The case count in a norovirus outbreak traced to oyster farms in British Columbia, Canada, has more than doubled in a week.

Public health officials initially reported the outbreak April 9 when there were about 40 known cases. This week the Public Health Agency of Canada reported the case count at 126. The cases of gastrointestinal illness linked to oyster consumption have been reported in three provinces: British Columbia (92), Alberta (9) and ontario (25).

People got sick between mid-March and early April, and those who did reported eating raw oysters from British Columbia. Not all cases have been tested, but testing of several of them confirmed the presence of a norovirus infection, the health agency said.

The Public Health Agency of Canada is collaborating with other federal and provincial public health departments on the investigation. The source of illness has been identified as raw oysters, but the cause of the contamination has not been identified.

Oyster farms in British Columbia that have been associated with illnesses in the outbreak have been closed as a part of the investigation and to prevent further illness, the Public Health Agency of Canada said in a notice.

Oysters are a known risk for causing foodborne illnesses if consumed raw. Norovirus and several other pathogens can be killed if oysters are cooked to an internal temperature of 90° Celsius or 194° Fahrenheit for a minimum of 90 seconds, according to public health officials.

Noroviruses are a group of viruses that can cause gastroenteritis in people, and usually include diarrhea and/or vomiting as main symptoms. Noroviruses are found in the stool or vomit of infected people. They are very contagious and can spread easily from person to person. Some foods can be contaminated at their source. For example, shellfish like oysters may be contaminated by sewage in water before they are harvested.

Acute gastrointestinal illnesses such as norovirus infection are common in North America and are very contagious, affecting all age groups. Pregnant women, people with compromised immune systems, young children and the elderly are at higher risk for developing serious complications.

Contaminated oysters remain on the market, according to Canadian officials, including at seafood markets, restaurants and grocery stores, and public health officials warn that people should be aware of the risks associated with consuming them raw or undercooked. Food contaminated with norovirus and other pathogens may look, smell and taste normal.

Advice to consumers
The following safe food handling practices will reduce your risk of getting sick:

Ensure oysters are fully cooked before consuming them

Discard any oysters that did not open while cooking.

Eat oysters right away after cooking and refrigerate leftovers.

Always keep raw and cooked oysters separate to avoid cross-contamination.

Do not use the same plate or utensils for raw and cooked shellfish, and wash counters and utensils with soap and warm water after preparation.

Wash your hands well with soap before and after handling any food. Be sure to clean and sanitize cutting boards, counters, knives and other utensils after preparing raw foods.

Norovirus can be transmitted by ill individuals and are able to survive relatively high levels of chlorine and and freezing temperatures. The Public Health Agency of Canada recommends foodservice operators and consumers clean and disinfect potentially contaminated surfaces with bleach, especially after an episode of illness. It also offered these tips.

After vomiting or diarrhea, immediately remove and wash clothing or linens that may be contaminated using hot water and soap.

If you have been diagnosed with norovirus illness or any other gastrointestinal illness, do not prepare food or pour drinks for other people while you have symptoms, and for the first 48 hours after you recover.

People with norovirus infections usually develop symptoms of gastroenteritis within 24 to 48 hours, but symptoms can start as early as 12 hours after exposure. The illness often begins suddenly. People can spread the virus before they begin exhibiting symptoms. Even after having the illness, a person can become reinfected by norovirus.

The main symptoms of norovirus illness are diarrhea, vomiting (in children more so than than adults), nausea and stomach cramps. Symptoms can also include low-grade fever, headache, chills, muscle aches and fatigue.

Most people feel better within one or two days, with symptoms resolving on their own. As with any illness causing diarrhea or vomiting, people who are ill should drink plenty of liquids to replac lost body fluids and prevent dehydration. In severe cases, patients may need to be hospitalized and given fluids intravenously. If you have severe symptoms of norovirus, consult your healthcare provider.