Diagnosis of Avian Metapneumovirus – Antibody determination
Testing serum samples at intervals (for example at the time of the clinical signs and 2-3 weeks later) provides the best basis for serological diagnosis. This is also applicable for monitoring vaccination responses.
Birds vaccinated with live and inactivated vaccine should have antibody titres in the higher range. However failure to detect antibodies following vaccination does not necessarily mean that the birds are not protected against challenge.
In chickens, high titres can also be found in unvaccinated birds without problems which creates doubts about the advantages of using serology to diagnose infections, in that species.
Methods that can be used for antibody determination are:
Avian Metapneumovirus infection can also be diagnosis by antigen determination.
Virus neutralization test (VN)
Serum is mixed with known aMPV virus strains grown in embryonated eggs, tracheal organ cultures or cell cultures. Inhibition of the growth of the virus allows identification of the antibodies. The test is carried out with the virus at a constant concentration against varying two-fold dilutions of suspect serum (so called “ß method”). The antibody titre is calculated on the basis of the highest of the serial dilutions of the serum that neutralise the infectivity of the virus.
Enzyme linked Immuno sorbent Assay (ELISA)
Specific antibodies in serum bind to the virus. The complex is detected by an enzyme-labelled anti-γ-globulin. After adding an enzyme substrate, a resulting coloured product is measured.
This is the most commonly used method for the determination of antibodies against aMPV and a variety of commercial kits are available. It is useful as a flock test to confirm adequate antibody responses to vaccination. However the choice of the aMPV strain used as antigen in the ELISA appears to be important. For example, ELISA’s based on subtypes A or B can detect antibodies raised against both A and B subtypes, although the heterologous antibodies may be detected to a lesser extent. Commercially available ELISA’s may or may not be sufficiently sensitive for detecting antibodies of other heterologous subtypes (C or D).