Most common cause of gastroenteritis
According to the World Health Organization (WHO) are Noroviruses the most common cause of viral gastroenteritis worldwide. Noroviruses account for around 267 million infections and over 200,000 deaths each year. It is estimated that 50%(!) of all foodborne outbreaks of gastroenteritis in the United States are related to Norovirus and approximately 90% of epidemic non-bacteria related gastroenteritis outbreaks all over the world.
Gastroenteritis is an infection of the stomach and the small intestines and results in a combination of diarrhea, vomiting and abdominal pain.
What is the Norwalk Virus?
The Virus was formerly named Norwalk virus after Norwalk, OH, where an outbreak occurred at Bronson Elementary school in 1968. It was later shortened to Norovirus. The Virus is also very fittingly called the winter vomiting bug.
Norovirus and cruise ships
The reason why it spreads more in winter than in summer is that because it travels from person to person through the air and in winter people spend more time indoors closer to each other. Cruise ships therefore present perfect conditions for Norovirus outbreaks. This is why reports about Norovirus outbreaks on cruises are so frequent. For example within a few weeks of spring 2013 confirmed the CDC three outbreaks with over 480 affected individuals. It has to be stressed out that although Noroviruses are often related with food these outbreaks are not necessarily related to the food on the ship. It is quite likely that some of the travelers were already infected when they entered the ships.
Noroviruses are very infectious
Outbreaks happen so often because Noroviruse are extremely contagious. The infectivity per virus particle is estimated to be the highest among all studied viruses to date. An individual only needs to take up a few single virus particles to get infected and infected individuals can release millions of Norovirus particles!
Food or Water?
In the U.S. Noroviruses are responsible for over 65% of human illnesses caused by food-borne pathogens. The consumption of raw shellfish (Oysters) is a known route of Norovirus infection. Foodborne outbreaks were reported worldwide. Many reports show that contamination of oyster and mussel cultures with Noroviruses can be directly linked to sewage. Again, every infected individual releases millions of virus particles. And so it is no surprise that Norovirus infections have also been linked to recreational water use (swimming snorkeling, etc.)
Shellfish like oysters, but also clams and mussels as well as scallops are filter feeders, which means they filter the surrounding water for food. For example one individual Asian clam, one of the most efficient filter feeders can filter up to 1.3 L (0.34 ga) per hour. Imagine a colony of thousands of clams. No wonder that pollutants are enriched in these animals. The good news is that cooking generally kills the viruses. Which puts Oysters, which are generally eaten raw in the focus.
Only Human Sources?
The Norovirus genus comprises viruses that infect cattle, pigs, dogs, rhesus macaques mice, and even lions. This is a growing list and closely related viruses can be present in different species. Human Noroviruses were for example observed in pigs and cattle.
How to prevent Norovirus infection?
Different strategies for the different routes of infection apply. General hygiene rules apply to avoid human to human transfer: Wash your hands thoroughly and often and don’t share knife, fork and spoon or drinking glasses.
Only use drinking water to wash food and cook seafood thoroughly to avoid infection from food.