Experimental Reproduction

Experimental PCV2-infection models indicate that PCV2 is an opportunist, depending on immunostimulation(Allan et al., 2000a and 2001; Krakowka et al., 2001) or coinfecting agents like PPV(Allan et al., 1999 and 2000b, Ellis et al., 1999; Hasslung et al., 2005; Kennedy et al., 2000; Kim and Chae, 2004; Krakowka et al., 2000; Opriessnig et al., 2004; Ostanello et al., 2005), PRRSV (Allan et al., 2000c; Harms et al., 2001; Rovira et al., 2002), or Mycoplasma hyopneumoniae (Opriessnig et al., 2004b) for PCV2-infection to progress to clinical PMWS. While most research groups have not been able to reproduce clinical disease in pigs inoculated singularly with PCV2, some groups have succeeded in this regard.

Experimental infection of gnotobiotic pigs. Krakowka et al. (2000) inoculated 1-day-old gnotobiotic pigs intranasally with PCV2 (n = 3), PCV2 and PCV1 (n = 4), and PCV2 and PPV (n = 4), and found that all PCV2/PPV developed clinical PMWS whereas the singular infected pigs and PCV1/PCV2-inoculated pigs did not. Microscopically, hepatic necrosis with severe granulomatous hepatitis and prominent bile retention was observed (Krakowka et al 2000).

Experimental infection of colostrum deprived cesarian derived (CDCD): Bolin et al. (2001) inoculated twenty-three 20- to 25-day-old CDCD pigs intranasally and intramuscularly with PCV2 and induced clinical PMWS in the majority of the pigs. Harms et al. (2001) inoculated 36 CDCD pigs with PCV2 (17 of the 36 pig were concurrently infected with PRRSV) and induced severe clinical disease consistent with PMWS in singular and coinfected pigs. Pogranichnyy et al. (2000) inoculated five 8-week-old CDCD pigs intranasally and intramuscularly with PCV2, necropsied all pigs at 35 DPI, and observed mild microscopic lesions associated with PCV2.

Experimental infection of colostrum deprived (CD) pigs: Allan et al. (1999) inoculated eleven 1- to 2-day-old CD pigs with PCV2 (5 of 11 pigs were coinfected with PPV, and 2 of 11 pigs were coinfected with a formalin-treated PCV2/PPV mixture) and 3 of the 11 pigs died from clinical PMWS (one singular and two coinfected pigs). Allan et al. (2000c) inoculated eight 1- to 2-day-old CD pigs intranasally with PCV2 (5 of 8 pigs were coinfected with PRRSV) and found enhancement of PCV2 but no clinical signs or gross lesions in dual-infected pigs. Allan et al. (2003) inoculated thirteen 4-day-old CD pigs intranasally with PCV2 (9 of 13 pigs were coinfected with PPV), and reproduced clinical PMWS in 1 of 4 singular PCV2-inoculated and 9 of 9 PCV2/PPV coinfected pigs.

Experimental infection of conventional pigs. Albina et al. (2001) inoculated four 6- to 7-week old specific pathogen free piglets intratracheally with a lung tissue homogenate obtained from two French naturally PMWS-affected pigs. Thereafter, 7 passages of 6 to 8 pigs per group were subsequently intratracheally and intramusucullarly infected with lymph node homogenates from the previous passage. PMWS was reproduced in 4 of 55 (7.2%) of the pigs (Albina et al., 2001). Clinical PMWS was recently induced in 3 of 5 non-immunostimulated and in 1 of 5 with keyhole limpet hemocyanin in Freund’s incomplete adjuvant (KLH/ICFA) stimulated 3-week-old conventional colostrum-fed pigs singularly-infected with PCV2 (Ladekjær-Mikkelson et al., 2002). Balasch et al. (1999) infected eight 8-week-old conventional pigs intranasally with tissue homogenates from naturally PMWS-affected pigs and reproduced PMWS-like lesions in the pigs. Allan et al (2000a) inoculated 28 conventional pigs at 13, 31, and 47 days of age oronasally with PCV2. Half of the pigs were vaccinated with a M. hyopneumoniae vaccine at 13 and at 31 days of age and with an APP vaccine at 47 days of age. Three of fourteen vaccinated pigs developed clinical PMWS.

 

PCV2 infectious genomic DNA

Fenaux et al. (2002, 2003) investigated the infectivity of an US PCV2 molecular DNA clone when infected directly into the liver or inguinal lymph nodes. Mild PCV2-associated lesions were observed in 4-week-old conventional pigs confirming the role of PCV2 in PMWS. Similar results (mild microscopic lesions and no clinical disease) were observed in another study using a European clone (Roca et al. 2004). Grasland et al. (2005) reported of the reproduction of PMWS in immunostimulated (injections of KLH/ICFA and thioglycollate medium) conventional pigs inoculated with a European PCV2 DNA clone.

 

Coinfections present in cases of naturally occurring PMWS

Ellis et al. (2000) demonstrated PPV and PCV2 coinfection in 12/69 cases of naturally acquired PMWS. A retrospective analysis of 484 PMWS cases in the Midwest United States revealed that PCV2 alone was found in only 9 of 484 (1.9%) cases investigated (Pallarés et al., 2002). PRRSV was detected in 251 of 484 (51.9%) cases, M. hyopneumoniae was found in combination with PCV2 in 172 of 484 (35.5%) cases, and SIV was detected in 26 of 484 (5.4%) cases (Pallarés et al., 2002). Aujeszky’s disease virus infection concurrent with PMWS was demonstrated in pigs associated with multifocal necrotizing tonsilitis and lymphadenitis (Rodríguez-Arrioja et al., 1999). Pogranichniy et al. (2002) did a case control study on pigs with a clinical history of wasting and microscopic lesions characteristic of PMWS (n = 31) and on control pigs without clinical signs or microscopic lesions typical of PMWS (n = 56). Among all viruses tested, PCV2, PRRSV, PPV, porcine enterovirus types 1-3, swine influenza virus (SIV), porcine respiratory coronavirus, TGEV, porcine endogenous retrovirus, porcine lymphotropic herpesvirus type 1, and bovine virus diarrhea virus, PCV2 had the strongest association with PMWS. The risk for PMWS was much higher if the pig was PCV2/PRRSV coinfected (Pogranichniy et al., 2002). Wellenberg et al. (2004) did a case-control study to investigate the presence of coinfections in the Netherlands. They compared 60 pigs affected with PMWS from 20 different farms and 180 pigs without clinical signs of PMWS. Concurrent PRRSV-infection was found in 83% of the PMWS-affected pigs and in 35% of the pigs from PMWS-free herds (Wellenberg et al., 2004).

 

References:

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