Toxic algae


Algae are a family of primitive, primarily aquatic, plants. Estimates of the number of species vary widely, ranging into the tens of thousands. Many species serve as an important source of food for marine life; however, about 40 produce toxic chemicals that may cause death to marine life and lead to human illness. Additionally, toxic algae produce blooms that make the ocean appear red under certain environmental conditions such as hurricanes. This is commonly referred to as red tide or harmful algal blooms (HABs).

Harmful algal blooms occur worldwide, although coastal waters are mainly affected. HABs are currently being studied by many organizations, including the National Oceanic and Atmospheric Administration (NOAA). The number of HABs has increased over the past 30 years due to increased amounts of certain kinds of pollution, which allow these species to thrive. Additionally, enhanced detection methods have made it easier to identify the toxins produced by these blooms that previously would have escaped discovery.

Episodes of red tide were first observed in Florida by Spanish explorers in the 16th Century. However, scientifically documented cases were first noted in the 1800s in Florida. Although red tides are perhaps the most famous of algal blooms, not all HABs discolor the water, nor are all discoloring blooms red.

Ocean-related human illnesses are usually caused by eating fish that have fed on toxic algae, or by breathing in certain toxins known as brevetoxins. Toxic algae cause five major types of seafood poisoning, including paralytic shellfish poisoning (PSP), neurotoxic shellfish poisoning (NSP), ciguatera fish poisoning (CFP), amnesic shellfish poisoning (ASP), and diarrheic shellfish poisoning (DSP). An additional form of poisoning caused by toxic algae, azaspiracid shellfish poisoning (AZP), was only recently discovered, with the first documented outbreak identified in 1995. However, there is limited available data on AZP. Diarrheic shellfish poisoning was first discovered in the 1960s, while ASP was first recognized in the 1980s. The other forms of poisoning have been known for centuries. These illnesses may all lead to serious illness and even death.

According to the U.S. Centers for Disease Control and Prevention (CDC), about 30 cases of marine toxin poisoning occur yearly in the United States. The true number is likely higher due to underreporting as there are no readily available blood tests to diagnose patients with seafood poisoning. Additionally, about one person dies every four years from toxic seafood poisoning.

Symptoms may resemble general food poisoning and include diarrhea, vomiting, and fatigue. Additionally, ciguatera fish poisoning is sometimes misdiagnosed as multiple sclerosis due to the presence of neurologic symptoms.

Algae-related poisonings have inflicted negative financial consequences on the fishing and tourism industries. For example, fisheries and shellfish beds often close during HABs, and seaside hotels and restaurants may suffer economic losses.


Toxic algae: Estimates of the number of species of algae vary widely, ranging into the tens of thousands. Algae play a vital role in aquatic ecosystems, providing food and oxygen for marine life. However, about 40 algae species produce toxic chemicals that may harm aquatic life and cause human illness. People eating fish will not be able to tell whether it is contaminated with toxins, although there are certain signs that fish may be contaminated if they are observed in the water (e.g., the fish are dead, discolored, or have open sores).

Red tide: Toxic algae produce blooms (an algal population explosion) that make the ocean appear red under certain environmental conditions. This phenomenon is commonly referred to as a red tide or a harmful algal bloom (HAB). Small marine organisms called dinoflagellates are mainly responsible for causing HABs and are found throughout the ocean, especially in and near coral reefs. Environmental experts prefer the term HABs when referring to such events, as not all blooms are red or necessarily produce any discoloration. Red tides commonly occur along Florida's coast and are caused by large amounts of the toxic algae Karenia brevis. Red tides may last for days, weeks, or months depending on wind, currents, and other environmental factors.

Studies demonstrate that a variety of environmental factors may trigger algae growth. Overfeeding may contribute to HABs when nutrients (e.g. phosphorus, nitrogen, and carbon) from lawns and farms flow into the ocean at rates that overfeed algae. Additionally, some HABs have been reported after unusual weather conditions, including high water temperatures, hurricanes, floods, and drought. Research is ongoing as to the causes of HABs, but scientists believe that favorable wind and water currents may contribute to the rapid growth of algae.

Harmful algal blooms may result in the widespread death of marine life, including fish, by reducing oxygen and blocking the sunlight needed for survival. Such events are also known as fish kills, and often cause dead fish to be scattered throughout the water, resulting in a foul odor. Fish may also appear discolored or have open sores.

Seafood poisoning diagnosis: Currently, there are no readily available blood tests to diagnose patients with seafood poisoning. Reporting regulations vary between states, and national reporting to the U.S. Centers for Disease Control and Prevention (CDC) is voluntary, making it difficult to keep track of the cases. Generally, symptoms may include neurologic side effects (e.g., tingling in fingers and toes) and stomach problems (e.g., diarrhea, nausea, and vomiting). Therefore, diagnosis is determined by the patient's symptoms and recent history of eating a specific type of seafood. If individuals have leftover shellfish available, then these may be tested for the presence of toxins. Currently, researchers from the Marine Biotoxins Program are developing a blood test for ciguatera fish poisoning (testing for the presence of ciguatoxin) to be used in hospitals.


General: Fish become contaminated with toxins by feeding on toxic algae. Humans who eat these contaminated fish may develop marine toxin diseases.

Red tide: Toxic algae, including microalgae (small single-celled organisms) and macroalgae (seaweeds), produce blooms that may make the ocean appear red under certain environmental conditions. This phenomenon is commonly referred to as a red tide or harmful algal bloom (HAB).

HABs occur throughout coastal regions of the United States. However, Florida's yearly red tides are the most well known.

Florida experiences HABs each year along its western shore. These blooms are composed of the toxic algae Karenia brevis, which produces brevetoxins. Brevetoxins kill fish and cause neurotoxic shellfish poisoning (NSP) in humans. Additionally, the cells of Karenia brevis may split and release toxins into the water. The wind may carry these toxins into the surrounding air, resulting in respiratory side effects. Typically, humans are exposed to these toxins on or near beaches during a Karenia brevis bloom. Blooms generally appear about 10-50 miles offshore along Florida's western continental shelf (an area extending from the western Panhandle to the Southwest Florida Keys). Many unanswered questions about these events are currently being researched, such as how these blooms are transported to shallow waters and the mechanism by which the HABs die off.

A study by Backer et al. in 2005 evaluated the effects of inhaled toxins in 28 lifeguards during Florida red tides. The study was performed over a span of four days; it measured lung function tests before and after the lifeguards' eight-hour shifts, during times when there were brevetoxins present and absent in the air. The study found that lifeguards exposed to the toxins experienced respiratory problems, including nasal congestion, eye and throat irritation, and cough. The most serious symptoms were wheezing and shortness of breath. No hospitalizations or deaths were reported.

Ciguatera fish poisoning: Donna Schroeder, a 65-year-old Texas woman, was misdiagnosed in July 2008 with general food poisoning when in fact she had ciguatera fish poisoning (CFP). MSNBC reported this case in February 2009. Schroeder reportedly diagnosed herself using the Internet after being sent home from the hospital. She first experienced symptoms, which included diarrhea, vomiting, and reversal of temperature sensation (cold objects feel hot and vice versa), within a few hours after her meal. Schroeder sued the restaurant and seafood supplier.

A 2006 U.S. Centers for Disease Control and Prevention (CDC) Morbidity and Mortality Weekly Report described four cases of CFP occurring in individuals who ate fish caught in the waters of South Carolina and Texas, which are areas not usually associated with the toxin responsible for CFP. Individuals experienced the following symptoms: diarrhea, stomach cramps, nausea, vomiting, low blood pressure, dizziness, itching, and reversal of hot and cold temperature sensation. All patients recovered from the poisoning. However, one individual still experienced tingling in her hands after 18 months.

There is no cure for CFP, and symptoms usually resolve without treatment. However, neurologic symptoms, such as reversal of temperature sensation (e.g., cold objects feel hot), may last for weeks, months, or years. There is mixed evidence supporting the use of the intravenous (IV) drug mannitol to help reduce neurological symptoms such as tingling and itching. A study by Palafox et al. evaluated IV mannitol in 24 patients with CFP. The study found that all symptoms were resolved in 17 of the 24 patients within 48 hours after the mannitol infusion. The author identified study limitations, such as the lack of comparison to another treatment, but concluded that IV mannitol should be considered as a treatment option in patients with CFP.

A study by Schnorf et al. evaluated IV mannitol vs. normal saline in 50 patients with CFP. The mannitol treatment showed no benefit compared to normal saline in patients after 24 hours. Additionally, more side effects, such as discomfort and burning pain along the vein used for infusion, occurred in the mannitol group.

Paralytic shellfish poisoning: The U.S. Food and Drug Administration (FDA) issued a health consumer warning in July 2008 against eating tomalley (the soft, green substance found in the body cavity of lobsters) from American lobsters due to the potential contamination with dangerous amounts of toxins that cause paralytic shellfish poisoning (PSP). The warning applied only to tomalley, as lobster meat is usually not affected by PSP toxins. The FDA announced this warning after regulatory authorities in Maine and New Hampshire conducted routine samplings of lobster tomalley. Officials determined that the PSP toxins were likely associated with an ongoing red tide episode in northern New England and eastern Canada.

Economic costs: According to the National Oceanic and Atmospheric Administration (NOAA), HABs incur yearly costs of about $50 million in the United States. The breakdown of these cost estimates include: public health ($20 million); commercial fisheries ($18 million); tourism ($7 million); and monitoring ($2 million).


General: There are five major types of poisonings that occur in humans due to algae toxin contamination of seafood; they include ciguatera fish poisoning (CFP), neurotoxic shellfish poisoning (NSP), diarrheic shellfish poisoning (DSP), amnesic shellfish poisoning (ASP), and paralytic shellfish poisoning (PSP). Azaspiracid shellfish poisoning (AZP) is an additional form of poisoning that has only recently been identified; there are limited data available on this illness. These diseases are believed to occur rarely, but the true number of cases may be underreported due to similarities with other food-borne illness. According to the U.S. Centers for Disease Control (CDC), about 30 cases of marine toxin poisoning are reported annually in the United States. About one person dies every four years from toxic seafood poisoning. The toxins that cause these illnesses are not affected by the heat from cooking and will not be destroyed by food preparation. Contaminated seafood will likely look, smell, and taste normal.

Ciguatera fish poisoning: Ciguatera is one of the most common types of seafood poisoning; it is caused by maitotoxin. Unlike the shellfish poisonings, CFP is caused by eating large, predatory tropical reef fish, especially barracuda, grouper, sea bass, snapper, and amberjack.

These fish are usually found in tropical areas such as Hawaii, Guam, the Virgin Islands, and Puerto Rico. In fact, barracuda sales and distribution have been banned in certain areas in the United States due to concerns of contamination.

Symptoms usually occur between 6-30 hours after eating the contaminated seafood. Common symptoms include diarrhea, vomiting, low blood pressure, irregular heart rate, weakness, itching, dizziness, nightmares, and hallucinations.

Additionally, patients may experience a unique symptom known as reversal of temperature sensation: cold objects will feel hot, and vice versa. Patients often recover within 1-4 weeks, with symptoms usually resolving on their own. However, neurologic symptoms, including temperature reversal, may last for weeks, months, or years. Evidence suggests that the intravenous drug mannitol may help to reduce neurological symptoms. CFP is generally not fatal.

Neurotoxic shellfish poisoning: Neurotoxic shellfish poisoning (NSP) is caused by brevetoxin, a poison that is released during harmful blooms of the algae Karenia brevis (HABs) which often occur along the Florida coast. Shellfish commonly affected include oysters, clams, and mussels found in western Florida and the Caribbean.

Individuals usually experience symptoms within 1-6 hours after eating contaminated shellfish. Symptoms include numbness, coughing, worsening of asthma, irritation of the eyes and nose, trouble breathing, tingling in the mouth, arms, and legs, difficulty walking, and diarrhea.

Additionally, patients may experience reversal of temperature sensation, as seen with CFP., Affected individuals generally recover within 2-3 days. Treatment involves supportive care, including fluid replacement; machines that provide breathing support are sometimes required. Reports of NSP-induced death are lacking.

Diarrheic shellfish poisoning: Diarrheic shellfish poisoning (DSP) is caused by the toxin okadaic acid, which most often contaminates scallops, mussels, and clams found in Japan, Europe, and the coast of Africa. No cases of DSP have been reported in the United States.

Individuals usually experience symptoms 30 minutes after eating contaminated shellfish. These symptoms may include diarrhea, nausea, vomiting, stomach cramps, and chills. Symptoms usually resolve within 3-4 days.

Amnesic shellfish poisoning: Amnesic shellfish poisoning (ASP) is caused by the toxin domoic acid, which most often contaminates mussels located in coastal areas of eastern Canada and the northeastern and western United States.

Poisoned individuals first experience stomach symptoms within 24 hours after eating contaminated shellfish. These symptoms may include vomiting, cramps, and diarrhea. Within 48 hours, patients may experience neurological symptoms such as confusion, headache, dizziness, seizures, and permanent short-term memory loss.

Paralytic shellfish poisoning: Paralytic shellfish poisoning (PSP) is considered a dangerous disease caused by the poisonous chemical saxitoxin. Shellfish that cause the disease include contaminated mussels, clams, scallops, oysters, crabs, and lobsters located in the colder coastal waters of the Pacific States, New England, and Central America.

Symptoms begin very quickly after eating contaminated shellfish; effects may be felt anywhere from 15 minutes to 10 hours following ingestion. Numbness and tingling usually begin with the mouth and spread to the face, neck, and arms. Patients may recover from PSP with proper medical support.

Headache, dizziness, nausea, diarrhea, and lack of muscular coordination may also occur with PSP. Individuals with severe poisoning may experience muscle paralysis. Patients with muscle paralysis usually need a machine to provide breathing support. Death may occur within 2-25 hours after the muscles used for breathing become paralyzed.

Azaspiracid shellfish poisoning: Azaspiracid shellfish poisoning (AZP) is a relatively new syndrome, with cases first identified in the Netherlands in 1995. The first reported U.S. cases of AZP occurred in Washington State in July 2008. Two individuals had eaten contaminated mussels that had been imported from Ireland. Reports of contaminated mussels in the United States are lacking. The toxin responsible for AZP is azaspiracid. The symptoms are similar to those of DSP and may include nausea, vomiting, diarrhea, and stomach cramps. Symptoms usually occur within three hours after eating contaminated shellfish., The illness generally lasts for 2-3 days, with no long-term effects reported to date.

Public health awareness: The public health department should be notified about cases of shellfish poisoning to identify the source of contamination and prevent further illness. Individuals are encouraged to freeze any uneaten shellfish for testing purposes. Consumers should look for harmful algal bloom (HAB) warnings, and contact the local health department before eating or collecting shellfish. Harmful algal bloom warnings may be issued by posting notices at affected beaches, or through newspaper, radio, or television announcements.

Monitoring: The U.S. Food and Drug Administration (FDA) is responsible for monitoring the safety of seafood. In 1997, the FDA established a seafood policy known as the Hazard Analysis Critical Control Point (HACCP) Regulation. The goal of this program is to enhance seafood safety. States test shellfish for toxin contamination with monitoring by the FDA. The FDA has also established agreements with some states to train health officials inspecting fish areas.

The Harmful Algal Bloom and Hypoxia Research and Control Act was passed on November 13, 1998 to establish enhanced monitoring, management, and research of HABs. The HAB Forecasting System, sponsored by the National Oceanic and Atmospheric Administration (NOAA), provides red tide and HAB warnings for the public. Detection methods include satellites, observation, and buoys.

The Sea-viewing Wide Field-of-view sensor (SeaWiFs) is an additional method used to detect HABs. This device uses a sensor from space to monitor the color of the ocean; it has provided advanced detection of HABs along the Florida coast.

These monitoring programs have been effective in decreasing the number of poisonings.


The Harmful Algal Research and Response: A National Environmental Science Strategy 2005-2015 (HARRNESS) is an ongoing national program dedicated to monitoring harmful algal blooms (HABs). HARRNESS includes enhanced detection methods for the prevention of HABs, for seafood monitoring, to improve reference materials for scientists and researchers, and for public education.

The Centers for Disease Control and Prevention (CDC) are examining the changes of lung function in individuals exposed to brevetoxins. Additionally, the CDC has recently established the Harmful Algal Bloom-related Illness Surveillance System (HABISS). This monitoring system uses a special program to collect data on the environmental effects of HABs in 13 states, including water sample information, toxin identification, and HAB geographic location. Future goals of HABISS include discovering links between weather conditions and HAB outbreaks, providing a web-based bulletin for HAB warnings, and expanding the program internationally.

Two researchers at the Massachusetts Institute of Technology were able to create the red tide toxin brevetoxin in 2007 through laboratory methods. This discovery should be a great help in future research on marine life and public health.


This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (

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