Our boar stud broke with PRRS one month ago (PCR positive and ELISA positive with very high titers). We gave 2 doses of Inmuno PRRS and then took new samples and grouped them in 3 pools of 5 samples each depending on the titer (high, medium, or low). We are waiting for the results, but is this the right way to make pools? And to introduce new boars, do we have to wait for the current animals to be negative and for the titers to drop?
ELISA positive results; How should they be interpreted?
First, it should be noted that ELISA readings (S:P ratio, commonly called titers) are not directly related to protective immunity.
Interpretation of ELISA serological test readings should not go beyond saying whether or not there has been contact with the virus (Christopher-Hennings J, et al. 2002).
Therefore, grouping the animals to be tested by high, medium, or low values should have no impact on the PCR values to be obtained.
Fig 1. ELISA technique interpretation with PRRS samples. Source: HIPRA
How many samples can a pool contain?
As for how many samples a pool can contain, it will depend on the expected viral concentration.
Under commercial conditions usually pools of 5 samples are taken, but when we want to increase the precision, as could be the case here where the individual value is very important, ideally we should reduce the number of samples to 3 per “pool” or even perform the analysis individually in order not to lose sensitivity (Rovira, A. et al. 2007).
In your opinion, how should be the new boars introduced in the boar stud?
As to whether it is necessary to wait until the boars are PCR negative to introduce new boars (negative boars, I understand), the answer is yes, but keep in mind that it is possible for there to be an individual that, even having a negative PCR result, can still be a carrier of the virus at the tonsillar level (Fombelle, W. et al. 2013).
These animals are very difficult to detect unless tonsil samples are taken, and in no case are they related to the fact that they are still positive for the ELISA serological test.
The most prudent thing to do would be to wait.
Fig 2. Interpretation of ELISA and PCR results in PRRS diagnostic. Source: HIPRA
In the case that negative and positive animals must be mixed, it would be necessary to wait for these tonsillar carriers to disappear and therefore additional time would have to be given to the negative PCR results.
But the practical problem is that this time is not defined because there can be wide individual variation.
It would be safest to wait at least 7 months (as done in PRRS elimination programs) from the time of infection before introducing new animals, despite having previously negative PCR results.
Visit our chapter of “Diagnostic and monitoring” to know more about how to approach and to interpret the disease through the diagnostic.
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Marcovetgrup S.L. – Spain
