What Is the Red Tide and How Does It Affect Health?

Red tide is a natural phenomenon that occurs in the sea and river mouths due to the presence of millions of microscopic algae, some of them toxic, which stain the waters from gold to red.

What Is the Red Tide and How Does It Affect Health?
The So-Called Red Tide in Yucatan: What Can We Do in Case of Red Tide Poisoning? Photo: Cofepris

In the last hours, Yucatan authorities warned about the presence of red tide, which, for the moment, does not represent any risk to the health of the population. However, last May, two children died from consuming shellfish contaminated by the so-called red tide, what is it and how does it affect health?

What is red tide?

The most common term used to denominate these "algal blooms" is "Red Tide", which could be very ambiguous, since, in the first term, this denomination describes all the events in which the water acquires certain tonalities due to the high biomass of marine algae; however, not all of them are harmful.

Secondly, it should be noted that many toxic or harmful events occur without changes in the color of the water due precisely to the low density in its biomass, but in concentrations of toxins sufficient to favor toxic and harmful events.

Harmful Algal Blooms (HAB) are events that occur globally, where the evidence indicates that their nature and extent have been expanding during the last decades, and the possible explanations for this type of spread are still under debate, which vary from natural mechanisms to the dispersion of species thanks to certain human activities.

How does red tide affect human health?

Red tide poisoning can affect the nervous, respiratory, and digestive systems, and can cause:

Respiratory arrest (in severe cases);
Paralysis of extremities.

Some species of microalgae are producers of potent toxins. Organisms such as bivalve mollusks (clams, mussels, mussels, mussels, cockles, oysters) or gastropods (sea snails) can accumulate toxins in their bodies by feeding on toxic microalgae. The toxins do not affect the mollusks, nor do they produce noticeable changes in their odor, color, or taste.

However, if these contaminated mollusks are consumed by humans, they can cause intoxication, the severity of which depends on the type of toxin and the dose ingested. Toxins are not inactivated by cooking, the addition of vinegar or lemon, or the consumption of alcohol. Nor are there any antidotes.

Worldwide, more than 2,000 cases of human poisonings are reported annually with a 15 percent associated mortality rate. Most of these cases have corresponded to people who collected mollusks on beaches or in areas near the coast to prepare them for family meals, or to fishermen who consumed mollusks collected at sea.

How to prevent red tide intoxications?

To avoid health risks, it is recommended:

Do not collect mollusks and/or shellfish on beaches for home consumption.

Respect the provisions, ordinances, and public notices of closures to the extraction and consumption of shellfish.

Do not buy seafood preparations (canned food, salads, paellas, etc.) in street stalls or places that do not have bromatological authorization.

Do not buy fresh shellfish "at the foot of the boat", in breakwaters, or those that may be offered by unauthorized collectors.

Do not buy shellfish that do not have the corresponding health certificate issued by an official agency.

Buy or consume shellfish only in fishmongers, restaurants, or food stores duly authorized.

What to do in case of red tide poisoning?

According to the authorities, in case of intoxication from consuming two-shell mollusks, such as clams, oysters, mussels, scallops, small mussels or tichindas, daisy mussels, goat's foot, and some sea snails contaminated by red tide, it is necessary to go immediately to the doctor.

Frequency of occurrence

HABs occur in a great variety of forms and with multiple impacts, the most important of which is undoubtedly when toxic phytoplankton is filtered from the water as food by bivalve mollusks such as clams, mussels, and oysters, whose capacity to filter large volumes of water can cause algal toxins to accumulate very quickly to levels that can produce symptoms of poisoning in humans, the main consumers of these types of products.

It is important to note that through the food chain other organisms are also involved in the transfer of biotoxins produced by these algae, such is the case of some species of the gastropods class such as snails.

Once this occurs, the intoxication syndromes manifested in the population can be but are not limited to, neurotoxic, diarrheal, paralytic, and amnesic, just to mention a few. The degree of toxicity of each type of event depends directly on the predominant species that causes it. To date, about 157 different species of algae have been identified on the Mexican coasts, of which only 45 are potentially toxic.

How are mollusks contaminated?

Mollusks can be contaminated by the variation of climatic conditions or by the increase of nutrients in the ocean that cause an accelerated reproduction of toxin-creating microorganisms.

More than 157 species of algae have been identified on Mexican coasts, of which 45 are potentially toxic.

During the presence of a red tide alert, authorities launch some recommendations on what to do before and during the alert:

Before: report to the authorities if the color of the marine waters changes to reddish and heed the official information that is disseminated.

During: do not consume mollusks. If the authorities place a red warning flag, do not swim on the beach and bury dead fish you find on the beaches to avoid contamination.