August 6, 2025
South Dakota Office of the Governor
500 East Capitol Avenue
Pierre SD 57501-8501
Dear Governor Rhoden,
The purpose of this letter is to respectfully request you take immediate action to help prevent the spread of the New World Screwworm (NWS) to South Dakotans and their livestock in conjunction with ongoing actions by the US Dept of Agriculture and other states.
WHAT IS THE NEW WORLD SCREWWORM?
The New World Screwworm scientifically known as Cochliomyia hominivorax, is a parasitic fly whose larvae (maggots) infest the living tissue of warm-blooded animals, causing a condition called myiasis. Unlike other fly species that feed on dead tissue, NWS larvae feed on the living flesh of mammals (including livestock, wildlife, pets, and rarely humans) and occasionally birds, making them particularly dangerous.
Adult female flies lay 250–500 eggs on the edges of open wounds or mucous membranes (e.g., nasal passages, eyes). Within 12–24 hours, eggs hatch into larvae that burrow into the tissue, feeding for about a week. Larvae then drop to the ground to pupate, emerging as adult flies after at least 7 days. The entire life cycle takes roughly three weeks.
The NWS was previously eradicated from the US in 1966 and Central America by the early 2000s using the sterile insect technique (SIT), but recent outbreaks in southern Mexico (2024–2025) and Central America (e.g., Nicaragua, Costa Rica) have raised concerns about northward spread.
DANGERS TO HUMANS AND LIVESTOCK
Dangers to livestock are associated with severe tissue damage. Larvae burrow into healthy tissue with hook-like mouthparts, enlarging wounds and causing severe pain, inflammation, and secondary bacterial infections. Infestations can lead to death within 1–2 weeks if untreated due to toxicity, trauma, or infection.
The dangers to humans are rare but serious. Human cases are uncommon but occur, particularly in rural areas such as our state where people work with livestock or have open wounds. However, there were 369 confirmed cases, including six infected people, reported in the Mexican states of Oaxaca and Veracruz through March according to “The New World Screwworm: History, Eradication, and Its Recent Resurgence – Texas Forest Country Living” (https://www.texasforestcountryliving.com/the-new-world-screwworm-history-eradication-and-its-recent-resurgence/ ), and at least 30 cases were reported in Nicaragua and 28 in Costa Rica in recent months, with a new case in Chiapas, Mexico, in July 2025.
Larvae can infest wounds (even as small as tick bites), surgical sites, or mucous membranes (e.g., eyes, nose, ears), causing pain, tissue destruction, and potential secondary infections. Infestations can be debilitating if they affect critical areas like the eyes or sinuses and may be fatal if untreated.
HOW ARE SCREWWORMS SPREAD?
There are several ways that NWS can spread to livestock and humans:
• Through direct contact. Infected animals with open wounds attract more NWS flies, which lay additional eggs, amplifying the infestation within a herd. The larvae’s feeding creates larger wounds, increasing the risk of secondary bacterial infections that can spread through contact or shared environments
• Through the movement of infected animals to uninfected areas. NWS spreads when infected livestock or wildlife (e.g., deer, feral hogs) move to new areas. Flies can travel up to 12 miles, but larvae can be transported longer distances inside hosts. This was a key factor in the 2024–2025 northward spread from Central America to southern Mexico and is of growing concern in Texas and other states.
• Through environmental contamination. Larvae that drop from hosts pupate in the soil, producing adult flies that can infest nearby animals. Contaminated vehicles or equipment can also carry flies or larvae to new locations.
• Through proximity to infected animals. Humans working with or near infested livestock (e.g., ranchers, veterinarians) are at risk if they have open wounds or are exposed to flies. Flies are attracted to wounds on humans just as they are to those on animals.
TOTAL ESTIMATED ECONOMIC IMPACT OF A NWS OUTBREAK IN SOUTH DAKOTA
NWS infestations cause economic losses through livestock mortality, reduced productivity, treatment costs, and labor demands. Drawing from the USDA’s Economic Research Service (ERS) analysis for Texas (estimating $1.9–$2.1 billion annual loss for 12.5 million cattle) and historical US data, the impact can be scaled to South Dakota’s smaller cattle population resulting in the following estimates:
Direct Livestock Losses: $285 million (mortality) + $500–$750 million (productivity) = $785–$1,035 million
• Treatment and Labor: $29–$96 million
• Wildlife/Hunting: $37.5–$45 million
• Market Disruptions: $100–$200 million
• Control Costs: $15–$30 million
• Total: $966.5 million–$1.406 billion annually
This estimate assumes a moderate-to-severe outbreak with rapid response. A less severe outbreak with effective containment could reduce losses to $500–$800 million, while a prolonged, uncontrolled infestation could exceed $1.5 billion due to cascading effects on trade and wildlife.
PREVENTION AND CONTROL
As the primary effort to eradicate the NWS, the USDA, through its Animal and Plant Health Inspection Service (APHIS), is organizing the production of sterile flies to combat the spread of NWS. This “Sterile Insect Technique (SIT)” involves releasing sterile male flies to mate with wild females, reducing the population. Through concentrated efforts over time, this eradicated NWS from the US in 1966 and the Florida Keys in 2017. The Panama-United States Commission for the Eradication and Prevention of Screwworm (COPEG) facility in Pacora, Panama, currently produces 20–100 million sterile flies weekly, but this is insufficient for the current outbreak.
In May 2025, the USDA announced a $21 million investment to renovate a fruit fly facility in Metapa, Mexico, to produce an additional 60–100 million sterile NWS flies weekly, aiming to double SIT capacity and push the NWS population southward.
On June 17, 2025, the USDA announced the groundbreaking of a sterile fly dispersal facility at Moore Air Base in South Texas to serve as a contingency for rapid response if NWS is detected in the U.S. Plans are also underway to design a domestic sterile fly production facility to ensure long-term capacity.
In addition to SIT actions, the use of insecticides and treatments, such as organophosphates, carbamates, pyrethroids, ivermectin, or nitenpyram can kill larvae or prevent infestation. Wounds must be cleaned, debrided, and monitored to prevent reinfestation.
On July 21, 2025, Texas Agriculture Commissioner announced the rollout of Swormlure-5, a synthetic bait engineered to mimic the scent of an open wound. This bait attracts NWS flies, which die on contact, aiding in population control. The Texas Department of Agriculture (TDA) is deploying this bait to combat NWS spread.
In June 2025, the USDA Secretary launched a “Bold Plan” to combat NWS, focusing on protecting the U.S. border, increasing eradication efforts in Mexico, and enhancing readiness through new facilities and surveillance. This includes stringent animal movement controls, trapping, and science-based methods to push the NWS barrier south.
ACTIONS REQUESTED
To assist in the USDA’s efforts, as well as those of other states, to eradicate the NWS and to protect South Dakotans and their livestock, the following actions are urgently requested:
• In conjunction with other states, urge the USDA to maintain the prohibition on the importation of live cattle, bison, and equines from Mexico until Mexico demonstrates it has eradicated NWS from within its borders
• Establish a South Dakota NWS Task Force under the SD Secretary of Agriculture that is focused on prevention and eradication of NWS. The suggested membership should include the SD Department of Agriculture and Natural Resources (DANR), Governor’s Office of Economic Development (GOED), SD Game, Fish and Parks, South Dakota Beef Industry Council (SDBIC), South Dakota Agri-Business Association and SDSU College of Agriculture, Food & Environment Sciences, and SD Animal Industry Board, as well as SD stock growers, individual managers of livestock markets, and producers. Actions of the TF should address livestock screening, coordination with other states, public and producer education about NWS, NWS surveillance via flytraps, wildlife surveillance (e.g., inspection of harvested game), and development of a SIT plan.
• Other recommended actions could include:
o Initiate notification of the state’s veterinarians to ensure they are inspecting livestock for any pest-infected wounds
o In consultation with the state veterinarian and if required due to the northward spread of NWS, impose a heightened health certification process for cattle entering South Dakota from southern states
o Coordinate SD actions with those of other states
o Speak publicly to South Dakotans on the NWS threat and actions in progress
Forewarned is forearmed. As a life-long rancher and beef producer, we are sure that you are aware of the threat posed by the potential spread of the New World Screwworm to the SD beef industry, livestock, and all South Dakotans. We trust that you will act swiftly to coalesce the resources necessary to prevent the spread of NWS and to eradicate the threat as required.
This letter was approved by a majority vote of the SD GOP State Central Committee on July 19th 2025.
Respectfully,
Jim Eschenbaum
Chairman, South Dakota Republican Party
