|
Aphthovirus Genome Map
|
|
Recombinant non structural (NS) proteins of FMDV can be used to differentiate infection from vaccination.
Studies have shown that the following NS proteins (Aphthovirus Genome Map) are immunogenic following infection (in decreasing order of immunogenicity)
3D
3A/3AB/3ABC
2C
2B
3C
L
Antibody can be measured by:
• Radio Immuno Precipitation (RIP)
• Immunoblotting
• ELISA
RIP - is extremely sensitive and enables very specific detection of antibody to individual NS proteins but is not suited to screening large numbers of sera.
Immunoblotting - an Electro Immuno Transfer Blot assay, extensively used
in South America, measures antibody by immunoblotting sera against five
NS proteins in a single test (Bergmann
et al., 1993). This is an extremely useful test for confirming the
status of suspect animals, but requires the production and supply of nitrocellulose
strips to which the NS proteins are bound.
ELISA - a variety of ELISAs have been described using the NS proteins 2C, 3ABC and 3D expressed in either baculovirus or E.coli. These are simple, rapid and can be performed in any laboratory familiar with ELISA technology. None has yet been approved by the OIE, although several have been extensively validated in a number of laboratories worldwide. Tests can measure antibody to a single antigen, such as 3ABC, or to a panel of proteins.
It is currently considered that the detection of an antibody response
to the polyprotein 3ABC, usually associated with a response to at least
one of the other NS proteins 3D, 2C or Lb, is conclusive evidence of previous
infection with FMD virus, irrespective of whether or not the animal of
origin has previously been vaccinated. Reliable and reproducible tests
to measure antibody to 3ABC have been described (DeDiego et al., 1997).
Measurement of antibody to 3ABC is extremely useful on a herd basis to
detect viral activity in vaccinated populations. In doubtful cases it
is advisable to measure antibody to several NS proteins simultaneously
(Mackay et al., 1998)
as the detection of an antibody to more that one NS protein is confirmation
of previous infection.
However, care must be taken in interpreting the results from individual animals as current evidence suggests that freedom from antibody to NS proteins does not, by itself, mean that an animal has never been infected with FMDV for two reasons:
• the duration of antibody to NS proteins following infection is
not well studied
• it appears that not all vaccinated animals which become
infected seroconvert to NS proteins.
For these reasons a negative result for antibody to NS proteins must be viewed with caution.
|
