Most of the attention on pig diseases associated with the porcine circovirus PCV2 has been devoted to the PMWS wasting syndrome in young pigs. However, recent successes with circovirus vaccines have suggested that they are also helpful in dealing with the later condition known as the porcine dermatitis and nephropathy syndrome.
PDNS was thought originally to occur separately from PMWS, but it is now regarded as part of the same systemic manifestations of a PCV2 infection. An important difference is in the timing of appearance. When they arise together in the same unit, the usual form is for PDNS to occur 12-14 weeks after PMWS, probably as a result of a damaged immune response. However, the later syndrome affects only a small proportion of the pigs, typically those in good condition.
Acute cases occurring in pigs in the wide age range of 7-22 weeks must be treated as involving a notifiable disease, because the signs resemble those of classical swine fever/hog cholera. A rare epizootic form seen in the UK particularly in 1999-2000 needed to be differentiated from both classical swine fever and African swine fever because of the presence of enlarged swollen, oedematous and haemorrhagic lymph nodes. Other differential diagnoses include acute erysipelas, bovine virus diarrhoea (BVD) infection, salmonellosis and similar septicaemias.
Signs on the skin
As a dermatitis, PDNS is seen as skin lesions. These can vary from small marks to patches particularly over the loins, hind legs and ventral abdomen, but in severe cases they will cover the whole body. Eventually these areas develop a purple-red colour with black centres. The pigs may be pale and lethargic, sometimes they have a fever.
In acutely affected pigs the skin signs are very pronounced. Mortality rates can be as high as 80% and perhaps even 90%. But the morbidity level is low, under 1%. Sporadic cases of PDNS also are observed and can happen at any stage between 5 weeks and 9 months old. The clinical signs of sporadic, enzootic and epizootic forms are shown in the chart on page 18.
Although there is no proof as yet, it is likely that PDNS results from disturbances of the immune system. High levels of the circovirus are thought to become attached to the major antigen-presenting cells in the tissues, which are known as dendritic cells. The infected dendritic cells retain their ability to process and present antigen to lymphocytes even with the PCV2 infection. However, they are less able to recognise the danger signals' presented by foreign antigens.
The effects on these cells may mean no efficient immune response develops against other pathogens. Therefore further infections can occur as they do in PDNS, with 3 different viruses (those of PRRS and swine influenza alongside PV2) being identified in one recently reported case. A further observation has been that PCV2 affects the infected macrophages (defence cells) so the protective system of the pig tries to stimulate both cellular and humoral immunity at the same time. As one will counteract the other, this is hardly likely to lead to a successful immune response.
But there has been some success internationally in controlling PCV2 infections through changes in management and the genetics used by pig units. Some countries report that the really susceptible family lines may have been eliminated from breeding stock, leaving the more resistant families. Such methods appear to have reduced morbidity and mortality by 50% at sites trying to control PCV-associated diseases, even before the recent introduction of vaccines to use either on the sow or on piglets. However, experience in Europe is that these vaccine products have greatly improved the disease situation.
What is more, from practitioner observations in the UK, the considerable reduction in the incidence of PMWS cases has been accompanied by the near-disappearance of PDNS. We can see already that vaccination has reduced the immuno-suppression and therefore the presence of the secondary infections associated with PDNS cases. PIGI