Duration and persistence of PRRS ELISA antibodies in PRRS-infected and vaccinated sow herds during an eradication program with Porcilis®PRRS

To define breeding sow herds as PRRS non-suspect, PRRS ELISA negative results are required according to the German surveillance program. IDEXX X3 ELISA is a commonly used test because of its high specificity and sensitivity. To control and eradicate PRRSv, the choice of PRRS MLV vaccine is important with respect to spread of vaccine virus and impact on virus circulation. Only little is known about duration and persistence of PRRS-specific antibodies, measured by ELISA, in positive herds after withdrawal of MLV vaccination. Such information is important for planning of time schedules, replacement rates and economic outcomes of PRRS eradications programs.

The study was done in a multiplier farm with about 300 sows and 2000 piglets up to 11 weeks (partly multi-site production) and PRRSv (-) replacement gilts from an external source. The farm had a long term PRRS non-suspect status. The herd was infected with a European PRRSv isolate in march 2008. To control clinical symptoms of PRRSv, all sows (first step mass-vaccination, afterwards 50-70 days p.i.; 21.03.2008-12.03.2013) and piglets (age 14 days; 21.03.2008-24.07.2008) were vaccinated with Porcilis®PRRS (i.m.). Vaccination of the clinically PRRS stable herd was stopped in March 2013. The PRRS status was continuously checked with ELISA, PCR and introduction of PRRS ELISA negative gilts (sentinels). Twenty eight (28) months after last vaccination and 27 month after last introduction of vaccinated gilts, all sows and boars (n=307) were sampled and tested by IDEXX X3 ELISA and PCR (Virotype LDL; Tetracore).

During the 28 month period, all sampled pigs (n=114) were PRRSv PCR (-). All piglets at the end of nursery (n=50) and sentinel gilts (n=52) were PRRS (-) in IDEXX X3 ELISA. The complete sampling of the sow herd on 17.07/24.07.15 showed PCR (-) results for all animals. Sentinel gilts (n=196) were ELISA (-), except for one sow (S/P 0.55). Seventy two (72) of the remaining PRRSv vaccinated sows (n=111; 64.9%) and 9 sows of the last group of PRRS vaccinated replacement gilts (n=17; 52.9%) were still PRRS ELISA (+).

The occurrence of a positive ELISA result in a non-vaccinated sow is in the range of the usual specificity (1/195; 0.5%). The high frequency (64.5%) of ELISA positive sows 28 months after the end of vaccination without reinfection is important for the planning of eradication programs. The results suggests that nearly all sows have to be replaced to achieve a PRRS ELISA negative status more quickly, which would be much higher than previously thought (40%).

Disclosure of Interest: None Declared.