Swedish authorities are investigating a national outbreak of E. coli O157:H7 with at least 50 cases.

Folkhälsomyndigheten (Public Health Agency) and Livsmedelsverket (National Food Agency) said it appears to be one of the biggest outbreaks of infections from the potentially deadly pathogen that the country has ever had. Sweden usually records about 30 to 50 domestic cases during July.

The bacteria behind the outbreak is Enterohemorrhagic E. coli (EHEC), which is also known as Shiga toxin-producing E. coli (STEC) and Verocytotoxin-producing E. coli (VTEC). The outbreak strain — EHEC O157:H7, clade 8 stx2a stx2c eae positive — is known for its high virulence.

Infections have been reported from several counties but mainly Uppsala and Västra Götaland.

Folkhälsomyndigheten told Food Safety News that through mid-August it had found 54 cases linked to a specific EHEC type through whole genome sequencing (WGS). An additional 50 cases/isolates are suspected and are under investigation to see whether they belong to the outbreak or not.

“There have been regional outbreaks during July that now seem to be connected according to cluster analyses by WGS. Since this link has been identified, the national outbreak team is investigating a possible common source linking all the cases together. The outbreak is ongoing,” said a spokeswoman.

“The national outbreak team is investigating if there initially was (or is) a common foodborne source behind these cases. Locally there could have been person to person spread via bathing in lakes.”

Cases have illness onset dates ranging from the beginning of July up to August. Adults, young adults and children have been affected. Gender distribution is fairly even, with slightly more men ill.

Folkhälsomyndigheten said information on hospitalization is not collected at national level. However, it has been reported that some outbreak victims developed a type of kidney failure called hemolytic uremic syndrome (HUS).

Sweden sees about 2 percent to 4 percent of the reported EHEC patients annually developing HUS. This condition can occur among persons of any age but is most common in children younger than five and older adults.

Authorities said as the E. coli O157:H7 has spread to different parts of Sweden, it is probably a foodborne outbreak but the source or sources are not yet known.

“Interviews are being conducted via the local departments of communicable disease control and these interviews are pieces in the national investigation puzzle that are now under investigation to see if a common link can be identified,” said the public health spokeswoman.

Primary sources of such outbreaks are raw or undercooked ground meat products, raw milk and fresh fruits and vegetables that are served raw.

The E. coli bacteria is most common during the summer months but a long heat wave in Europe this year may have had an impact on the spread of infection. The pathogen is destroyed by cooking foods until all parts reach a temperature of 70 degrees C or higher.

In Sweden, more than 500 cases are reported each year and about half domestically infected. The largest E. coli O157 outbreak in the country was in 2005 with 135 cases. Iceberg lettuce was the probable source of those outbreak infections.

Symptoms include abdominal cramps and diarrhea that may become bloody. The incubation period can range from three to eight days. Most otherwise healthy adults recover within 10 days.

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