Elimination of porcine reproductive and respiratory syndrome virus infection using an inactivated vaccine in combination with a roll-over method in a Hungarian large-scale pig herd

Abstract

Background Porcine reproductive and respiratory syndrome virus (PRRSV) causes severe economic losses worldwide and only four countries in Europe are free from PRRSV. Complete depopulation–repopulation is the safest and fastest, but also the most expensive method for eradicating PRRSV from a population. Another possible way to eliminate an endemic PRRSV infection is to replace the infected breeding stock by gilts reared isolated and protected from PRRSV on an infected farm. With this method it is possible to maintain continuous production on the farm. The authors report the first successful elimination of PRRSV in a Hungarian large-scale pig farm by using an inactivated vaccine and performing segregated rearing of the offspring. Case presentation The study was performed on a PRRSV infected farm (Farm A) with 1475 sows. The clinical signs of reproductive failure had been eliminated previously by using an inactivated vaccine (Progressis®, Ceva). At the beginning of the elimination programme, gilts intended for breeding were vaccinated at 60 and 90–100 days of age. After that, gilts selected for breeding were vaccinated at 6 months of age, on the 60–70th day of pregnancy and at weaning. Approximately 1200 piglets from vaccinated sows were transported at 7 weeks of age to a closed, empty farm (Farm B) after being tested negative for PRRSV by a polymerase chain reaction (PCR) method, and then were reared here until 14 weeks of age. At this age, all pigs were tested by PRRS ELISA. Seronegative gilts (n = 901) were subsequently transported from Farm B to a third, closed and empty farm (Farm C), and (having reached the breeding age) they were inseminated here after a second negative serological test (ELISA). At the same time, Farm A was depopulated, cleaned and disinfected. All pregnant gilts were transported from Farm C to Farm A after being re-tested negative for antibodies against PRRSV. Follow-up serology tests were performed after farrowing and results yielded only seronegative animals. Based on the subsequent negative test results, the herd was declared PRRSV free by the competent authority. Conclusions The presented farm was the first during the National PRRS Eradication Programme of Hungary to eradicate PRRSV successfully by vaccinating the sows with an inactivated vaccine and performing segregated rearing of the offspring. Production was almost continuous during the whole process of population replacement.

Progress in PRRSV Infection and Adaptive Immune Response Mechanisms

“…Acute infection, persistent infection, and disappearance are the three main stages of PRRSV infection. Susceptible pigs are not reinfected when placed in a herd containing infected pigs that were confined for more than 200 days [ 34 , 35 ]. PRRSV can be treated in large, infected herds through confinement management, and it can finally be removed by the immune system of pigs.…”

Research Progress on the Development of Porcine Reproductive and Respiratory Syndrome Vaccines

“…This achievement was accomplished by administering an inactivated vaccine to sows and adopting segregated rearing for their offspring. Notably, the production remained nearly uninterrupted throughout the entirety of the population replacement process [ 64 ]. Several studies have indicated that the utilization of a combination of the MLV vaccine and inactivated vaccine provides enhanced protection against PRRSV infection in pigs.…”

PRRSV Elimination in a Farrow-to-Finish Pig Herd Using Herd Closure and Rollover Approach

“…It is well established that PRRSV elimination is a powerful strategy to restore a high production level in pig farms. There are three PRRSV elimination strategies applied in the field: (1) whole herd depopulation and repopulation; (2) test and removal; and (3) herd closure and rollover (also called load–close–exposure, LCE) [ 20 , 21 , 22 ]. The depopulation strategy shows the highest efficiency and lowest risk but the cost is too high for most pig herds.…”

“…The herd closure and rollover strategy typically involves three steps: (1) Introducing the PRRSV-negative gilts in one batch. To minimize the cost, the amount of PRRSV-negative gilts should be enough for the sow replacement during herd closure; (2) Herd closure until the herd reaches a provisional negative status, which usually takes 210–250 days; (3) Expose all of the animals to the PRRSV live virus at the same time [ 22 , 24 , 25 , 26 ]. The epidemiological basis of this strategy is that if the sow population is infected by PRRSV at the same time and recovers simultaneously, the entire population will establish sterilizing immunity against field strains.…”

“…To make sure that all of the sows and gilts are exposed simultaneously, natural exposure may be replaced with simultaneous vaccination of MLV or inoculation with serum containing resident virus [ 22 , 24 , 25 , 26 ]. Yet, deliberate exposure is still a controversial step.…”