Dutch and UK Hepatitis A cases linked - study

The outbreak investigation was of two closely related foodborne clusters of hepatitis A virus (HAV).

Investigations following the traceback of the implicated mussels led to matching of the nine primary and two secondary Dutch cases to three primary and two secondary cases in the UK with identical HAV sequences within the same time period.

“The rising numbers of reported foodborne HAV outbreaks and RASFFs are likely to promote interest in diagnostic sample typing. This would assist source tracing in outbreak investigation and may result in an improved estimate of the impact of contaminated food on public health,” ​said the report authors.

Patient clusters​

Analysis of mussel supply chains identified the most likely production area. International enquiries identified a cluster of patients near this area with identical HAV sequences with onsets predating the first Dutch cluster of cases. 

The most likely source was a case who returned from an endemic area in Central America, and a subsequent household cluster from which treated domestic sewage was discharged into the suspected mussel production area.

Mussels from this area were also consumed by a separate case in the UK sharing an identical strain with the second Dutch cluster. 

The investigation came after identification of a cluster consisting of four seemingly unrelated Dutch hepatitis A cases with an identical but unique HAV sequence (genotype IA) in August 2012. In November that year, another cluster of seemingly unrelated hepatitis A cases, again with an identical HAV IA sequence, was identified.

The HAV IA sequence of the second cluster was closely related (459/460 nt similarity) to the strain that caused the first cluster.

Small patient number and non-endemic area​ 

From 1 August 2012 to 18 February 2013, 89 hepatitis A cases were reported to the RIVM. Of these, 24 cases acquired their infection from an unknown source in the Netherlands with no travel history.

In the same period, the RIVM received 79 samples from notified cases for typing. In total, 62 strains could be successfully typed, and 15 of these were from hepatitis A cases with an unknown infection source within the Netherlands. These 15 cases diverged into the summer and autumn clusters, each with an identical sequence which was further investigated in this study, and six unrelated cases.

The summer cluster of cases infected with RIVM-HAV12–070 consisted of three male and one female primary cases, with ages ranging from under 10 to under 70 years of age, and onset of illness within 10 days in August 2012. One primary case needed hospitalisation and two secondary cases were reported, both relatives of primary cases.

The autumn cluster cases were infected with RIVM-HAV12–124 and consisted of two male and three female primary cases, ranging from over 40 to under 70 years of age, and onset of illness within 12 days in November 2012.

Three RASFF notifications on HAV were identified in 2012 and two were related to the same batch of frozen strawberry cubes from China. Traceback to find a possible link to the cluster cases indicated that strawberries from this batch had been on the Dutch market partly in the same period of the incubation period of the patients.

The strawberries had been used as a decoration for consumer-ready packed ice cream sold by one of the supermarket chains identified by two of six cases. Unfortunately, no sequence information for the HAV strain detected on the frozen sliced strawberries was available.

All cluster cases had bought mussels at local supermarkets, but no batch numbers or original package labels were available. Therefore traceback was targeted at suppliers within the incubation periods of the patients.

Within this period the suppliers had sourced from a Dutch growing area and from other countries within the EU.

From the questionnaire data available, mussels, raw spring onions, raw onions, prawns, iceberg lettuce and leafy green lettuce were recalled by at least eight of the nine cases as having being consumed within the incubation period.

In the two to six weeks before the onset of illness in August and November 2012, all nine hepatitis A cluster cases consumed mussels, compared with 32 of 125 (26%) and 13 of 143 (9%) for control groups during the 4 weeks before August and November 2012, respectively

Although other food items could not be excluded, the known association of shellfish with HAV outbreaks informed the mussel traceback investigations. 

Source: Eurosurveillance, Volume 21, Issue 3, 21 January 2016​

“International linkage of two foodborne Hepatitis A clusters through traceback of mussels, The Netherlands, 2012”​

Authors: IL Boxman, L Verhoef, H Vennema, S Ngui,​IH Friesema, C Whiteside, D Lees, M Koopmans