Caliciviruses (noroviruses)
Caliciviruses are naked, (+) single-stranded RNA viruses. They are roughly 25 nm to 40 nm in diameter and possess an icosahedral symmetry. Two human calicivirus (HuCV) genera belonging to this family are responsible for human gastroenteritis: Norovirus and Sapovirus. Noroviruses (NoV) include the Norwalk (NV) and Norwalk-like viruses (NLV), whereas sapoviruses (SaV) include the Sapporo-like viruses (SLV); all these viruses are currently grouped under the term noroviruses (I mention this because many people use all of these names!). Because these viruses cannot be grown in culture, there are as of yet no serological groupings possible; they are however divided into genogroups (5 NoV genogroups & 5 SaV genogroups) and genotypes. All these viruses present very narrow host specificity (humans; experimental infections in other primates has been difficult).
Noroviruses occur throughout the world and represent 60 to over 90% of non-bacterial gastroenteritis outbreaks (developing & developed world), which occur as outbreaks mostly during the cooler months of the year3. In fact, NoV are the most important cause of waterborne and foodborne diseases. Norovirus infections affect young children and adults alike, normally as community-wide outbreaks. Noroviruses are highly infectious and infections spread very rapidly [secondary contact infection is common (> 50 to 90%)]. Outbreaks also often occur in institutionalized settings (school, day care, hospitals, orphanages, hotels, cruise ships, nursing homes, the military, camps etc.). Transmission occurs primarily through faecal-oral route, although airborne and fomite transmission may also likely occur (being in the same room as someone with projectile vomiting is an important risk factor, and the virus can be recovered from the vomitus).
Contaminated water (drinking, swimming & ice) and food (undercooked shellfish, *****FRUITS & VEGETABLES****, frosting, pasta, sandwiches & cold cuts) are the main culprits so far identified. Viral particles are resistant to a wide variety of detrimental treatments including chlorination, low pH, solvents, heat, cold, ethanol, detergents and quaternary ammonium; they can however be inactivated by glutaraldehyde, iodine-based disinfectants and fresh bleach. Surfaces can be efficiently disinfected using a combination of detergent and bleach.
Norovirus infection spans a wide spectrum of presentations varying from asymptomatic to severe dehydration. When compared with childhood acute gastroenteritis, especially when compared to rotavirus acute gastroenteritis, NoV gastroenteritis usually presents as a milder, self-limiting disease; this is generally (but not always) a distinguishing feature of NoV infection.
****THE INCUBATION PERIOD IS LESS THAN 50 HOURS, FOLLOWED BY A SUDDEN ONSET OF VOMITING AND/OR WATERY DIARRHEA (USUALLY BOTH). STOOLS USUALLY SHOW NO BLOOD, NO MUCOUS AND NO LEUKOCYTES.****
The symptomatic period is short, lasting 1 to 3 days. Other symptoms may include nausea, malaise, abdominal cramps and dehydration; there is usually no fever. As previously mentioned, disease is self-limited. Symptomatic illness is linked with carbohydrate malabsorption and steatorrhea due to a decrease in microvilli enzymatic activity [alkaline phosphatase (regulation of fat absorption; bi-directionnal), sucrase (sucrose fructose + glucose; leads to malabsorption, gas & diarrhoea) & trehalase (decreased trehalose hydrolysis diarrhoea)], and correlates with blunting and broadening of the intestinal villi, crypt cell hyperplasia, cytoplasmic vacuolization (fat?), as well as polymorphonuclear and mononuclear infiltration into the lamina propria.