Clinical signs of PRRS are distinctive though not diagnostic. Clinical severity can vary from mild to severe and the disease can present as different syndromes attributable to immunosuppression by the field virus. Diagnosis should be confirmed by a variety of tests discussed below.
Serum, lung (tissue or wash fluid) are the best samples to submit to the lab from acute cases while tonsil and lung wash fluid are better for chronically affected animals. Weak newborn piglets are better for virus isolation than mummified or aborted foetuses. To find this virus the Polymerase Chain Reaction (PCR) or virus isolation can be used. The latter is laborious, time consuming and not routinely conducted by many labs.
Virus isolation (VI)
Pigs can be viraemic for 4 weeks but still it is not very easy to sample the right animals. Pigs with clear infection can best be sampled 4-6 weeks after onset. Be aware that MLV-vaccinated piglets can also be positive as the vaccine-virus remains long present in the vaccinated piglet. If needed vaccine virus can be differentiated from field viruses by sequencing the ORF segments. Another practical method to separate field virus from Porcilis PRRS vaccine virus is by the type of cell’s where they grow on. Field virus grows only (>95 %) on Porcine Alveolar Macrophages (PAM cells) where European-strain live PRRS vaccine virus grows only (>95 %) on so called MARC cells. Piglets vaccinated with this vaccine have a positive VI result from 2-6 weeks post vaccination. Adult animals have much shorter viraemic periods and consequently detection and diagnosis can be challenging.
This technique detects the genetic part of the virus and is therefore very specific. It is an easy technique as compared to VI but it detects any genetic part including that of dead virus. PCR tests are very sensitive detecting also very low quantities of virus.
Both for VI and PCR the demonstration of the virus alone is not very meaningful. Most animals harbour the virus without clinical consequences so finding the virus must be associated with clinical signs to assert the diagnosis. For example when virus is detected in aborted foetuses or fresh weak-born piglets, PRRS is likely to be the cause of the same. Also here we have to be prudent that the PCR can be positive due to the vaccine virus.
Serology is probably the most widely used diagnostic intervention but has some deficiencies. First of all titres are not directly related to protection. Tests only detect virus neutralizing antibodies. Piglets after vaccination will not all show seroconversion. Antibody levels in sows, that repeatedly came in contact with the field or vaccine virus, will gradually become low in titre or even negative.
So an individual pig that is seronegative can still be persistently infected with virus.
The IFA/IPMA test is the gold standard . Results are expressed in 2 log titres. Every step in titres means a doubling of the titre. The most used test however is the ELISA. The results are expressed as a SP ratio. Cut off level is considered to be 0.4. Every increase by 0.4 is correlated to one titre step in the IPMA test.
After repetitive sow vaccination the expectation is to have low titres where there is no further circulation of field-virus This can be expected 5-6 months after start of vaccination program in the herd. At the start with sow-vaccination there are still booster effects due to the relatively novel heterologous contact.
After piglet vaccination you can expect values similar to those after contact with field-virus.
The ELISA test becomes positive 1-2 weeks after contact or vaccination. Highest titres appear around 5-6 weeks. After this, titres go down again and many animals can become seronegative in time (after some months) The sensitivity of the ELISA is around 95% so the chance of an individual false-negative result is 5%.
After vaccination of piglets with MLV vaccine not all will seroconvert. It is very common that 20-40% do not seroconvert though were protected in challenge studies.
When pigs are infected or vaccinated with homologous PRRS-virus no increase of titre will appear. So after repetitive vaccination (dead or MLV-vaccine) titres will not increase. When titres do increase this is an indication that the sampled pigs had contact with heterologous field-virus.. Dead vaccines elevated titres to approximately 1.5. Higher titres indicate PRRS field-virus contact. MLV vaccines push titres up to around 3.5. Antibody levels from natural exposure can persist for 4-10 months after infection.