Page 284 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 253

a note on prions PrPsc (Figure 4.28) based on brain samples, par-
ticularly obex samples.
Unlike viruses, bacteria and parasites, prions
are entirely proteins. The misfolding of cellu- Benefits and limitations of rapid
lar prion proteins (PrPc) into pathogenic prion ‘pen side’ tests
proteins (PrPsc) and subsequent accumulation
of PrPsc leads to spongiform diseases such as Point of care or ‘pen side’ diagnostic tests need
scrapie, in sheep, bovine spongiform encepha- to be quick, simple to use and easy to interpret
lopathy (BSE) in cattle, and chronic wasting with little training. These tests must also be
disease (CWD) in deer. completely self-contained with no maintenance
Anti-mortem diagnosis based on combina- or calibration required. Rapid diagnostic tests
tion of neurological manifestations and real-time which can detect pathogens in as little as five
quaking-induced conversion (RT-QuIC) assay minutes are considered to be a good ‘pen side’
using various samples are being investigated, screening option to support more comprehensive
currently, the diagnosis of prion diseases is diagnostic laboratory services. However, despite
based on combination of neuropathology in the success in the laboratory, in the hot, humid areas
central nervous system and western blot assay where the test kits are likely to be used, ques-
targeting the detection of proteinase K resistant tions remain as to how reliable these tests are.
Few commercially available kits are validated for
field use in the populations of interest and they
are rarely developed to handle harsh climatic
conditions. However, the benefit of using pen
side tests (for example, DirectigenTM flu A+B,
Figure 4.29) is that a rapid result is available
and that the test can be performed with minimal
equipment and with limited training. The limita-
tions of these tests relate largely to the difficulty
in determining the diagnostic sensitivity (DSe)
and specificity (DSp) in the population of inter-
est. Although most of the pen side kits available
do provide data to support claims of high DSe
and DSp, test performance should really be
established for the specific population of inter-
est. This can be done by comparing the results
obtained with the kit test and those obtained
using a ‘gold standard’ test such as classical cul-
Figure 4.28 Western blot assay demonstrating ture or a traditional serological screening test.
pathogenic prion proteins, PrPsc, in brain homog- In the laboratory setting there may be scope
enates. The brain homogenates (10%) originated to develop and validate new ‘in-house’ diagnos-
from pathogenic prion infected and non-infected tic tests but this is rarely justified when there are
C57BL6 mice were either non-treated with pro- a wide range of commercial kits readily available
teinase K (-PK) or treated with proteinase K (+PK) for the detection of antibodies (that is, serologi-
for 1 h at 37°C and assayed. Photo: Sabine Gilch, cal test) or microorganisms (antigen detection
University of Calgary, Canada. test) in clinical samples (Table 4.6). Although

Vet Lab.indb 253 26/03/2019 10:25