A Shigella outbreak in Denmark that sickened more than 40 people was likely caused by imported fresh mint.

In August and September, 44 Danes became ill including 30 women and 14 men between 0 and 75 years old. Thirteen people were hospitalized. Those sickened mostly live in Hovedstaden.

A case was defined as a patient with either a culture-positive sample for Shigella sonnei or people PCR-positive for the ipaH gene, which is a marker for all Shigella species and enteroinvasive E. coli, from August 2020 onwards.

The Statens Serum Institut (SSI) investigated the outbreak with the Danish Veterinary and Food Administration (Fødevarestyrelsen) and DTU Food Institute.

Patient interviews and traceback
Inquiries showed the majority of patients had eaten fresh mint bought at local greengrocers or bazaars in and around the Copenhagen area from mid to late August. Because of the herb’s short shelf life, it is thought there is no longer any contaminated product on the market and no risk of more people being infected.

Among the 36 interviewed patients, 24 had eaten fresh mint before disease onset. Of the 24 cases, 22 had bought it in a greengrocer or bazaar. It was not sold in retail chains. A total of 12 different places were mentioned, which indicated the source of infection was a locally traded food.

Traceback was difficult as the mint sellers had several suppliers during the outbreak period and had bought different types of mint packed in a variety of ways. A lack of invoices for purchase of the mint also hampered the investigation. The duration of the outbreak indicated there was probably only one or a few contaminated batches.

The Danish Veterinary and Food Administration has seized mint lacking traceability details and has taken some test samples with results pending.

Luise Muller, an epidemiologist at SSI, said the outbreak was an opportunity to remind consumers that fresh herbs must always be washed thoroughly before eating them.

Infections not common in Denmark
SSI checked with European health agencies but no other countries have reported an increase in Shigella infections in the same time period.

Infections with Shigella are not common in Denmark and are most often acquired in connection with travel abroad. Interviews with those affected showed they had not been traveling before they became ill.

Previous outbreaks in Denmark include one in 2007, when more than 210 people became ill from eating contaminated imported baby corn. Another outbreak in 2009 involved 10 people and was traced to fresh, raw sugar peas (sugar snaps) imported from Kenya.

Most people infected with Shigella develop diarrhea, fever, and stomach cramps starting a day or two after they are exposed to the bacteria. Shigellosis usually resolves in five to seven days but some people may experience symptoms from a few days to four or more weeks. People who are in poor health or who have weakened immune systems are more likely to get sick for a longer period of time.