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Could a worm screw up cattle markets?

Kenan Layden - FFMC - Thu May 22, 6:10AM CDT

You likely have read a news headline like the one below:

USDA Press Release May 11, 2025: Secretary Rollins Suspends Live Animal Imports Through Ports of Entry Along Southern Border, Effective Immediately1

When I read these headlines, my brain thinks the following: new world screwworm is bad, it is not currently in the U.S., it once was in the U.S., and we don’t want it back in the U.S. Then my brain starts popping up questions that I can’t answer: Why are screwworms so bad? How were they eliminated? Can we eliminate mosquitoes the same way? Are these screwworms coming back?

My background is in education. I taught high school and college courses for 11 years and continue to serve as an adjunct instructor at the local college. The questions above are an assignment I would give students. Answer these questions, with sources, then relate how it affects you, the producer, and cattle markets. So, I decided to do the first part of the assignment!

Why is new world screwworm so bad?

Screwworms are parasitic flies that lay larvae which eat healthy flesh. The adult female has the potential to lay thousands of eggs near wounds or external openings on livestock. These eggs hatch, the larvae eat the flesh of livestock for 5-7 days then they fall out, burrow into the ground, and emerge as adults in about 5 days. Thousands of maggots can infest livestock leading to large open wounds and infection. There is a huge time and resource cost associated with inspecting and treating infected cattle.2,3

How is screwworm controlled now?

Screwworms were eliminated in the U.S., Mexico and Central America – but have had a resurgence in the two latter areas. Control uses a sterile insect technique. Facilities have been constructed which raise millions of flies on artificial diets. The offspring are sterilized through radiation. Sterile males are released in large numbers in affected areas. The wild female flies mate only once and lays eggs which do not hatch if mated with a sterile male.2,3

Why can’t we do the same with mosquitoes?

Mosquitoes are harder to control than screwworms. Mosquitoes mate more than once, females are picky about mates (possibly avoiding sterile males) and it’s harder to raise mosquitos in man-made facilities. Mosquitos require more sterile males to be released since they lay more eggs, have shorter life spans and denser populations. Also, with thousands of species, mosquitos would need separate facilities – not just one like with New World Screwworm. These reasons make it less cost-effective and limit viablity.3

Are screwworms coming back?

Maybe? In 2023 screwworm detections in Panama exploded from an average of 25 cases per year to more than 6,500.4 Screwworm detections have been continually being detected further north becoming problematic in southern Mexico in 2024.

Areas of central America with almost no cases now have active cases. Below is a map showing the spread from USDA Animal and Plant Health Inspection Service.5 This map really hits home the severity of this problem.

Screwworms were essentially eradicated everywhere shown on this map but are now creeping closer to the U.S.

New World Screwworm map

Why are screwworms coming back?

Screwworms were never eradicated from the world. Populations exist in South America and some Caribbean islands. Moving livestock and goods from these areas to Central America is a great way to reintroduce the pest back into areas. (That’s why U.S. imports are stopped) Also, if you haven’t had to look for an eradicated pest for 20-plus years it is easier for the invader to go unnoticed and continue to spread. 6

If you search this issue online (which you should do), a lot more possible reasons point to why screwworm may be returning. Viewer discretion is advised for pictures.

Answering my initial press release questions led me down a rabbit hole of information that shows screwworm is a serious issue and not just a passing news story.  

Contact Kenan Layden by dialing 701-401-9600 or emailing klayden@agmarket.net as well. Call 844-4AG-MRKT to contact anyone with the AgMarket.Net team.

  1. United States Department of Agriculture. (2025, May 11). Secretary Rollins suspends live animal imports through ports of entry along southern border, effective immediately [Press release]. https://www.usda.gov/about-usda/news/press-releases/2025/05/11/secretary-rollins-suspends-live-animal-imports-through-ports-entry-along-southern-border-effective[](https://www.hstoday.us/subject-matter-areas/border-security/u-s-suspends-live-animal-imports-at-southern-border-ports-effective-immediately/)
  2. Animal and Plant Health Inspection Service. (2023, February 9). New world screwworm story map. United States Department of Agriculture. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/new-world-screwworm-mapping
  3. Dyck, V. A., Hendrichs, J., & Robinson, A. S. (Eds.). (2021). Sterile insect technique: Principles and practice in area-wide integrated pest management (2nd ed.). CRC Press. https://doi.org/10.1201/9781003035572
  4. Animal and Plant Health Inspection Service. (2025, May 13). New World screwworm outbreak in Central America and Mexico. United States Department of Agriculture. https://www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/screwworm/outbreak-central-america
  5. Animal and Plant Health Inspection Service. (2025). [Map of New World screwworm detections by country with ports] [Image]. United States Department of Agriculture. https://www.aphis.usda.gov/sites/default/files/2025-05/nws-detections-by-country-with-ports.png
  6. Centers for Disease Control and Prevention. (2025, May 14). Resurgence of New World screwworm in Central America and Mexico: What healthcare providers need to know. https://www.cdc.gov/coca/hcp/trainings/resurgence-new-world-screwworm.html

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