Norovirus is a very common and highly contagious virus that causes acute gastroenteritis, an inflammation of the stomach and intestines. This illness is often referred to as the “stomach flu” or “stomach bug,” though it is not related to the influenza virus. Norovirus is a leading cause of vomiting and diarrhea, with 19 to 21 million cases reported annually in the United States. Outbreaks commonly occur during colder months, typically between November and April in the Northern Hemisphere.
Understanding Norovirus Structure
Norovirus particles are remarkably small, making them difficult to detect without specialized equipment. The virus measures approximately 27 to 34 nanometers in diameter. It is classified as a non-enveloped virus, meaning it lacks an outer lipid membrane that many other viruses possess. This absence of an outer lipid layer contributes to its resilience, allowing it to survive in various harsh environmental conditions, including resistance to many common disinfectants.
The outer shell of the norovirus is a robust protein capsid, which encases and protects its genetic material, a single strand of ribonucleic acid (RNA). These capsid proteins are responsible for the virus’s interaction with host cells, enabling it to attach and initiate infection. This durable capsid allows the virus to persist on surfaces and in food, facilitating its widespread transmission.
How Norovirus Transmits
Norovirus spreads easily and rapidly through various pathways. One primary mode is direct person-to-person contact, such as sharing food or eating utensils, or direct care of an infected individual. The virus can also become airborne through tiny particles released during vomiting, which can then be inhaled by others.
Contaminated food and water are common sources of norovirus outbreaks. Foods like raw oysters, other shellfish, and fresh produce are susceptible to contamination if handled by an infected person or grown in contaminated water. Touching contaminated objects or surfaces, often called fomites, and then touching one’s mouth, is another common route for infection. It takes only a small number of virus particles, estimated to be as few as 18 to 1,000 viral particles, to cause illness. The virus’s ability to survive on surfaces for days or even weeks further contributes to its rapid dissemination in various environments.
The Infection Process and Symptoms
Once norovirus enters the body, typically through the mouth, it travels to the digestive system. The virus primarily targets cells lining the small intestine, leading to inflammation and disruption of normal digestive processes. This cellular invasion and damage result in the characteristic symptoms of the illness.
Symptoms of norovirus infection usually appear suddenly, within 12 to 48 hours after exposure. Common symptoms include nausea, forceful vomiting, watery diarrhea, and abdominal cramps. Some individuals may also experience a low-grade fever, headache, and body aches. These symptoms can be severe, leading to frequent episodes of vomiting and diarrhea throughout the day.
The illness generally lasts for a short period, with most people recovering within one to three days. However, rapid fluid loss from vomiting and diarrhea can lead to dehydration, particularly in young children, older adults, and individuals with underlying health conditions. Signs of dehydration include decreased urination, dry mouth and throat, or dizziness when standing.
Strategies for Prevention
Preventing norovirus infection relies on strict hygiene practices. Thorough handwashing with soap and water is the most effective defense, especially after using the restroom or changing diapers, and before eating or preparing food. Alcohol-based hand sanitizers are less effective against norovirus compared to soap and water.
Careful food handling and preparation are also important. Fruits and vegetables should be rinsed thoroughly under running water, and shellfish should be cooked completely to eliminate viral contamination. If someone in a household is ill with norovirus, contaminated surfaces should be cleaned and disinfected using a bleach-based cleaner, as the virus can persist on surfaces for extended periods. Avoiding direct contact with sick individuals and staying home for at least 48 hours after symptoms have stopped can help prevent further transmission.