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Neutralisation Test
Liquid Phase Blocking ELISA
Differentiation of Infection
from Vaccination
VIAA AGID
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Both the virus neutralisation test and the liquid phase blocking ELISA measure antibody to the structural, capsid proteins of FMD virus. The detection of antibody to the structural proteins of FMD virus does not distinguish animals that been vaccinated from those that have been infected. Differentiation of these two categories of animals is important during eradication campaigns and in international trade.
Replication of FMD virus during infection of the host results in translation of the entire protein coding region
of the genome. The resulting polypeptide then cleaves itself to produce the structural proteins, which assemble to form new virions, and the non-structural (NS) proteins, which are involved in polyprotein processing and altering the host cell functions. At least some of these non-structural proteins are immunogenic and induce the production of specific antibody. Vaccines consist of purified FMD viral particles mixed with buffer and adjuvant. The immune response to vaccination is therefore limited to the production of antibody to the structural proteins of the virus. Weak and transient antibody responses are sometimes detected to the small amounts of non-structural proteins that contaminate the purified viral harvest. Antibody to NS proteins is therefore more likely following either repeated vaccination or the use of vaccines containing unpurified virus (Differentiation of Infection from Vaccination).
Responses to NS proteins were classically detected by measurement of antibody to the virus infection associated antigen (VIAA AGID). This is a mixture of NS proteins comprising mainly the RNA polymerase protein 3D. VIAA was first recognised as the third line of precipitation in agar gel immunodiffusion (AGID) tests between sera from infected animals and FMD viral harvests. Detection of antibody to VIAA by AGID has been superseded by the use of ELISA (Alonso et al., 1990), but the use of VIAA tests to differentiate infection from vaccination has been largely discredited due to:
• poor sensitivity and specificity of the AGID
• the fact that vaccinated animals, particularly multiply
vaccinated animals, produce antibody to VIAA
• the demonstration that at least some purified viral particles
contain an exposed copy of the 3D protein, explaining the
induction of antibody to 3D in vaccinated animals.
More discriminatory tests have now been developed which use recombinant FMD NS proteins expressed in vitro
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