Page 272 - The Veterinary Laboratory and Field Manual 3rd Edition

Microbiology 241

Properties of viruses Methods used to diagnose viral
infections
Viruses are obligatory parasites requiring living
cells for replication. Similar to other microorgan- In common with the diagnosis of other diseases,
isms they are infectious, many are host specific the identification of a viral disease depends on
and not all are pathogenic. Mature viruses, also assessment of clinical signs and consideration
called virions, vary considerably in size. For of the epidemiology of the disease condition or
example, porcine circoviruses are the small- outbreak. As indicated in Figure 4.15, a thor-
est animal virus with the size of approximately ough clinical evaluation allows the selection of
–9
17 nm (×10 m) in diameter, foot-and-mouth suitable samples for laboratory diagnosis.
disease (aphthovirus) virions are approximately As shown in Figure 4.16, various compo-
27 nm in diameter, while the poxviruses (larg- nents of the virus structure can be detected
est animal virus family) are nearly 300 nm in or quantified using modern laboratory assays.
diameter. Recently, Pandora viruses with the size This is often based on specific antigen detection
of 1000 nm have been described inhabiting pro- using serological techniques (see Chapter 6).
tozoa rebutting the fact that the viruses are not Appropriate sample collection is a critical step
visible under a light microscope. that will impact the ability of the laboratory

Figure 4.15 Steps in viral disease diagnosis starting from the evaluation of clinical signs leading to labora-
tory confirmation.

Figure 4.16 Modern laboratory assays are available targeting various structural components of the envel-
oped and naked viral particles.

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