Page 281 - The Veterinary Laboratory and Field Manual 3rd Edition

250 Susan C. Cork and Roy Halliwell

These tests are particularly useful for the diag- a notE on tHE aGar GEL IMMunodIFFuSIon
nosis of viral infections (see also Chapter 6). tESt (aGId)
Antibodies can also be used for the identification The basis for the AGID test (see Figures 6.8a–
of an unknown virus in the laboratory (see Table c) is the concurrent migration of antigen and
4.6). Some viruses will agglutinate erythrocytes, antibodies towards each other through an agar
this is a characteristic that can also be used for gel matrix. The test is commonly used for the
virus characterization (for example, haemagglu- detection of antibody to avian influenza viruses,
tination is used to identify sub-types of avian paramyxoviruses, haemorrhagic enteritis virus
influenza virus). and equine infectious anaemia virus. When the
The serological methods commonly employed antigen and the specific antibodies come in con-
in viral disease diagnosis include the following: tact, they combine to form a precipitate in the gel
matrix resulting in a visible line. The precipitin
• agglutination and precipitation line forms where the concentration of antigen
• complement fixation (CFT) and antibodies is optimum. Differences in the
• serum neutralization (SN) and virus neutral- relative concentration of the antigen or antibod-
ization (VN) ies will shift the location of the line towards the
• inhibition of cytopathic effects (CPE) in tis- well with the lowest concentration or result in
sue culture the absence of a precipitin line. Electrolyte con-
• haemagglutination and haemagglutination- centration, pH, temperature, and other variables
inhibition (HA and HI) also affect precipitate formation.
• enzyme-linked immunosorbent assay (ELISA)

Complement fixation
Agglutination and precipitation
The complement fixation test (CFT) is an immu-
Serological reactions such as agglutination and nological test that can be used to detect the
precipitation are in vitro reactions that remain presence of either specific antibody or specific
popular methods for the diagnosis of diseases antigen in serum. It was widely used to diagnose
and for identification of specific antigens and viral infections that are not easily detected by
antibodies. The main difference between these culture methods (see also Chapter 6). However,
two serological reactions pertains to the size of in clinical virology the CFT has been largely
the antigens. In the case of precipitation, anti- superseded by other serological methods such as
gens are soluble molecules while in the case of the ELISA and by molecular methods of patho-
agglutination; antigens are large, insoluble mol- gen detection such as the polymerase chain
ecules (see also Chapter 6). Another difference reaction (PCR).
between precipitation and agglutination is that
the agglutination reaction is often more sensi- Neutralization tests
tive than the precipitation reaction because a lot
of soluble antigens and antibody molecules are Neutralization tests are still frequently used in
required to form visible precipitation. However, virology. There are two neutralization techniques
it is possible to make a precipitation reaction commonly employed: SN and VN and these are
more sensitive by converting it into agglutina- often considered together. SN tests employ the
tion reaction. This can be achieved by attaching use of a known virus to which unknown (test)
soluble antigens to large, inert carriers such as serum is added (Figure 4.27). If antibodies for
erythrocytes or latex beads. the virus are present in the serum, the virus will

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