Why in 5 weeks old piglets respiratory disease due to PRRSV appears despite vaccination of sows with a MLV PRRS vaccine every 4 months, and which have high level of antibodies?

Answered by: Tomasz Stadejek
Respiratory symptoms in piglets can be caused by a number of pathogens. In order to identify a causing factor it is necessary to conduct laboratory diagnosis.
Confirmation of PRRSV role can be performed through PCR analysis of lungs from sick animals. If such material is not available, serum obtained from pigs from different age groups can be tested by PCR or ELISA. Detection of PRRSV in serum of sick animals indicates viremia and can be considered as a proof of the virus’ role in respiratory disease in a given age group.
Appearance of antibodies against PRRSV in pigs at about 2 weeks from the start of the symptoms also supports such diagnosis. However, analysis of serological results is complex and we have to keep in mind that maternal antibodies can be detected in pigs for several weeks. This is why it is so important to test sera from groups of pigs of different age (e.g. 4, 6, 8 weeks of age etc.) and to assess the dynamics of seroconversion (compare the levels of antibodies in different age groups).
If diagnostic results confirm the role of PRRSV in the clinical disease in piglets, it may suggest that the vaccination program in sows is inefficient. It can be caused by insufficient cross protection against the local strain, poor colostrum management as well as vaccination errors.
The impact of limited cross protection can be diminished with the increase of frequency of mass vaccination in sows, from 3 to 4 per year. Vaccination every 3 months will likely increase the level of passive immunity.
The level of antibodies in vaccinated sows is difficult to correlate with the level of immunity. Antibodies detected in ELISA are directed against capsid protein and they are not protective. Moreover, sows from stable farms, where PRRSV does not circulate, and which were vaccinated with modified live vaccine multiple times, can have low or undetectable in ELISA levels of antibodies, and still be immune

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