Toxic marine algae




In aquatic environments, lakes, reservoirs, seas and oceans, the first level of the food chain is mainly represented by organisms consisting of a "single cell (unicellular), algae or cyanobacteria (formerly referred to as blue-green algae, although in reality they are treats, as their name implies, bacterial species), which can live freely in water (phytoplankton) or stay on the bottom in shallow areas, where there is sufficient light for photosynthesis (phytobenthos). About 2% of known species it can represent a danger to the health of the environment, animals and humans.

The reasons are of different nature:

  • production of toxic substances or toxins, in some cases even fatal to other organisms including man
  • exaggerated growth and density, so as to form a thick patina on the surface of the water (blooms) and to color the water in which they grow. During blooms, algae can clog the gills of fish, causing them to die from asphyxiation, but massive deaths of aquatic organisms can more frequently be caused by a decrease in dissolved oxygen in the water during the decomposition of dead cells. Accumulations of smelly masses can also form on the beaches which, even in the absence of toxicity, create inconvenience to the population residing on the shores of the affected lakes or on the coast, also causing significant economic damage in tourist areas.

During the last forty years there has been a significant increase, on a global scale, in the blooms of algae, including toxic ones (ie those species that produce toxins).

There are many factors behind the spread of algae. Some are natural, such as the dispersion of algae species by currents and typhoons capable of displacing masses of sediment containing cysts (resistant cells).

Many others, linked to human activities (anthropogenic) such as the discharge of floating water from ships from one sea to another, the importation of shellfish for aquaculture, the construction of large aquaculture facilities in coastal areas. The factors related to human activities also contribute to the phenomenon of "eutrophication, ie the presence of very high levels of nutrients, such as phosphorus and nitrogen, which stimulate the growth of algae.

Finally, a role is played by climate change, due to the increase in temperature in the oceans, the variation in the acidity of the waters and the greater frequency of extreme events such as floods and droughts.

Many marine algae, and the toxins they produce, have been known for some time and it is erroneously thought that they only affect the tropical area. Although in the past most of the reports concerned that geographical area, in recent years the phenomenon of algae blooms affected temperate areas, including Europe.

The toxins produced cause various types of disturbances to human health, both due to contact associated with bathing, sports and recreational activities generally carried out in water, and due to the ingestion of marine organisms that feed on algae and accumulate the toxins, thus resulting contaminated.

In fact, during the blooms, fish, molluscs and crustaceans that filter the water and / or feed on algae can accumulate toxins inside them, without showing apparent damage, and still be toxic for consumers.

The presence in fish products of some of the best known toxins has been monitored by law in many countries (including Italy) since the 70s and 90s of the last century. Furthermore, in 2009 the EFSA established the quantity that can be consumed within 24 hours without appreciable health risks (the so-called acute reference dose) for various toxins (okadaic acid, azaspiracidi, yessotoxins, saxitoxins, pectotoxins and domoic acid) produced by different algae.

With regard to the effects associated with bathing, the best known case in Italy is that which occurred in 2005 in Liguria, when for the first time around 200 people had health problems after having frequented the beach in front of Genoa. The case was widely reported in the local and national press both for the number of people involved and because it took a few days before the "Regional Agency for the Protection of the Environment of Liguria (ARPAL) concluded the analyzes and it could be assumed that the the cause of the disturbances could be the flowering of an alga known as Ostreopsis ovata.

Ostreopsis ovata

Ostreopsis ovata

Ostreopsis ovata it is a marine micro-algae that lives in fresh or salt water, in close contact with the seabed or fixed to a solid support such as a rock (benthic) and is typical of tropical and subtropical areas.

This type of algae has dimensions between 30 and 70 micrometers, so it is invisible to the naked eye. In particular climatic conditions, however, it proliferates to form large reddish-brown patches that are easily recognizable in the sea.

It grows in shallow coastal areas on the surface of red and brown benthic macro-algae and on rocky bottoms.

It appeared in the Mediterranean around 1970; blooms of O. ovata have been reported in Italy since 1989. Since the beginning of the 2000s there have been several cases of health problems for bathers in various Italian, French and Spanish coastal areas, but the most important case was certainly the one that affected the Liguria, near Genoa.

In 2005, in fact, about 200 people who had visited the beaches and the coast experienced respiratory problems, eye irritation, irritation of the upper airways, cough and fever. Although most cases resolved spontaneously after a few hours, many were hospitalized for 1-3 days. After this case, the press continued to monitor and report other episodes of "toxic alga" blooms that occurred in the following years.

The number of people with disorders attributable to toxic algae, however, continues to decrease, also thanks to the control exercised by the responsible regional structures, aided by guidelines on the management of the risk associated with the blooms of Ostreopsis ovata on the Italian coasts, issued by the Ministry of Health in May 2007, following the Ligurian episode, and updated in 2014 by the "Istituto Superiore di Sanità".

With rare exceptions, for several years almost all the Italian coasts have been affected by blooms of OR. ovata which reach their highest densities between July and October, depending on the region.

Protected and shallow areas, such as semi-closed bays, inlets, areas located behind breakwater barriers, are more at risk of blooms and have higher abundances than areas exposed to wave motion, due to the scarce movement of water (hydrodynamics).

So thatOstreopsis ovata it flourishes in our latitudes, it is also necessary that some conditions occur:

  • stable weather for several days, high atmospheric pressure
  • reduced wave motion, calm sea, standing water for protection due to natural or artificial reefs
  • relatively high water temperature, 25 ° C or more

During the blooms the water becomes cloudy and mucilaginous filaments or aggregates or brownish colored foams can be seen floating.

Ostreopsis ovata produces a series of toxins, called ovatoxins, and a very low percentage of palytoxin, all chemically similar to each other (often referred to as a group of palytoxin-like toxins).

The palytoxins produced in tropical regions by corals of the genus Palythoa (hence the name) have given rise to even fatal poisoning following the consumption of contaminated marine organisms, while in the Mediterranean such effects have never been reported. This is because, despite the chemical similarity, the data available so far, albeit limited, suggest that the ovatoxins are much less toxic than palytoxins.

There are also no known cases of human intoxication following the ingestion of water during bathing activities, but massive deaths of many species of aquatic organisms have been reported, either as a consequence of toxins or due to the conditions of absence of oxygen (anoxia) caused by the large mass formed by algae.

How can you be exposed to the toxin produced by O. ovata?

How can you be exposed to the toxin produced by O. ovata?

The main routes of exposure to ovatoxins are:

  • skin contact, through activities such as swimming or playing in the water
  • inhalation, breathing small droplets of aerosols formed by particles of algae carried by the air. This route of exposure can involve not only bathers proper but also those who stop or walk near the beach and / or on the seafront, as happened in the case of Genoa
  • ingestion, eating contaminated food or drinking water. In the latter case, even accidentally drinking water while swimming or playing in the water or falling from a boat. As regards ingestion, it is reiterated that although some fish or crustaceans, such as sardines, anchovies and crabs, an integral part of the Mediterranean diet, can accumulate the toxin in high concentrations, so far no effects associated with the " ingestion of fish products contaminated by it
Symptoms and health effects

Symptoms and health effects

To date, effects on human health have only been observed during very intense blooms of Ostreopsis ovata, in the presence of strong wind towards the shore and storm surges, probably due to the inhalation of the droplets carried by the wind (aerosol).

Swimmers or people stationed along the shoreline have experienced signs and complaints (symptoms) which include:

  • irritation of the upper airways
  • respiratory disorders
  • pharyngitis
  • cough
  • headache
  • nausea
  • cold
  • eye irritation and conjunctivitis
  • skin irritation or dermatitis
  • He retched
  • diarrhea
  • altered heart beat (arthymia)
  • weakness of the limbs
  • fever above 38 degrees

Typically, these disorders spontaneously regress after 24 to 72 hours without further complications, in less than 12 hours following a doctor's supervision and prescription of nonsteroidal anti-inflammatory drugs.

In the event that the problems worsen after leaving the beach, it is advisable to go to the emergency room.

Serious complications associated with poisoning by this alga have not yet been reported.

In marine-coastal environments, exposure to algae toxins can also occur by eating molluscs, fish and crustaceans that have accumulated them. However, poisoning due to these phenomena has never been reported in the Mediterranean.



The ascertainment (diagnosis) of disorders due to the toxins produced by Ostreopsis ovata it is mainly linked to the observation of their appearance in conjunction with the exposure to the algae blooms.

In the guidelines of the Istituto Superiore di Sanità (ISS, 2014) a definition of "case"for each of the three routes of exposure to palytoxin-like toxins, even if several routes of exposure are involved during bathing, and the disorders described could also occur simultaneously.

Very briefly, we have a "case"When there are at least two of the following disorders described for each route of exposure:

  • oral intoxication, general malaise, gastro-intestinal symptoms, muscle aches, muscle cramps and limb weakness, modest changes in heart function, breathing problems with potential subsequent blue discoloration of the mucous membranes (cyanosis)
  • skin exposure, dermatitis with the appearance of reliefs, bubbles and / or red spots on the skin (maculo-papular and / or erythematous) redness on the skin of the whole body even in the absence of contact, numbness and weakness in the limbs, dizziness, muscle aches, pain in the chest, breathing difficulties, fever, neurological disorders
  • inhalation exposure, cough, cold, runny nose (rhinorrhea), pain in the pharynx (pharyngodynia), difficulty in breathing (dyspnoea), fever (greater than or equal to 38 ° C), headache, watery eyes, nausea / vomiting, red skin ( rash)

A basic assumption is that the person has been exposed to the alga Ostreopsis ovata and / or toxins (must have frequented coastal areas in the previous 24 hours or carried out amateur or professional and / or recreational fishing activities in areas affected by O. ovata). If there are potential cases without reporting a flowering in the area, it is necessary to alert the territorial structures for verification. In the event that the intoxication is caused by the consumption of fish products, which may not be local, it is still necessary to alert the local health authorities for a check.



Generally the disturbances caused by exposure to Ostreopsis ovata spontaneously regress after 24-72 hours without further complications; drugs (always prescribed by the doctor) can alleviate any discomfort or speed up recovery.

Sometimes it is even enough for people to move a few tens of meters from the beach to eliminate or mitigate the disturbances. This recommendation is especially valid for people already suffering from other types of respiratory disorders, such as, for example, asthma.

If, on the other hand, the disturbances persist or worsen even after leaving the beach, it is advisable to go to the emergency room.



With the entry into force of the decree of 30 March 2010, in Italy it has become mandatory to carry out monitoring in areas at risk of the appearance of the alga Ostreopsis ovata and other potentially toxic algae.

The Regional Agencies for Environmental Protection (ARPA) must carry out surveys every year in the marine coasts where the presence of these algae has already been highlighted in order to keep the phenomenon under control and, if necessary, promptly notify the municipalities and the Health Authorities. Local (ASL) of competence.

The Ministry of Health, in May 2007, produced guidelines on the management of the risk associated with algae blooms Ostreopsis ovata in the Italian coasts. These guidelines were updated in 2014 by the ISS and in 2018 they were integrated into the bathing water legislation.

To mitigate the possible harmful effects linked to contact with toxic microalgae and to reduce the risk of unjustified closures of bathing areas, it is necessary to monitor the density of the algae and take various actions depending on the density of the flowering.

The control is carried out by visually inspecting the most suitable places for blooms, or those in which they have already occurred, and with the monitoring activity (collection and periodic analysis of water samples).

Three phases have been identified on the basis of which to establish the frequency of monitoring and the actions to be taken by the authorities in charge: routine phase, alert and emergency.

The definition of the phases is based on a combination of algae cell density, meteorological and marine conditions that favor the formation of blooms and the formation of aerosols formed by small particles of alga.

In the routine stage, monitoring is performed from June to September every 15 days and, if the cell density is less than 10,000-30,000 cells per liter, the health risks are negligible and there is no action to be taken. In the case of blooms or if there were reports of possible effects (phenomena of irritation of the skin and eyes, however in a limited number and reversible within a few hours), monitoring must be intensified and the authorities must initiate the necessary health prevention measures and activities foreseen in the alert phase.

During the alert phase the population must be informed about possible health risks and adequate information should also be provided to professional and amateur fishermen about the risks associated with skin contact (e.g. manipulation of nets and harvesting of shellfish, especially in the presence of wounds); and with the consumption of fish products collected in the area affected by flowering.

Recreational activities that give rise to aerosol formation in demarcated areas such as jet skis, water skiing, windsurfing, rowing, etc. may be discouraged.

Take the emergency phase if the density of algae continues to increase, if floating foams form on the surface and climatic conditions favor the formation of aerosols (strong winds from the sea and sea storms), if cases of health effects associated with "exposure to" have been reported and confirmedOstreopsis ovata (dermatitis, conjunctivitis, upper respiratory tract disorders).

During the emergency phase there is a risk of more serious effects deriving from the involuntary or accidental ingestion of water, therefore you must proceed to:

  • define the extension of the area affected by flowering
  • activate the health surveillance plan
  • intensify the frequency of observations and the monitoring of total and dissolved toxins in water
  • determine toxins in fish products
  • inform the health authorities, for appropriate management measures
  • inform the citizen, and prevent dangerous exposures with signs on the beach, bulletins, local information systems, brochures, ordinances and communication of these activities to the Ministry of Health and the Ministry of the Environment and Protection of the Territory and the Sea.

It is considered necessary by the Mayor to prohibit bathing and other recreational activities and it would also be advisable to quantify the toxins in the aerosol.

What to do in case of unreported flowering

If there is an unreported flowering, with filamentous or mucilaginous formations, it is necessary to:

  • avoid bathing or recreational activities in that area
  • do not stand on the beach, in case of strong wind from the sea that can carry aerosols containing algae or cell fragments, as long as the weather conditions do not change
  • do not eat shellfish, personally collected in the flowering area
  • report the event to the beach managers (if private) or to the area ARPA, so that all the precautionary measures described are implemented
  • get away from the water and the exposed area for a few hours, in case of irritation of the eyes or skin, so that the disturbances subside spontaneously. In case of persistence, it is advisable to go to the nearest emergency room
Palytoxins and aquariums

Palytoxins and aquariums

The palytoxins are a family of about 20 compounds, including palytoxin produced by tropical corals of the genus Palythoa (P. toxica And P. tuberculosa) from which it takes its name. This toxin was isolated in 1971 in Hawaii; ovatoxins are some of the variants produced by Ostreopsis ovata. Each toxin in the family has different toxicological characteristics: palytoxin is recognized as one of the most toxic marine substances, while currently available studies suggest that ovatoxins are much less toxic than palytoxin and other variants.

There palytoxin it is resistant to high temperatures, so it is not denatured by boiling or other types of cooking. In fact, cases of palytoxin poisoning, even fatal, have occurred in tropical and subtropical areas following the consumption of contaminated fish and crustaceans.

There palytoxin it acts by altering the functioning of a pump, located on the cell membrane, which controls the passage of sodium and potassium through the membrane itself, a fundamental function for the transmission of the nerve impulse.

The disorders that palytoxin causes initially affect the gastrointestinal system and include:

  • metallic taste in the mouth
  • nausea
  • vomiting and diarrhea

Subsequently causes:

  • burning / tingling in the mouth or throat
  • contractions of skeletal and heart muscle
  • cardiovascular problems
  • destruction of red blood cells(hemolysis)

In fatal cases, seizures and delirium have also occurred.

To reduce the risk of intoxication following the ingestion of contaminated organisms, the EFSA (the European Food Safety Authority) has established a maximum limit, protective for the health of consumers, of content for palytoxins in the pulp assuming the consumption of 400grams of product, (EFSA, 2009). It should be noted that for the ovatoxins produced by Ostreopsis ovata in the waters of the Mediterranean there are not yet sufficient data to establish these limits.

In addition to the waters of tropical areas, you can come into contact with the palytoxin even in our homes whenever there is a marine aquarium containing corals zoanthids, including the species Palythoa believed to produce and contain more toxin. In Europe and also in Italy there have been reports of health effects in professionals and hobbyists in contact with corals, especially when cleaning aquariums.

The most frequent cases concern the inhalation of the steam coming from the boiling of aquarium rocks in fresh water during the cleaning of the aquariums: the people involved accused cough, breathing difficulties and chest pains which, in some cases, led to hospitalization in hospital. In the Netherlands, an "entire family remained intoxicated: oxygen and anti-inflammatory drug therapy were required for prolonged periods before the definitive disappearance of the various ailments (from 1 to 3 months). Cases of mild intoxication caused by" were also reported. inhalation of the aerosol produced by the aerator, when the toxin produced by the coral was dissolved in water. In addition, effects were observed on internal organs, especially on the heart even in those who handled tropical corals with wounds and small lesions on the hands that facilitated the absorption of palytoxin and entry into the bloodstream. In all cases observed, the zoanthid coral samples present in the aquariums produced palytoxin.

If you want to keep aquariums with tropical organisms, therefore, it is good to know which species are present. In the case of corals that produce palytoxins (Palythoa toxica or Palythoa tuberculosa but also other zoanthids), it is necessary to clean the aquariums with a series of precautions:

  • always protect your hands with resistant gloves, possibly long, in order not to have direct contact with the toxins and avoid making small wounds, which would favor the absorption and entry into the bloodstream of the toxins themselves
  • wear protective masks and goggles
  • avoid using hot water, or worse, boiling the corals, so as not to produce toxic vapors that could be inhaled. Palytoxin degrades in an acidic or basic environment, so it can be neutralized by immersing the coral for 30 minutes in a solution of household bleach (sodium hypochlorite) greater than or equal to 0.1% (1 part of household bleach in 10 parts of water)

There is no antidote for palytoxin intoxication. Therefore, in case of problems it is essential to go to the doctor describing them precisely, pointing out the possibility of having come into contact with corals.



Funari E, Manganelli M, Testai E. Ostreopsis cf.ovata: guidelines for the management of blooms in coastal marine environments in relation to bathing and other recreational activities. Rome: Higher Institute of Health; 2014. (ISTISAN Reports 14/19)

Centers for Disease Control and Prevention (CDC). Harmful Algal Bloom (HAB) - Associated Illness (English)

EFSA Panel on Contaminants in the Food Chain (CONTAM). Scientific Opinion on marine biotoxins in shellfish - Palytoxin group.EFSA Journal. 2009; 7: 1393 

European Food Safety Authority (EFSA). Scientific Opinion of the Panel on Contaminants in the Food Chain on a request from the European Commission on Marine Biotoxins in Shellfish - Summary on regulated marine biotoxins. EFSA Journal. 2009; 1306: 1-23  

Intergovernmental Oceanographic Commission of UNESCO. Harmful Algal Bloom Program (English)

Higher Institute for Environmental Protection and Research (ISPRA). The Ostreopsis seaweed

In-depth link

In-depth link

Decree of 30 March 2010. Definition of the criteria for determining the prohibition of bathing, as well as methods and technical specifications for the implementation of Legislative Decree 30 May 2008, n. 116, transposing Directive 2006/7 / EC, relating to quality management of bathing water. (GU General Series n.119 of 24-05-2010 - Ordinary Suppl. n. 97)

Decree of the Ministry of Health of 19 April 2018. Amendment of the decree of 30 March 2010, containing: "Definition of the criteria for determining the ban on bathing, as well as methods and technical specifications for the implementation of Legislative Decree 30 May 2008, no. 116, transposing Directive 2006/7 / EC, relating to the management of bathing water quality (GU General Series n.196 of 08-24-2018)

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