Experimental infection of wild boar with pseudorabies virus isolates of different virulence

The wild boar is considered as a potential carrier for Pseudorabies virus (PrV) and could also function as a reservoir for the infection. So far, however, little is known about the susceptibility of the wild swine, especially the wild boar, for PrV. Although in recent years PrV has repeatedly been isolated from wild pigs in the USA, Italy and Germany, data on the virulence of these PrV isolates are sparse. Molecular characterisation of PrV isolates obtained from wild boars in Germany has revealed a unique DNA pattern which had not been observed before in domestic animals. In addition, studies on the virulence of these PrV isolates were required to evaluate the potential risk of PrV infections for the wild boar and also for the domestic pig population. Therefore, the objective of this study was to (1) determine the susceptibility of the European wild boar (Sus scrofa, L.) for PrV, and (2) to characterise the virulence of a PrV isolate from the wild boar for the wild boar itself and for domestic pigs. In separate trials for each PrV isolate, four juvenile wild boars were intranasally infected with 104 or 106 TCID50 and kept together with two control animals. Infections with PrV-Bartha and the wild boar isolate BFW1 showed no or very mild symptoms. By contrast, wild boars infected with PrV-Kaplan showed severe clinical disease which included anorexia, acute shortness of breath, foamy salivation, vomitus, pruritus, blindness, irritability, incoordination and paralysis. After two animals had died on day 7 p.i., this trial was ended. The virus was isolated from nasal and genital swabs for PrV-Bartha between day 1 and 5 p.i., between day 1 and 7 p.i. for PrV-Kaplan, and between day 5 and 12 p.i. for PrV-BFW1. Characteristic titres of excreted virus were found in each trial. Maximal titres ranged between 1.5 and 4.5 log10. While antibodies were first observed on day 7 p.i. with PrV-Bartha and Kaplan as measured by an indirect ELISA, a specific humoral immune response was detected only after 12 days p.i. after infection with PrV-BFW1. Neutralising antibody titres reached 1:22 $\pm 10$ in PrV-Bartha- and 1:6 $\pm 7$ in PrV-BFW1-infected animals. Virus isolation from tissues succeeded only after infection with PrV-Kaplan (tonsil, spleen, mediastinal lymph node, salivary gland, lung, brain, spinal cord, accessory genital glands). Following immunosupression, in animals infected with PrV-Bartha, viral DNA was detected in tonsils and salivary glands by PCR, while PrV-specific amplicons were obtained from the tonsils, lung, brain and spinal cord after infection with PrV-BFW1. Two groups of 6 or 8 domestic pigs each were infected with 104, 106 or 107 KID50 of PrV-BFW1. Similar results were obtained with the lower dose, as described for the wild boar. With the higher dose, however, all animals developed respiratory symptoms, while only some pigs showed convulsions for a short period, unphysiological movements and nausea. Antibodies were first detected on day 7 p.i. by ELISA while neutralising antibodies reached titres of 1:4 $\pm 17$. Virus or viral DNA were only found in the tonsils. Immunosuppression failed to reactivate latent PrV. It can be concluded that the European wild boar is susceptible to infection with PrV isolated from domestic pigs. Clinical symptoms depend on the virulence of the PrV isolate, the dose and route of infection. The wild boar PrV isolate, BFW1, seems to be well adapted to the wild boar whereas it leads to a differentiated clinical and antibody response in domestic pigs depending on the dose of infection.