FIP was first recognized as a specific clinical entity in the late 1950’s. This timeline was based on decades of meticulous necropsy records kept by pathologists at the Angell Memorial Animal Hospital. There was a steady increase in the incidence of the disease in the 1960’s onward, and it is currently one of the leading infectious causes of death among young cats from shelters and catteries.
The reason for the sudden emergence of FIP is not known, but there are at least two possible explanations. First, it is noteworthy that FIP appeared within a decade of the initial descriptions of transmissible gastroenteritis (TGE) of pigs in North America. The causative virus of FIP is closely related to TGEV of pigs and canine coronavirus (CCV), although they are still genetically distinguishable. However, mixtures between these three viruses are known to occur. At least one strain of canine coronavirus can induce mild enteritis in cats and enhance a subsequent infection with FIPV, indicating a special closeness to feline coronaviruses. Therefore, CCV may be a more likely parent of FECV in this scenario. Another related possibility is that the FIP mutation occurs only in a relatively new strain of FECV, and that this new strain only evolved in the 1950’s. Coronaviruses such as FECV are continuously mutating as a result of the manner in which their genetic material (RNA) is replicated. Therefore, genetic change, either among themselves or through genetic mixing with closely related coronaviruses from other species, could have either allowed a coronavirus of another species to take up host in cats or to alter a strain that existed prior to the 1950s.
An alternative non-genetic explanation may involve changes in how cats were viewed as pets and their husbandry. There was a dramatic shift in the status, keeping, and breeding of cats as pets after WWII. The numbers of pet cats greatly increased, pure breeding and cattery rearing became increasingly popular, and more cats, and in particular kittens, found themselves in shelters. These large multiple cat indoor environments are known to favour feline enteric coronavirus (FECV) infection and FIP. Interestingly, feline leukaemia virus (FeLV) infection also became rampant among indoor multiple cat households during this period, and FeLV infection was a significant enhancing factor for FIP until it was pushed back into nature as a result of testing, elimination/isolation, and eventual vaccination in the 1970s and 1980s.