The entrance of gilts is today still one of the main causes of destabilisation of PRRS in swine farms and a source of the entrance of new PRRS virus strains. Therefore, it is crucial to check and ensure that protocols for the entrance of future breeders are being performed correctly in order to succeed in the fight against the disease.
The entrance of every single batch of gilts has to pursue two objectives: on the one hand the performance of good quarantine protocols is needed to ensure that newly arrived gilts will not introduce new heterologous strains of PRRS in swine. On the other hand performance of good acclimatisation protocols prior to service is fundamental to ensure that gilts are well immunised and are not shedding PRRS virus. Monitoring the whole process is essential in order to achieve these objectives.
Focusing on the acclimatisation process, there are two different and widely used strategies to protect against PRRS in swine. On the one hand, we can use modified live virus PRRS vaccines. The vaccination protocol is usually two doses before the first insemination, separated by 3 or 4 weeks, although some vaccines have demonstrated efficacy with a single dose prior to mating. The objective of vaccines is to induce protection against the farm’s strain and also against any strains that may enter into the farm. On the other hand, direct exposure to the wild virus. The purpose of this practice is to develop comprehensive homologous protection from the farm’s strain, although recent studies have demonstrated that occasionally some heterologous strains may induce better protection than the homologous one.
Different methods are used to perform exposure to the wild virus. However, these practices involve a risk, since infection from other pathogens, different from PRRS in swine, may be disseminated, provoking an undesired effect. On the other hand, a certain quantity of virus is needed to immunise the animals, and this is much more complicated to guarantee with these techniques, subsequently also making it more difficult to achieve the objective of immunising the animals. Different acclimatisation protocols can be applied depending on the status of replacement gilts and if there is virus circulation on the farm of destination. If gilts arrive at the farm PRRS- negative, without any previous contact with the virus, and the virus is not circulating on the farm of destination, we have to vaccinate the gilts if the farm is serologically positive. If the farm is serologically negative we have just to make sure the gilts remain negative without vaccination.
If gilts arrive negative to PRRS in swine and the virus is circulating on the farm of destination, we have to vaccinate the gilts and then expose the animals to the farm’s strain of the virus. Prior to introducing animals to the breeding herd, we must ensure that there are no viraemic and no excreting animals. If gilts have been in contact with the strain which is circulating on the farm (homologous strain) and are therefore PRRS-positive, firstly we have to vaccinate the gilts. Then, prior to introducing animals to the breeding herd, we must ensure that there are no viraemic and no excreting animals. In situations where gilts arrive PRRS-positive and we know that they have been in contact with virus that is different from the strain present on the farm, we have to avoid the entrance of these animals. In this type of situation there is a high risk of introducing a new strain on to the farm and this could provoke a destabilization of PRRS in swine farm, resulting in a huge economic impact for the farm.
During the gilts’ acclimatisation process we have to know their status both at the time of arrival and when they are introduced on to the farm. To accomplish this, we will use serological and PCR techniques. At the time of arrival and after 15 days, to confirm the expected status of the gilts we will use serology and PCR. Serologically positive gilts means that they have been in contact with the PRRS virus previously. PCR positive gilts means that these animals are infected and are currently viraemic and that the animals may be disseminating the virus. The ideal situation in all cases is to obtain animals that are negative for PRRS in swine.
During acclimatisation, gilts will be vaccinated. In order to evaluate the immunisation process, samples will be taken from the animals to perform ELISA, three or four weeks post-vaccination in order to confirm that this immunisation has actually taken place. One week prior to joining the breeding herd, and for animals positive to serology, the batch of animals will be evaluated using PCR in order to confirm that there is no viral shedding at this time. If they are PCR positive, replacement animals have to remain in the acclimatisation building until it is demonstrated, by means of PCR, that there is no viral excretion. The whole process should last at least 8 to 12 weeks in order to achieve a good acclimatisation process.
In order to have good productivity it is essential to stabilise PRRS in swine farms. Proper management of the entrance of gilts is one of the most important stepts for the control of PRRS virus circulation in the breeding herd.