1. CALICIVIRUSES (CVs) E-mail Web site
PETER H. RUSSELL, BVSc, PhD, FRCPath, MRCVS
Department of Pathology and Infectious Diseases, The Royal Veterinary College,
Royal College Street,
London NW1 OTU.
Web site
This lecture is the part of THE VIROLOGY COURSE, (The aim of this foundation course is to link the structure, diagnosis and control of the different families of veterinary viruses in an up-to-date and well-paced manner). This lecture has been updated by Dr Janice Bridger.
COURSE OBJECTIVES - hyperlinks to course.htm
BOOK LIST - hyperlink to book.htm
USEFUL WEBSITES- hyperlink to websites.htm
INDEX OF ABBREVIATIONS - hyperlink to index.htm

2. LEARNING OBJECTIVES Students should be able to:
Explain the background to the control measures for feline calicivirus disease
Describe the emergence of rabbit haemorrhagic disease virus (RHDV) and the past and present control measures in the UK

3. FAMILY CHARACTERISTICS:
- Small (35nm in diameter), non-enveloped spherical virion Family name derived from the 32 cup-shaped (calix = cup) surface depressions which give a distinctive morphology in the electron microscope. Composed of one major structural protein (unusual).
- (+) sense, single-stranded (ss) RNA. Once classified with Picornaviruses but replication strategy differs as several sub-genomic mRNAs produced rather than one.
- Reasonably resistant allowing survival in the environment.

4. FELINE CALICIVIRUSES (FCV)
DISEASE & PATHOGENESIS
Incubation period usually 2-4 days. Upper respiratory disease, acute or subacute, subclinical/mild/severe. Usually lethargy, anorexia, pyrexia, respiratory tract signs, vesicles & ulcers of the oral epithelium (2-5 mm). Also, dyspnoea, ocular & nasal discharges, conjunctivitis, rhinitis, tracheitis, pneumonia, stiff gait.
Strains differ in tropism & virulence; some cause upper & lower respiratory disease, others only upper respiratory disease, some associated with subclinical infections. Morbidity high, mortality low; often seen in kittens within hours of infection. Mortality increased by concurrent viral or bacterial infections. Recovery followed by prolonged carrier state.

5. FELINE CALICIVIRUSES (FCV)
HOST RANGE :
Domestic cats & cheetahs. No zoonotic potential known or alternative hosts.

6. FELINE CALICIVIRUSES (FCV)
EPIZOOTIOLOGY:
Present in about 50% of cats with upper respiratory disease. Clinical disease often seen in the acute form in young kittens when passive immunity declining. Clinical disease rare in cats over 1 year due to active immunity.
Most common in dense cat populations (catteries, breeding colonies & multiple cat households). Infections with different strains probably occur throughout life sub-clinically; an important source of infection. Unlikely that many cats escape infection. FCV can exist in many cat populations without causing serious problems.
Up to one third of adult cats remain persistently infected (carriers), shedding virus asymptomatically more or less continuously from oropharynx for several years; high-, medium-, or low-level excretors. Persists in tonsil; sometimes chronic oral lesions. Carrier state may be self-limiting or life-long;direct contact with naso-lacrimal or oral secretions probably the primary means of virus spread. Worldwide infection.

7. FELINE CALICIVIRUSES (FCV)
ANTIGENICITY:
Considered to be a single serotype but strains form an antigenic mosaic reacting with other strains to different degrees. Some show lack of cross-protection. FCV-F9 reference strain broadly cross-reactive.

8. FELINE CALICIVIRUSES (FCV)
DIAGNOSIS:
Clinically cannot be distinguished from feline rhinotracheitis (herpes virus) although tends to be less severe. Diagnosis depends on lab tests. Readily isolated in cell cultures with cytopathic effects (CPE).

9. FELINE CALICIVIRUSES (FCV)
CONTROL:
Cannot be eradicated from the cat population because of carriers. Maternal antibody in colostrum is vital & protects kittens from clinical disease for the first 4-8 weeks of life (antibody half life is 15 days), then susceptible to infection from asymptomatic, clinically-ill cats or carrier mothers. Clinical signs of variable severity; more severe disease in kittens with low maternal antibody levels.
Attenuated live virus vaccine or inactivated vaccine (containing broadly cross-reactive strains) in combination with herpesvirus & panleukopenia virus. However, some evidence that vaccines do not protect against all strains. Duration of protection 12 months or longer. Vaccination of the whole cat population should be the aim.
Carriers should not be in a breeding colony or at the least should not come into contact with kittens. Cattery owners should insist on recent vaccination and contact between pens should be limited.

10. (VIRAL HAEMORRHAGIC DISEASE VIRUS)
RABBIT CALICIVIRUS
(VIRAL HAEMORRHAGIC DISEASE VIRUS)
DISEASE & PATHOGENESIS: systemic disease: clinical signs variable but include depression, anorexia, muscular tremors, incoordination. At post-mortem, generalized congestion particularly in lungs and trachea, liver necrosis, haemorrhages. Mortality common.

11. RABBIT CALICIVIRUS (VIRAL HAEMORRHAGIC DISEASE VIRUS)
ANTIGENICITY:
No antigenic relationship to other caliciviruses apart from a similar virus in hares (European Brown Hare Syndrome Virus - EBHSV).

12. RABBIT CALICIVIRUS (VIRAL HAEMORRHAGIC DISEASE VIRUS)
HOST RANGE:
Rabbits & hares - the rabbit & hare viruses are thought to be variants of the same virus.

13. RABBIT CALICIVIRUS (VIRAL HAEMORRHAGIC DISEASE VIRUS)
EPIZOOTIOLOGY:
The disease in rabbits was notifiable in the UK until autumn The disease was originally reported in China in 1984, spread through Europe & in 1992 was reported in housed UK rabbits with fatal consequences. Current thinking is that the rabbit virus (presumably a less pathogenic variant of the virus which has been causing VHD recently) is present in the wild rabbit population in the UK (from serological surveys of stored rabbit sera) and affords some protection to the virulent virus.
Analysis of hare sera for virus antibodies suggests that the virus has been present in the UK hare population since 1971.

14. RABBIT CALICIVIRUS (VIRAL HAEMORRHAGIC DISEASE VIRUS)
CONTROL:
Until 1996, it was classified as a notifiable disease in the UK and affected animals were slaughtered. Now there are effective vaccines - either formalin-inactivated whole virus or based on the recombinant capsid protein.

15. ENTERIC CALICIVIRUSES
OTHER CALICIVIRUSES
ENTERIC CALICIVIRUSES
These are the subject of current research as they have only been recognized recently. Found in association with calf scours & cause similar pathology to rotaviruses(hyperlink to 3621) . No vaccine at present.

16. OTHER CALICIVIRUSES
VESICULAR EXANTHEMA OF SWINE VIRUS (VESV) & SAN MIGUEL SEA LION VIRUS (SMSV)
VESV causes clinical signs which are indistinguishable to foot and mouth disease virus in pigs - hence its relevance. However, the disease has not been recorded since the late 1950's. It is thought to have originated by feeding swine sea lion carcasses contaminated with the antigenically related virus, SMSV. An example of cross-species infection with important implications.

17. Summary
Feline calicivirus (FCV) is one of the two major causes of viral upper respiratory tract disease in cats; carriers are common.
The rabbit calicivirus emerged as a fatal disease of rabbits in the 1980's, was notifiable until Now there is an effective vaccine.