Control of porcine reproductive and respiratory syndrome virus (PRRSV) infection is a complex task comprising at least four different lines of action: diagnosis and monitoring herd management, biosecurity and immunization. In general terms, immunization through vaccination is the easiest and safest way to stabilize a herd. In the present paper, the practical use of PRRSV vaccination will be revised.
What can I expect from PRRSV vaccination?
In most instances PRRSV vaccines will induce just “partial protection” to vaccinated pigs. This means that vaccinated pigs could be infected if confronted with a PRRSV strain different enough to the vaccine but the existing immunity will control the infection sooner and stronger that if the animal had been not vaccinated. Usually, vaccination results in good clinical protection against the reproductive form (reduction of abortions, mummies, etc. caused by PRRSV) and in a fair protection for piglets, that is usually seen as a reduction of mortality and of secondary respiratory infections.
In virological terms, vaccination reduces the duration of the viremia in case that the vaccinated animal got infected and decreases viral shedding. Therefore, vaccination may contribute to reduce transmission of the infection. For this reason, vaccination is useful to stabilize the herd (ceasement of vertical transmission from sows to foetuses during pregnancy).
However, reduction in viral circulation after vaccination could be influenced by the strain. In general terms, PRRSV strains with higher virulence tend to be more able to surpass the protection afforded by vaccines.
Remember: Since the immunity conferred by PRRSv vaccines is partial, the efficacy of a vaccine will be related to the biosecurity of your farm. The higher the number of new lateral virus introductions, the higher the probability to confront a more virulent or different strain entering your farm.
PRRSV vaccination: who, when and how
Vaccination against PRRSV can be used with four different purposes: a) to stop an outbreak; b) to acclimatize gilts; c) recall vaccination of sows or, d) to immunize piglets. Let’s review these four uses.
a) Emergency PRRSV vaccination
When a reproductive PRRSV outbreak occurs, the clinical phase will last between 1 and 4 months depending on the characteristics of the farm if no intervention is done. Therefore, to minimise the impact of the disease is a peremptory objective. This can be achieved by means of an emergency vaccination of sows. The basis of this strategy is to induce a basal level of immunity against the virus in all present sows at the same time.
There are a number of points to be considered for the success of the emergency vaccination:
- First: the sooner, the better. It has to be kept in mind that the development of immunity after vaccination will take about 3 weeks. So, the later the vaccination is performed, the higher the proportion of infected sows at the moment of vaccination. Remember: the vaccine is only useful when applied to naïve sows.
- Second: emergency vaccination is to be performed only with modified live (attenuated) PRRSV vaccines. It has been shown that primo-immunization develops faster and stronger with live vaccines. However, you have to remember that live vaccines replicate in the animal and therefore, sows will become transiently viremic as a consequence of the vaccination (apply this knowledge to the interpretation of PCR results in the following weeks after vaccination). Also, if applied in late pregnancy, viremic piglets may be born (although with no clinical consequences).
b) Vaccination for acclimatizing gilts
Acclimation of PRRSV-free gilts is best done through vaccination. For this purpose consider always a first vaccination with a live vaccine and check gilts serologically 14 days after vaccination in order to see that they have been correctly vaccinated. In some instances, when infectious pressure in the destiny farm is high, consider revaccination 4 weeks after the first vaccination and at least 3 weeks before first insemination. If your replacements are serologically positive to PRRS because of a previous infection when younger, a single vaccination (again with an attenuated vaccine to assure a high level of immunity) would suffice. Remember: first immunization has to be done always with a live vaccine.
Recently, devices for the intradermal application of a live vaccine in gilts and sows have been marketed. Intradermal vaccination seems to be at least equivalent to intramuscular vaccination for PRRSV and offer advantages related to the handling and restraining of gilts and sows.
c) Recall vaccination of sows
Recall vaccination can be done using modified live or inactivated vaccines. In both cases, the most usual protocols consider 3 (sometimes 4) vaccinations/year, usually in a blanket vaccination strategy. For the selection of vaccines, consider your risk of infection. When the pressure of infection is not very high, booster vaccination with inactivated vaccines is an option if you don’t want to introduce a live virus in your sow stock.
If a protocol fails to keep the immunity of your herd after 3-4 vaccinations per year, first check for things you may be doing wrongly (inadequate storage of vaccines, wrong vaccination technique, etc.) since this is a common cause. Remember: re-vaccination of already seropositive sows is not always translated to increases in the S/P ratios in the ELISA.
d) Vaccination of piglets
Vaccination of piglets has been controversial for many years for several reasons. To understand how and when vaccination of piglets can help, it is essential to remember that if viremic piglets are born, vaccination of already infected animals will be of no use. Also, you have to consider that at least 3-4 weeks for the development of immunity are needed. In other words, vaccination of piglets is to be applied when in a stabilized herd PRRSV infections take place after 6-7 weeks of age; if weaners are infected earlier, the success of vaccination will be probably scarce.