A study on the severity and relevance of porcine circovirus type 2 infections in Dutch fattening pigs with respiratory diseases

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Abstract

This study was set up to get more insights in the severity and relevance of porcine circovirus type 2 (PCV2) infections in Dutch fattening farms in an endemic PCV2-situation with no clinical signs of post-weaning multisystemic wasting syndrome (PMWS). In part A of the study, in total 29 commercial fattening farms with varying percentages of pneumonia and pleurisy at slaughter were examined. Blood samples were collected at random by cross-sectional sampling; 10 in the age of 10–12 weeks, 10 at the age of 16 weeks and 10 blood samples at the end of the finishing period (20–22 weeks of age). Serum samples were examined for the presence of PCV2 IgM and IgG antibodies and for antibodies against other porcine lung pathogens. In part B, 8 “high” and 8 “low” herds were selected. The 8 “high” herds were defined as herds having high percentages of lung lesions (pneumonia) at slaughter, and the 8 “low” herds had low percentages of pneumonia at slaughter. For both the “high” and “low” herds, 3 pigs showing signs of respiratory distress were selected for necropsy (n = 48). Lung tissue samples were examined post-mortem for macroscopic and histopathological lesions, and for the presence of bacteria and viruses. The results of part A showed that, pigs at 16 weeks of age with IgM antibodies against PCV2 had a lower probability of having pleuritis at slaughter (OR 0.34, P < 0.000). Pigs in the age category of 20–22 weeks, and with IgM antibodies against PCV2, also had a lower probability of having pneumonia at slaughter (OR 0.29, P = 0.032). In part B lobus apicalis pneumonia, PCV2 in macroscopically unaffected lungs, Pasteurella multocida, Mycoplasma hyopneumoniae, and swine influenza viruses were all found significantly more often in “high” than in “low” pigs at autopsy. High PCV2 DNA loads (>104 PCV2 DNA copies/mg) were found in lungs of 14 (58%) “high”, and in 7 (29%) of the “low” pigs (P = 0.13). In 11 of the 19 affected lungs from “high” pigs, high PCV2 DNA loads were found in combination with one or more other lung pathogens, while this was found only in 5 of the 17 affected lungs from “low” pigs (P = 0.02). This study confirms the hypothesis that PCV2 plays a role in pneumonia and pleurisy in 10–24 weeks old fattening pigs, not only in herds with a high prevalence of PMWS, but also in herds with no clinical signs of PMWS.

Introduction

Porcine circovirus type-2 associated diseases (PCVAD) are an important concern to swine producers around the world. In addition to post-weaning multisystemic wasting syndrome (PMWS), porcine circovirus type 2 (PCV2) has also been associated with porcine dermatitis and nephropathy syndrome (PDNS), porcine respiratory disease complex (PRDC) and proliferative and necrotizing pneumonia (PNP) (Allan et al., 1999, Opriessnig et al., 2004, Opriessnig et al., 2006, Opriessnig et al., 2007, Wellenberg et al., 2004a, Wellenberg et al., 2004b, Grau-Roma and Segalés, 2007). In the US, PCV2 associated pneumonia was the most common presentation of PCV2 associated diseases (Opriessnig et al., 2007).

PCV2 has often been associated with lung lesions in pigs co-infected with PRRSV, swine influenza viruses (SIV) or Mycoplasma hyopneumoniae. These agents appeared to create a synergistic environment that resulted in increased amounts of PCV2 and unusual severe respiratory disease outbreaks. However, there have also been reports on uncomplicated PCV2 pneumonia cases in western Canada (Opriessnig et al., 2007). These lesions included necrotizing and ulcerative bronchiolitis, granulomatous inflammation in the alveolar septa and mixed inflammation and fibroplasias in the lamina propria and peribronchiolar areas.

More insight is warranted on the role of PCV2 as etiological agent in porcine respiratory diseases such as PRDC and PNP in endemic situations. In addition, little or no information is available about whether PCV2 can be associated with pneumonia in pigs from Dutch swine herds, as described for US and western Canada.

The purpose of this study was to get more insight in the severity and relevance of PCV2 infections in Dutch pigs with symptoms of respiratory distress in an endemic situation (situation with no or low prevalence of PMWS: Wellenberg and Segalés, 2006). In studies on the association between PCV2 infections and respiratory disease, the role of other concurrent co-infections cannot be excluded and therefore, this study was not only focused on the detection of PCV2 infections, but also on the presence of other infectious agents associated to respiratory diseases in swine.

The results of this study may be used to gather more evidence on the hypothesis that PCV2 plays a role as etiological agent in respiratory diseases such as PRDC and PNP.

Section snippets

Materials and methods

This study was divided in to two parts: A and B. In part A, blood samples were examined for the presence of antibodies directed against pathogens associated with porcine lung lesions. In part B, pigs from 8 herds, with high percentages of lung lesions (pneumonia) at slaughter (“high”), and pigs from 8 herds, having low percentages of pneumonia at slaughter, were examined. These pigs, from both groups of herds, were selected on showings signs of respiratory distress, and further examined

Part A

The overall technical data are presented in Table 1. Data from pigs from 29 herds were collected but three herds were excluded in the data presented in Table 1. Their data sets were not complete (the slaughterhouse percentages of 2006 were not available).

The mean number of slaughtered pigs per herd in 2006 was 1651 and this number varied between herds from 669 to 7600. The recorded lung lesions and pleuritis in slaughtered pigs varied per herd between, 0.1–17.7% and 0.6–35.8%, respectively. The

Discussion

In Europe, the economic losses in swine industry related to respiratory diseases are very high. PRRSV, SIV, M. hyopneumoniae, P. multocida and A. pleuropneumoniae, are all known contributors to this disease complex. PCV2 also has been found in association with porcine respiratory diseases, but its role as aetiological agent is still not clear (Allan et al., 1999, Opriessnig et al., 2004, Opriessnig et al., 2007, Grau-Roma and Segalés, 2007). More insights are warranted on the role of PCV2 as

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