13-06-2025
County officials warn of possible fly infestation
Although no Texas cases had been reported as of Tuesday, Johnson County Commissioner Larry Woolley warned of a potential screwworm fly infestation in mid to late summer.
In May, the U.S. Department of Agriculture suspended importations of cattle, horses and bison from Mexico into the U.S. to prevent New World screwworm movement north following detection in Veracruz, Mexico.
'A lot of people may not remember the last time one of these happened,' Woolley said during Monday's Johnson County Commissioners Court meeting. 'I was in high school. I remember it vividly. It's not a pretty site and it's not something to be taken lightly.'
Commissioner Kenny Howell agreed.
'I remember that too,' Howell said. 'It was a huge problem.'
Although the closest detection is still 700 miles away, the New World screwworm is of increasing concern across the southern U.S., as it can potentially cause losses in the livestock and wildlife-related industries if it moves across the Mexican border and reestablishes.
The USDA Animal and Plant Health Inspection Service is working with Mexican officials to train personnel and expand surveillance and eradication efforts in Mexico to better address the movement of the fly northward.
Should the New World screwworm enter the U.S., this fly is estimated to cause losses, conservatively, of $2.1 billion to cattle and $9 billion to the hunting/wildlife industry in Texas.
Where the larvae of other flies feed on carcasses, the larvae of these flies feed on the live flesh of animals including people.
'All it takes is a cut or a drop of blood to attract them,' Woolley said. 'They are flesh-eating larvae called screwworms.'
The larvae of the New World Screwworm flies burrow into the flesh of warm-blooded animals resulting in damage, possible secondary infections and potentially death, according to the Centers for Disease Control and Prevention.
'The control method then and now is the distribution of sterile male flies into the population, and that's something they're working on now,' Woolley said.
Typical insecticide use on the New World screwworm is not as effective as with other insect pests due to its wide host range and occurrence on wildlife. Eradication is the best way to control screwworms. However, control/removal of the fly when found on hosts is critical to reducing the local population so that the sterile fly program succeeds more quickly and reduces animal suffering.
The New World screwworm is incapable of overwintering in regions that experience low temperatures or extended dry periods — they don't do well below 45 degrees and certainly die if frozen, however, the larvae can burrow about 6 inches into the soil, changing the temperatures that they experience.
This fly poses a serious threat to livestock and wildlife because it deposits eggs into the wounds of living animals, thereby leading to larval infestations. Because they can host on any living mammal, they can also affect pets and humans.
This metallic green-blue fly with large orange eyes is about twice the size of a common house fly. New World screwworms do not bite animals; their damage comes from laying eggs in the open wounds of living, warm-blooded animals — though rarely birds. The immature fly larvae, or maggots, feed on the living tissues of hosts, burrowing into the animal's tissue, causing tremendous damage and often resulting in the host's death.
Historically, ranchers have altered breeding dates to avoid birthing during the fly season. Common livestock management practices and events, such as castration, dehorning, branding and birthing in all animals and antler velvet shedding in deer, often resulted in infestations. Infestations can occur at the site of any recent wound, like a scrape, lesion or even a tick-feeding site.
Other wounds of common infestation include those from ear tagging or shearing, as well as mucous membranes and antler shedding in deer. Newborn mammals are prone to infestations at the healing umbilical cord site.
If a wound on a living animal is found to have larvae in it, a veterinarian must be contacted. Find an accredited veterinarian or Texas Animal Health Commission, TAHC, region office. The veterinarian is mandated by law to collect and submit the larvae for identification to the USDA's National Veterinary Services Laboratory.
Also, the animal should be immediately confined for wound care, and all larvae should be removed and placed into alcohol. After removing the larvae, treat the infestation with topical pesticides labeled for use on the animal to kill any larvae that may not have been removed and reduce the likelihood of re-infestation. The wound should be monitored until it fully heals. When dealing with livestock, all other animals in the herd should be checked for wounds and larvae.
Anyone who finds fly larvae infesting a living animal, called myiasis, must report this to a state veterinarian. The protocol for reporting can be found on the Texas Animal Health Commission website or at the USDA-APHIS website.
An informational webinar on the topic is available at and as a link through the county's website.
'I encourage people to watch that to be educated on the possible problems this may pose,' Woolley said. 'It can affect livestock, pets, people.'
