ELISA: Useful tool in monitoring of live mycoplasma vaccines
For non-vaccinated flocks, mycoplasma monitoring is an effective tool to prevent spread of disease through early detection and control of infection. Recent serological data on live mycoplasma vaccinated flocks have shown that ELISA can be effectively used to indicate successful vaccination or confer a field challenge and circumvent egg production losses due to mycoplasma infections.
By Dr Bart van Leerdam, BioChek, Reeuwijk, the Netherlands
The purpose of monitoring flocks, assumed free of infection, is to confirm negative status or early detection of infection with Mycoplasma gallisepticum (MG) or Mycoplasma synoviae(MS) after challenge. Early detection in multiplier flocks is very important in order to prevent disease spreading. When a flock becomes positive, the various measures taken in order to prevent the spreading of the disease and the confirmation tests are usually very costly. Therefore, the test used for monitoring should be both highly sensitive for early detection and highly specific for confirmation.
Vaccination as a control tactic
Vaccination against MG or MS is an option in situations where maintaining flocks free of infection is not feasible, such as on multi-age commercial layer farms. Both killed vaccines (bacterins) and live vaccines are currently in use. Bacterins can reduce the decline in egg production associated with MG, but they do not prevent infection. Live attenuated or naturally mild strains are used in some countries and also reduce the decline in egg production. In addition, live vaccines may be helpful in displacing field strains on multi-age sites.
Live vaccination monitoring
The use of live mycoplasma vaccines has become increasingly popular to minimise mycoplasma induced egg production losses (MG) and displacing field mycoplasma strains from multi-age farms. The live MG vaccines have not only been shown to effectively protect against egg production losses but also provide good protection against colonisation by field strains, which inactivated MG bacterins fail to provide. For live MG vaccines there are three commercially available products; F-strain, 6/85 and Ts-11. From a serological perspective, there are major differences between the vaccines.
The F-strain establishes a permanent colonisation in vaccinated flocks, which is reflected in the serology. F-strain vaccinated flocks show positive serology after 8-12 weeks of age and will seroconvert to 100% positivity. The more attenuated strains of Ts-11 and 6/85, have a more limited potential to spread from bird to bird. The main serological differences on ELISA between 6/85 and Ts-11 is that 6/85 will show no or very limited seroconversion after vaccination, while the Ts-11 will show partial, fluctuating seroconversion (30-70% positive).
Suspect of field infection
The advantage of the negative serology after vaccination with 6/85 is that the flock can be easily monitored for field infection. In case of Ts-11, differentiation of vaccination serology and field challenge serology will have to be done by evaluation of mean flock titers with baselines and evaluating % positives. Flocks that show mean titers above baseline and are 100% positive are suspect of field challenge. For the F-strain vaccinated flock, evaluating serology for evidence of challenge may be more difficult as vaccinated flocks normally show 100% positivity. For F-strain, mean titers well above established baselines indicate challenge. Live vaccination against MS is a relatively new topic when compared to live MG vaccination.
The only available commercial live MS vaccine available is the MS-H vaccine. The experience from Australia, South Africa and Europe, suggest that the MS-H vaccine has characteristics similar to Ts-11. This is also reflected in the serology, where the MS-H serological pattern is very similar to Ts-11.The serological interpretation of the various live mycoplasma vaccines has been summarised in Table 1 for the various MG vaccines and Table 2 for the live MS-H vaccine.The successful use of live mycoplasma vaccines require that live mycoplasma vaccines must be applied before a flock becomes infected with a field strain. Therefore, it is recommended to monitor for the absence of antibodies, immediately prior to vaccination. Testing prior to vaccination will confirm negative status of the flock, thus ensuring optimal application of the live vaccines.
The significance of live vaccination monitoring
The experience from various parts of the world, suggest that the BioChek system can be used to monitor the success of live mycoplasma vaccinations and to differentiate vaccinated flocks from those flocks undergoing a mycoplasma challenge. The BioChek ELISAs for MG and MS have a distinct advantage over the RSAT test, which has been shown to have poor sensitivity to Ts-11 or MS-H vaccinated flocks.A common serological misconception about live mycoplasma vaccines is that observed rising antibodies following field challenge indicate vaccine failure. This may not be so. Suspect serology can only indicate the presence of a field challenge. Suspect serology with the absence of clinical symptoms, might just indicate that the vaccinated birds were challenged and displayed successful resistance to colonisation with a field strain of Mycoplasma. Infections with field strains can only be confirmed by differential culture/PCR techniques for wild type mycoplasma.
Rising antibody titers detected through monitoring however, will indicate the need for enhanced biosecurity and effective timing and application of live vaccines to keep flocks free from challenge. Field experiences involving the use of live mycoplasma vaccines also suggest that the live vaccines may be viable tools for displacing and eradicating mycoplasma field strains on multi-age poultry farms. This is due to a decrease in serological response to normal baselines following prolonged use of live vaccines, indicating the loss and exclusion of field challenge.
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