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Can porcine circovirus type 3 cause persistent infection in pigs?
  1. Shao-Lun Zhai and
  2. Yun Xi
  1. Animal Disease Diagnostic Center, Institute of Animal Health, Guangdong Academy of Agricultural Sciences, Guangzhou, China
  2. Department of Clinical Laboratory, The Third Affiliated Hospital of Sun Yat-sen University, Guangzhou, China
  1. email: zhaishaolun{at}163.com
  2. email: xiyun1993{at}163.com

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Porcine circovirus type 3 (PCV3) is a recently described virus belonging to the family Circoviridae. It is related to porcine circovirus type 2 (PCV2) – one of the most economically important viruses for the pig production industry worldwide.

PCV3 was originally identified by metagenomics analyses of tissues from pigs suffering from porcine dermatitis and nephropathy syndrome (Fig 1), reproductive failure, myocarditis and multisystemic inflammation.1,2 The absence of other common pathogens that could be causing these disease conditions prompted the suspicion that PCV3 might be involved in their aetiology. However, the virus has also been detected in clinically healthy pigs.3

Fig 1: Porcine circovirus type 3 infection in a Chinese pig with severe skin lesions

Although the virus primarily infects pigs, it has, somewhat surprisingly, also been detected in other animals such as dogs, mice, cattle and even ticks.4-7

PCV3 has now been reported worldwide and has been detected in a diverse range of samples from domestic pigs and wild boars, including the heart, liver, spleen, lungs, kidneys, blood, gut, brain, tonsils, faeces, lymph nodes and colostrum.8-15 However, the pathogenesis of PCV3 remains unclear due to the failure of virus isolation in cell culture systems.1,15

PCV3 has been detected in pigs of varying ages – from stillborn fetuses to older sows.16-18 Despite this, evidence of persistent infection has not been systematically established.

In a paper summarised on p 619 of this issue of Vet Record, Klaumann and colleagues report a longitudinal study on the infection dynamics of PCV3 in conventional Spanish pig farms.19 Their findings reveal that PCV3 was widespread in clinically healthy pigs on the four farms sampled, but the viral load was low. This suggests that subclinically infected pigs may be responsible for maintaining the circulation of PCV3 within herds.

What you need to know

  • Porcine circovirus type 3 (PCV3) is widespread in clinically healthy pigs but the viral load is low, suggesting the presence of subclinical infection.

  • In most cases, PCV3 infection is either intermittent or occurs only once during a pig’s lifetime.

  • Persistent PCV3 infections can occur, but this is not a frequent occurrence.

Klaumann and colleagues also found that persistent infection with PCV3 did occur in pigs, but the occurrence of this was fairly low.19 Persistent infection with PCV3 was observed during the period between lactation (two to four weeks old) and growing/fattening (10 to 25 weeks old). Unfortunately, the sampling period did not cover the whole lifetime of the pigs, so information regarding infection dynamics during gestation and adulthood (over 26 weeks old) was not obtained.

However, a recent study in Korea16 found that PCV3 could be detected in samples from the lungs and heart of stillborn fetuses with a gestational age of 60 to 100 days, and also in tissue samples from three- to seven-day-old suckling piglets. This suggests the possibility of vertical transmission of PCV3.

Another study by Klaumann and colleagues showed that the frequency of PCV3 genome detection in wild boar was significantly higher in adults (over 24 months old) than in subadults (between 12 and 24 months old) or juveniles (less than 12 months old).14 Some of these wild boars were positive for PCV3 throughout the course of the study. Taken together with this data, the results of Klaumann and colleagues’ latest study19 confirm the ability of PCV3 to persistently infect pigs.

Klaumann and colleagues also found that PCV3 infection was either intermittent or occurred once during the lifetime of most pigs.19 These findings indicate that PCV3-specific antibodies can provide pigs with long-term protection against reinfection. As such, it seems unlikely that intermittent infection with PCV3 would be due to reinfection. This, therefore, suggests that PCV3 may establish latent infections.

Although Klaumann and colleagues shed light on the circulation of PCV3, no particular infection dynamics could be inferred from the frequency of detection of PCV3 in this study. Therefore, further research is needed – particularly regarding the immune response against PCV3.

References

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