We have no idea if US Beef is infected. The USDA prohibits anyone from testing their cows and ended their spot-testing program (which was a joke).
So unlike Europe we never reckoned with the problem and don't know if it is currently spreading. For human exposure we would not yet be far enough along to know if the US Beef supply either is or was contaminated.
Personally I have basically stopped eating beef for this reason.
It is estimated that hundreds of thousands of infected cattle entered the food chain in the 1980s. The total number of cases of vCJD is at less than 250 as of 2018.
Feeding practices that led to BSE have changed, the USDA is still testing for BSE, and if the disease was spreading, we'd probably notice it at some point.
Personally, I'd be far more worried about cattle farming practices spawning some sort of super-resistant flesh-eating bacteria than anything related to BSE.
Also worth noting that only four of those cases occurred in the US, and none of those have been traced to US beef; in each of these cases, the patients had spent a significant time living outside of the US (and two of the patients lived in the UK, whose cows were known for having BSE).
The USDA does test a sample of cows, but prohibits companies from doing testing on their own cows probably because the tests would likely be inaccurate and provide an unwarranted impression of safety:
> NOT A FOOD SAFETY TEST
> BSE tests are not conducted on cuts of meat, but involve taking samples from the brain of a dead animal to see if the infectious agent is present. We know that the earliest point at which current tests can accurately detect BSE is 2-to-3 months before the animal begins to show symptoms. The time between initial infection and the appearance of symptoms is about 5 years. Since most cattle that go to slaughter in the United States are both young and clinically normal, testing all slaughter cattle for BSE might offer misleading assurances of safety to the public.
> ...
> Why doesn't USDA test every animal at slaughter?
> There is currently no test to detect the disease in a live animal. BSE is confirmed by taking samples from the brain of an animal and testing to see if the infectious agent - the abnormal form of the prion protein - is present. The earliest point at which current tests can accurately detect BSE is 2 to 3 months before the animal begins to show symptoms, and the time between initial infection and the appearance of symptoms is about 5 years. Therefore, there is a long period of time during which current tests would not be able to detect the disease in an infected animal.
> Since most cattle are slaughtered in the United States at a young age, they are in that period where tests would not be able to detect the disease if present. Testing all slaughter cattle for BSE could produce an exceedingly high rate of false negative test results and offer misleading assurances of the presence or absence of disease.
> Simply put, the most effective way to detect BSE is not to test all animals, which could lead to false security, but to test those animals most likely to have the disease, which is the basis of USDA's current program.
The ban might not be warranted (I don't believe it is myself), but it is important to be aware that testing for BSE may not be accurate at the time most cows are slaughtered.
Thereby combining all of the potential isolated, individually bad disasters into one enormous fully-correlated watershed moment when all companies are simultaneously found to be selling prion-containing beef...
We did find out. The US has had a total of 4 cases of vCJD, all of which were contracted overseas.[1] For comparison, the UK has had over 170 cases.[2]
So unlike Europe we never reckoned with the problem and don't know if it is currently spreading. For human exposure we would not yet be far enough along to know if the US Beef supply either is or was contaminated.
Personally I have basically stopped eating beef for this reason.