At least six people fell ill with diarrhetic shellfish poisoning after eating mussels harvested in the United Kingdom earlier this year.

The mussels contained high levels of heat-stable okadaic acid (OA)-group toxins. Without a validated test for such toxins in human samples, the diagnosis was made based on clinical symptoms, according to a study published in the journal Eurosurveillance.

Researchers said given diarrhetic shellfish poisoning is a self-limiting illness that might be under-reported and has low awareness among clinicians, the actual number affected in the outbreak was likely to be higher.

Unwell after eating mussels
In June, Public Health England South West was told by the local authority of three diners who were unwell following consumption of mussels in a restaurant five days earlier. The local authority found the restaurant had received a batch recall notice, also five days earlier, from the shellfish producer for the mussels because of elevated toxin levels but this was not seen before they were served.

PHE South West then received a report from the neighboring county about the first gastrointestinal illness linked to mussels from this producer. A multi-agency outbreak control team was led by the PHE South West health protection team.

Thirteen people that reported to have been unwell after eating mussels were contacted. Completed questionnaires were received from seven of them, of which three were confirmed and three were probable cases. They ate at four separate venues. One person did not meet the case definition as symptom onset was more than 24 hours after consumption.

Those ill ranged in age from 37 to 76 years old with three of them male and three females. All reported eating steamed mussels. Five ate mussels as a main course and one as a starter. Reported portion sizes ranged from 11 to 50 mussels.

The incubation period ranged from eight to 17 hours and median duration of symptoms was 2.5 days. One case sought medical advice but there were no hospitalizations.

Outbreaks of DSP are rare. Seventy cases were identified in 2013 following consumption of mussels harvested around the Shetland Islands and 49 people fell ill in 1998 after eating UK-harvested mussels in London. Outbreaks have been recorded in recent years in China, the United States, France and Canada.

Shellfish monitoring findings
A routine shellfish monitoring program is in place at the affected site. The water column is sampled every two weeks from April to September and cell counts of potentially harmful algal species are measured. Shellfish flesh samples are tested for seleced European unio (EU)-regulated biotoxins every four weeks during April to September each year. Lipophilic toxin determination, including that for OA-group toxins, is carried out.

The shellfish producer gave the outbreak control team a list of businesses who had received affected mussels. They were not tested by the producer for toxins. A large volume of the product was distributed to seafood wholesalers, restaurants and pubs.

Some companies not linked to known cases, including wholesalers, retailers, restaurants and pubs, said they had sold some of the affected produce. None of it was still in circulation at the time of outbreak response.

Measured densities of Dinophysis spp. in the water column increased rapidly from being undetectable 16 days before outbreak notification to 1,600 cells per liter seven days before, coinciding with harvesting time of the affected batch and exceeding the Food Standards Agency trigger level of 100 cells per liter.

Total OA-group lipophilic toxins in mussel flesh was 338 µg OA equivalents (eq) per kilogram, seven days before outbreak notification. This exceeded the maximum permitted limit of 160 µg OA eq per kg set by European Commission (EC) regulation 853/2004. The lowest-observed-adverse-effect level of OA is 45 to 50 µg OA equivalents per person. The shellfish bed was closed for harvesting, which is standard practice in England.

“A rapid increase in concentrations of Dinophysis spp. cells within the waters of the production site may have contributed to the outbreak, in tandem with shellfish harvesting occurring before official control results were reported and site closure. Transdisciplinary research is required to predict future risk and inform monitoring, particularly given likely changes in the distribution of potentially-toxic species particularly if temperature of ocean water increases,” according to researchers.