Revisiting Diaplacental Transmission of PCV2 and PCV3 , By Kevin Vilaca, Southwest Ontario Veterinary Services

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Diaplacental (across the placenta) transmission of PCV2 is an important means of spread of PCV2 from sows to piglets. Diaplacental transmission often results in latent PCV2 infections at birth. PCV2 viremia in the pregnant female during the early stages of pregnancy increases the probability of early transmission from sow to piglet. PCV2 viremia and diaplacental transmission is usually of greatest concern in young sows Ref 1. PCV2 infection of the suckling and young growing pig can also be associated with high levels of PCV2 environmental contamination in the farrowing pen. We know that detection of PCV3 DNA in aborted fetuses indicates that a diaplacental route of infection for PCV3 is also possible. Subsequent transmission of PCV3 to naïve littermates can occur in the early stages of life. The presence of PCV2 and PCV3 DNA in sow colostrum is most common when the sows are experiencing a high viral load in serum. This presents yet another potential source of infection to the very young piglet.

These researchers wanted to revisit the detection rates of the PCV2 and PCV3 in suckling piglets and finisher pigs under field conditions. In this study, tissue samples were procured from 185 piglets in 16 farms located in Germany and Austria . Tissue samples of thymus, inguinal lymph node, myocardium, lung and spleen came from piglets that were crushed by their mothers within the first week of life. Samples were  pooled and analyzed for PCV2 and PCV3 by qPCR. Oral fluid samples (OFS) from grow-finish pigs were collected and examined the same way. This type of field study makes use of pigs that die “on farm” is becoming more common in the context of European disease research as they look for ways to study diseases in commercial farms rather than laboratory settings where there may be more welfare concerns associated with laboratory experimentation.

The researchers found the following:

  • Overall breeding herd prevalence
    • PCV2 – 15/16 herds with positive litters
    • PCV3 – 9/16 herds with positive litters
  • In piglets
    • PCV2 was highly prevalent (litters: 69.4%; piglets: 61.6%)
    • PCV3 prevalence was low (litters: 13.4%; piglets: 13.0%).
    • PCV2 plus PCV3 co-infections were not associated with higher viral loads in the tissue samples of the piglets indicating that the infection of a pig with both viruses does not enhance viral replication in the very young pig. Overall, the viral loads in the tissue samples in this study appeared to be in the low range, indicating subclinical disease rather than PCV-associated reproductive failure.
  • In finisher
    • PCV2 was highly prevalent ( 72.6% of OFS were positive)
    • PCV3 was also highly prevalent (67.2% of OFS  were positive)
  • Sow vaccination against PCV2 was identified as a protective factor concerning PCV2 in piglets (OR: 0.279; CI: 0.134-0.578; p < 0.001)
  • Porcine reproductive and respiratory syndrome virus (PRRSV) vaccination of sows was identified as a protective factor concerning PCV3 in piglets (OR: 0.252 CI: 0.104-0.610; p = 0.002).

Take Home Messages:

  • PCV2, but not PCV3, is ubiquitous in suckling piglets. The researchers commented that the number of PCV2 or PCV3 DNA-positive piglets might be overrepresented in this newer type of study as the presence of the pathogens in individual pigs may have impacted the health of the piglets, making them more prone to being crushed. In previous studies pre-suckle sera were examined while in this case the researchers used pooled tissue samples. Viremia might not always be present in PCV2-infected piglets.
  • PCV2 and PCV3 infections in older growing pigs was ubiquitous.
  • PCV2 vaccination of sows was identified as a protective factor concerning PCV2 in piglets
  • PRRS vaccination of sows was identified as a protective factor concerning PCV3 in piglets. The researchers commented that the effects of PRRS as a disease cofactor may have increased the probability of PCV3 infection. That hypothesis needs to be confirmed.

Ref 1: Eddicks M., Beuter B., Stuhldreier R., Nolte T., Reese S., Sutter G., Ritzmann M., Fux R. Cross-sectional study on viraemia and shedding of porcine circovirus type 2 in a subclinically infected multiplier sow herd. Vet. Rec. 2019;184:189. doi: 10.1136/vr.105069.

Ref 2:  Matthias Eddicks , Roland Maurer , Pauline Deffner , Lina Eddicks , Wolfgang Sipos , Sven Reese , Vojislav Cvjetković , Roman Krejci , Tanja Opriessnig , Mathias Ritzmann , Robert Fux   Cross-Sectional Study on the Prevalence of PCV Types 2 and 3 DNA in Suckling Piglets Compared to Grow-Finish Pigs in Downstream Production  Pathogens . 2022 Jun 10;11(6):671. doi: 10.3390/pathogens11060671.