9 Monitoring

Monitoring, based on detection of the virus by RT-PCR and antibodies by ELISA, are essential to:

• Detect early problems associated with PRRS
• Classify the farm by the PRRS virus status
• Verify that a control program is being implemented
• Assess the success of a control program

There are different systems to classify swine herds according to their PRRS status; however, all of them share at least three clear categories according to the breeding herd:

• NEGATIVE. Negative status corresponds to a negative consistent result confirmed by ELISA.
  Main objective: to maintain the status maximizing external biosecurity in order to avoid lateral introductions of PRRS virus.
• POSITIVE STABLE. There is no vertical transmission from sows to piglets. Therefore, no viraemic piglets are born and there is a negative flow of pigs from farrowing to weaning.
  Laboratorial confirmation can be done sampling umbilical cords or blood from weak-born piglets or bleeding weaning piglets.
  Main objective: to maximize external biosecurity in order to avoid lateral introductions of the virus and secondarily, to evaluate the possibilities and risks to become negative.
• POSITIVE UNSTABLE. There is vertical transmission from sows to piglets. Circulation of the virus is active in farrowing units: viraemic piglets are born and there is a positive flow of pigs from farrowing to weaning. Losses in an unstable farm may lead to five piglets less during the productive life of a sow.
  Main objective: to stabilize the herd and to elucidate the source of the virus.

Classification of farms must be based on consistent and constant sample monitoring.

It is important to sample enough animals from several litters. Moreover, viraemic farrowings are not constant in time, so it is strongly recommended to obtain consecutive samples.

For example, 30 animals at least –focusing on the worst or smallest pigs- four times over a period of 90 days will determine a provisional negative status, or six cords in weaned piglets during 4 consecutive weeks will determine stability/instability.

The size of the farm and the incidence of farrowing should be considered as well.

Problems monitoring vaccinated sows by ELISA

Compliance of vaccination or simply monitoring of the breeding herd could be done using ELISA. In multiple vaccinated herds, all sows should be positive by ELISA.

However, both European and American swine veterinarians often report a variable percentage of sows becoming or remaining negative by ELISA despite being vaccinated multiple times.

After vaccination, seroconversion usually occurs at 7-14 days, maximum peak at 28-42 days and later on there is a plateau. Finally, animals can loss antibodies (to be seronegative) between 4 to 18 months later.

Therefore, a sow can be negative by ELISA before revaccination because of losing antibodies. Most probably, this animal will seroconvert again after the revaccionation.

But, how can we explain that a sow remained negative after revaccination? Obviously, this situation creates uncertainty about the real immunological status of vaccinated sows and the performance of the ELISAs.

It is important to note that this problem has been observed for several MLV vaccines; it is not MLV specific.

Possible causes of negative ELISA results in MLV multivaccinated sows:

• Inappropriate storage and/or vaccine administration and/or administration of unauthorized mixtures.
• Idiosyncrasy of the farm. It does not occur in all farms.
• Idiosyncrasy of animals. It does not occur in all animals in the same farm.
• When several ELISAs have been compared using the same apparently negative sample, it has been demonstrated that the major cause behind negative results are the ELISA. Therefore, when a negative result is found the first recommendation should be to analyze the samples in more than one ELISA
• Vaccination protocol. Could the continuous administration of the same antigen induce a state of anergy? If occurs, it would affect less than 5% of animals; however, further investigations are needed to clarify this point and its consequences.