The current tests to identify strains of TSE involve inoculation of a test species with brain extracts from infected animals then comparison of brain lesions after injection and/or analysis of PrP proteainase K sensitivity (see below).

Below is a cartoon based on work by Lasmezas et.al. illustrating the type of results that are obtained with strain typing using proteainase K sensitivity of the PrPs in test animal brain samples, in this case mice.

• This work clearly shows that normal control PrP^C is loss with digestion by Proteinase K (first lane without bands). In contrast bands corresponding to various forms of PrP^Sc remain in samples collected from mouse brains after infection with different strains of TSE.
• The work found similarities between mice injected with sheep Scrapie and a form of human sCJD; i.e. band 2 was more intense than 3 or 4 and very little high molecular weight PrP proteins (band 1)
• The work found similarities between BSE and vCJD PrP^Sc; i.e. band 2 and 3 very intense and more so than band 4 and quite a bit of a high molecular weight PrP protein (band 1)
• mice injected with tissue from a iatrogenic CJD of human origin with similar but slightly different than Scrapie or spontaneous CJD injected mice

  This and other studies with similar results gave evidence indicating that BSE probably gave rise to vCJD.

In this study the similarities between Scrapie and sCJD indicate either a common source or a similar effect of different strains of PrP^Sc.

This type of analysis is the current litmus test for 'Strain Typing' of various TSEs.

However, this test is not without limitations

1. Strain typing is time consuming, expensive and laborous
• tissue must be collected and prepared
• mice must be injected and maintained with daily feeding and observation
• dependant upon the infectuous strain, test species and mode of infection, an incubation time of 6mo to 2yr is needed for the disease to develop (intracranial injection shortens the time needed compared to intraperitoneal or oral)
• analysis requires killing the test animals (usually mice or hampsters), collecting brain, prepping the tissue and analysis by either histology or gel electrophoresis
2. When injecting test animals with PrP^Sc from different animals to test for strain identities, similarities in PrP protease resistance implies but does not prove that the TSEs arose from the same source
• in the example above BSE and vCJD are now believed to be linked (cause-effect relationship)
• in contrast Scrapie is currently not known to be contageous to humans
  • similarites between Scrapie and sCJD are probably cooincidental, indicating that the PrP^Sc protein may only be able to assume a limited number of configurations associated with TSE
  • eventually two or more mutations with PrP mis-foldings or spontaneous mis-foldings will eventually result in the same shape of PrP protein and hence pass on similar diseases to a test animal
  • the number of TSE strains possible is not currently known
• the fact that Scrapie is not believed to be contageous to humans indicates that species specificity is not limited to the mis-folding effect on PrP alone
3. PrP^Sc vary in their degree of sensitivity to Proteinase K; this could lead to a false negative if the protease is not dosed properly
• control brains contain PrP 27-30 (not PrP^Sc 27-30)
• proteinase K must be added at a dose to digest all of the normal PrP^C to avoid false positives
• proteinase K may require tests at various doses to identify those strains with low proteinase resistance
4. Species barriers exist that prevent testing of some TSE strains without serial passaging through an intermediate species or subsequent passaging through the same species until the disease manifests at a level adequate for analysis
• this adds cost and time to the analysis
• this also increases the possiblility of creating a new strain as a result of serial passaging through either different or the same species of test animal
• click on image to learn more about strain fidelity