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Diagnosing Viral Diseases in our Patients
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Introduction
We all do it...diagnose viral infections. Sometimes we just rely on our clinical judgement. In other cases, we may back this up with a specific test which we either perform ourselves or send off to a lab. But do we know actually know what we are doing? What are we testing for? How do the tests work? How should we interpret positive and negative results? Here we will describe the methods used to diagnose viral infections and think about common pitfalls in their interpretation, using important examples from small animal practice to illustrate key points. Is any of this important? YES. If you get this stuff wrong, you will interpret tests wrongly, and possibly carry out inappropriate clinical procedures on someone’s precious pet!.
Diagnostic Tests
All viral diagnostic tests can be divided into two categories, either those that detect parts of the virus or those that detect antibody.
1. Virus detection
Virus isolation (VI) relies on the ability of some viruses to replicate in cell cultures causing cell damage (cytopathic effect - cpe). It works best for viruses that are stable outside the host (e.g. canine parvovirus, feline calicivirus and cowpox virus), and poorly for very fragile viruses (e.g. feline coronavirus). VI requires specialist facilities and can be slow.
Viral antigen detection. This relies on the ability of viral antigens within clinical samples (e.g. blood and tissue sections) to specifically react with antibodies raised to that particular antigen in the laboratory. The antigen must be immobilised to a solid surface. In some clinical specimens, the viral antigen may already be immobilised on a glass slide within infected cells such as in a tissue smear or tissue section. In other cases, where the antigen is free in a biological solution, the antigen must first be captured onto a solid surface with a specific laboratory antibody (antigen capture or ‘sandwich’). Once immobilised, antigens can be detected by incubation with a specific laboratory antibody that contains a “molecular beacon” so it can be seen.
A good example is feline leukaemia virus in which p27 is detected in the blood of infected cats, either by immunofluorescence on a blood smear, or by rapid immunomigration (RIM) kits used in general practice for “pen-side” diagnosis.
Polymerase chain reaction (PCR). Unlike other methods, PCR detects viral genetic information rather than protein. PCR is fast, often more sensitive than other methods, can quantify viral load, and if sequenced, allows the source of infections to be investigated.
Histopathology. For some viruses, the pattern of the histopathological changes are so specific that it can allow a definitive diagnosis to be made. Examples include FIP, intranuclear inclusions (herpesviruses, canine adenovirus), or intracytoplasmic inclusions (cowpox, rabies Negri bodies, canine distemper). The presence of viral antigen may be confirmed by immunohistology.
Electron Microscopy (EM). Perhaps the purest example of virus antigen detection is direct visualisation by EM. It is still used for cowpox diagnosis, or for the demonstration of virus particles in faeces.
Haemagglutination (HA). Some viruses have the seemingly peculiar ability to agglutinate (clump) red blood cells from species other than the viruses natural host. Such clumped red cells can be seen with the naked eye. The best examples of HA in small animal diagnosis are the parvoviruses of cats and dogs. [...]
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