Development of the disease
Porcine circovirus type 2 (PCV2) is passed via direct contact and is shed in most bodily fluids. The oral and nasal routes are the most common routes of virus transmission. The virus is capable of replicating in T and B lymphocytes and macrophages, as well as in porcine embryos.
The virus diminishes the immune system by depleting the B and T lymphocytes and increasing the number of macrophages, while also changing the effector function and cytokine production of these macrophages. The first signs of infection occur around 7 days after virus inoculation.
Porcine Circovirus Diseases (PCVD) are slow and progressive diseases with a high fatality rate in affected pigs post-weaning.
Signs are usually observed from about 6-8 weeks of age onward:
- Weaned pigs lose weight and gradually become emaciated
- Rough hair coat
- Pale sometimes jaundiced skin
- Sudden death
- Enlarged peripheral lymph nodes – inguinal lymph nodes are often very prominent
- May show diarrhea
- May show respiratory distress caused by interstitial pneumonia
- Rarely incoordination
Post-weaning mortality is likely to rise to 6-10% but is sometimes much higher. Mortality peaks in affected piglets around 9-12 weeks of age. Signs of disease are present between 2-6 months of age.
Clinical cases may keep occurring in a herd over many months. They usually reach a peak after 6-12 months and then gradually decline.
Table 1 from Segales et al., 2012 includes proposed terminology for PCVDs, together with their case definition, based on clinical and laboratorial findings.