Top Ten: What to Remember About Caliciviridae

Genome 

The calicivirus genome comprised of is single stranded, positive sense RNA. The naked genome is infectious, and it is linear and nonsegmented.

 Morphology

The name "Caliciviridae" is derived from the Latin calyx or the Greek kalyx, meaning "cup". Caliciviruses are named for the 32 "cupped" depressions on the surface of virions. Caliciviruses have spherical capsids completely formed from a single capsid protein, and with regular surface structure. The capsid is icosahedral and the viruses lack an envelope.

Classification and Taxonomy

The Caliciviridae family is made up of four genera - Norovirus, Sapovirus, Lagovirus and Vesivirus. Of these, only noroviruses and sapoviruses infect humans. Recently, a fifth genus has been proposed for this family, with Bovine Enteric Calicivirus as its representative species.

Caliciviruses were formerly a separate genus within Picornaviridae, as they bear a superficial resemblance to enlarged picornaviruses. Also, Heaptitis V virus was formerly classified in Caliciviridae, but now its the sole member of its own family, Hepeviridae. 

Transmission

Transmission occurs through person-to-person contact and fecal-orally from contaminated food and water. In fact, caliciviruses may cause up to 90% of food-related gastroenteritis outbreaks. The viruses are very stable outside the host, so contamination of surfaces by infected individuals can lead to exposure. 

The extremely efficient nature of calicivirus transmission raises concerns about public safety in a global market economy. The explosive nature of norovirus outbreaks, as an example, indicate that infection is often acquired from a common source, such as a community water supply. Also, a recent example of aninternational norovirus gastroenteritis outbreak caused widespread distribution of contaminated raspberries from a single source in eastern Europe demonstrates the dangers associated with viral transmission.

Diagnosis

Caliciviruses cannot be diagnosed on clinical grounds alone. They are still commonly identified by electron microscopy performed on fecal specimens. Sometimes, sufficient virions are excreted for detection via direct EM, especially in hospitalized patients from whom specimens can be obtained early during infection, when larger amounts of virions are being excreted. However, caliciviruses and especially noroviruses are usually diagnosed with immune electron microscopy, using the convalescent sera from recovering patients.

EIA can also directly detect antigens from caliciviruses found in the stools of infected individuals. Most recently, RT-PCR is possible thanks to the complete sequencing of several human calicivirus genomes.   

Clinical features

Caliciviruses cause epidemics of acute gastroenteritis by infecting the intestinal brush border of the upper intestinal tract. The viral infection causes blunting and atrophy of the intestinal villi, leading to vomting and diarrhea. The illness has a rapid onset (incubation period of 2 days) and is self-limited (usually resolves in 3 days). The disease is usually very mild, and only those who are immunocompromised usually require hospitalization for rehydration therapy. 

Management and prevention

There is no specific antiviral treatment or vaccination available for caliciviruses. Treatment is usually symptomatic, including oral rehydration therapy to replace fluids and electrolytes. Transmission can be prevented through good personal hygiene practices, such as handwashing.

Epidemiology

Caliciviruses have a worldwide epidemiology, and multiple antigenic types have been found circulating simultaneously in the same region. They do not show seasonal incidence, although peaks have been observed during cooler periods. Calicivirus outbreaks tend to occur in closed populations, for example in child care centers and cruise ships.

Noroviruses are responsible for over 85% of all nonbacterial outbreaks of gastroenteritis. They are often affect the elderly, especially those living in institutional settings such as nursing homes.

Sapoviruses, in contrast, frequently infect children under the age of 5. Children at day care centers and institutions are at greatest risk of SaV-associated infection and transmission.

The above chart demonstrates the disparate age distributions of norovirus and sapovirus-caused acute gastroenteritis (Fields  Virology 2007).

Host range

As a family, caliciviruses exhibit a broad host range, infecting a wide variety of vertebrates and invertebrates including felines, rabbits, and even insects. Human caliciviruses, however, have an incredibly narrow host range; in fact, the are the only known human viruses that do not even infect chimpanzees!

Research challenges 

Despite three decades of intensive research, scientists have been unable to grow noroviruses or sapoviruses in cell culture. For this reason, the biological mechanisms of human caliciviruses are not very well studied or understood. This prevents the development of an effective live or attenuated vaccine, as well as an understanding of the family's molecular biology.