On April 24, the U.S. Department of Agriculture reported the fourth confirmed case of mad cow disease in the U.S., the first since 2006. In an official statement, the department’s chief veterinary officer John Clifford said that the animal (a dairy cow from central California), “was never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health.” That’s good news: it means that the USDA protection system—banning cattle that cannot walk on their own and testing them for disease—worked. But this latest discovery also suggests that a new case will appear every few years and that a few such cows may have entered the food system in the past.

What’s interesting about this latest cow case is not so much that the animal had bovine spongiform encephalopathy (BSE), the medical name for mad cow disease, but the type of BSE it had. Biochemical tests on its brain tissue showed that the cow had an “atypical” variety of BSE—that is, the disease did not come from tainted feed, which triggered the U.K. epidemic in the late 1980s and led to deaths from the human version of the sickness, known as variant Creutzfeldt-Jakob disease (vCJD). Most nations, including the U.S., have instituted regulations that limit the odds of cattle eating contaminated feed, an approach that has succeeded in containing the classic form of BSE.

Instead, the Californian cow had the “sporadic” form, which arises not because of contaminated feed but because of other, unknown reasons. Humans also suffer from sporadic forms of this brain-wasting ailment, which is caused by malformed proteins called prions. And despite decades of work trying to find environmental and genetic triggers for it, scientists have come up empty. Part of the reason lies in the rarity of this random form of prion disease: it hits about one in a million people—mostly in their later years.

No one knows the prevalence rate of sporadic BSE—there just isn’t enough data. The USDA’s targeted surveillance program tests roughly 40,000 cows per year. But to determine if sporadic BSE exceeds the human rate of one per million, researchers would need to conduct tests on three million randomly selected older cattle, according to a 2006 analysis by researchers led by Paul Brown, a longtime prion disease expert from the National Institutes of Health.

But one in a million is a reasonable assumption to make for cows if sporadic prion disease is truly random. So let’s work the numbers. According to the USDA’s livestock slaughter summary report (pdf) of April 2012, 34.1 million cattle were slaughtered last year, and 8.7 percent were dairy cows (which typically end up as ground beef rather than as steaks). According to the paper by Brown and his colleagues, about 10 percent of sporadic human cases occur in middle-aged individuals whose cattle-equivalent age is about seven to 13 years, when dairy cows typically face slaughter. (In contrast, beef cattle are usually killed before they reach two years old.) So that means about 0.3 sporadic cases of BSE (or 34.1 million x 0.087 x 0.10, divided by one million) can be expected to occur each year, or one every three to four years.

That’s a very low rate. And even if a couple of slaughtered dairy cows with sporadic BSE slipped into our food supply over the past 10 years, the risk of contracting the human form of the illness is extraordinarily low. At the height of the European mad cow epidemic, when millions of people unwittingly ate risky beef, only 222 became infected and developed vCJD since surveillance began in 1990. (Three others got sick via blood transfusions from asymptomatic donors.) A theorized second wave of human cases, based on genetic profiles, has not materialized—and at this point, given that so much time has passed, probably never will.

If sporadic BSE has been with us for decades, then cows with the illness probably entered the food system in the past (especially before the U.S. implemented more stringent rules in 2003). They did not cause any known problems for human brain health. That’s even more reason to think that the latest case doesn’t mark some sort of silent BSE time bomb. Reactions such as a South Korean retailer suspending the sales of U.S. beef there are overblown.

In fact, we should expect to discover a sporadic BSE case every few years. Not finding them would mean the U.S. surveillance program needs to be tightened. A watchful eye on our herds is important, because the behavior of prion diseases is notoriously difficult to predict, especially if the prions jump species and establish new strains. Then, all bets are off.