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Food Focus August 2007

Shellfish collecting – keeping your harvest food-safe

Collecting kaimoana from the sea is a favourite tradition for many New Zealanders. However, if you collect shellfish from an area that is contaminated with pathogens, chemicals or algae toxins, the resulting illness can be very serious. But there are ways to minimise the risk of illness so you can still enjoy the experience of harvesting from nature

You can be exposed to several types of illness from shellfish, depending on what they have been feeding on or in. Biotoxins from phytoplankton (microscopic algae that are part of the staple diet of bivalve shellfish) cause toxic shellfish poisoning. Pathogens often come from water contaminated by animal or human sewage or farm run-off, and chemical contaminants in the water, such as heavy metals, fuel, paints and solvents, can also cause illness.

Bivalve shellfish (with two shells) pose a greater risk than other seafood because they filter a lot of seawater for food particles, and accumulate whatever toxins or pathogens are in the water. Bivalves include mussels, tuatua, toheroa, oysters, cockles, pipi and scallops.

Grazing shellfish, eg: paua, pupu (catseyes) and sea eggs (kina), pose a much lower risk, especially if the gut is discarded.

Toxic shellfish poisoning

Toxic shellfish poisoning is caused by biotoxins from algal blooms. Three forms of biotoxin are regularly found in New Zealand shellfish.

Paralytic shellfish poisoning (PSP) results from eating shellfish contaminated with saxitoxins and related compounds. Symptoms include numbness and a tingling (prickly feeling) around the mouth, face and extremities, followed by difficulty swallowing or breathing, headache, dizziness, and double vision. These occur within 12 hours of consumption and people overseas have died from this illness.

Amnesic shellfish poisoning (ASP) 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 diarrhetic shellfish poisoning (DSP). Symptoms are mainly gastrointestinal and usually of short duration.

The good news is that popular collecting sites are tested regularly for biotoxins. When the shellfish are contaminated with biotoxins, District Health Boards put up warning signs and may alert local newspapers, television, radio stations and local networks including Iwi and fishing/boating clubs. NZFSA’s website (www.nzfsa.govt.nz ) has a section on marine biotoxin alerts.

Pathogens and heavy metals

Pathogens such as norovirus, hepatitis A, Shigella, Vibrio and Salmonella, which are present in sewage, can accumulate in bivalve shellfish, causing gastro-enteritis in people within a few hours or days of consumption. Hepatitis A may take longer to show up (about a month). Complications from pathogens can include damage to your blood, liver and immune system.

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

Vigilance is your best protection

To protect yourself and family from eating contaminated shellfish, pay close attention to the areas where you collect shellfish. Avoid areas that have warning signs advising against collecting shellfish – these have been put up for a good reason! Avoid collecting where pipes or culverts run down to the beach, including storm water drains. Don’t collect after heavy rainfall, wait until the rivers or estuaries have run clear for several days. Avoid areas where animals have been grazing nearby, or near wharves or marinas where boats may have discharged sewage or chemicals, such as anti-fouling paint or diesel.

Store and prepare shellfish carefully

Once collected, how you store your harvest can also help reduce the risk of contracting an illness. It’s important to:

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

• store on ice if transporting in a chilly-bin (be careful as freezing will kill them)

• refrigerate as soon as possible after collection (within four hours), and keep covered in the fridge with a clean wet towel (not in an airtight bag or container).

When you prepare your shellfish:

• do not eat any that have died during storage

• do not cook or eat any with broken shells

• avoid cross-contamination from pathogens – use clean hands, chopping boards, knives etc, and keep them separate from other foods and utensils.