Isolation of Feline Coronavirus type I from clinical samples

Abstract

The non-virulent feline enteric coronavirus (FECV) may cause a transient and mild enteric infection in cats but does occasionally mutate into a highly virulent form, the systemic replicating feline infectious peritonitis virus (FIPV). Feline infectious peritonitis (FIP) is after half a century of research still one of the most studied infectious diseases in cats. Because of its complex nature, FIP remains one of the leading infectious causes of death of cats all around the world for which there is no conclusive routine diagnostic test, no vaccine, no cure and no clear explanations about how virus-host interaction leads to clinical disease. Feline coronaviruses are divided into two serotypes: type I and II, with type I being the most prevalent of the two. Feline coronavirus research has been inhibited by the inability to establish cell lines susceptible for propagation of type I FCoV strains. However recently, a cell line which enabled the growth of type I feline enteric coronavirus to high titres was developed. This cell line is called fDC-SIGN Huh7, a human cell line susceptible for type I FCoV transfected with a feline DC-SIGN, a nonspecific receptor which is suspected to play an important role during virus entry. During this study it was tried to propagate field strain FCoV from clinical samples on this fDC-SIGN Huh7 cell line, in order to isolate type I field viruses. With these isolates, challenge studies could be performed to learn more about pathogenesis and efficacy of prototype vaccines. Samples from different lesions, organs and excretions from FIP-infected cats were collected, processed and inoculated on fDC-SIGN Huh7 cells. Virus strain UCDp was inoculated on fDC-SIGN Huh7 cells to serve as a positive control. Presence of virus was determined by using diagnostic methods like detecting cytopathogenic effect (CPE) and performance of immunofluorescence assay (IFA) and polymerase chain reaction (PCR). Each method revealed the same conclusion; the laboratory strain UCDp could be propagated on fDC-SIGN-Huh7 cells, while all other cells inoculated with sample were tested negative. Hence, it was concluded that the fDC-SIGN-Huh7 cell line is not suitable for the propagation of type I FCoV from clinical samples, at least not with the samples and conditions used in this study. This finding can be explained by several factors. It might be that UCDp has specific cell culture adapted virus characteristics and that field virus strains do not have the capability to infect this cell line. Another explanation could be that there was simply no or not enough infective virus in the samples in the first place, probably due to presence of defective particles and neutralizing antibodies. More knowledge is needed in order to obtain more reliable results and hopefully future studies will offer a way to grow type I FCoV field strains in a cell culture.