Australian researchers have examined an outbreak of paralytic shellfish poisoning in 2015 that caused at least four cases in Tasmania.

Paralytic shellfish poisoning (PSP) is a rare illness caused by eating shellfish containing paralytic shellfish toxins (PST). PST are produced by certain types of algae during algal blooms and bio-accumulate in bivalve filter-feeding shellfish such as oysters, mussels and scallops. Contaminated seafood looks and tastes normal.

The toxins are not destroyed by cooking or freezing.

Routine monitoring of commercial shellfish in 2015 detected a large bloom of Alexandrium tamarense on the east coast of Tasmania, which can cause PSP.

In October 2015, four cases of PSP were identified in adults who ate wild mussels from the east coast of Tasmania and had onset of numbness or tingling of the face and muscle weakness from 30 minutes to 12 hours later. Two cases were hospitalized overnight but both recovered, according to researchers writing in the Communicable Diseases Intelligence journal.

A permanent public health alert recommends people in the state do not eat recreationally harvested wild shellfish due to the risk of poor water quality in unmonitored areas. Routine monitoring covers commercial shellfish but not most recreationally foraged wild shellfish.

Bivalve shellfish marine farming and commercial wild harvest bivalve shellfish fishing are monitored by the Tasmanian Shellfish Quality Assurance Program (TSQAP) under the Tasmanian Biotoxin Management Plan. Toxin concentrations above the regulatory limit of 0.8mg/kg result in closure of the marine farm until levels drop below the limit on two consecutive tests.

Five suspected cases were notified to public health services but one reported illness did not meet the case definition and was excluded. The four confirmed cases between Oct. 2 and 12, 2015, ate wild mussels gathered from Little Swanport, an area known to be affected by high levels of PST.

The median number of mussels consumed was 14, with individual patients eating six to 35. Cases were aged between 51 and 61; three were male. Time from exposure to onset of symptoms was 30, 60 and 90 minutes, and 12 hours.

Little Swanport harvest area, adjacent to where wild mussels were gathered by the cases, was closed due to elevated PST levels from Sept., 23 until Nov., 20, 2015. A single wild mussel sample from the same location as the four cases in mid-October had a PST level of 35 mg/kg, more than 40 times the limit.

Water temperatures on the east coast of Tasmania in 2015 were four degrees warmer than average due to the El Niño Southern Oscillation, which caused warm currents travelling south to move closer to the land mass. The contribution to the harmful algal blooms is unclear.

PSP is a notifiable disease in Tasmania. Prior to this outbreak, there were anecdotal accounts of cases in the 1980s and 1990s and one documented case in 2011.

“Because the disease is rare clinicians and the public have limited awareness of PSP. Mild cases may have not sought medical attention nor been reported. Clinicians also need to be made aware of the need to report suspected cases of PSP to public health units and be encouraged to collect urine specimens or leftover uneaten shellfish for PST testing,” said researchers.

Following the outbreak, a fact sheet on PSP was created and permanent signage put up at coastal sites where shellfish are frequently recreationally foraged. There are no Australian labs accredited for human testing of urinary PST.

Researchers said several states in Australia are at risk of cases of PSP.

“We recommend active surveillance across Australia in collaboration with Departments of Primary Industries, researchers and other marine regulatory bodies to manage the social, economic, environmental and health risks associated with harmful algal blooms and shellfish poisoning,” they added.

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