Pathogenesis of PCVD

The exact pathogenesis of PCVD is still not exactly known. It has been agreed that PCV-2 infection is necessary, but its presence doesn't automatically equal PCVD. PCVD is a disease complex, which requires multiple components to develop.

The immune response seems to play a major role in the outcome of PCV-2 infections. Natural cases of pigs with PCVD have extensive lymphoid lesions (lymphocyte depletion and granulomatous inflammation) and altered cytokine expression patterns in peripheral blood mononuclear cells (PBMC's) and lymphoid organs.

Several key factors of the pathogenesis of PCVD are now known including:

• the accumulation of PCV-2 in dendritic cells, a specific class of immune cells involved in pathogen surveillance (Vincent et al., 2003) in subclinically infected but otherwise healthy pigs;
• the pivotal roles of immune stimulation and viral upregulation in clinical disease expression (Krakowka et al., 2001);
• the presence of high PCV-2 levels in clinically ill and significantly lower levels in healthy pigs (Ladekjaer-Mikkelsen et al., 2002; Brunborg et al.,2004);
• PCV-2 infection in early life being a risk factor for the development of clinical disease (Rose et al., 2003; Lopez-Soria et al., 2005); and
• the hallmark histological lesions of granulomatous inflammation and lymphoid depletion (Krakowka et al., 2000; Segales & Domingo, 2002) in clinically sick animals.

Clinical PCVD appears to be triggered by a number of infectious and noninfectious factors. Virtually all commercially raised pigs are subclinically infected with low levels of PCV-2 (Larochelle et al., 2003; Harding, 2000) yet most remain healthy and do not develop clinical disease because triggering factors are absent. The exact mechanism(s) of the triggering factors is not known, but simultaneous infections with other pathogens including PRRS virus, Mycoplasma  hyopneumoniae, swine influenza virus (SIV), and parvovirus, swine influenza virus (SIV), and parvovirus, or the absence of good production practices exacerbate clinical disease.