In a sow farm positive for PRRS should vaccines be done blanket or weekly at 60 days pregnant?

This is quite a difficult question to answer, as it depends of a wide range of factors, such as:

Herd structure, herd size, herd organization, PRRS virus gilt acclimation strategies and the number of herds situated in the area (i.e. how high the PRRSV infection risk is).

Sow vaccination programmes against PRRS virus

In a previous question answered by Enric Marco on this site, the different possibilities of sow vaccination were described: from PRRSV blanket vaccination 4 times a year, to the 6/60 program.

Enric Marco also analyzed the pros and cons of each strategy, so reading this previous entry is highly recommended.

Fig 1. Sow vaccination programmes from less to most intensive one for PRRS virus immunization.

PRRS virus status of the herd

Another important issue is the PRRSV status. For instance, we’d need to know the following information of the herd:

• Is the sow herd stable -antibody positive and virus negative- and weans PRRSV negative piglets?
• Is the PRRS virus circulating in the farrowing unit? Are there clinical signs of PRRSV in the nursery?

PRRSV blanket vaccination

Let’s do a brief review of this vaccination programme:

• Objectives:

As I understand it, then the rational for the use of blanket vaccinations 2-4 times a year is partly to avoid that some sows have a low PRRS virus level of immunity and partly to secure that the sows are protected against infection with more diverse PRRSV strains.

• Scientific studies:

In general, there is a lack of public available controlled, independent studies that document the advantages and disadvantages of using this system in the field, especially under European conditions.

• Personal experiences:

Many practitioners and producers in Denmark that use PRRSV blanket vaccination are satisfied with the results, whereas others do not believe in the system or have stopped, because they experienced too many PRRS virus problems in connection to the vaccinations.

The differences in outcome of PRRSV blanket vaccination can probably be explained by the differences between the herds as listed above, but since very few controlled field studies with appropriate controls have been published, these differences in perceptions are not supported by statistical sound data.

• Extensive use of PRRS virus sow mass vaccination, should you worry?:

Theoretically, from a virologist point of view, I am a bit worried about the extensive use of mass (sow) vaccination, partly because it sustains (vaccine) PRRS virus circulation in the farrowing unit and allows naïve piglets to be exposed to the vaccine virus which in turn may push the evolution of the vaccine virus; and partly because it will increase the risk of PRRS virus recombination between vaccine and field virus strains.

Also remember that for most of the PRRS MLV vaccines, the use in sows in late gestation is off-label use!

I am sorry that I was unable to give more clear recommendations; however, hopefully the answer can help you in your final decision on which strategy to implement in your herds.

If you want to know more about the different protocols for PRRS virus immunization of sowa, Chapter No. 4, “Immunisation of gilts and sows”.

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