This article focuses on some of the key aspects of the PRRS vaccine as a tool required for active animal immunisation. From an economic and practical perspective, vaccination is a feasible tool for all kinds of breeders when compared to other immunisation systems.
In recent studies comparing active immunisation with modified live virus (MLV) PRRS vaccines and live virus inoculation (LVI), it was shown that when MLV vaccines are used to stabilise the breeder population after an outbreak, the mean time required to recover production levels and the impact on production are shorter and less severe than when LVI is used. Furthermore, the economic losses associated with an outbreak –quantified according to the piglets that are not bred- are greater with LVI than with MLV PRRS vaccine (Linhares et al., 2013). Finally, the use of MLV ensures correct contact of all the animals with PRRSV; however, when LVI is used the quantity of administered virus cannot be guaranteed, and therefore we cannot ensure proper immunisation of all sows.
CONTROL OF PRRS
Numerous strategies have been described for the control of PRRSV on individual farms. Successful control of the disease depends on a combination of the following measures:
Animal management (sow replacement, unidirectional animal flow, etc.)
Active immunisation (PRRS vaccine)
Although it is known that it does not prevent infection, vaccination is used in order to reduce the clinical onset of the disease and to reduce viral excretion. When it is used in nulliparous sows there is a reduction in viraemia, dead piglets (before and after birth) and congenitally infected piglets (Scortti et al., 2006). Moreover, live piglets have a higher birth weight and a higher survival rate than piglets from non-vaccinated nulliparous sows (Rowland, 2010). The use of MLV PRRS vaccine in multiparous sows infected with PRRSV effectively helps to reduce abortions and time to return to oestrus, increasing the birth rate and number of weaned piglets (Pejsak et al., 2006). During an acute PRRS outbreak or endemic infection, the use of attenuated live PRRS vaccines in growing pigs helps to reduce viral excretion and respiratory syndrome, also increasing the growth rate (Cano et al., 2007). The use of MLV in the field has been shown to be effective in the control of both reproductive and respiratory disease.
Currently, the greatest challenge related to the control of PRRSV is the heterologous protection provided by a given PRRS vaccine against the strains found on farms, as the virus presents high genetic and antigenic variability. Clinical heterologous protection can be defined as the clinical protection conferred by vaccine strains against the different field virus strains.
IMMUNE RESPONSE AGAINST PRRSV
It is very important to understand how immunity against PRRSV is developed in animals after vaccination in order to be able to design the best immunisation strategy. It is well known that PRRSV can strongly modulate the immune response, resulting in unusual characteristics in both humoral and cell component (Mateu and Díaz 2008; Darwich et al., 2010).
Innate response generically recognises and responds to pathogens. PRRSV can act as an antagonist of the pig’s defence mechanisms in this initial phase. It can also interfere in the correct presentation of the antigen and T-cell activation. PRRSV modulation of the immune system is variable and depends on each strain.
HUMORAL ADAPTIVE RESPONSE
The adaptive response is specific to each antigen and is characterised by immunological memory. Humoral response to PRRSV is characterised by the early appearance of largely non-neutralising antibodies. Neutralising antibodies usually appear 2-4 weeks post-infection, but are occasionally not even detected (Díaz et al., 2012). Variability of both the quantity and time of appearance of neutralising antibodies has been described.
CELL ADAPTIVE RESPONSE
When cell-mediated response is evaluated using the ELISPOT assay (quantity of IFN-γ secreting cells (IFN-γ-SC)), it takes place 2-3 weeks post-infection. Its evolution is slow, erratic and of a low level compared to other porcine pathogens. The cell response generated against PRRSV appears to be a strain-dependent phenomenon (Díaz et al., 2012). In summary, adaptive cell response is weak and characterised by late, reduced production of neutralising antibodies and a poor cell-mediated response. In an environment poor in neutralising antibodies, cell-mediated response could be used to evaluate immune response after using PRRS vaccine and more or less effectively predict protection against the virus (Díaz et al., 2006, 2012; Lowe et al., 2005; Zuckerman et al., 2007).