Feline coronavirus (FCoV) belongs to the family Coronaviridae of the Order Nidovirales [de Vries et al, 1997]. These viruses are large, spherical, enveloped, positive-sense single-stranded RNA viruses [Lai and Holmes, 2001].
With a genome of 27 to 32 kb, encoding an ~750-kDa pp1ab replicase polyprotein, four structural proteins (S for Spike, M for Matrix, N for nucleocapsid, and E for Envelope) and up to five accessory non-structural proteins, coronaviruses (CoVs) are the largest RNA viruses known to date [Brown and Brierly, 1995; de Vries et al, 1997].
A significant characteristic of these viruses is their capability to undergo recombination [Lai, 1996; Lai and Holmes, 2001].
FCoV, together with canine coronavirus (CCoV) and transmissible gastroenteritis virus (TGEV) of pigs, belongs to group I of the coronaviruses, defined by both antigenic and genomic properties.
Feline coronavirus may itself be subdivided serologically and by nucleotide sequencing into two types. Type I virus is the most prevalent [Hohdatsu et al, 1992; Addie et al, 2003; Vennema, 1999; Kummrow et al, 2005; Shiba et al, 2007]. Type II virus is less common and results from recombination between type I feline coronavirus and canine coronavirus involving the spike gene [Herrewegh et al, 1998]. Most research studies have been conducted on type II since, unlike type I virus, it can be readily propagated in cell cultures [Pedersen et al, 1984]. Both types of virus can induce FIP. Previously, feline coronavirus strains have also been subdivided into two distinct “biotypes”: Feline Enteric Coronavirus (FECV) and Feline Infectious Peritonitis Virus (FIPV) [Pedersen, 1987]. However, since all FCoV may induce systemic infection as demonstrated by RT-PCR studies such descriptions are perhaps best avoided and have not been used in these guidelines.
Feline coronavirus is an enveloped virus that can survive up to seven weeks in a dry environment [Scott, 1988]. Therefore, FCoV can be transmitted indirectly readily, e.g. via litter trays, shoes, hands and clothes. Indirect transmission may also occur at cat shows. However, FCoV can be readily inactivated by most household detergents and disinfectants.
Feline coronavirus infection is extremely common in domestic cats and wild felidae may also be seropositive. Infection is particularly common in multi-cat households where the seroprevalence may reach 90 to 100% [Horzinek et al, 1979; Addie and Jarrett, 1992; Sparkes et al, 1992; Addie, 2000; Kummrow et al, 2005; Herrewegh et al, 1995; Foley et al, 1997; Kiss et al, 2000]. A substantial proportion of FCoV infected cats go on to develop FIP, a fatal disease [Pedersen, 1995b] that is especially common in multi-cat environments [Addie & Jarret, 1992]. In some studies up to twelve percent of FCoV infected cats subsequently die from FIP [Addie et al, 1995a; Fehr et al, 1995]. The prevalence of FIP will depend on the population of cats, particularly their age, and local differences are likely to apply.
Some breeds of cats (e.g. Persians) and individual lines within breeds are more likely to be affected by FIP. [Kiss et al, 2000; Pesteanu-Somogyi et al, 2006]. Age is an important risk factor for FIP and 70% of cats that develop disease are less than one year old [Rohrer et al, 1993; Hartmann, 2005]. However, the disease has been observed in cats up to 17 years of age. It has also been suggested that the prevalence of FIP is higher in sexually-entire cats [Pesteanu-Somogyi et al, 2006].
Since any stress experienced during FCoV infection may predispose a cat to develop FIP e.g. surgery, visit to a cattery, moving, co-infection with FeLV [Poland et al, 1996; Rohrer et al, 1993], stress management is an important part of control.
In breeding catteries, kittens usually become infected at a young age, often prior to weaning. The mother is often the source of infection, particularly if the litter has been reared in isolation. The exact age at which kittens become infected appears to vary. It may not occur until 5-6 weeks of age, associated with the loss of maternally derived immunity, but in some situations very early infection (as early as 2 weeks of age) has been detected [Lutz et al 2002].
Faeces are the major source of FCoV and the major mode of transmission is believed to be the faecal-oral route, with litter boxes representing the main source of infection in groups of cats. Contamination via saliva may occur in groups of cats in close contact or sharing feeding bowls [Addie & Jarrett, 2001]. Transplacental transmission has been described from a queen that developed the disease during pregnancy [Pastoret & Henroteaux, 1978] but is rare [Addie & Jarrett, 1990].
Susceptible cats are most likely to be infected with FCoV from asymptomatic cats. Although transmission of infection from cats with FIP may occur, it is important to note thatthis does not necessarily lead to disease. Indeed, transmission of FIP is considered unlikely under natural conditions although it has been demonstrated experimentally.
Folllowing natural infection with FCoV cats begin to shed virus in the faeces within one week [Pedersen et al, 2004] and shedding continues for weeks to months. A small proportion of cats may shed virus for life (also called carriers) [Addie & Jarrett, 2001] and at high levels [Horzinek & Lutz, 2000]. Whilst a cat remains infected, faecal excretion of virus appears to be continuous [Addie & Jarrett, 2001].