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Porcine circovirus type 2 (PCV2) is the essential infectious agent of postweaning multisystemic wasting syndrome (PMWS), a multifactorial disease affecting pigs worldwide. This disease causes severe economic losses associated to increased mortality rates and reduced production parameters in nursery and fattening units. Hallmark lesions of PMWS affected pigs are found in lymphoid tissues and research performed during last 10 years indicates that PMWS is a disease of the immune system. It is worthy to remark that PMWS may affect a proportion of farms and, within affected herds, a variable percentage of pigs (usually 4-20%) may display clinical signs compatible with PMWS. However, PCV2 infection takes place in almost all farms, independently of the PMWS status, and virtually in all pigs at a given moment of their life. Therefore, from a numerical point of view, the proportion of PCV2 subclinical infections is much larger than the number of pigs affected by PCV2 associated diseases.
Most immunological studies have focused on the outcome of PMWS affected pigs. Data come from naturally occurring disease but precise timing of immunological alterations in regards PCV2 infection is virtually unknown. To date, there is little doubt that PMWS is an acquired immunodeficiency syndrome of pigs. Interestingly, experimental studies have shown that a PCV2 subclinical infection has also an impact on the pig immune system. Infected pigs develop specific humoral and cellular responses, although with a relatively slow kinetics. In addition, PCV2 subclinically infected pigs show an increase on plasmatic interferon (IFN)-α shortly after infection, and interleukin (IL)-10 (an immunomodulatory cytokine) levels in serum seem to correlate with longer viremia duration. Moreover, the transient PCV2-specific IL-10 response during the viremic phase of infection seems to coincide with an inversion of the IgM/IgG ratio. In conclusion, all together, available data indicate that PCV2 has the ability to modulate the immune response, not only during the obvious overt disease but also in a subclinically infection.
To evaluate the effects of PCV2 subclinical infection on production parameters is relatively difficult by means of experimental infections. Our particular research group has performed around 12 trials infecting globally more than 250 pigs intranasally inoculated with PCV2. Most pigs of these experiments were subclinically infected. Only one of these studies, in which piglets (caesarean-derived, colostrums-deprived) were infected with PCV2 at the age of 7 days, showed significant differences in weight compared to control animals after 29 days post-infection. One of the major constraints of experimental studies evaluating productive parameters is the low number of animals used, preventing therefore, the possibility to assess differences in terms of productivity. An alternative way to study the impact of the subclinical infection is through PCV2 vaccination in PMWS-free PCV2 infected farms, since it will provide a much higher number of animals to monitor and the usual environment (facilities, co-infections, etc) that pigs face with.
PCV2 vaccination has become very popular during last 3-4 years, even to the point to consider such vaccination as almost compulsory in a high proportion of pig farms worldwide. Data from field studies have shown that all commercial vaccine products show clear efficacy. This result has been recently emphasized by means of a meta-analysis work in which 66 trials published in 24 studies were included. Improved average daily gain and feed conversion rate, decreased mortality and cull rates and reduced medication cost are most evident benefits observed in nursery-growing pigs from vaccinated farms. Besides, viral genome load in serum and within PMWS-like lymphoid lesions are also reduced in vaccinated pigs. Importantly, these improvements have not only been observed in PMWS affected farms, but also in farms with no diagnosis of the disease and farms without experiencing PMWS-like clinical signs. Therefore, benefits of vaccination do not only come from preventing PMWS, but mostly from preventing the effects of PCV2 subclinical infection.
