4 Immunisation of gilts and sows

Two of the most frequent methods to immunise gilts are:

A) Inoculation/contact of live-resident virus:

Immunisation with live-resident virus is usually performed by using contact to presumably infected material, such as feces or animals, or by sera inoculation from a viremic piglet. Both strategies are potentially dangerous and the outcome is unpredictable.

• Unknown quality of the product:
  Pathogenic agents beyond PRRS virus could be found in the material/animal used.
• Unknown quantity of PRRS virus in the product:
  The amount of PRRS virus in the material/animal used to immunise are usually not measured.
  If measured, the amount is usually low or insufficient to infect and to induce an immune response in all animals at the same time.
• Risk of introduction of infected gilts in breeding herd:
  Due to the insufficient concentration of transmissible virus in the material, some gilts could not get infected at first chance, but by contact from those initially infected lately.
  When this happens, the infection time is unknown, so it can not be guaranteed when infection has really occurred.
  Consequently, gilts could be introduced to the breeding herd being still viraemic and/or shedding virus. It is important to note that immunisation is done with a virulent strain and therefore, introducing a shedding gilt in the breeding herd is extremely dangerous.
• More than one strain could circulate in the farm:
  Inoculation or contact with live-resident virus aims to immunise gilts with the strain circulating in the farm to afford homologous protection.
  However, more than one strain could be circulating in the farm. Under these circumstances, the material used to immunise gilts could not contain the most important strain in the farm in terms of clinical or prevalence.

B) Vaccination with MLV vaccines:

The immunity afforded by commercial vaccines is always heterologous –due to the high genetic variability of PRRS virus, all field infections should be considered as heterologous-.
However, if supplier guidelines are followed, vaccines provide clear advantages compared to inoculation/contact of live-resident virus:

• Known quality of the product:
  Commercial vaccine contains only PRRS virus.
• Known quantity of PRRS virus in the product:
  Pharmaceutical companies guarantee the concentration of PRRS virus that can be found in a commercial vaccine. This information is always stated in the label.
• No risk of introduction of infected gilts in breeding herd:
  The time zero of immunisation is known –gilt are immunised when they receive the shot-, therefore the risk to introduce a viremic gilt in the breeding herd is negligible.
  More importantly, live vaccines are based on attenuated virus that can induce viremia and can be excreted but are non-virulent.
• The product remains constant:
  Unlike inoculation/contact of live-resident virus, the commercial vaccine composition is constant and guaranteed by the pharmaceutical company.

Whatever the chosen method, seroconversion by ELISA (appearance of antibodies) should be checked in gilts 2-3 weeks later in order to assess that they have been truly immunised.
In case of inoculation/contact of live-resident virus, viraemia or presence of virus in oral fluid should be also analyzed before introducing gilts in breeding herds.
Nevertheless, if inoculation/contact is used, long acclimation times are strongly recommended in order to minimise the introduction of shedding gilts in the breeding herd.

Immunisation of sows:

Vaccines induce only partial immunity against heterologous challenge but they are effective to reduce abortions, mortality of new-born piglets and other reproductive disorders related to PRRS virus infection in sows.
Regarding virological parameters, vaccines helps to reduce transmission of the infection, since they can contribute to reduce viraemia and shedding.
As result, vaccines are a useful tool to stabilize the herd -to cease vertical transmission from sows to fetuses- and to minimise the impact of an outbreak:

• Minimising the impact of an outbreak:
  It is usually achieved vaccinating animals in the breeding herd with a live vaccine, all at same time.
  Vaccine will not have an effect in already infected animals, but in this case the final goal is to induce immunity to those animals that have not been infected yet.
• Recall vaccinations:
  There are several vaccination protocols to immunise sows against PRRS virus.
  All of them seek to maintain the immunity reached when gilts and to avoid the existence of negative subpopulations. Recall vaccination can be done using MLV or IV.
  Currently, the most common protocol applied is based on 3-4 vaccinations/year, usually in a blanket vaccination strategy (all animals at the same time).

Compliance of vaccination can be done by testing antibodies using ELISA since it should be expected all vaccinated animals will be positive.

Nevertheless, from time to time swine practitioners in Europe and North-America report that multiple vaccinated sows yield negative results in ELISA.

Obviously, this creates uncertainty about the real status of the sows and the performance of the ELISAs. Please, see point 9 “Monitoring” in the “Diagnostic and monitoring” section for more data.