In this study, we report the successful eradication of a large-scale farrow-to-finish pig herd in Hungary where herd replacement was achieved by applying the roll-over method with the use of an inactivated vaccine: gilts were born and reared on the farm under PRRS-free conditions, then segregated and reared on a separate farm. A PRRS eradication programme compulsory for every swine farm was launched in Hungary in 2014. When evaluating the PRRS status in Hungary, it turned out that the majority of large-scale pig farms follow a farrow-to-finish technology. The all-in-all-out procedure was not carried out consequently during the production phases in a number of the infected herds, which lead to PRRSV persistence and circulation .
Stricker  pointed out that it is impossible to reach PRRSV free status if animals are retained in the different production phases. The most plausible, but at the same time the most expensive method of PRRSV eradication is depopulation-repopulation. But the management of Hungarian breeding herds wanted to achieve eradication on the PRRSV endemic farms with the lowest possible costs and economic losses.
It is crucial to stabilise the sow herd and to establish the PRRSV status on the infected farms [5, 21]. A stable PRRS status in an endemic herd can be achieved by vaccination, while determining a PRRS stable status of a herd requires regular laboratory tests (PCR, ELISA, sequencing). Moreover, ensuring high levels of both external and internal biosecurity is an important element in reaching a stable herd status . Analysis of data in PRRSV infected herds established that a PRRS stable status can be achieved in an average of 26.6 weeks after introducing MLV vaccination. In these stable herds, PRRSV can still be detected but at a low level and with low frequency.
By utilising the above principles and practical experience, several countries performed successful local and regional eradication with the use of MLV vaccines and biosecurity measures [19, 23]. PRRS eradication of the Hungarian herd was carried out in a complex way, applying strict biosecurity measures and vaccination with Progressis®. Immunisation with inactivated vaccines was previously recommended on PRRSV endemic farms as a therapeutic tool . Although some publications mention that immune response is relatively weak after inactivated vaccination , a strong protection was shown to develop after vaccinating previously infected animals [2, 9]. Papatsiros  reported a similar experience in a farrow-to finish herd, where an inactivated vaccine was used for 2 years and lead to significant improvement in reproductive parameters. However, in spite of vaccination, PRRSV was present in the finishing unit. This phenomenon was observed in the Hungarian herd as well.
For successful elimination of PRRSV in farrow-to-finish type farms it seems necessary to break the chain of infection between the nurseries and finishing in at least one production cycle, and to perform regular laboratory testing to check if the finishers are maintaining the PRRSV free status [18, 25].
In the Hungarian farm the sow herd was stabilised by applying an inactivated vaccine between 2008 and 2014. With this method piglets could maintain a PCR-negative and seronegative PRRSV status during until out of nurseries (until 80–90 days of age). However, nearly 100% of them turned seropositive to PRRSV 3–4 weeks after arrival in finishing unit, indicating a persistent PRRSV circulation.
The results confirm the conclusion of Linhares  that it is possible to successfully implement a vaccination and biosecurity programme even in heavily infected farms to guarantee PRRSV free status until leaving the nurseries.
In 2014, at the beginning of the eradication programme, the age distribution of the sows in the Hungarian farm was far from the ideal, the majority of the sow herd consisted of animals > P7. Gilt replacement was increased in the aged sow herd until 40% over the years. Offspring of older sows (> P3) were selected for herd replacement at the start of the repopulation process. According to literature data, the highest level of immunity can be expected in these sows .
Reason for pasting of Farm B into elimination programme was the capacity of quarantine facility on Farm C was not enough for 300 piglets. Laboratory examinations are key elements in controlling the progress of the eradication programme. For the purpose of PRRS monitoring, PCR and ELISA tests are performed regularly, at defined time points and contribute to the validation of the progress of PRRSV eradication .
When designing the schedule and determining the number of samples for laboratory testing, the main goal is to be able to identify infected individuals in time and with high probability.
Toman  reported successful eradication with the use of an MLV vaccine, but sequencing had to be performed from PCR positive animals. In our case, sequencing was not necessary because we used an inactivated vaccine, and this reduced the cost and increased the data safety of eradication. Once the animals were tested negative with PCR, serological tests were sufficient for the control of the replacement gilt population reared on the closed farms B and C. ELISA tests were always performed before transports and at the end of the quarantine period. The last two serological tests on Farm A were performed in order to declare a PRRS free status.
The most important technical measures leading to the successful acquisition of the PRRS free status in the pig population on the farm were: establishment of a PRRSV stable sow herd by regular vaccination with Progressis® and increased culling of older sows. Further measures also contributed to the success, such as discontinuing fattening in the infected farm and transportation and breeding of animals based on negative results of laboratory tests performed during the eradication process.
By applying these steps and implementing strict internal and external biosecurity measures it was possible to achieve and maintained a PRRSV free herd status throughout every phase of production.