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Red tide (Dynamics and public health aspects)

Red tide (Dynamics and public health aspects)

Red tide (Dynamics and public health aspects)

Red tide refers to the abnormal discoloration of seawater due to the sudden proliferation of microscopic organisms called dinoflagellates. Red tides can be classified into two: toxic and non-toxic. The first nontoxic red tide in the Philippines was recorded in 1908 and was caused by Peridinium sp.; while toxic red tide in the country was first experienced in 1983. Since then, all recorded red tides have been poisonous. Today, more than 1,300 Filipinos have suffered from Paralytic Shellfish Poisoning (PSP). About 71 have died. Most victims were fishermen and their immediate dependents. About 18 provinces have reported red tides since 1983 when the first red tide was documented off the coast of Western Samar. The dinoflagellate causing poisonous red tides in the Philippines is called Pyrodineum bahamense var. compressum. These microorganisms live in coastal waters and lagoons under conditions of high salinity and will not survive in freshwater bodies.

Red tides are found not only in the Philippines but also occur in other parts of the world.

Worldwide distribution of paralytic shellfish poisoning (PSP) - Distribution of PSP in the Philippines

Pyrodineum has seeds found on the muddy bottom of shallow coastal waters. The seeds or cysts are resistant to harsh conditions and may remain viable for 1,000,000 years. Under favorable conditions (e.g., right temperature and amount of nutrients), the seeds can germinate into a free-floating form suspended in seawater. During the day, they are found near the surface where they receive sunlight for their photosynthetic activities. At night, they occur deeper in the water column where they receive nourishment. When conditions change from good to bad, they either die or revert back to their cystic stage.

Sometimes, blooms occur. That is, the dinoflagellates suddenly proliferate rapidly on a massive scale. Possible causes include a change in climate (from prolonged drought to sudden rains) and pollution due to improper disposal of human wastes and bad farming practices. None of these, however, have been proven to be a direct cause of algal blooms. Therefore, scientists are unable to predict red tides.

Tracking the distribution of red tides is very difficult because the bloom tends to move. Red tides are only visible when the algal bloom occurs on the water surface during daytime.

Effects on human health

Dinoflagellates serve as food to shellfish. If a shellfish ingests too much, then the shellfish could possibly contain amounts of the red tide poison lethal to humans. People can get sick after eating seafood products contaminated with the red tide organism. The most common known to cause poisoning are bivalve shellfishes like mussels, oysters; scallops, cockles and limpets. Fish, shrimps, squids, crabs, lobsters and products grown in ponds are generally safe even when caught from red tide-infested waters.

Filter-feeding bivalves affected by red tide

The disease following consumption of contaminated shellfish is called paralytic shellfish poisoning (PSP). Symptoms develop within 12hours following the meal and include tingling or burning sensation in the lips, gums, tongue and face; progressing to the neck, arms, fingertips and toes. Severe cases will suffer from an inability to walk, breathe, swallow and speak and some patients may die from the inability to breathe spontaneously. The poisoning is caused by the neurotoxins from the dinoflagellates which are concentrated by filter-feeding shellfishes. The poison is stable to heat and, therefore, is not destroyed by cooking. It is soluble in water and can be present in shellfish broth. The amount of poison present in shellfish is dependent upon the amount of dinoflagellate filtered by the shellfish. The Philippine standard considers any sample containing 80 micrograms of red tide poison in 100 grams of shellfish meat as dangerous to human health.

There is no known antidote to red tide poison. The best way to avoid the illness is not to eat contaminated shellfish for as long as red tides are present. Patients with mild symptoms can force themselves to vomit to expel the poison from the stomach. Patients with moderate to severe disease must seek medical advice and hospitalization.