Post-weaning multisystemic wasting syndrome (PMWS) is a complex disease syndrome in
swine, affecting nursery and fattening pigs. Although ongoing evidence suggests that
porcine circovirus type-2 (PCV2) is the causal agent of PMWS, the host immune system
appears to have a crucial role in the PMWS pathogenesis of PCV2-affected pigs. Owing to
difficulties in producing a biologically pure form of PCV2 devoid of the other viral agents
commonly present in swine tissues, we decided to use a tandem-cloned PCV2 DNA
providing highly pure grade reagent in order to monitor the virulence of PCV2 alone or
with an immunostimulating co-factor, granulocyte-macrophage colony-stimulating factor
(GM-CSF). A single intramuscular injection of tandem-cloned PCV2 DNA into 5-weekold
piglets produced plasmid to viral genome progeny and infectious particles as early as 8
days post-injection in all the organs tested (the lung, the tonsil and the inguinal, mesenteric,
bronchial and upper-right axial lymph nodes). The initial plasmid load was not
detected with the help of primers designed to specifically detect the acceptor plasmid, thus
confirming the replication of the viral genome. Despite the presence of a high level of
PCV2 genome copies in the lymphoid organs – the tonsil and the lung – and the presence
of infectious particles, no detectable clinical manifestations or pathological lesions were
observed in the transfected pigs over the period of observation, regardless of whether they
had been co-injected with plasmid containing GM-CSF DNA or had received plasmid
containing PCV2 DNA alone. GM-CSF encoding DNA injection had no significant effect
on viral replication or on the production of viral particles and appearance of the disease.