Utah agriculture agency warns screwworm could spread north; readies sampling kits and movement controls

A Utah Department of Agriculture and Food (UDAF) staff member warned that Cochliomyia hominivorax, commonly called the screwworm, is moving north through Central America and Mexico and could reestablish in the United States, with potentially heavy economic and animal-health costs.

The presenter described the fly’s life cycle and clinical signs in animals, outlined containment tools used by federal and state authorities—including quarantine, movement controls, larval sampling and the sterile‑insect technique—and said UDAF is assembling sampling kits and working with partners to update response plans.

The warning follows increased detections in Panama beginning in 2023 and the November finding of a local case in Mexico; the presenter said authorities recently confirmed a case roughly 370 miles from the U.S. border in a calf with no travel history, indicating local transmission in that part of Mexico. "If this is not treated, it is fatal to the animals," the UDAF staff member said, noting that the pest affects all warm‑blooded animals, including people.

Why it matters: Screwworm infestations destroy tissue, reduce production and can cause animal death; historical eradication of the pest from the U.S. in 1968 required a nationwide campaign. The presenter cited an estimate that a reintroduction could cost billions; historically, eradication efforts and movement restrictions have been used to halt outbreaks.

How the fly spreads and what it does: Females lay eggs on wounds and mucous membranes; eggs hatch in roughly 10–12 hours and larvae feed in wounds for about five to seven days before dropping to pupate in soil. Warmer climates speed development; under favorable conditions an egg can mature to an adult in as few as 18 days, though development can take up to three months in cooler conditions. The presenter noted larvae cannot pupate when soil temperatures remain below about 46°F, which should reduce overwintering risk in much of Utah.

Surveillance and detection: The presenter said U.S. authorities are limiting some live‑animal imports from Mexico and requiring dog inspections within five days of transport, and that the Centers for Disease Control and Prevention (CDC) and USDA must coordinate on canine movement rules. Laboratories identify suspect larvae microscopically—NVSL (USDA’s National Veterinary Services Laboratory) in Ames, Iowa, is the primary confirmatory lab—and UDAF plans to distribute sampling kits (forceps, tubes, instructions and shipping materials) and post an order form and guidance online. Collected larvae should be placed in 70% alcohol, not formalin, for submission.

Control tools described: Treatment of wounds and infested animals (topical sprays, systemic macrocyclic lactones such as ivermectin and doramectin), quarantine and movement controls, cleaning and insecticidal treatment of livestock trailers and premises, and use of sterile males produced by irradiation. The presenter described the sterile‑insect technique: sterile males are released to reduce fertilized eggs because females typically mate only once. Panama’s sterile‑fly facility can produce about 117 million sterile flies per week; a new U.S. facility at Moore Air Force Base in Texas is slated to produce a far larger number (presenter cited a target of roughly 6.1 billion per week once operational in about six months). Typical release goals cited were about 2,300 sterile flies per square mile per week in an affected area.

State preparations and likely impacts: UDAF is updating response plans, increasing research funding for surveillance tools and baits, and coordinating with the Division of Wildlife Resources and public‑health partners to track possible human cases. The presenter said Utah currently benefits from higher beef prices after southern ports were closed to Mexican cattle, but noted potential burdens if the pest arrives locally: increased labor and treatment costs, decreased production, restricted movement and possible export limitations. The agency indicated it would consider tighter health‑certificate windows, treatment or permitting requirements for animals imported from higher‑risk areas, and 2–3‑mile control zones around infested premises based on the fly’s usual movement.

Laboratory and tracing procedures: UDAF described a case classification scheme—suspect, presumptive positive for imported animals within 10 days, and confirmed after NVSL verification—and said tracing will examine the previous 10 days of animal and human movements to identify sources and exposed herds.

What producers should do now: Prevent wounds (delay elective procedures during fly season), inspect animals frequently, treat and submit suspect larvae promptly, maintain clean trailers and handle carcass disposal carefully (incineration or insecticidal treatment where pupae are seen), and report suspicious animals to wildlife or veterinary authorities. The presenter said sampling kits and guidance will be made available and that UDAF staff can be contacted for follow‑up.

The presentation closed with an offer from the presenter to provide slides and contact information for questions and noted continuing coordination between state and federal agencies on surveillance and response plans.