Norovirus infection

Introduction

Noroviruses are a group of viruses that cause an intestinal flu-like illness. They're a major cause of outbreaks of infection in closed and crowded environments, such as hospitals, nursing homes and cruise ships.

Typically, people with norovirus infection develop diarrhea and abdominal pain and begin to vomit within 24 to 48 hours of exposure. Signs and symptoms last one to five days, and most people recover completely without treatment. However, in some people — especially infants, older adults and people with underlying disease — vomiting and diarrhea can be severely dehydrating and require medical attention.

Noroviruses spread primarily through consumption of food or water contaminated by fecal matter or through close contact with infected people. The infection is highly contagious, and food handlers who contaminate food prior to consumption are a common means of transmission.

Noroviruses are named after the "Norwalk virus," which caused an outbreak of illness in Norwalk, Ohio, in 1968. Norovirus infection is the most common nonbacterial cause of gastroenteritis, a condition involving inflammation of the stomach and intestines.

An estimated 23 million cases of norovirus infection occur in the United States each year. Noroviruses are hardy and difficult to eradicate, although certain precautions can help prevent the virus from spreading.

Signs and symptoms

Signs and symptoms of norovirus infection vary, but generally involve abdominal cramps and nausea, followed by vomiting and watery diarrhea. Either vomiting or diarrhea can be present alone.

The incubation period for norovirus infection is usually 24 to 48 hours after first exposure to the virus, and signs and symptoms usually last one to five days. The most common norovirus-induced illness is gastroenteritis, involving inflammation or infection of the stomach and intestinal tract.

Signs and symptoms of norovirus infection include:

• Nausea
• Abdominal pain
• Abdominal cramps
• Watery or loose diarrhea
• Weight loss
• Malaise
• Low-grade fever

Some people with norovirus infection may show no signs or symptoms. However, they may continue to shed the virus in their feces for several days up to several weeks following infection, potentially infecting other people.

Causes

You can contract norovirus infection by touching or ingesting anything contaminated with the virus, which is shed in the feces of infected humans and animals. Noroviruses are highly contagious and can spread easily in a number of ways.

Outbreaks of infection tend to occur in crowded places — with retirement centers, nursing homes, hospitals, cruise ships and child care centers as common settings.

Many possible points of contamination
Contamination of food can happen at any point during its production. Oysters and raspberries contaminated at the site of harvest have been responsible for outbreaks. Food handlers can transfer the virus to any type of food product just before it is served. Food items that allow the virus to spread evenly — such as cake frosting or salad dressing — are common culprits.

Transmission also can occur during sporting activities. In one instance, during a football game, some members of one team acquired norovirus infection after an acute food-borne outbreak had occurred in the other team. It's believed that the virus shed in some players' bodily fluids — including saliva, vomit or diarrhea — was transmitted to others by either aerosolization or direct contact. Aerosolization occurs when virus particles become airborne, making them easy to inhale.

Methods of norovirus infection include:

• Eating contaminated food, which accounts for between one-half and two-thirds of all food-borne illness
• Drinking contaminated water
• Ingesting infected mucus or breathing airborne norovirus particles
• Touching your hand to your mouth after your hand has been in contact with a contaminated surface or object
• Having close contact with an infected person and then transmitting the virus from your hands to your mouth

Noroviruses are difficult to eradicate because they can withstand hot and cold temperatures as well as most disinfectants. Further, modern lifestyles may be contributing to the incidence of norovirus infection. More older adults are living in communal settings. Eating out at restaurants, where food is handled by many people, is becoming more common. Finally, more people than ever are traveling and facing an increased risk of exposure in crowded settings — hotels, airplanes and cruise ships.

Risk factors

Risk factors for becoming infected with norovirus include:

• Exposure to contaminated food or water
• Exposure to an infected person
• Having an impaired immune system, such as people with AIDS and transplant recipients
• Living in a household with improper or unsanitary food-handling procedures
• Living with a child who attends preschool or a child care center
• Traveling to or staying in hotels, vacation resorts, cruise ships or other destinations with many people in close quarters
• Living in close quarters, such as in nursing homes and retirement centers

When to seek medical advice

Seek medical attention if you develop diarrhea that doesn't clear within several days. Also call your doctor if you experience severe vomiting, abdominal pain or dehydration.

Screening and diagnosis

Specific diagnostic testing isn't usually necessary to identify the exact viral cause of viral gastroenteritis. Your doctor will likely be able to diagnose your illness by asking about your symptoms as well as your exposure to other ill people. If there has been an outbreak of norovirus infection, your doctor or the public health department may ask you additional questions about your activities in order to help identify the source of contamination.

If necessary, noroviruses can be identified in stool samples under an electron microscope. A reverse transcriptase polymerase chain reaction test (RT-PCR assay) also can detect noroviruses in food, in water, in stool samples and on environmental surfaces. These tests isolate and replicate the suspected virus' genetic material for analysis.

Complications

For most people, norovirus infection clears up within a few days and isn't life-threatening.

In some cases, severe dehydration, malnutrition and even death can result from norovirus infection, especially among children and among older and immunocompromised adults in hospitals and nursing homes.

Treatment

There's no specific treatment for norovirus infection, and recovery generally depends on the health of your immune system. In most otherwise healthy people, the illness usually resolves within a few days.

However, outbreaks among infants, children and older, immunocompromised people may cause severe complications and even death without prompt measures.

The goals of treatment in severe cases are to replace lost fluids and to alleviate symptoms of severe diarrhea and vomiting.

If you've encountered a severe case of norovirus infection, you'll need to replace fluids and electrolytes — minerals such as sodium, potassium and calcium that maintain the balance of fluids in your body — lost to persistent diarrhea. This can be done either by drinking lots of liquids, or in cases of serious fluid loss, by receiving fluids through a vein (intravenously). These precautions will help keep your body hydrated and functioning properly.

Prevention

Norovirus infection is highly contagious.

Follow these suggestions to help prevent norovirus infection from spreading:

• Practice good hygiene, including thorough hand washing.
• Don't consume water and food that may be contaminated. Throw out any food you suspect may be contaminated.
• Disinfect virus-contaminated areas with a solution of detergent and chlorine bleach (sodium hypochlorite).
• Routinely disinfect surfaces and furniture in common areas, such as lobbies and dining rooms. Where possible, allow bleach to stay on surfaces longer than 10 minutes.
• Clean vomit with a two-step process: Collect visible debris with absorbent material, and then disinfect any surfaces that may have come in contact with vomit.
• Avoid consuming raw shellfish, which may have been harvested from infected waters.
• Avoid contact sports if you have acute gastroenteritis.

If you work in a food-handling job and you've been sick with norovirus infection, don't return to work until 48 to 72 hours following cessation of your signs and symptoms so that you avoid infecting other people.