As a tsunami of stomach bugs strikes schools across the US, here’s how to tell the difference between the two main culprits — shigella and norovirus

If you or a loved one has been struck down by a stomach bug in recent weeks, you’re not alone.

Cases of norovirus – or the ‘stomach flu’ – reached a 12-month high in February and are still wreaking havoc, particularly in schools.

At the same time, cases of Shigella bacterial infections are on the rise and health officials are concerned about an antibiotic-resistant strain that is growing in the US.

The two bugs – which are the leading cause of gastroenteritis in American schools – have very similar symptoms, making it tough to differentiate them.

But there are ways to tell the difference between the two:

The above graph shows the number of shigella outbreaks reported this year (red line) and previously. The season got off to an early start but has since declined. It remained below pre-pandemic levels

This map shows the number of outbreaks reported by state. It was most common in Virginia, California, Ohio and Michigan – with cases still ongoing. Schools in Virginia are advising parents to keep children at home for 48 hours after clearing symptoms to help stop the spread of norovirus

Dr Marci Drees is the chief infection prevention officer at ChristianaCare in Delaware, who regularly deals with patients with the diseases.

Asked how to tell the diseases apart, she told ABC6: ‘Norovirus tends to start off with a lot of vomiting and then progress to diarrhea, or sometimes they both start at once.

‘[But] shigella tends to be more diarrhea and not so much vomiting. You might have stomach cramps.’

Diarrhea caused by shigella tends to be more watery or bloody, medics say, while that caused by norovirus tends to follow a bout of vomiting.

The two can also be told apart by how long they last. Norovirus cases normally clear up within three days, but shigella tends to last for four to seven days. 

In serious cases, it can take weeks to months for the bowels of patients with the bacterial infection to return to normal.

Other differences include that shigella patients are more likely to have a fever than those with norovirus.

The diseases may also strike at different types of year.

Norovirus is also known as the ‘stomach bug’ because of its propensity to spike in the cold November to April months, before dropping.

Shigella, on the other hand, tends to transmit at a constant rate throughout the year.

Norovirus causes up to 21 million cases in the United States every year, 109,000 hospitalizations and 900 deaths, statistics suggest.

Cases spiked earlier than normal this year, data suggests, reaching a peak of 30 outbreaks a week in early January.

But they have since declined. They remained below the levels registered between 2012 and 2020 before the pandemic.

Virginia, California, Ohio and Michigan have faced the most norovirus outbreaks this year, data suggests.

Earlier this month, schools in Chesterfield, outside Richmond, Virginia, told parents to keep children home for an extra 48 hours after their symptoms cleared — suggesting norovirus was behind the illness.

Shigella, on the other hand, causes fewer diseases every year than norovirus.

Surveillance data from the Centers for Disease Control and Prevention (CDC) suggests the bacterial infection causes about half a million cases every year leading to 5,400 hospitalizations and 38 deaths.

Last month a ‘serious public health alert’ was issued over cases of the superbug nationwide that were becoming resistant to antibiotics, making them harder to treat.

About five percent of all cases now involve resistant strains, compared to none only six or seven years ago.

Medics say that in most cases the best treatment for sick children is plenty of rest and to ensure they are hydrated.

Some patients may also be able to eat small amounts of bland food, they suggest, such as soup, rice, pasta or bread.

Children will only need to see doctors if they start to show symptoms including bloody diarrhea, prolonged fever, severe stomach cramps or dehydration.

In these cases, tests will be carried out on stool to determine whether an infection is caused by shigella or norovirus.

Because shigella is caused by a bacteria, it can be treated with a course of antibiotics.

But there is no similar treatment for norovirus, which is caused by a virus, with doctors instead focusing on managing symptoms.