Porcine Circovirus: Is Clinical Disease Just the Tip of the Iceberg?

Porcine Circovirus is an extremely successful little virus, being found in all pig populations globally, with no successful examples of eradication in any literature. We have learnt to live with this virus and are lucky to have PCV2 vaccines that have successfully controlled PCV2 associated disease (PCV2-AD) PMWS, PDNS etc. since the first vaccine’s release around 15 years ago.  

We frequently find PCV2 in samples if we look for it, even in vaccinated herds, but how clinically relevant is it if we are not seeing typical PCV2-AD?
Going back to the basics of virology, viruses require host cells in order to replicate themselves, and these cells are often severely damaged or killed when the virus copies are released. PCV2 viruses most commonly infect cells of the pig’s immune system, so when the PCV2 virus replicates it will kill cells of the immune system. Although when we find PCV2 in our pigs it is not a surprise, and just finding it doesn’t necessarily mean it is a primary problem, we should consider the levels of virus we are finding. For example, we could take a blood sample from an apparently healthy pig and find 106 PCV2 viruses, equating to a million PCV2 copies per ml of blood; in finisher pigs there is estimated to be between 6 – 7 litres of blood, which would equate to 6-7 billion PCV2 viruses per pig. When those viruses will have all been produced within an immune cell the level of damage to the pig’s immune system, and its capacity to fight other infections, must surely be impacted. This is supported by the literature which suggests that the most common type of PCV2 infection is subclinical, where there is no PMWS etc. but there are suboptimal growth rates and increased susceptibility to other infections.  

Subclinical disease is a common and costly reality
Alarcon et al. 2013 estimated the cost of subclinical PCV2 disease to be £8.10 per marketable pig and £82.30 per mortality.  

The prevalence of new strains is unsettling
In addition to the impact of subclinical infection with PCV2, it is also worrying to consider that PCV2 is the most rapidly mutating virus of its type on the planet (single-stranded DNA viruses), and in addition to this, around 1 in 3 PCV2 strains found on farms are recombinant, meaning different PCV2 viruses have merged together to make a new strain. The first genotype of PCV2 identified was PCV2a, and until recently this was the only genotype included in PCV2 vaccines. However, the dominant strains have now shifted from PCV2a, to PCV2b and then to PCV2d. Interestingly PCV2d is so similar to PCV2b (Up to 97.7%) it was originally called mutant-subtype PCV2b. Although PCV2a based vaccines have been protecting against PCV2-AD, the virus is still detectable in most pig flows and in addition studies have shown that vaccination can influence the genomics of PCV2 populations on a pig farm, meaning current vaccines could be driving the evolution of the virus.  

Studies have shown a dual genotype vaccine can provide better clinical protection than a single genotype vaccine
A recent scientific paper by Bandrick et al., (2022) has proven that a vaccine which is matched to the farm genotype will give better biological protection e.g. a PCV2a vaccine will protect better against PCV2a infection than it will against PCV2b infection. Furthermore, the paper demonstrated that a vaccine which contains multiple PCV2 genotypes gives better biological protection against PCV2 pathologies than a vaccine which only contains a single genotype. The CircoMax range, from Zoetis, are the only PCV2 vaccines that contain multiple genotypes (PCV2a and PCV2b antigen) providing protection against PCV2a, PCV2b and PCV2d, representing the three PCV2 strains in the UK. Currently in the market is CircoMax Myco which is a combined one dose product providing protection against PCV2a, PCV2b, PCV2d and Mycoplasma hyopneumoniae (M.hyo) in a single shot. Coming soon will be CircoMax, the only PCV2 vaccine, not combined with M.hyo, that is licensed to protect against PCV2a, PCV2b and PCV2d. 

Would you like to know what PCV2 genotype you have circulating on your farm?
We offer a complimentary genotyping service which can be organised through your consulting/prescribing vet. Please email us at pigteamuk@zoetis.com if you would like to arrange this or discuss PCV2 genotypes further. 

Click here to learn more.

Dr Laura Hancox, Zoetis National Veterinary Manager 

CircoMax® Myco contains inactivated recombinant chimeric porcine circovirus type 1 containing the porcine circovirus type 2a open reading frame 2 (ORF2) protein, inactivated recombinant chimeric porcine circovirus type 1 containing the porcine circovirus type 2b (ORF2) protein and inactivated Mycoplasma hyopneumoniae, strain P-5722-3. POM-V.  CircoMax® contains two inactivated recombinant chimeric porcine circovirus type 1s, containing open reading frame 2 (ORF2) of porcine circovirus type 2a and 2b respectively. POM-V 

Use medicines responsibly (www.noah.co.uk/responsible). Further information can be obtained from the product SPC or from Zoetis UK Limited, 1st Floor, Birchwood Building, Springfield Drive, Leatherhead, Surrey, KT22 7LP. www.zoetis.co.uk Customer Support: 0345 300 8034. Date of preparation: October 2023. MM-27176