Feline infectious peritonitis (FIP) is a deadly infectious disease that affects cats. FIP is caused by Feline Coronavirus (FCoV), a highly prevalent virus among cats in shelters and catteries. Since 2018, the antiviral GS-441524 has been used to treat FIP. Prior to its approval for use in the U.S. in mid-2024, unlicensed antivirals(mostly GS-441525) were obtained by cat owners to treat their cats. Other antivirals, such as molnupiravir, were also used during the large-scale outbreak caused by the emergent FCoV genotype FCoV-23. Although reports of cats relapsing after extended antiviral treatment exist, little is known about whether these cats were still infected and shedding the virus. This study aims to determine if antiviral resistance sequence markers have emerged among FCoV in the U.S. I will focus on coronavirus genes known to be targets of current antivirals, including nsp3,nsp5, and nsp12. These genes will be amplified and sequenced using next-generation sequencing. I will analyze the genetic diversity of these genes in FCoV variants collected before (2012 to 2018) and after (2019 to present) the widespread use of antiviral treatments for FIP, and from cats that underwent antiviral treatment and succumbed to FCoV infection. If the increasing widespread use of antivirals to treat FIP has promoted the emergence of antiviral resistance, I expect to detect positive selection in genetic mutations related to antiviral use after 2018. The future goal is to develop diagnostic tools for the early detection of resistant variants, to prevent their spread, and to guide accurate treatment.