Porcine circovirus 2 immunology and viral evolution

Abstract

Porcine circovirus 2 (PCV2) has and is still causing important economic losses to pig industry. This is due to PCV2-systemic disease (PCV2-SD), formerly known as postweaning multi-systemic wasting syndrome (PMWS), which increases mortality rates and slows down the growth of the animals, as well as other conditions collectively included within the so-called porcine circovirus diseases (PCVD). PCV2-SD affected pigs are considered to be immunosuppressed, with severe lymphocyte depletion and evidence of secondary infections. However, PCV2-infected pigs not developing the disease are able to mount humoral and cellular immune responses and clear the virus or limit the infection. On the contrary, insufficient amounts of neutralizing antibodies have been linked to increased PCV2 replication, severe lymphoid lesions and development of PCV2-SD. Central role in controlling PCV2 infection are played by the antigen specific memory T cells. These cells persist long term post-infection or vaccination and are able to expand rapidly after recall antigen recognition. Most farms in the main pig producing countries are applying vaccination against PCV2 to prevent the disease and improve the farm performance. Vaccines do not induce sterilizing immunity and PCV2 keeps on circulating even in farms applying vaccination. This, together with the high mutation rate of PCV2, world-wide fluctuations in the genotype dominance and emergence of novel genetic variants, warrant close molecular survey of the virus in the field.

Porcine circovirus 2 (PCV2) population study in experimentally infected pigs developing PCV2-systemic disease or a subclinical infection

“…The direct consequences of viral replication, especially in immune cells, and the deregulation of immune signaling can cause a severe immunosuppression in infected animals. Actually, PCV2-SD is characterized by a significant lymphocyte depletion in lymphoid tissues and alteration of cytokine and chemokine expression profile 17,18 . Based on these premises, two components are likely to contribute to the observed scenario: the viremia level (i.e.…”

Natural viral co‐infections in pig herds affect hepatitis E virus (HEV) infection dynamics and increase the risk of contaminated livers at slaughter

“…Another swine virus, the porcine circovirus of type 2 (PCV2), is known to modulate the immune response as well. PCV2 can cause PCV2‐ systemic disease also named post‐weaning multisystemic wasting syndrome, which leads to severe B and T lymphocyte depletion in blood and lymphoid tissues (Kekarainen & Segales, ). PCV2 DNA genome is able to inhibit the production of IFN‐α by stimulated plasmacytoid dendritic cells (Vincent et al, ).…”

A randomized controlled study on the efficacy of a novel combination vaccine against enzootic pneumonia (Mycoplasma hyopneumoniae) and porcine Circovirus type 2 (PCV2) in the presence of strong maternally derived PCV2 immunity in pigs

“…Those include disorders such as PCV2 - Systemic Disease (PCV2-SD) (formerly known as postweaning multisystemic wasting syndrome) which is characterised by growth retardation - wasting, anaemia, possible respiratory problems and diarrhoea, as well as lymphocyte depletion and immune suppression. Moreover, PCV2 is present in the aetiology of porcine dermatitis and nephropathy syndrome, and clinical manifestations of pneumonia, reproductive failure or intestinal disorders [5–7]. However, subclinical PCV2 infection occurs frequently and although clinically “silent”, gradually results in significant economic losses due to growth retardation [8].…”

“…For almost a decade (since 2007), control of PCVD has been also based on vaccination of sows and/or piglets [3, 9]. Nevertheless, those two pathogens persist, circulate and interact with other pathogens in the host, making their clearance extremely difficult [7, 10]. …”