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Pfiesteria in Seafood and Your Health - Free Information from EHSO

Pfiesteria in Your Seafood and Your Health


What is Pfiesteria ?

Pfiesteria piscicida (fee-STEER-ee-uh pis-kuh-SEED-uh) is a toxic dinoflagellate that has been associated with fish lesions and fish kills in coastal waters from Delaware to North Carolina. A natural part of the marine environment, dinoflagellates are microscopic, free-swimming, single-celled organisms, usually classified as a type of alga. The vast majority of dinoflagellates are not toxic. Although many dinoflagellates are plant-like and obtain energy by photosynthesis, others, including Pfiesteria, are more animal-like and acquire some or all of their energy by eating other organisms.

Until recently, 'Red Tide' was perhaps the most notorious outcome due to a marine toxin. These phenomena are associated with outbreaks of the dinoflagellate Ptychodiscus brevis. Throughout the world there are two-thousand known species of dinoflagellates and about twenty of these have been demonstrated to produce toxins. These marine toxins cause a variety of diseases in humans as well as mammals, fish and birds. The toxins are known to bioconcentrate in the marine food web, and, accordingly, shellfish and fish are frequently the source of human exposure. These marine toxins are predominately neurotoxins, but other effects have been identified. The toxins are, in general, tasteless, odorless and heat stable, making them difficult to easily identify or detoxify. Most recently another dinoflagellate, Pfiesteria, has been identified as the organism responsible for massive fish kills in North Carolina and Maryland and has raised public health concerns.

Discovered in 1988 by researchers at North Carolina State University, Pfiesteria piscicida is now known to have a highly complex life-cycle with 24 reported forms, a few of which can produce toxins. Three typical forms are shown on the right. A few other toxic dinoflagellate species with characteristics similar to Pfiesteria have been identified but not yet named. These are referred to as "Pfiesteria-like organisms," and they occur from Delaware to the Gulf of Mexico.

How does Pfiesteria affect fish ?

Pfiesteria normally exists in non-toxic forms, feeding on algae and bacteria in the water and in sediments of tidal rivers and estuaries. Scientists believe that Pfiesteria only becomes toxic in the presence of fish, particularly schooling fish like Atlantic menhaden, triggered by their secretions or excrement in the water. At that point, Pfiesteria cells shift forms and begin emitting a powerful toxin that stuns the fish, making them lethargic. Other toxins are believed to break down fish skin tissue, opening bleeding sores or lesions. The toxins or subsequent lesions are frequently fatal to the fish. Fish may also die without developing lesions. As fish are incapacitated, the Pfiesteria cells feed on their tissues and blood. Pfiesteria is NOT an infectious agent like some bacteria, viruses, and fungi. Thus, fish are NOT killed by an infection of Pfiesteria, but rather by the toxins it releases, or by secondary infections that attack the fish once the toxins have caused lesions to develop.

Is Pfiesteria the only cause of
fish lesions and fish kills ?

A lesion is any sore, wound, or area of diseased tissue. There are many possible causes for fish lesions other than Pfiesteria and Pfiesteria-like organisms. These include physical injury in nets or traps, bites by other fish or birds, chemical pollutants, generally poor water quality, and infectious disease agents such as certain viruses, bacteria, and fungi. A fish kill is a situation in which many fish -- more than a few dozen -- die over a short period of time -- hours or days. Pfiesteria and Pfiesteria-like organisms are only one cause of fish kills on the southeast and Gulf coasts. Other causes include a lack of dissolved oxygen in the water, sudden changes in factors such as salinity or temperature, sewage or chemical spills, blooms of other kinds of harmful or toxic algae, infectious disease agents, and other environmental changes.

How long do toxic Pfiesteria outbreaks last ?

Toxic outbreaks of Pfiesteria are typically very short, no more than a few hours. After such an event, Pfiesteria cells change back into non-toxic forms very quickly, and the Pfiesteria toxins in the water break down within a few hours. However, once fish are weakened by the toxins, Pfiesteria-related fish lesions or fish kills may persist for days or possibly weeks.

Where has Pfiesteria been found ?

Pfiesteria piscicida is known to occur in brackish coastal waters from the Delaware Bay to North Carolina. Other Pfiesteria-like organisms occur along the southeast coast from Delaware to the Gulf of Mexico. These organisms are believed to be native, not introduced species, and are probably common inhabitants of estuarine waters within their range. These microbes have not been found in freshwater lakes, streams, or other inland waters.

Pfiesteria piscicida has been implicated as a cause of major fish kills at many sites along the North Carolina coast, particularly the New River and the Albemarle-Pamlico estuarine system, which includes the Neuse and Tar-Pamlico Rivers. Millions of fish have died from Pfiesteria in North Carolina. In 1997, Pfiesteria or Pfiesteria-like organisms killed thousands of fish in several Eastern Shore tributaries of the Chesapeake Bay, including the Chicamacomico and Manokin Rivers and King's Creek in Maryland, and the lower Pocomoke River in Maryland and Virginia. Pfiesteria piscicida is the probable cause for a 1987 fish kill in Delaware's Indian River. Fish kills in coastal aquaculture operations in Maryland and North Carolina have also been linked to Pfiesteria and Pfiesteria-like organisms. Lesioned fish found in association with Pfiesteria or Pfiesteria-like organisms have been documented in several Maryland and Virginia tributaries of the Chesapeake Bay, in many coastal areas of North Carolina, and in the St. John's River in Florida.

Can Pfiesteria cause human health problems ?

Pfiesteria is not a virus, fungus, or bacterium. It is not contagious or infectious, and cannot be "caught" like a cold or flu. There is no evidence that Pfiesteria-related illnesses are associated with the consumption of finfish, shellfish, or crustaceans such as crabs, lobsters, and shrimp. Any human health problems associated with the microbe stem from its release of toxins into river and estuarine waters.

Preliminary evidence suggests that exposure to waters where toxic forms of Pfiesteria are active may cause memory loss, confusion, and a variety of other symptoms including respiratory, skin, and gastro-intestinal problems. It has been shown that similar human health effects can be caused by exposure to Pfiesteria toxins in a laboratory setting. To date, other Pfiesteria-like organisms have not been shown to cause human illness.

The Centers for Disease Control and Prevention, in cooperation with state health departments in Delaware, Florida, Georgia, Maryland, North Carolina, South Carolina, and Virginia, have established a surveillance system to collect reports of human illness thought to be related to exposure to Pfiesteria and Pfiesteria-like organisms in estuarine waters. This and other ongoing research efforts are expected to further delineate the nature, extent, and duration of any Pfiesteria-related human health effects.

Is it safe to eat seafood ?

YES. In general, it IS safe to eat seafood.

  • There has never been a case of illness from eating fish or shellfish exposed to Pfiesteria.
  • There is no evidence of Pfiesteria-contaminated fish or shellfish on the market.
  • All seafood products and processing facilities are required by law to have programs to ensure the safety of the fish and shellfish they sell. Seafood from restaurants, supermarkets, and other retailers is considered safe.
  • There is no evidence that illnesses related to Pfiesteria are associated with eating fish or shellfish.

To be on the safe side, the following common-sense precautions are recommended:

  • Comply with state closures of water bodies and public health advisories. Do not harvest or consume fish or shellfish from areas that are closed by the state.
  • Do not handle or consume fish that you have harvested that are dead or dying; that exhibit sores, peeling, lesions, or other signs of disease; or that were acting abnormally when caught. If you notice significant numbers of fish that are dead or that exhibit lesions or other signs of disease, avoid contact with the fish and water, and promptly report the incident to your state's environment or natural resource agency.

Is it safe to swim and boat in coastal waters ?

Swimming, boating, and other recreational activities in coastal waters are generally safe. To be on the safe side, the following common-sense precautions are recommended:

  • Comply with state closures of water bodies and public health advisories. Do not go into or near the water in areas that are closed by the state.
  • If you notice significant numbers of fish that are dead or that exhibit lesions or other signs of disease, avoid contact with the fish and water, and promptly report the incident to your state's environment or natural resource agency.

Is Pfiesteria a "harmful algal bloom" ?
How is it related to red and brown tides ?

Most species of algae are not harmful. Algae are the energy producers at the base of the ocean's food web, upon which all other marine organisms depend. However, a few species of algae and other microbes can become harmful to marine life and to people under certain conditions. Scientists call such events "harmful algal blooms." Brown tides, toxic Pfiesteria outbreaks, and some kinds of red tides are all considered types of harmful algal blooms. Some harmful algal blooms, like toxic Pfiesteria outbreaks, can cause detrimental effects when the microbes are at low concentrations in the water and cannot be visibly detected. In other cases, like certain red and brown tides, harmful effects occur when the algae reach high concentrations that discolor the water. However, not all algal blooms that discolor the water are harmful -- many red tides appear to have no negative effects on marine life, people, or the environment.

Some kinds of algal blooms, like toxic Pfiesteria outbreaks and some kinds of red tides, are harmful because the algae produce one or more toxins that poison fish or shellfish, and can pose human health risks when people come in contact with affected waters. These toxic algal blooms may also kill seabirds and other animals indirectly as the toxins are passed up the food chain. Certain kinds of these toxic algal blooms can cause human health problems via contaminated seafood, like Ciguatera Fish Poisoning, Amnesic Shellfish Poisoning, and Paralytic Shellfish Poisoning. However, there is no evidence that Pfiesteria-related illnesses are associated with eating fish or shellfish.

Most algal blooms are not toxic, but they are still considered harmful if they reduce the amount of light or oxygen in the water, consequently killing sea grasses, fish or other marine life. Blooms of macroalgae -- seaweed -- can also be harmful if they damage underwater habitats such as coral reefs or sea grass beds.

What causes toxic Pfiesteria outbreaks ?

The exact conditions that cause toxic outbreaks of Pfiesteria to develop are not fully understood. Scientists generally agree that a high density of fish must be present to trigger the shift of Pfiesteria cells into toxic forms. However, other factors may contribute to toxic Pfiesteria outbreaks by promoting the growth of Pfiesteria populations in coastal waters. These factors include warm, brackish, poorly flushed waters and high levels of nutrients.

Nutrients such as nitrogen and phosphorus are thought to encourage the growth of Pfiesteria populations by stimulating the growth of algae that Pfiesteria feeds on when in its non-toxic forms. Some evidence suggests that nutrients may also directly stimulate the growth of Pfiesteria, but more research is needed to show this conclusively. At this time, the precise role that nutrients and other factors may play in promoting toxic outbreaks of Pfiesteria is not clear, and is an area of active research.

Excess nutrients are common pollutants in coastal waters. Chief sources of nutrient pollution in coastal areas are sewage treatment plants, septic tanks, polluted runoff from suburban landscape practices and agricultural operations, and air pollutants that settle on the land and water.

What is being done about Pfiesteria ?

State and federal agencies are working closely with local governments and academic institutions to address the problems posed by Pfiesteria. Federal agencies involved in the effort include the U.S. Environmental Protection Agency, the National Oceanic and Atmospheric Administration, the Centers for Disease Control and Prevention, the National Institute of Environmental Health Sciences, the Food and Drug Administration, the U.S. Geological Survey, and the U.S. Department of Agriculture. Together with state departments of health and natural resources, these agencies are working to:

  • manage the risk of human health effects by monitoring and rapid response through river closures and public health advisories
  • direct funding and technical expertise to Pfiesteria-related research and monitoring
  • make current and accurate information widely available to the public
  • understand and address the causes of Pfiesteria outbreaks, especially the possible role of excess nutrients

Whom should I contact to report fish lesions, fish kills, or possible human exposure to Pfiesteria ?

A few fish with lesions or even a few dead fish are not cause for alarm. However, if you notice a significant number of fish -- more than a few dozen -- that are dead, dying, behaving abnormally, exhibiting lesions, or showing other signs of disease, please contact your state's department of environment or natural resources. If you experience health problems after being exposed to fish, water, or air at the site of a fish kill or suspected toxic Pfiesteria outbreak, contact your physician and your state or local public health agency at once. Several states have set up Pfiesteria hotlines, listed below.

Where can I get more information about Pfiesteria ?

State Pfiesteria, Fish Kill, or Related Health Effects Hotlines:

North Carolina1-888-823-6915

On the Internet:


State: Academic Institutions:

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