New videotutorial on Replacement Gilts: Key points for the control of PRRS in swine

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.

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Inter-herd transmission of PRRS in pigs: Where is the risk?

Many factors have been associated with PRRS transmission between herds. Amongst these factors, the introduction of infected animals and the use of contaminated semen are considered to be the most important since PRRS in pigs is mainly transmitted through direct contact with infected pigs and their secretions such as saliva, nasal discharges, semen or feces. However, airborne transmission and contaminated transport can be also considered to be an important source of PRRS infection between herds.
Biosecurity is the key to prevention of the inter-herd transmission of PRRS in pigs. Sometimes, the list of biosecurity rules and measures to avoid the entry of new pathogens into the farm can be endless and almost impossible to implement fully. For this reason, evaluation of the different risk factors and their importance in terms of the transmission of PRRS between herds is crucial. Once we identify the most dangerous factors for the introduction of PRRS into the farm we will be able to establish and focus on limited biosecurity rules but with the highest level of effectiveness for the protection of your herd. At the same time monitoring the PRRS status on the farm farm can also help us to determine the PRRS control program.

PRRS in pigs is transmitted mainly through direct contact with infected pigs and their physiological fluids, such as saliva, nasal secretions, semen or feces. So, we can consider the entry of infected pigs to be the most important cause of the spread of PRRS between farms and herds. Other main causes for the introduction of PRRS into a farm are contaminated semen and vehicles.
Nowadays, most boar stations in Western Europe and North America are PRRS-free and strict controls are applied in order to ensure that semen is completely PRRS-free. Unfortunately, in most countries we pay less attention to the control and disinfection of vehicles, so currently, we could consider this to be the second main risk for the transmission of PRRS in pigs from one farm to another.
In addition, airborne transmission can also play a very significant role in PRRS infection between farms, especially in high-density pig production areas. At this point, air filtration should be considered as the best biosecurity barrier to avoid PRRS infection from neighboring farms. Different air filtration systems have shown good results in several studies and field trials, but up to now no one can be considered 100% effective for the prevention of farm infection.

Indeed, different fomites, birds or insects or even humans, can be also considered a risk for the transmission of PRRS in pigs, but with a lower epidemiological importance than those mentioned above: animals and semen, vehicles and air.

  
Finally, regional PRRS control or eradication plans and programs can also play a very important role in the reduction of transmission of PRRS from farm to farm and even from region to region. So far these have only been introduced in some regions of the USA and Canada, but they have provided the first results on the reduction of the number of PRRS outbreaks and they will probably be extended to other areas of Europe and Asia in the near future.