5 Immunisation of piglets

The use of commercial vaccines should be the only possible option to immunise piglets.

To vaccinate piglets, two important points must be considered in order to choose the best moment:

• The development of immunity will require at least 3 weeks.
• Maternal antibodies can interfere with the vaccines.

In a stable farm, vaccination of piglets can be considered if infection occurs after six weeks of age. Only under these circumstances, will be possible to adjust the vaccination timing to the development of immunity and the interference with the maternal antibodies.

Maternally derived antibodies and potential interference with vaccination:

Piglets are born defenceless. Unlike human and other species, no antibodies are transferred through the placenta from the sow to the piglets before birth.

Thus, piglets only receive passive immunity after birth, through to the ingestion of colostrum, which is secreted and ingested during the first hours after parturition.

The antibodies present in colostrum are known as maternally derived antibodies (MDA). Composition of colostrum is approximately 60% Immunoglobulin G (IgG), 30% Immunoglobulin A (IgA) and 10% Immunoglobulin M (IgM).

On the contrary, milk, which also contains antibodies, is mostly composed of IgG (70%) and IgA (30%).

In sows, MDA are induced during the gestation after the exposition to an infection and/or by vaccination.

In piglets, MDA may persist up to twelve weeks; the duration depends on the amount of colostral antibody taken and on the type of antigen against antibodies react.

During first days/weeks of life, MDA are essential for the survival of the piglet. Their main function is to neutralize pathogens.

Unfortunately, due to this ability they are also capable of interfering with vaccines. MDA has been observed to interfere with several vaccines, such as Influenza virus, Aujeszky’s disease virus, Classical swine fever virus and PCV2.

Also, this interference has been proven for Equine viral arteritis, a disease caused by a virus belonging to Arterivirus.

In the case of PRRS virus, few studies have addressed this topic:

Results show that MDA affect both post-vaccination humoral and cellular immune responses.

How to overcome or reduce MDA interference?

• Postponing the vaccination until it is assumed that there is no interference:
  MDA levels decline over time until they do not interfere with the vaccine. Vaccination should be administered as late as possible to avoid interference with MDA and at the same time well in advance (2-3 weeks at least) before infection is expected; otherwise, the animal would not have time to build up an adaptive immunity.
  Seroprofiles at different ages are mandatory to determine when MDA level falls and when antibodies due to infection appears.
• Choosing more powerful vaccines:
  More powerful vaccines could be created by increasing the antigenic titer and/or with proper adjuvants. One of the weaknesses of PRRS vaccines is their low amount of antigen; obtaining high titers of virus in cell cultures is a main issue in PRRS vaccine production.
• Using alternative routes of administration:
  Although no conclusive data is available, it could be expected that intranasal or intradermic vaccination could overcome the MDA interference.
• Administering two doses of vaccine in a short period:
  In those animals in which the first dose was neutralized by MDA, second dose would result in a response to the vaccine.