Norovirus is a highly contagious virus that causes sudden, severe vomiting and diarrhea, known as acute gastroenteritis. It is a common cause of foodborne illness and outbreaks in various settings, including healthcare facilities, restaurants, schools, and cruise ships. People of all ages can become infected, and symptoms appear 12 to 48 hours after exposure, lasting one to three days. While most individuals recover completely without specific treatment, norovirus can lead to dehydration, particularly in young children, older adults, and those with weakened immune systems. The virus spreads easily through direct contact with an infected person, contaminated food or water, or by touching contaminated surfaces.
Understanding Norovirus Appearance
Norovirus particles are small, measuring about 27 to 38 nanometers in diameter. A nanometer is one billionth of a meter, making these viruses invisible to the naked eye. Their small size contributes to their wide spread.
The virus is classified as non-enveloped, lacking an outer lipid membrane. Instead, its genetic material is encased within a tough protein shell known as a capsid. This capsid is characterized by its distinct icosahedral symmetry, meaning it has 20 triangular faces and 12 vertices, resembling a 20-sided die.
The icosahedral shape provides a robust and compact structure for the virus. This outer protein shell is composed of a major capsid protein called viral protein 1 (VP1). Specialized imaging techniques, such as electron microscopy, allow scientists to visualize the norovirus structure. This helps researchers study its shape and component arrangement, which is important for developing treatments or vaccines.
How Its Structure Aids Survival
The non-enveloped, robust capsid of norovirus contributes to its resistance and survival in various environments. Unlike enveloped viruses, norovirus does not rely on a fragile outer lipid layer, making it less susceptible to many common disinfectants that target fats. This structural stability allows norovirus to endure conditions that would inactivate many other pathogens.
Norovirus is resilient to environmental stressors, including freezing, thawing, and high temperatures up to 60°C. It can also withstand acidic conditions, such as stomach acid, enabling it to pass through the digestive system and cause infection. This hardiness allows the virus to persist on surfaces, in food, and in water for extended periods, sometimes for weeks or even months.
The persistence of norovirus on contaminated surfaces and in contaminated food or water contributes to its high transmissibility. Even a small number of viral particles, as few as 10 to 100, can be sufficient to cause an infection. This structural integrity, combined with its low infectious dose, makes norovirus a challenging pathogen to control in community settings and during outbreaks.