Diagnosis of PRRSV infection

Laboratory diagnosis of PRRSV is done by serology, virus isolation, reverse transcriptase polymerase chain reaction or nucleotide or amino acid sequencing

Serology

Serological testing can be used to determine the status of a herd: infected or free. Paired samples are requiredto confirm the involvement of PRRSV in the clinical problem seen in the herd at a particular point in time.

ELISA test
ELISA is used routinely in laboratories. It is used for diagnosis and to determine the epidemiology of the PRRSV infection. Once time of infection is known, a tailor-made control strategy can be suggested. The epidemiology in a population should be checked 1-2 times a year, as it is a dynamic factor.

Immune Fluorescence or IPMA test
These tests are mostly used in research laboratories and are regarded as more sensitive and specific than ELISA. With some IFT or IPMA tests, it may be possible to differentiate between infections due to NA and EU serotypes.

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Virus isolation (VI)

Serum is the specimen of choice for VI. VI can only be accomplished successfully when the serum is cooled rapidly to -20°C after separation. Serum samples are placed on porcine primary macrophages, which can be scored for CPE approximately seven days later.

Abortions
VI is used to determine the involvement of PRRSV in abortions. Serum should be taken from surviving piglets in an affected litter. Stillborn piglets or aborted fetuses are less suitable for VI; in these cases samples for VI can be taken from lymphoid tissue (i.e., tonsils or lymph nodes).

Respiratory disease
Lung samples and lung lavages from pigs with signs of respiratory disease can be used for virus isolation in order to confirm whether PRRSV is involved in the clinical picture seen.

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The Reverse Transcriptase Polymerase Chain Reaction (RT-PCR)

Together with ELISA, RT-PCR is widely used in cross sectional and serial sampling to determine PRRSV exposure in fattening pigs.

RT-PCR is also indicated for samples that cannot be used in cell cultures, such as semen, and samples in which PRRSV infectivity has been reduced, such as autolytic tissue.

The correlation between pathological lesions and positivity to PCR or to other diagnostic techniques gives stronger evidence that PRRS plays a role in the condition

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PRRSV nucleotide or amino acid sequencing

Sequencing is most commonly done on the ORF 5 region (See PRRSV structure). The region is highly variable and encodes the information for a protein that resides on the surface of PRRSV and interacts with the cells that the virus infects.

The use of sequence information to compare PRRSV isolates is a valuable tool to monitor PRRSV strains within a farm over time. It can differentiate between resident and newly introduced viruses and between vaccine and field strains. Sequence information cannot be used to determine biological properties (virulence, antigenic similarity, vaccine efficacy) of PRRSV.

Proper use of the assays provides valuable information for PRRS control. Improper use or "over-interpretation" can generate poor decisions resulting in failure of PRRS control efforts.

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laboratory assistant