Detection of pseudorabies virus genomic sequences in apparently uninfected 'single reactor' pigs.

With a pseudorabies virus (PrV) gB ELISA, performed on 480,000 pigs on 8,900 Swedish farms, approximately 1,300 cases were observed with only one single animal reacting positively. These animals were termed 'single reactors' (SR). In order to find explanations for this peculiar phenomenon, the presence of PrV was investigated in organs of immunosuppressed and non-immunosuppressed SR animals. The virus was not detected by immunohistochemistry, virus isolation or co-cultivation. An in situ DNA hybridization test detected PrV gC gene sequences in the olfactory bulb of one sow. A nested polymerase chain reaction (PCR) assay revealed gB, gE and gD gene sequences of PrV in the tissues of trigeminal ganglia, olfactory bulb, tonsils and brain. The nucleotide sequences of the amplicons revealed 98 to 100% homology with the corresponding sequences of PrV. The large latency transcript (LLT) was not detected in the organs of the SR pigs. Transmission of the SR phenomenon to animals in contact or to the next generation was not observed. Considering the present observations and the facts that (i) PrV vaccination is not applied in Sweden; (ii) the SR animals occur not only in the South, but also in Northern Scandinavia, which has no history of PrV infection and (iii) viral reactivation was not observed under natural conditions or after experimental immunosuppression, it is concluded that the SR phenomenon should hardly be considered as a typical PrV latency. The present findings show that certain herpesviral genomic sequences exist in apparently uninfected individuals.