Feline calicivirus (FCV) disease: FCV infection can be inapparent or associated with various acute and chronic disease syndromes in cats. Infection of immunologically naïve animals leads to upper respiratory tract disease accompanied by glossopharyngeal and/or nasal ulceration (12, 13). Infection with more virulent FCV strains can cause pyrexia, depression, dyspnoea, and pneumonia (12). Less commonly, a shifting lameness is seen following acute infection (2, 6). Acute FCV infection has also been reported to cause abortion in pregnant queens (33). After FCV infection, many cats will secrete virus for up to 3 weeks after resolution of clinical signs. Some cats will become asymptomatic carriers, shedding virus for prolonged periods (34). FCV persistently infects the tonsillar epithelium (7, 35), however, the factors that lead to long term persistence are unknown. An uncommon sequela of persistent FCV infection is chronic gingivostomatitis, which may be causally associated with coexisting feline immunodeficiency virus or feline herpesvirus-1 infection (14, 15, 25, 31, 36).

Recently, hypervirulent FCV strains have been associated with a hemorrhagic fever-like syndrome that causes high morbidity and mortality in cats (21, 26). These epizootics of severe disease have occurred in animal shelters or environments where cats are housed in relatively close confinement. In the reported outbreaks infection was spread inadvertently by animal handlers (21, 26). Because FCV is highly contagious and easily spread by fomites and animal handlers, these outbreaks can cause havoc in larger veterinary hospitals and clinics and are, thus, of major concern to veterinarians. These outbreaks are similar to the dramatic outbreaks of Norwalk virus gastroenteritis, a human calicivirus, that have occurred in nursing homes and cruise ships where fomites were also an important factor in spread (1, 5, 18).

FCV vaccines: Although modified-live and inactivated vaccines are available against FCV, there is a significant rate of vaccine failure. Vaccinated animals are largely protected from acute disease symptoms, but are not protected against infection. Upon challenge, most vaccinates become infected and will secrete the challenge virus post-exposure, thus acting as transient virus reservoirs (9). In addition, vaccinated animals can become chronically infected. Although some vaccine failures can be explained by emergence of vaccine-resistant strains, it appears that the vaccine does not protect against development of the chronic carrier state. In addition, the modified live vaccines will occasionally cause mild disease symptoms in vaccinated animals. In the most recent outbreak of highly virulent FCV disease, prior vaccination did not protect against severe disease (26). There is an urgent need for better vaccines and a clearer understanding of the factors involved in virus immunity and persistence.

FCV antigenicity: There is significant antigenic variation in the capsid protein amongst FCV field isolates, which in part explains the poor efficacy of available vaccines. Several studies have attempted to correlate FCV capsid antigenic variation with disease presentation, but have failed thus far to identify any pattern that correlates with disease (10). However, these studies have not included analyses of the newer hypervirulent strains of FCV. Sequence analyses of the capsid protein gene have identified two hypervariable regions designated regions C and E (27). These hypervariable regions appear to contain most of the identified neutralizing epitopes (11, 20, 28, 32). However, other non-neutralizing epitopes lie outside of these regions in more conserved regions of the capsid protein.

FCV molecular biology: Feline caliciviruses are small non-enveloped viruses (~35 nm diameter) that contain a polyadenylated linear ~7.7 kb (+) stranded RNA genome. The genomic RNA contains 3 open reading frames (ORFs). Only ORF1 is translated from the incoming full-length genome, as translation of ORFs 2 and 3 occurs from a subgenomic 2.4 kb mRNA. ORF1 is co-translationally cleaved by a virally-encoded protease to yield at least 5 viral nonstructural proteins. These include the 76 kDa viral proteinase-polymerase, a putative 39 kDa NTPase, 2 potential membrane-associated proteins (p32 and p30) of unknown function, and the 13 kDa Vpg protein which is covalently linked to the 5'-end of the full-length genome and the subgenomic RNA. The Vpg protein likely acts as a eukaryotic cap ortholog. The viral genome is encapsidated by 180 copies of a single mature viral capsid protein (VP1) that is synthesized as a 76 kDa precursor protein (29, 30).

Calicivirus capsid structure: Calicivirus capsids are composed of 180 copies of a single mature capsid protein. The capsid structures of the human Norwalk virus (23) and the San Miguel sea lion virus (unpublished) have been determined to atomic resolution. These structures reveal that the capsid protein has two domains, one that forms the T=3 icosahedral capsid shell, and a second that lies on the surface of the shell and contains the variable regions of the capsid protein, the antigenic sites, and the likely receptor interaction site(s) (22).

Tropism and host range: Caliciviruses have narrow host ranges and FCV infection is believed to be restricted to cats. Recently, however, FCV-like viruses that are antigenically and genetically similar to FCV have been isolated from dogs with mild enteritis (8, 19, 24). A large epidemiologic study of risk factors associated with FCV infection in cats found that contact with dogs was a risk factor (4), raising the possibility of cross-species infections. The San Miguel sea lion calicvirus, in addition to infecting sea lions, can infect other marine mammals and pigs (3). Studies of FCV and canine calicivirus have shown that host range restriction is determined early during the infectious cycle as virus can be recovered from non-permissive cells when they are transfected with viral genomic RNA (20, 21).

Binding and uptake of FCV: Little is known about the binding and uptake of FCV into susceptible cells. A previous study examined the binding of FCV strain CFI/68 to Crandell-Reese feline kidney cells (CRFK) and found that binding occurred between pH 6 to 8, required divalent cations, and was rapid and saturable with a maximum of 1.5 to 3 × 103 viruses bound per cell. FCV binding was reduced by pretreatment of the cells with neuraminidase plus O-glycanase suggesting that the presence of an O-linked sugar molecule on the cell surface might be important (17). However, the cell receptor for FCV has not been further defined.
FCV likely enters host cells by endocytosis as neutralization of the low pH of endosomes by pretreatment of cells with the lysosomatropic drug chloroquine inhibits FCV infection (16). Viruses commonly enter cells by endocytosis prior to penetrating the cellular membrane barrier. In our ongoing studies we are investigating the endocytic route of entry used by FCV.

FCV – a model for human calicivirus disease: Feline caliciviruses are related to similar caliciviruses that infect dogs, mink, sea lions, rabbits, and humans. The human caliciviruses, Norwalk disease virus and the Sapporo-like viruses, are the most common cause of acute adult-onset viral gastroenteritis. Human caliciviruses are highly contagious, persist in the environment, are antigenically variable, and are difficult to manage. For these reasons they have been identified by the NIH as potential bioterrorism agents. The study of human caliciviruses is hindered by the inability to propagate these viruses in tissue culture cells. As FCV is easily grown in tissue culture and shares many of the biological properties of human caliciviruses it has emerged as a model agent for studying the caliciviruses generally.

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