Aim The severity of classical swine fever virus (CSFV) infection is believed to be determined by different factors, including virulence of the specific strain as well as factors related to the host, e.g. age, genetic background and health status of the pig [1, 2]. In recent Danish experiments, the CSFV strain Eystrup, usually classified as highly virulent, showed very different clinical signs. These observations formed the background for the present study, comparing CSFV Eystrup infection in Danish pigs through parallel experiments with pigs of different age and genetic profile. In addition, we also wanted to use these experiments to introduce a scoring system to standardize the parameters of clinical disease in future animal experiments. The present study focuses on the clinical outcome of 3 animal experiments. The immunological and virological examinations carried out will be published separately. Methods Experiment I: 12 pigs, 6 weeks of age, originating from 2 litters of the institute’s own SPF herd. The pigs were randomly divided into two groups each of 6 pigs – a control group and a virus-inoculated group. Experiment II: 12 pigs, 12 weeks of age, originating from the institute’s own SPF herd. The 6 control pigs from experiment I were applied as controls in experiment II, too. The virus-inoculated group, six pigs were littermates to the pigs from experiment I. Experiment III: 10 pigs, 6 weeks of age, of different genetic background than the pigs in experiment I and II, originated from the University of Copenhagen, Faculty of Life Sciences. The litter comprised normal pigs as well as pigs of a new phenotype, initially characterized by juvenile hairlessness and thin skin, which was discovered in a Danish pig herd. All 10 pigs were virus inoculated. The experimental set-up for the 3 experiments is showed in table 1. On post infection day (PID) 0, the principal pigs were inoculated oronasally with a dose of 105.2 – 105.8 TCID50 CSFV strain Eystrup. Each group of pigs was kept in separate isolation units. Individual pigs were examined daily by clinical examination for symptoms of CSF infection and body temperatures were recorded. To obtain a semi-quantitative measure for clinical disease, all pigs were evaluated in a clinical scoring (CS) system previously described by Mittelholzer et al. . A maximum score of 27 (severe CSF symptoms) could be obtained in the present study. Blood samples were collected from all pigs at predetermined days for immunological and virological evaluation. The duration of the experiments was 4 weeks. At PID 28/29, all pigs were euthanized and subjected to post mortem examination. Table 1 Experiment control pigs virus-inoculated pigs age/weeks origin I 6 6 6 VET DTU II 6 12 VET DTU III - 10 6 LIFE KU Table 1. Experimental set-up for all 3 experiments. VET DTU= National Veterinary Institute, Technical University of Denmark. LIFE KU= Faculty of Life Sciences, University of Copenhagen. Results All inoculated pigs became infected with CSFV after inoculation, as determined by virus detection. Experiment I: Virus-inoculated pigs showed mild clinical symptoms characterized by slight lethargy, pyrexia and occasional obstipation within the first week, i.e. PID 5-7. Max individual CS was 4 and mean body temperature was 40.0°C for the same days. Experiment II: Virus-inoculated pigs showed mild clinical symptoms characterized by pyrexia, soft feces and transient purple discolorations at extremities within the first two weeks after inoculation. Max individual CS was 5, and the highest mean body temperature was 40.0°C. Experiment III: Clinical symptoms were observed few days after inoculation, soft feces from several pigs were observed from PID 2. At PID 5, one pig (pig 52) was lethargic with pyrexia, anorexia and dyspnea. Further progression of disease, including watery diarrhea, ataxia, intermittent convulsions and purple discoloration of extremities, developed over the next days. Lagging a few days behind, the remaining pigs showed clinical symptoms similar to those of pig 52. Max individual CS was 12 and highest mean body temperature was 40.3°C. Conclusions The sparse clinical symptoms in experiment I and II compared to the more aggravating clinical symptoms in Experiment III suggest that severity of infection may vary when different groups of pigs are infected with the same strain of CSFV. Although, the present results may indicate that the genetic background plays an important role for the clinical outcome of this infection, the variations in microbial load of the pigs in Experiments I and II, and III, respectively, may also have contributed to the observed differences. Finally, the CS system was found to provide a valuable tool to compare levels of clinical disease between experimental animal groups.