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Atypical porcine pestivirus (APPV) is a positive-sense ribonucleic acid (RNA) enveloped virus belonging to the Pestivirus genus, a group of viruses known for their high socio-economic impact. Since its discovery in 2015, APPV has been established as the causative agent of congenital tremor type A-II (CT A-II). The overarching aim of this study was to investigate host-pathogen interactions with regards to the clinical, pathological and immunological outcomes with a focus on the British pig industry. As no commercially available diagnostics are currently available, it was initially necessary to establish techniques capable of detecting the virus and APPV-specific antibodies. To aid in diagnostic assay development and to characterise host-pathogen interactions in an initial in vivo viral amplification was undertaken, Three one-week-old piglets were inoculated with tissue homogenate supernatant from three British CT A-II outbreaks. Peak viraemia detected by RT-qPCR was observed by nine days post inoculation (DPI). APPV shedding was detected from seven DPI in oropharyngeal and rectal swabs and from nine DPI in nasal swabs. APPV was found in all tissue types, with the highest viral loads detected in lymphoid tissues. To understand APPV transmission and the infection dynamics in naturally occurring CT A-II outbreaks, two farrowing groups from a Scottish CT A-II outbreak were studied. The exploratory study (first farrowing group) determined the relationship between clinical status, virus presence, and antibody response over time. There was no significant relationship between clinical status and viral load at either two or eight weeks of age, nor was there a difference between the two age groups. Similarly, no relationship was observed between clinical status and level of APPV-specific antibodies present at either age. However, there was a significant reduction in antibody levels between two and eight weeks regardless of clinical status indicating a probable maternal origin of the antibody. Viral loads among different tissue types varied significantly: brain and Peyer's patches showed less detectable virus than lymph nodes, thymus and tonsil. APPV BaseScope ISH analysis of the CNS revealed a high concentration of APPV RNA in the cerebellum, hippocampus and the spinal cord identifying APPV cell and tissue tropism (neuronal and lymphatic). A longitudinal cohort study followed piglets from a second farrowing group from 2.5 weeks of age until slaughter. This study aimed to understand virus-host interactions, including the potential for viral persistence. In line with establishing diagnostic techniques, ear tissue was assessed for its suitability as a diagnostic sample. The viral load in both clinical and non-clinical animals was highly variable. APPV was detected in serum at 2.5 weeks of age, suggests either vertical transmission (most likely for clinical animals) or early postnatal horizontal transmission by direct contact. At 10 weeks, 24% of piglets were viremic for the first time, indicating horizontal transmission. Of all the initially infected animals, 97% were APPV RNA free at slaughter, indicating that none exhibited persistent viraemia. Maternally-derived APPV-specific antibodies were found to be greatly reduced in the majority of piglets (89%) at slaughter. However, APPV-specific antibody levels increased fivefold demonstrating not only exposure of the whole farrowing group to the virus and seroconversion, but also a lack of persistent infection, even in clinical animals, differentiating APPV from closely related pestiviruses. A lack of relationship between clinical signs, APPV viral load in serum or ear tissue at 2.5 weeks and the length of time to slaughter suggests a limited impact of APPV on growth of the animals. There was also no relationship between the clinical status at 2.5 weeks and viral load in serum; however, there was a positive relationship between clinical status and APPV viral load in ear tissue. A weak positive correlation was found between viral loads in serum and ear tissue, indicating that ear tissue is a suitable sample type for the diagnosis of APPV infection by RT-qPCR. Interestingly, APPV was also detected in high amounts in raw (non-extended) semen, indicating the potential for venereal transmission, although further investigation is needed. In natural cases of CT A-II, APPV has been found in comorbidity with other porcine pathogens. 'Classical' pestiviruses are also known to induce immunosuppression of the host, which may lead to enhanced disease or increased susceptibility to co-infections. To investigate the potential effect of APPV on the immune system and to determine its contribution to disease during co-infection with another pathogen, 10-week-old piglets from the Scottish CT A-II field outbreak were co-infected with porcine respiratory and reproductive syndrome virus (PRRSV). Although no significant differences in antibody level or viral load were found between APPV positive and negative groups, there was a clear interaction between APPV and PRRSV, suggesting that although APPV did not enhance or prolong PRRSV viraemia, viral interference might have occurred. Significant differences in febrile response were observed between the APPV positive and APPV negative PRRSV challenged groups. APPV was also found to increase lung consolidation, a recognised sign of PRRSV infection, suggesting that APPV may play a role in enhancing pathology. Following the Scottish CT A-II field outbreak investigation, a more comprehensive epidemiological survey of 108 Scottish pig farms was conducted to determine the presence of APPV and the extent of exposure to the virus within the Scottish pig population. APPV was detected by RT-qPCR in 4.7% of samples within the study, with viral ribonucleic acid (RNA) detected in one or more sample on 33 out of 108 farms. Additionally, of the 1,077 samples tested, 48.8% were antibody positive by ELISA and 93 out of 108 farms tested had at least one seropositive sample. A broad investigation into APPV presence within semen was conducted: 475 pooled semen samples submitted from 41 commercial stud units were tested for the presence of APPV. No virus was detected in any of the pools tested. This project is the first step to understanding APPV associated CT A-II in the British pig industry, providing insights into the pathogenesis and immunogenicity of the virus, alongside the development of validated diagnostic techniques for the detection of British APPV strains.