Global AgriTrends

06.08.26

New World Screwworm Confirmed in Texas, background, thoughts and expectations

USDA confirmed a positive case of New World Screwworm in Texas June 3rd. This is reportable disease under global rules and was completely expected. However, unlike many other reportable diseases, this one is unlikely to affect markets in a significant way. It does not affect the safety of the food supply and is more easily treated. But producers in the southern states will need to adjust how they monitor and treat cattle, and USDA will now shift into eradication mode.

https://www.aphis.usda.gov/news/agency-announcements/usda-confirms-presence-new-world-screwworm-united-states

Some thoughts on New World Screwworm in America:

- How will U.S. beef and cattle exports be affected? The confirmation of New World Screwworm in Texas will likely spur cattle futures market uncertainty but is unlikely to have any significant impact on supplies or demand. Exports are unlikely to be affected (Mexico is still exporting beef to Japan and Korea), but live cattle exports could be banned by some nations. Of keen interest will be Canada. The U.S. shipped over 490,000 head of feeder cattle to Canada last year. There could be regional restrictions by Canada.

- How do cattle, beef, and futures markets react? Uncertainty is typically negative in futures markets. But the markets surged HIGHER on the news, possibly (incorrectly) predicting heavy culling? And also realizing that the border is now less likely to open to Mexican feeder cattle imports. This situation is unlikely to affect either the supply or demand of beef in America. Cash and spot prices are expected to continue to trade the existing fundamentals: production is down -5.6% YTD, and demand remains stellar.

- Will this affect consumer demand for beef? NWS poses no risk to beef or food systems. We do not see any significant shift in demand.

- How does this affect cattle and what will producers need to do? The case recently found in Texas included larvae implanted by flies on the navel of a newborn calf. These flies seek any wound in which to lay larvae. Once detected the wounds can be treated with insecticide. And there are pour-ons, tags, and other means to reduce fly contact with livestock. But careful monitoring of wounds will be necessary, especially after tagging, branding, castrating, and dehorning. Even a scratch from a fence or thorn provides an entry point for screwworm infection. Cattle will need to be monitored more closely and quickly treated when wounds are found.

- Will this mean an opening to the border for Mexican feeder cattle? As USDA moves to eradicate NWS, the border is likely to remain closed. But eradication efforts will be very difficult based on 1) flies continuing to spread in Mexico and along the border, and 2) wildlife populations that are not treated for infections.

- How far will this fly migrate into the U.S.? Note that NWS is a tropical fly. At 60 degrees F flies stop flying. And at 51 degrees F they die. This will be a seasonal problem in Texas and other Southern states, but the problem is unlikely to move into Central or Northern regions, except when transmitted via cattle movements. The furthest north cases back in the prior infestation in the 1960s was in Arkansas.

- What can be done to prevent the spread and eradicate the flies? The last NWS cases in America were eradicated in 1996 using the Sterile Insect Technique (SIT): Scientists mass-produced screwworm flies, male flies were sterilized using radiation, millions of sterile males were released by air, female screwworm flies mate only once, when they mated with sterile males, no offspring were produced, and repeated releases caused the population to collapse.

- How was it eradicated from America in the 1960s? The former USDA SIT eradication campaign began in the Southeast in the 1950s, expanded across the Southwest, and eliminated self-sustaining U.S. populations by 1966. The program then pushed south through Mexico and Central America. Mexico was officially declared screwworm-free in 1991.

- Why has it come back after all those years? The U.S. and Panama have been releasing around 100 million flies per week on the Darian Gap (narrow neck) region of Panama for decades. It has been effective until recent years when it appears to have lost efficacy for some unknown reason. The current SIT fly program by USDA to prevent spreading across the 1,954 (3,145 km) mile U.S. border with Mexico appears to be a very difficult prospect.

- What about hogs, horses, sheep and other animals? These flies will lay eggs in the wound of any animals, or even humans. Mexico has reported human cases of screwworm. All animals will need to be carefully monitored with insecticides applied promptly to wounds.

More from USDA on New World Screwworm at: https://www.aphis.usda.gov/animals/animal-health/livestock-and-poultry-disease/stop-screwworm

Brett Stuart