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Food Focus February 2009

Serve your succulent shellfish ‘hot’ in the shell

For those who love to eat fresh, briny oysters or sweet, succulent mussels with nothing but a squeeze of lemon juice – and there are many – summer is the perfect time to indulge. Take care, though, when collecting and eating shellfish straight from the sea

Certain shellfish are more risky than others as a potential cause of foodborne illness. Bivalve shellfish (those with two shells), such as oysters, mussels, pipis, scallops and cockles, are the most risky to collect and eat. They get their food by filtering large quantities of water through their systems.

If these shellfish are living in polluted waters they can concentrate pathogens – the microorganisms that can cause disease. These pathogens build up in the shellfish and make people sick if they eat the contaminated flesh.

Pathogens such as norovirus, hepatitis A, Shigella and Salmonella are typically found in sewage, and can accumulate in bivalve shellfish. If people eat the contaminated shellfish, they can develop diarrhoea and vomiting or more serious illnesses, such as hepatitis A.

Unsafe levels of contamination from heavy metals and other chemicals are very rare in New Zealand shellfish. The areas most likely to be contaminated are those near slipways, marinas and discharge outlets for sewage.

Grazing shellfish such as paua, pupu and sea eggs, by contrast, are not filter feeders so pose a much lower risk, especially if the gut is thrown away.

Marine biotoxin alerts

As well as the dangers from polluted water, bivalve shellfish also filter and concentrate toxic phytoplankton and algae, which can cause toxic shellfish poisoning.

Shellfish and seawater samples around New Zealand are tested each week to ensure they are not contaminated with biotoxin from blooms of algae. District Health Boards will put up warning signs and alert local media and networks including Iwi and fishing and boating clubs when shellfish are not safe to eat. NZFSA’s website has a marine biotoxin alert page.

All commercial growing areas have strict monitoring programmes for shellfish toxins and bacteria to ensure they are safe to eat.

Reducing the risks of getting ill

There is sometimes no way to know if shellfish are contaminated – there may be no change in how they look, smell or taste. Short of giving them up entirely, there are ways to minimise the risks.

The factors that increase the risk of becoming ill from shellfish include the type of shellfish eaten, any contamination, and your immunity.

People with low immunity such as young children, the frail elderly, pregnant women and people with a chronic illness, are more at risk from food poisoning and should consider this before eating shellfish.

Do not collect wild shellfish for eating from water known to be affected by algal blooms, from areas where animals have been grazing nearby, or near wharves and marinas where boats may have discharged sewage or chemicals, such as anti-fouling paint or diesel. Avoid collecting from where pipes or culverts run down to the beach, including storm water drains, and from harbours, boat moorings, marinas or ports. Don’t harvest from waterways that have health warnings in place – look for signs along the shoreline. Also avoid gathering shellfish for several days following heavy rainfall – wait until the rivers or estuaries run clear.

Once you have collected your shellfish, it is also important to store, handle and prepare them correctly to help prevent illness. Remember to:

• keep shellfish alive and cool to keep them fresh, and use within two days of harvest

• discard any that have died during storage, or have broken shells – fresh shellfish should close their shells when you tap them

• store on ice if transporting in a chilly bin, but be careful as freezing will kill them

• refrigerate as soon as possible after collection, on a shelf below any cooked food, and keep covered with a clean damp towel

• keep raw shellfish separate from cooked foods and follow good hygiene practices – use clean hands, chopping boards, benches and utensils

• if you are freezing shellfish, shuck them and freeze in small amounts, and thaw in the fridge for 24 hours before you cook them.

Cook shellfish until they are piping hot throughout, and eat while hot. Raw or lightly steamed shellfish can still contain live pathogens, and you may be putting your health at risk by eating them.

Toxic shellfish poisoning

The chemicals that cause toxic shellfish poisoning are produced by certain species of toxic algae, which are released into the shellfish when they eat the algae. Three forms of biotoxins are regularly found in New Zealand shellfish.

Paralytic shellfish poisoning results from eating shellfish contaminated with saxitoxins and related compounds. Symptoms occur within 12 hours of eating shellfish, and include numbness and tingling around the mouth, face or extremities, difficulty swallowing or breathing, dizziness, double vision, and in severe cases, paralysis and respiratory failure. People overseas have died from this illness.

Amnesic shellfish poisoning is caused by domoic acid in shellfish. Symptoms are mainly gastrointestinal (vomiting, diarrhoea, etc), especially at low levels, however about a quarter of cases experience neurological problems including memory loss, which may be significant and permanent. Gastrointestinal symptoms first appear within 24 hours and neurological difficulties within 48 hours.

Okadaic acid and related compounds cause diarrheic shellfish poisoning. Symptoms are mainly gastrointestinal, and people generally recover within three days.