- What typically indicates a positive EIA test?
Part 3
Along with the RT/PCR analysis, David’s saliva was also collected for viral cultivation. In general, no single
diagnostic test is sufficient for antemortem diagnosis, since the results will depend on the sensitivity of the
assay, the quantity of virions present at the time of testing, and the timing of the assay, since release of
virions in the saliva can vary. As it turns out, the result was negative for viral cultivation from the saliva. This
is not surprising to David’s doctor, because one negative result is not an absolute indication of the absence of
infection. It may be that the number of virions in the saliva is low at the time of sampling. It is not unusual to
repeat the test at intervals to enhance the chance of detecting higher virus loads.- Should David’s doctor modify his course of treatment based on these test results?
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6.4 Viroids, Virusoids, and Prions
Learning Objectives
- Describe viroids and their unique characteristics
- Describe virusoids and their unique characteristics
- Describe prions and their unique characteristics
Research attempts to discover the causative agents of previously uninvestigated diseases have led to the discovery
of nonliving disease agents quite different from viruses. These include particles consisting only of RNA or only of
protein that, nonetheless, are able to self-propagate at the expense of a host—a key similarity to viruses that allows
them to cause disease conditions. To date, these discoveries include viroids, virusoids, and the proteinaceous prions.
Viroids
In 1971, Theodor Diener, a pathologist working at the Agriculture Research Service, discovered an acellular particle
that he named a viroid, meaning “virus-like.”Viroidsconsist only of a short strand of circular RNA capable of self-
replication. Thefirstviroiddiscovered wasfoundtocause potatotuberspindle disease,whichcauses slowersprouting
and various deformities in potato plants (seeFigure 6.24). Like viruses, potato spindle tuber viroids (PSTVs) take
control of the host machinery to replicate their RNA genome. Unlike viruses, viroids do not have a protein coat to
protect their genetic information.
Clinical Focus
Chapter 6 | Acellular Pathogens 261