Feline herpesvirus and calicivirus infections: What's new? (Proceedings)
Acute and chronic upper respiratory tract disease caused by feline herpesvirus (FHV) and feline calicivirus (FCV) is well known to practitioners. Clinical signs common to both FHV and FCV include nasal discharge, sneezing, conjunctivitis, depression, anorexia, fever, and hypersalivation. Damage to respiratory tract epithelium and the nasal turbinates may lead to chronic rhinitis/sinusitis. FHV is more often associated with corneal ulceration and FCV with oral ulceration. However, it is virtually impossible to distinguish the pathogens on clinical signs alone, and mixed infections are not uncommon. Some novel clinical syndromes have been associated with these pathogens in recent years, and will be the focus of this lecture.
Feline calicivirus belongs to the Vesivirus genus of the Caliciviridae family, a large family that also includes rabbit hemorrhagic virus, and norovirus ("Norwalk virus", a major cause of gastroenteritis in humans). These are small, non-enveloped, RNA viruses.FCV isolates are numerous and belong to a single serotype, but there is considerable genetic variation among strains. RNA viruses have a plastic genome due to their error-prone replication. This allows them to exploit new niches, as they are highly adaptable. Consequences of this adaptability include the emergence of novel virulent strains, and the dilemma of which representative strains should be chosen for the production of vaccines. Only some of the currently known isolates are related closely enough to vaccine strains to be neutralized by vaccine-induced antibodies.
FCV is shed in secretions from the oropharynx, conjunctiva and nose. Transmission is most efficient by direct cat-to-cat contact and via fomites. Aerosol transmission is less important, as sneezed macrodroplets do not travel far (less than 4 feet). A major source of infection is asymptomatic carrier cats that shed virus continuously. FCV shedding is not influenced by stress. There is a high prevalence of FCV in healthy cats (up to 24%, depending on the assay). Carrier cats may shed for months to years (even life-long), although one study showed that 50% of infected cats ceased shedding within 75 days. Re-infection after recovery is possible.
FCV is relatively resistant in the environment, with persistence reported up to 4 weeks. Effective disinfectants include 5% bleach (diluted 1:32), peroxygen compounds (Trifectant™, Vetoquinol; Virkon® S, Antec) and glutaraldehydes. Quaternary ammonium products are not effective against FCV. Thorough hand washing is important for infection control. Hand sanitizing products are not equally effective; products containing 1-propanol and 70% ethanol appear to be the best.
Although primarily considered an upper respiratory tract pathogen, FCV has been associated with a wide range of clinical presentations, including:
There are no specific treatments for FCV; therapy is aimed at supportive care and control of secondary bacterial infections. Recent investigation of virus-specific antiviral phosphorodiamidate morpholino oligomer (PMO) for treating kittens during outbreaks of severe caliciviral disease show promise (Smith, Iversen et al. 2008).