Suid herpesvirus 1 (SuHV1, syn. Aujeszky's disease virus [ADV] or pseudorabies virus [PrV]), which belongs to the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus is the causative agent of Aujeszky's disease (AD, pseudorabies), a notifiable disease, that causes substantial economic losses to the swine industry in countries, where AD is present. Members of the family Suidae (true pigs) are the only natural hosts for PrV, although the virus can infect numerous other mammals including ruminants, carnivores and rodents. Despite the tremendous progress that has been made in controlling and eliminating PrV in domestic pigs, there is mounting evidence that PrV infections are more widespread in wild swine across the world than originally thought. Unfortunately, our understanding of the extent of PrV infections in these wild populations and of the threat to domestic swine is still fragmentary. This review aims at giving a global perspective on PrV infections in wild swine by scrutinizing the current state of knowledge concerning (i) the global occurrence of PrV infections in free-living populations of wild swine, e.g., wild boar and feral swine, (ii) the molecular characterization of wild swine PrV, (iii) infection characteristics of PrV in populations of wild swine, (iv) the risk of spillover infections to domestic pigs, (v) potential risk-mitigating measures, focusing on further research needs.

SUMMARYPseudorabies virus (PrV) infections appear to be more widely distributed in the European wild boar (Sus scrofa) population than assumed. In Europe, attempts to isolate and characterize the causative agents have been limited so far. We therefore collected and examined a total of 35 PrV isolates obtained from wild boar or hunting dogs in Germany, France, Spain, Italy, Slovakia and Hungary between 1993 and 2008. Restriction enzyme analysis of genomic DNA using BamHI showed that all isolates, except one, belonged to genogroup I but different subtypes were evident. For further investigations of the phylogenetic relationships, a 732-bp fragment of the glycoprotein C (gC) gene was amplified by PCR. Sequence analysis revealed about 40 variant positions within this fragment. Comparison of the nucleotide sequences supported the separation into a clade containing isolates from North-Rhine Westphalia, Rhineland-Palatinate (Germany), France and Spain (clade B) and an apparently more variable clade comprising isolates from Brandenburg, Baden-Wurttemberg, Saxony, Saxony-Anhalt (Germany), Slovakia, Hungary, Italy and France (clade A).

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Porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 gene sequences were generated by RT-PCR from 55 field isolates collected in Illinois and eastern Iowa. Spatial and temporal patterns of genetic variation in the virus were examined on a local geographical scale in order to test the hypothesis that the genetic similarity of PRRSV isolates (measured as their percentage pairwise ORF5 nucleotide similarity) was positively correlated with their geographical proximity. Levels of genetic variability in the Illinois/eastern Iowa PRRSV sample were similar to levels of variability seen across broader geographical regions within North America. The genetic similarity of isolates did not correlate with their geographical distance. These results imply that the movement of PRRSV onto farms does not generally occur via distance-limited processes such as wind or wildlife vectors, but more typically occurs via the long-distance transport of animals or semen. Genetic distances between PRRSV isolates collected from the same farms at different times increased as the time separating the collection events increased. This result implies rapid movement of new genetic types of PRRSV into and out of farms. PRRSV ORF5 displayed a pattern of third-codon-position diversity bias that was not evident in a geographically comparable sample of pseudorabies virus (a swine alphaherpesvirus) gC gene sequences. This result provides evidence that PRRSV ORF5 is experiencing stabilizing selection against structural novelty. Despite high genetic variability at all geographical levels, PRRSV ORF5 nevertheless contained potentially antigenic regions that were invariant at the amino acid level. These regions should make effective vaccine targets if they prove to be immunogenic.

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Between 1995 and 1998, we designed a series of studies in which we attempted to determine the main routes of transmission involved in the natural infection of pseudorabies virus (PRV) indigenous to free-ranging feral swine (Sus scrofa). Naturally infected feral sows transmitted the infection to uninfected feral boars, with which they had been commingled for a 6-wk period. Pseudorabies virus was isolated from boar preputial swabs, but not from nasal swabs. Three of the same PRV-infected feral sows did not transmit the infection to domestic boars during a 16 wk commingling period, despite the fact that they became pregnant. Feral boars, naturally infected with PRV, transmitted the virus to domestic gilts while penned together during 6 wk. Pseudorabies virus was isolated from vaginal swabs, but not from nasal swabs of gilts, after 2 and 3 wk of commingling. When the same infected boars were commingled with either feral or domestic boars for 13 wk, PRV transmission did not occur. None of the exposed boars developed neutralizing antibodies or yielded virus from their preputial or nasal swabs. Our results indicate that PRV indigenous to feral swine is preferentially transmitted to feral or domestic swine of the opposite sex by the venereal route. This mode of transmission differs from that seen in the natural transmission of PRV prevalent in domestic swine, where contaminated secretions, excretions and aerosols are responsible for the spread of the virus. Based on these results, we feel that as long as feral swine do not come into direct contact with domestic swine, PRV-infected feral swine probably pose only a limited risk to the success of the National Pseudorabies Eradication Program. The fact that PRV is usually transmitted from feral to domestic swine at the time of mating would indicate that the isolation of domestic herds by the use of a ''double fence,'' should be adequate protection against reinfection with PRV.

Restriction fragment length polymorphism (RFLP) analysis and partial-genome DNA sequencing are commonly used to infer genetic relationships among pathogens. This study compares the application of both techniques to the analysis of 16 pseudorabies virus isolates collected during a 1989 outbreak. Genetic distances derived from RFLP and DNA sequence data were not significantly correlated with geographic distances between farms from which isolates were collected. RFLP-based genetic distance was, however, strongly correlated with temporal distance between isolates (days separating time of isolation). Sequence-based genetic distance was significantly correlated with temporal distance only when synonymous changes (nucleotide changes not leading to amino acid changes) were considered separately. Conversely, non-synonymous changes were correlated with the host species of origin of the viral isolate. These results indicate that selectively-neutral genetic changes most accurately reflect historical relationships, but that non-neutral changes most accurately reflect the biological environment of the viral isolate (e.g. host immune system).

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