The resurgence of a hepatitis A outbreak linked to strawberries underlines the importance of complete traceback of implicated products during outbreaks, according to researchers.

Following outbreaks linked to frozen strawberries in Sweden and Austria in 2018, 65 cases of the same hepatitis A virus (HAV) strain were detected in Germany between October 2018 and January 2020.

The HAV subgenotype IB strain caused outbreaks in Sweden from June to July and Austria from July to September 2018, affecting 20 people in Sweden and 14 in Austria. In Sweden, the outbreak strain was detected in frozen strawberries and the contaminated batch was withdrawn from sale. Traceback investigations from Sweden and Austria identified a Polish producer as the source for implicated frozen strawberries.

German outbreak
Shortly afterward, cases with the identical virus sequence appeared in Germany. The Federal Office of Consumer Protection and Food Safety (BVL) and all federal public health authorities were informed by the Robert Koch Institute (RKI) and sequencing of samples from hepatitis A cases was intensified.

A first wave of cases started in 2018 and a second wave began in July 2019. The first case-control study was conducted by RKI and included all 21 primary outbreak cases in Germany with disease onset in 2018 and 237 people as controls, according to the study published in Eurosurveillance.

Overall, 30 cases including 27 confirmed and three probable, from 11 states had disease onsets between Aug. 29 and Dec. 22, 2018. Of confirmed cases, three were likely secondary infections.

The second case-control study was done by the State Office for Health and Social Affairs Berlin and included the first 11 outbreak cases of the second peak in the city and 103 controls.

During the second wave, 33 cases including 31 confirmed and two probable from seven states had disease onsets between June 13 and Sept. 29, 2019. One probable and one confirmed case likely were secondary infections. Twenty were in Berlin and five from the neighboring state Brandenburg. Eight cases were notified from five other states in Germany and one person reported travel to Berlin.

Frozen strawberries from Poland or Egypt?
Of all 65 patients, the median age was 48 years with a range of 1 to 77 years old and 45 percent were female. More than three quarters of patients were hospitalized.

From interviews with 46 cases, 34 reported definite and four had possible consumption of items containing frozen strawberries. Frozen strawberry cake was the most commonly mentioned such product; 27 cases reported definite and five had possible consumption.

Of 27 people with definite consumption of such cake, 26 gave details on the type, with 25 identifying strawberry cake(s) from one brand spontaneously or in product picture-assisted recall. Some of the cakes are ready to eat after thawing and do not require oven cooking.

Trace back investigations revealed the Polish producer involved in the outbreaks in Sweden and Austria received frozen strawberries from Egypt via a German wholesaler that also delivered them to a cake manufacturer. A retention sample of frozen strawberries of that batch provided by the German distributor was negative for HAV.

Evidence needed to prompt recall
A sample of frozen strawberries from a patient in Berlin tested negative for HAV. Samples of two frozen strawberry cakes from the implicated brand also tested negative. Detection of HAV in food samples, especially berries, is known to be difficult, according to researchers.

Food authorities have not issued product recalls from the German market in relation to the outbreak. Researchers said preventive measures should not solely rely on microbiological findings but include epidemiological evidence.

Contamination of berries can occur in various ways. The most likely route is contaminated water used for irrigation or processing of fruits. The outbreak subgenotype IB strain is similar to circulating strains in Egypt which might indicate that production and/or contamination of strawberries could have occurred in that country and not in Poland as suggested by the Swedish and Austrian investigation, according to the study.

One person had disease onset in January 2020. They likely consumed a frozen strawberry cake of the implicated brand in the incubation period as it had been bought in summer 2019. It can take months for symptoms to begin.

The Netherlands reported two cases with an identical sequence: one with disease onset in September 2018 after travel to Germany and one in May 2019. Both had eaten strawberries. Italy also had two cases in August and September 2019; both had consumed frozen berries.

“Many uncertainties remain regarding distribution routes, mechanism of contamination, role of other frozen strawberry products in the outbreak and, if one or multiple shipments were involved,” said researchers.