Screwworms And The Death Of America’s Beef Industry

Screwworms have been detected in Texas. You probably don’t spend a lot of time thinking about screw worms.

You should.

Screw worms are parasitic worms found in Panama and Colombia. They lay their eggs in the skin of live animals, including cattle. The eggs then hatch, and the larvae (maggots) infest and feed on the living tissue of warm-blooded animals.

“The name screwworm refers to the maggots’ feeding behavior as they burrow (screw) into the wound, feeding as they go like a screw being driven into wood. Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly damage to the animal.

Adult screwworm flies are about the size of a common housefly (or slightly larger). They have orange eyes, a metallic blue or green body, and three dark stripes along their backs.

Report mammals and birds with the following signs:

• Irritated behavior

• Head shaking

• The smell of decay

• Presence of fly larvae (maggots) in wounds”

• Eggs hatch within 12-24 hours, and the larvae burrow into living flesh using hook-like mouthparts:

• Larvae feed for 5-7 days, enlarging the wound dramatically, causing severe pain, tissue destruction, foul-smelling discharge, secondary infections, and often death if untreated.

• Mature larvae drop to the ground, pupate, and emerge as adults. The full cycle can complete in as little as a few weeks in warm conditions, allowing rapid population growth.

  United States Department of Agriculture

Screwworms have been migrating north since mid-2025 at least.

There are also cases in which screw worms have infected people. To prevent that, one of the things that has to be done is that any outdoor laundry drying on lines has to be ironed to kill the eggs and avoid human cases. If you are infected, you then have to dig the developing maggots out of your skin.

Screwworms used to be a huge threat to the cattle industry in this country before we launched a massive effort to eradicate them. Annual losses in the 1930s-1950s reached tens to hundreds of millions of dollars (equivalent to much more today) from deaths, reduced weight gain, lower milk production, treatment costs, and labor. Infestations hit cattle hardest, but affected sheep, goats, horses, and wildlife as well. Newborn calves were especially vulnerable.

We eradicated screwworms in the United States in the 1960’s primarily through the use of something called the Sterile Insect Technique (SIT). SIT worked like this:

• Mass-rearing huge numbers of screwworm flies in facilities.

• Sterilizing the males (usually as pupae) with radiation (gamma or X-rays), which doesn’t kill them but renders their sperm non-viable.

• Releasing the sterile males in overwhelming numbers into the wild (often by airplane).

• Wild females mate only once in their lifetime. When they mate with a sterile male, they lay infertile eggs, producing no offspring. This progressively crashes the wild population.

There was a small outbreak of screwworms in the Florida Keys in 2017. It was quickly contained using the same method.

The U.S. cattle herd is already the smallest it has been since 1951. We suspended beef imports from Mexico in 2025.

Larval feeding causes rapid, severe tissue destruction: Maggots burrow into living flesh in any open wound (e.g., from branding, castration, dehorning, tick bites, scratches, or umbilical cords in newborns). Wounds enlarge quickly, attract more flies, lead to secondary infections, sepsis, and often death within 7–14 days if untreated.

One infested animal can spread the issue rapidly in a herd due to the fly’s mobility and high reproductive rate (females lay hundreds of eggs per batch; full life cycle in weeks in warm weather). In a full outbreak, significant portions of herds could be affected, with historical infestation rates around 20% in exposed populations.

The screwworm is already in Texas. That one state accounts for 14% of all U.S. cattle.

There have been several major screwworm outbreaks over the past century. The first recorded outbreak occurred from 1933-1936. Texas alone reported 3 million cases in 1935. The screwworm was believed to have claimed 180,000 head of cattle in 92 of Texas’s 254 counties, according to Charles Scruggs’ The Peaceful Atom and the Deadly Fly. By 1936, annual damage totaled $225.7 million in 2024 dollars.

Mortality and infestation rates vary. At the peak of the 1930s outbreak, about 12 percent of animals were infected, with a mortality rate of 12.4–19.4 percent. At the peak of the 1970s outbreak, the infestation rate in surveyed areas of Texas was 20.6 percent in cattle and 9 percent in sheep and goats. Texas is a particularly suitable environment for the parasite, given the state’s seasonal warmth, abundant potential targets and proximity to the southern border. During the 1970s outbreak, Texas had more cases than any other state in the U.S.

We have had a lot of warning that this threat was resurging. We don’t seem to have taken advantage of that to get ahead of it. We might want to move now with a sense of urgency. We are talking about the death of the American beef industry.