The screwworm is a fly larva that eats living flesh instead of dead material.
Females lay their eggs in open wounds of warm-blooded animals like cattle but wildlife, pets and occasionally even humans can be infested.
A government program to breed sterile male flies and drop swarms of them from planes to mate with wild females had kept screwworm contained at the southern end of Panama for decades.
So far, there are four confirmed cases in Texas.
The USDA said the latest were a calf and a dog in La Salle and Andrews counties respectively.
The initial screwworm cases were discovered last week in two calves within a few kilometres of each other in southern Texas.
Scientists expect a handful of new cases could pop up in the coming days and weeks but it does not mean screwworm is spreading rapidly, said Edward Burgess, a University of Florida entomologist who studies the fly.
"When that first case is seen, everyone is being vigilant and their eyes are on it more intensely," Burgess said.
"And when you are looking for something, you are more likely to see it."
An agriculture department statement on Monday said officials are sampling suspected cases and working to eradicate the pest entirely.
The USDA and the US cattle industry have been racing to prevent an infestation since screwworm was detected in Mexico late in 2024.
Screwworm was a yearly warm-weather scourge of cattle ranchers before being eliminated in the US in the 1960s.
So far, its reappearance has not greatly affected beef prices, which are already near record levels because there are fewer cows in the US.
Although the parasite attacks live cattle, it does not infest meat or fruit.
There are also a dozen government-approved medications to treat livestock.
Canada temporarily stopped importing cattle, horses or other livestock from Texas on Friday.
The parasites prefer humid areas where temperatures are at 25C, making them more of a summer problem up north.
Burgess said the long-term solution - breeding sterile male flies - is months away.
Since wild female flies mate just once, if that encounter is with a sterile male, outbreaks can eventually be halted as the flies die out.
The USDA is working to both increase sterile fly production in plants outside the US and build a massive fly factory in Texas.
