Combination of ELISA and RT-PCR in serum

Commercial ELISAs and conventional or quantitative RT-PCR are the most routinely used diagnostic tests. In combination, they can be used not only to detect an infection, but also to determine approximately when it occurred and to monitor the immune status at individual or at farm level. If sequencing is also performed, we can even establish the possible sources of the virus.

¹ Excluding the effects related to low sensitivity and/or low specificity.

² In adults, viraemia is short; so, this pattern may be already observed after 14 days post-infection. In young pigs, viraemia is longer; so, this pattern may be observed after 14-28 days post-infection.

LABORATORY CONFIRMATION IN REPRODUCTIVE FAILURE.

Seroconversion measured by ELISA can be used to demonstrate the involvement of PRRS virus in reproductive failure. However, seroconversion can be only observed in a previously negative animal/farm. Thus, in some circumstances, serum from sows should not be considered as one of the first choices to diagnose reproductive failure due to PRRS virus: 1) it is difficult to detect the virus in blood by RT-PCR, as viraemia in sows is usually of short duration; 2) seroconversion by ELISA cannot be demonstrated in a previously positive or vaccinated animal/herd.

 

Nor should samples from aborted, mummified or autolysed foetuses be used as the main sample in reproductive failure, since PRRS virus is relatively labile and it rapidly disappears from these samples.

In reproductive failure where PRRS virus is one of the main pathogens suspected to be involved, best samples to use are: blood samples from sows (to determine seroconversion by ELISA, when possible) and/or blood samples from weak-born piglets (to determine PRRS virus viraemia by RT-PCR). 

LABORATORY CONFIRMATION IN A RESPIRATORY OUTBREAK.

Seroconversion measured by ELISA can be used to demonstrate the involvement of PRRS virus in a respiratory outbreak. However, seroconversion can be only observed in a previously negative animal/farm. Series of samples from various ages for detection of antibodies by ELISA is useful to determine the age of the infection and, consequently, the best time to analyse for the presence of virus in the blood. RT-PCR is useful because viraemia is longer in young pigs. Through IHC or ISH combined with histopathology, the link between PRRS virus and lesions can be analysed in tissues, especially in lungs.

The presence of concurrent infections can considerably complicate the diagnosis of PRRS in a respiratory outbreak. PRRS virus infection can have no effect, but it can also cause a serious respiratory outbreak. 

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