In the United States, the CDC has defined 4 different biosafety levels for handling biological materials of different risk levels. These mean different requirements as to what containment practices have to be used in order to work safely with those materials. This is all outlined in the CDC’s Biosafety in Microbiological and Biomedical Laboratories, currently on its 5th edition (BMBL5) as of December 2009.
Section IV describes the containment practices for each biosafety level and Table 2 (on the last page of Section IV) gives the criteria for inclusion in each category, which I’ll summarize in a very broad stroke as follows:
- BSL-1: Things that don’t usually cause disease in humans, like regular old E. coli.
- BSL-2: Things that either only cause mild disease (like the flu) or are unlikely to be transmitted by inhalation (like hepatitis).
- BSL-3: Things that could kill you if you inhale them, like tuberculosis.
- BSL-4: Things that could easily kill you and maybe lots of people, and are not treatable, like Ebola.
Based on this, you’d think prions would probably fall into BSL-2. They’re very scary, but aren’t transmitted by inhalation.
In practice, what we’ve heard anecdotally is that most institutions in the U.S. treat most animal prions at BSL-2 but human prions (whether from human tissue or from transgenic animals expressing HuPrP) at BSL-3. Michigan State’s biosafety protocols for prions are available online and they are more or less in agreement with what I’ve just said. At Michigan State, animal prions are all considered BSL-2 with the exception of BSE (since that’s the one animal prion strain that has been shown to be transmitted to humans); BSE and human prion tissues are considered BSL-3 “under most experimental conditions” though under certain circumstances only require BSL-2.
It appears that the choice of BSL-3 for human prions is a choice made by the institutions themselves, and is not explicitly required by CDC regulations. BMBL5 Section VIII-h deals specifically with prions. Here’s what it has to say about recommended biosafety levels:
In the laboratory setting prions from human tissue and human prions propagated in animals should be manipulated at BSL-2. BSE prions can likewise be manipulated at BSL-2. Due to the high probability that BSE prions have been transmitted to humans, certain circumstances may require the use of BSL-3 facilities and practices. All other animal prions are manipulated at BSL-2.
So according to CDC, even BSE and human prions can be handled at BSL-2 most of the time, consistent with its definition in Section IV of the BSL-2 vs. BSL-3 distinction as resting on whether the agent can be infectious by inhalation. But this “certain circumstances” phrase is a bit ambiguous: which circumstances, exactly, require BSL-3?? Only two specific circumstances called out later in the document – BSE and autopsies – and even there, CDC recommends “a BSL-2 facility utilizing BSL-3 practices” as a minimum. Regarding human tissue:
Unfixed samples of brain, spinal cord, and other tissues containing human prions should be processed with extreme care in a BSL-2 facility utilizing BSL-3 practices.
And regarding BSE:
The most prudent approach is to study BSE prions at a minimum in a BSL-2 facility utilizing BSL-3 practices.
It appears, then, that no procedures strictly require BSL-3 facilities according to federal regulations, though of course the above practices are presented as a “minimum”. Presumably the institutions utilizing BSL-3 facilities for all human prions have made that decision according to their own assessment of what approach they think is best.