Genetic drift and genetic shift during feline coronavirus evolution

Feline coronaviruses (FCoVs) can be divided in strains that cause feline infectious peritonitis (FIP) and strains that cause a subclinical to mild enteric infection, feline enteric coronavirus (FECV). Furthermore, tissue culture adapted FIPV strains exist, that have lost the ability to cause disease. This phenomenon is accompanied by (but not necessarily caused by) deletions in the 7b gene (Herrewegh et al., 1995). The currently available vaccine virus is an example of the attenuated strains. FIPV and FECV cannot be distinguished serologically or morphologically and were therefore called biotypes of the feline coronaviruses; the two viruses differ only biologically. The object of the research was to demonstrate that the biotypes are genetically different and to find out where the (important) differences are located. A genetic comparison was made of an FECV strain and an FIPV strain from one focal outbreak. The biotypes of these strains were established by experimental infection (Hickman et al., 1995). The genetic comparison revealed that strains of different phenotypes can be genetically more closely related than strains with the same phenotype from other geographic backgrounds (Vennema et al., 1995, 1998). This observation and the fact that the FIP cases followed a widespread infection with FECV, suggested that the FIPV emerged as a variant of FECV (Hickman et al., 1995). To find evidence for this hypothesis we infected 19 cats with the same strain of FECV and looked for clinical signs of FIP which would be indicative of the appearance of a mutant (Poland et al., 1996). The animals were immunosuppressed through feline immunodeficiency virus (FIV) infection to increase the chances of finding a mutant in a relatively small number of cats. It was established that FIV positive cats had 10 to 100-fold higher FCoV viral titers in their faeces, shed virus for a longer period of time and produced lower levels of antibodies after a longer delay than the control cats. After 8 and 10 weeks two of the cats came down Veterinary Microbiology 69 (1999) 139±141