Genetic characterization of Porcine Circovirus 2 field isolates from Italian farms

Porcine Circovirus Type 2 (PCV2) is an important pathogen related to several disease syndromes in pigs, collectively named PCVD (PCV disease). PCV2 strains are currently classified into four genotypes: PCV2a, PCV2b, PCV2c and PCV2d. The latter is considered an emergent genotype. It has been, indeed, increasingly isolated worldwide, mainly in cases of suspected vaccine failure, rising concerns about vaccine protection and possible ongoing genetic shift.

In order to study the circulation of PCV2 strains related to PCVD outbreaks in Italian farms, 46 samples with high viral loads were completely sequenced. Samples had been conferred, between January 2014 and October 2015, to IZSLER diagnostic laboratories from 35 different herds. All the farms except 2 declared to apply a vaccination program against PCV2, and were recording an increase in clinical cases compatible with PCVD. PCV2 detection and quantification were performed by Real-Time PCR. PCV2 full-length genome sequence was achieved by Sanger method from tissue samples with more than 10 exp 8 genome copies/g or sera and oral fluids with PCV2 loads higher than 10 exp 6 genome copies/mL. Phylogenetic analysis was accomplished using the distance-based Neighbor-Joining method.

PCV2a was found in 4 samples, from 2 (5,7%) vaccinated herds. PCV2b was the prevalent genotype and was identified in 24 samples from 21 farms (60%); only one was not vaccinated. The emergent genotype PCV2d, was detected in 15 samples from 10 herds (28,6%); only 1 had suspended the vaccination program. Finally, 3 samples from 2 vaccinated farms (5,7%) had mixed sequences of genotypes PCV2b and PCV2d. Considering the distribution of genotypes over the time, a substantial increase of PCV2d circulation in Italy during 2015 can be observed. Indeed, in 2014, among 21 investigated herds, 2 were infected by PCV2a, 21 (85,7%) by PCV2b and only 1 by PCV2d. On the contrary, in 2015 none of the investigated herds had PCV2a, PCV2b was recovered in 3 (21.43%), 2 had a concomitant circulation of PCV2d and PCV2b, while 10 (64.2%) were infected by PCV2d.

Data strengthen the hypothesis of PCV2d as an emergent genotype. Besides, the considerable increase in proportion of PCV2d-infected herds recorded in this study in 2015 suggests the existence of an ongoing genetic shift between PCV2b and PCV2d. The role of vaccination pressure and natural selection is not clear yet and should be further investigated. Supporting diagnostic data with sequence analysis is extremely important in this context.

Disclosure of Interest: None Declared.