics are indicated if an infection is present. Livestock can also be protected by regular spraying or dipping with insecticides, or by subcutaneous injections of ivermectin and related compounds.

In endemic areas, animals must be inspected for screwworms every few days. NWS myiasis is often fatal in untreated cattle within 14 days.

Whenever possible, procedures that leave wounds (castration, dehorning, branding, ear tagging) should not be performed during screwworm season, and sharp objects should be removed from livestock pens. No vaccine is currently available for NWS.

Producers can find a list of approved treatment and prevention strategies on the FDA website.

Early detection, prompt reporting and treatment — backed by coordinated surveillance along the border — will be critical to keeping this treatable pest contained. Ranches are tightening calving seasons, upgrading working facilities and revisiting parasite control plans with their veterinarians. The core message to the fight against NWS: nothing replaces “eyes on animals.”

Your Next Reads:

• “Report, Don’t Hide It”: Experts Urge Rapid Action When Suspecting New World Screwworm
• What Products are Available to Prevent and Treat New World Screwworm?
• How Will U.S. Producers Maintain Business when New World Screwworm Invades?
• The Smell You’ll Never Forget: A Calf Infested with New World Screwworm