An 11-year-old cow in Alabama tested positive for an “atypical” strain of the prion disease Bovine Spongiform Encephalopathy (BSE), also known as mad cow disease, according to the US Department of Agriculture.

The cow tested positive for the strain, called L-type BSE, during routine surveillance at a livestock market where the animal had started exhibiting clinical signs. The USDA stressed that the case posed no health threat and would not change the country’s international risk status, and thus it would not cause any beef trade issues.

“This animal never entered slaughter channels and at no time presented a risk to the food supply or to human health in the United States,” the USDA said in a statement.

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BSE, like all spongiform encephalopathy diseases, involves malformed prion proteins that spur normal proteins to deform and clump in the brain and spinal cord. In cows, this leads to tremors, balance issues, and changes in behavior, such as excessive aggression and hyper-activity.

Classic BSE, or C-type, primarily spreads among cattle through feed contaminated with meat or bone from an infected animal. That was the case in the mad cow outbreaks that devastated UK herds from the 1980s to early 2000s. The C-type also spread to people who ate the brain, spinal cord, or digestive tract of infected animals, causing the human counterpart of BSE, Creutzfeldt-Jakob disease (CJD).

Because of those outbreaks, there is now strict surveillance of US livestock, a ban on using mammalian protein in livestock feed, and other safeguards.

But two atypical strains of BSE still pop up randomly worldwide—although very rarely. These atypical strains—L-type and H-type—are thought to occur spontaneously in older animals that have had no prior symptoms or exposures. However, it can’t be entirely ruled out that they can be spread in feed. Animal studies suggest that, like C-type, they can jump to other cows and potentially humans.

The full differences between the three types of BSE are still not clear. The designations simply relate to the prion proteins’ molecular weights relative to C-type: L-type has a lower-weight prion compared to C-type, while H-type has a higher-weight prion. These atypical types also create different lesion patterns in the brains of infected cows.

What is clear is that these atypical types are very rare. Only a few dozen cases have ever been reported worldwide. The Alabama cow is the fourth atypical case ever detected in the US.

Apart from those, there has only been one case of C-type BSE in the US. It was detected in 2003 in a cow on a Washington farm that had been imported from Canada.