By Dr. Michael Payne, UCD School of Vet Med and Director CDQAP and Dr. Nicki Humphrey, Cattle and Swine Specialist, Animal Health Branch, CDFA – Originally Published in CDQAP’s newsletter

Livestock producers remain understandably concerned about reports that the New World Screwworm (NWS) continues to creep towards the U.S. – Mexico border. Recent briefings by USDA, CDFA, and a webinar hosted by the NMPF, however, provide assurance that surveillance and response efforts are moving forward with all deliberate speed. Here are some highlights:

How close is Screwworm pest to the U.S.?

Northward fly movement has occurred in the Mexican state of Tamaulipas, about 150 miles from the Texas border. The previous closest detection site, about 70 miles from the U.S. border in the state of Nuevo Leon, now appears inactive. The USDA map on the next page depicts the relative concentrations of reported cases in Mexico.

Is the U.S. – Mexico border still closed?

Yes. Because virtually all long-distance spread of the pest results from transportation of infested animals, the current border closure provides critical protection to U.S. livestock.

What other actions are being taken to combat the pest?

USDA Audits in Mexico – USDA staff conduct regular NWS audits in various areas throughout Mexico. Because different Mexican States have variable animal health expertise and resources, the audits help direct corrective actions.

Sterile Fly Release – Sterile flies are being released over areas in Mexico where surveillance traps show continued infestation. The map on the next page shows the blue “polygon” dispersal area targeted for fly release along the Mexican border, into the state of Tamaulipas and 50 miles into Texas. In Texas, some limited ground release of flies is being conducted as a proactive, precautionary measure.

U.S. Sterile Fly Facilities – New sterile fly production and dispersal facilities are being brought online rapidly. The fly dispersal facility site at Moore Air Base in Texas is just now becoming operational, with an eventual capacity of dispersing up to 100 million sterile flies a week. Currently sourced from North America’s only existing production facility in Panama, eventually flies will be supplied by a new facility, also located at Moore Air Base. That facility is scheduled for completion in fall of 2027, with a projected starting capacity of 100 million flies per week ramping up over time to 300 million flies per week.

Mexican Sterile Fly Facilities – Two dispersal facilities are now operational. They are based in Tuxtla in the southernmost state of Chiapas, and in Tampico, close to the northern infestations most threatening the U.S. The USDA is also investing $21 million to support Mexico’s renovation of an existing fruit fly facility in Metapa, Mexico, converting it into a NWS sterile fly production facility. That facility is projected to produce an additional 60-100 flies per week and is expected to be operational as early as this summer.

What will happen if the fly is detected in Texas?

The USDA’s draft NWS Response Playbook provides guidance for dealing with incursions of NWS. See the diagram below. An Infested Zone will be established with at least a 12.4-mile radius around the pest detection, with implementation of animal movement controls and enhanced animal and fly surveillance. Inter- and intra-state movement from these zones would be managed with use of movement permits, veterinarian inspections, and as needed, treatment. Texas is working to develop a certified inspector training and those who receive the training may eventually be used to conduct inspections for within state movements. No treatment would be required when moving cattle directly to slaughter; however, cattle would need to be thoroughly inspected and moved to slaughter within 72 hours of the inspection. Additional layers of surveillance zones would be established, totaling up to 124 miles from the original detection. Sterile flies will be released as quickly as possible following confirmation of infestation and will continue until it is eradicated.

Will the entire state of Texas be quarantined?

Hopefully not. The current response plan is to limit quarantine and animal movement restrictions to local or regional areas when possible. For example, should the fly cross the border from the Mexican state of Tamaulipas into southern Texas, movement restrictions would not necessarily include the northern Texas panhandle or other areas that might have large concentrations of dairy youngstock from other states. State Animal Health Officials are also working together to ensure movements that are regularly happening across state lines can continue with appropriate risk mitigation.

How is the California border being monitored?

In addition to coordinating closely with Texas and federal response planners, CDFA is also monitoring California’s southern border. Fly traps in Imperial and San Diego counties have yielded some 1500 suspect flies, all which were confirmed negative as NWS by the national NVSL laboratory.

What should producers be doing to protect their herds?

Response plans call for sterile fly release as soon as possible following confirmation of infestation and will continue until it is eradicated. Obviously, however, no releases can begin until suspect cases are first reported. Producers should remain alert for unusual looking wounds or maggots and report them to their herd vet or local CDFA office. Producers should not treat any maggot-infested wound without first consulting with their herd vet. Because of the high risk of umbilicus and surgical sites, it is important to treat naval and castration and dehorning sites thoroughly and promptly. Wounds can be treated with commercially available permethrin pesticides.

Has there been any progress on treatment?

USDA has collected information on topical pesticide products available to prevent or treat NWS, some of which can be used in both lactating and non-lactating cattle. In addition, the FDA maintains webpages for both producers and veterinarians. Three prescription-only injectable or pour-on products recently received conditional approval for systemic treatment of NWS. None of these products, however, are approved for use in female dairy cattle older than 20 months. Because these treatments would result in very prolonged milk withdrawal times, the herd veterinarian should be consulted before use.

Where can I get more information?

There are several excellent NWS websites available, including those from USDA, CDFA, and Texas A&M.