'Mad cow' disease, or bovine spongiform encephalopathy (BSE), is one of the scariest and strangest conditions of the past 20 years.

The degenerative disease is spread through infected cow meat and blood transfusions, and causes rapid brain decay. It's always fatal, and there's no cure. But most terrifying of all, doctors have never been able to test for it, and still aren't sure how many people could be infected, which is why many people who lived in the UK during the '80s and '90s still aren't allowed to donate blood.

But that might be about to change, with two teams of scientists developing blood tests that can detect the human variant of the disease, Creutzfeldt-Jakob disease (vCJD), with 100 percent accuracy, and years before symptoms arise.

Both blood tests have only been trialled in small sample groups so far, so it's too soon to say for sure that they'll be useful for the greater population.

But the results have been published alongside each other in Science Translational Medicine (here and here), and they signal one of the first major breakthrough in the disease in years.

For those young enough not to remember the big 'mad cow' disease scare of the '90s, the condition seemed to appear out of nowhere, causing fatal degeneration of the brain and spinal cord.

The first symptoms are usually depression and hallucinations, which are followed rapidly by dementia and loss of motor control. Within a year of symptoms arising, most patients will be dead.

But the more researchers found out about the condition, the more scary it seemed.

It's now known that the disease is spread through misfolded proteins known as prions. These weird molecules start out as harmless brain proteins, but become mutated somehow and turn into highly contagious pathogens that recruit any other prions they come into contact with, grouping together in clumps that damage the brain.

These abnormal prions infect cows when cattle eat the offal of other cows, and the same thing happens in cannibalistic human populations, which also develop prion diseases.

Once it became clear that these abnormal prions were being spread through the meat of infected cows, there was a huge sterilisation of the cattle industry.

In the UK, where the condition has killed 177 people, more than 4.4 million cows were slaughtered during the eradication program between 1986 and 1998.

Even worse, prions can't be easily killed by heat or radiation, so scientists still don't really know how to stop them, and we have no way of knowing how many people out there are still infected.

It's not yet clear how long prions can remain dormant in blood - in cows it often takes 2.5 to 5 years for symptoms to appear, and incubation periods as long as 50 years have been reported in humans.

Recent studies have suggested that as many as one in 2,000 people in the UK might carry abnormal prions - and thousands of people around Europe could still be infected - but it's not known when or even if any of these carriers will ever develop vCJD.

Even now in the US, Australia, and many other countries, people who lived in the UK or parts of Europe between 1980 and 1996 can't donate blood. The UK also won't use local blood to manufacture certain products, such as albumin, in an attempt to prevent secondary infections in the country.

The only way to know for sure how many people are still infected is to come up with a blood test for abnormal prions, and for more than a decade, researchers have been struggling to develop one. But these two new studies could finally represent real progress.

"There is new technology to go forward, and it looks promising," Jonathan Wadsworth, a biochemist who studies prion at University College London, but wasn't involved in the study, told Kelly Servick over at Science. "These are definitely very welcome papers."

Both new tests take a slightly different approach, but have the same basic idea - they mimic the progression of the disease in an amplified environment so that tiny traces of abnormal prions in the blood become easily detectable.

First of all, they separate prion proteins and culture them alongside normal proteins, and use sound waves to agitate the prions.

This results in more fresh clumps of abnormal prions breaking off, and converting their neighbours, so that eventually there's a whole lot of mutated prions clumped together.

These are then pulled apart and cultured with normal proteins, and the process is repeated over and over again until the number of abnormal prions is easily detectable.

The first test, developed by Claudio Soto from the University of Texas, trialled the approach on blood from 14 vCJD patients and 137 controls, some of which were healthy and some of which had other neurodegenerative diseases. The test was 100 percent accurate at identifying vCJD.

The second test, created by Daisy Bougard from the University of Montpellier in France and her team, accurately identified 18 vCJD patients out of 256 samples.

The French test also identified vCJD in two patients before they even showed symptoms - a first for prion disease.

Together, the two tests identified a total of 32 cases of vCJD from 391 control samples, with 100 percent accuracy.

"That’s very encouraging," Wadsworth told Science.

Both of these studies are based on small sample sizes - too small to tell if the tests might be useful in large-scale blood transfusion screening - but the researchers are now hoping to test in bigger groups and continue to tweak the technique.

In fact, Soto told Scientific American that he's already partnered with a startup called Amprion to develop their technology, and is currently working on getting approval in the US and Europe to use the test for blood screening. He predicts the test to be available commercially within one to two years.

That might be a little optimistic, but so far progress is looking good.

And while vCJD, or 'mad cow' disease, might not be the looming threat it was 20 years ago, the reality is that we stil have no idea how many people are silent carriers of the disease, and what kind of risk they pose - if any - to populations today.

But without a cure for vCJD, being able to clear blood samples of abnormal prions once and for all would be a huge step forward, and would also make it possible for tens of thousands more people to donate much-needed blood supplies.

You can read the two papers here and here.