Our project is a continuation of our successful studies to understand the virus causing the large outbreak of FIP in Cyprus in 2023 (Cyprus-23). We previously showed that this virus was unusual as it both originated in dogs and was present in two forms – either the expected ‘long’ form, or a form with a ‘short’ spike protein. The ‘short’ form was highly active and could infect both cat and dogs cells in the laboratory, but not human cells. To follow up these studies, we have also studied two other viruses from the USA from 2013 and 2016. These follow up studies suggested that the ‘long’ form was spread from cat to cat, but the ‘short’ form was not —but that the ‘short’ form was the one causing FIP. These viruses, along with new viruses that we have found more recently, and new samples that will be collected during the project period, will be examined to extend these phase I studies. We will also make new reagents and develop techniques to quickly identify ‘long’ and ‘short’ viruses, as well as viruses that are similar to Cyprus-23 - to identify these viruses and control outbreaks quickly so that owners and veterinarians can begin treatment with antiviral drugs.