Experimental Inoculation of Conventional Pigs with Porcine Reproductive and Respiratory Syndrome Virus and Porcine Circovirus 2

Abstract

Postweaning multisystemic wasting syndrome (PMWS) is a disease of nursery and fattening pigs characterized by growth retardation, paleness of the skin, dyspnea, and increased mortality rates. Porcine circovirus 2 (PCV2) has been demonstrated to be the cause of PMWS. However, other factors are needed for full development of the syndrome, and porcine reproductive and respiratory syndrome virus (PRRSV) infection has been suggested to be one of them. Twenty-four conventional 5-week-old pigs were distributed in four groups: control (n ‫؍‬ 5), PRRSV inoculated (n ‫؍‬ 5), PCV2 inoculated (n ‫؍‬ 7), and PRRSV and PCV2 inoculated (n ‫؍‬ 7). The two groups inoculated with PRRSV showed growth retardation. Pigs inoculated with both PRRSV and PCV2 had increased rectal temperature. One of these pigs developed wasting, had severe respiratory distress, and died. The most important microscopic lesion in pigs inoculated with PCV2 was lymphocyte depletion with histiocytic infiltration of the lymphoid organs, more severe and in a wider range of tissues in doubly inoculated pigs. Interstitial pneumonia was observed in the three inoculated groups. PCV2 nucleic acid was found by in situ hybridization in larger amounts and in a wider range of lymphoid tissues in PRRSV-and PCV2-inoculated than in PCV2-inoculated pigs. TaqMan PCR was performed to quantify the PCV2 loads in serum during the experiment. PCV2 loads were higher in doubly inoculated pigs than in pigs inoculated with PCV2 alone. These findings indicate that severe disease can be reproduced in conventional 5-week-old pigs by inoculation of PRRSV and PCV2. Moreover, these results support the hypothesis that PRRSV infection enhances PCV2 replication.

Lack of an effect of a commercial vaccine adjuvant on the development of postweaning multisystemic wasting syndrome (PMWS) in porcine circovirus type�2 (PCV2) experimentally infected conventional pigs

“…No characteristic PMWS lesions [8,23] were found at the experimental termination day, despite the occasional presence of the viral genome in some lymphoid tissues. The only clinical evidence of the viral infection was an intermittent hyperthermia, between days 16 and 19 PI, which coincided with the onset of viremia and the highest viral load in the serum, which is in concordance with the results from other PCV2 experimental trials [7,24].…”

“…In addition, no significant differences were observed in PCV2 viremia quantification, thus excluding any potentiation effect on viral replication by the adjuvant in this particular trial. It is known that PMWS affected pigs have a higher viral load than subclinically PCV2 infected pigs [12,13,15,16,24]. Therefore, it has been proposed that the use of products that stimulate the immune system, such as vaccines or nonspecific immunostimulators, may potentiate PCV2 replication leading to PMWS [4,12,14].…”

“…To quantify PCV2 DNA in PCR positive serum samples from 7, 14, 21, 49 and 69 DPI, a real-time fluorescent-probe PCR was used as previously described [24]. The assay was performed in duplicate.…”

“…However, the severe clinical expression observed in field cases has been difficult to reproduce experimentally [1], suggesting that the disease is probably of multifactorial aetiology and/or that not all hosts are susceptible to the development of the disease. Experimentally, a synergistic relationship is observed when PCV2 is coinoculated with porcine parvovirus (PPV) or porcine reproductive and respiratory syndrome virus (PRRSV), resulting in a more severe expression of PMWS [2,3,9,11,24]. These studies did not elucidate the synergistic mechanism between those viruses and PCV2, but it is suggested that the activation of the cells from the monocyte/macrophage lineage by PPV and PRRSV could support an increase of PCV2 replication [2,3].…”

Quantitative Detection of Porcine Torque Teno Virus in Porcine Circovirus-2–Negative and Porcine Circovirus–Associated Disease-Affected Pigs

“…Porcine circovirus-2 alone was not assumed to be sufficient to cause PCVAD and needed cofactors to cause the disease. 3,18 Recently, TTV has been proposed as a candidate that aggravates PCV-2 infection and causes clinical manifestation of the disease.…”