Pfiestria piscicida: A Case Study

By: Dr Natalie Carroll, Purdue University

Confirmed fish kills in North Carolina were observed in 1994 in the Pamlico and Neuse Estuaries. Fish losses were reported to be in the millions. Reports of dead fish found floating in North Carolina tidal waters caused a great deal of concern. In the summer of 1997, thousands of fish were killed in the Pocomoke River on Maryland's Eastern Shore. Consequently, the public was banned from a five-mile stretch of the scenic waterway. Local watermen had begun reporting gaping red sores on fish almost a year previously but the fish kills and public ban brought this issue to national attention.

Research has shown that Pfiesteria piscicida, a single-cell microorganism, is responsible for the fish kills in North Carolina, and is suspected to be the cause of fish kills in Maryland as well. Pfiesteria piscicida has a complex life cycle that may include 24 flagellated, amoeboid, and encysted stages or forms. Pfiesteria may live for years in tiny, cyst-like shells buried in river bottom sediment, then hatch when conditions are right. Conditions supporting and/or encouraging Pfiesteria are a combination of warm water temperatures (70F); increased levels of phosphorous, ammonium, and suspended solids; moderate to low salinity levels; and increased rainfall or runoff. When large numbers of fish swim into an area where Pfiesteria are present their excreta triggers encysted cells to emerge and become toxic. Other stages of Pfiesteria can also become toxic in the presence of fish excreta (amoeboid and flagellated cells). The small cells swim toward the fish prey and give off potent toxins which make the fish lethargic and often cause bleeding sores and hemorrhaging. Once fish are incapacitated, Pfiesteria feeds on the sloughed epidermal tissue, blood, and other substances that leak from the sores. When the fish are dead, flagellataed stages transform to amoeboid stages and feed on the fish remains or, if conditions become unfavorable for the Pfiesteria, the Pfiesteria cells make protective outer coverings and sink to the bottom of the river as dormant cyst stages. All of these changes can take place in a matter of hours.

Pfiesteria outbreaks in North Carolina were shown to occur in waters that were heavily nutrient enriched. Possible sources of nutrients flowing into the water include sewage treatment plants, fertilizer runoff, chicken and hog manure, phosphate mines, and municipal wastewater treatment plants where effluents are rich in phosphorus and nitrogen.

The primary contributor to the problem in North Carolina, however, seems to be the state's large confinement hog-farming operations. After the outbreak in Maryland, a leading environmental group called for reforms in the handling of manure from the Eastern Shore's millions of chickens. Chicken waste is often applied to fields as fertilizer. Rain washes the nitrogen and other nutrients in the manure into the surrounding waters. The Pocomoke River, at its headwaters, drains the largest chicken-producing county in the nation. Maryland's Delmarva Peninsula houses some 625 million chickens. Governor Glendening of Maryland has announced that farmers may soon be subject to regulations on animal waste disposal.

Maryland's top farm official has been quoted as saying that poultry farmers have been responsible in their handling of chicken waste. A spokesman for the poultry industry rejects the suggestion that chicken manure is responsible for the Pfiesteria outbreak, saying bird waste is well-managed. Farming advocates also note that if regulatory measures target only one possible source, the regulations might unfairly cause producers to go out of business. Farmers work on small profit margins under current management practices.


1. Do you think that news reports, researchers, and politicians unfairly blame farmers without looking at other possible causes?

2. If large poultry or hog operations are shown to be the primary cause of the Pfiesteria outbreaks, should the producers be required to get rid of these animals? or reduce them to a certain number that will produce less waste?

3. The chicken industry spokesman said that "bird waste is well-managed", implying that there is no cause for concern. Do you believe this statement? Can you think of assurances from another industry that were later proven false? How much of the public perception is formed by what we read and see and how much is formed by previous experiences?

4. Should large, corporate, farms be allowed only in less populated states? Does a land owner have the right to use their land as they wish? Where should we set the boundaries between personal rights and the rights of society?

Pfiesteria piscicida: Part 2

Researchers in North Carolina have not identified a definite link between Pfiesteria and human illness in the natural environment. However lab workers and researchers in North Carolina have suffered from cramps, difficult breathing, and loss of short-term memory after being exposed to high concentrations of the microbe in a poorly ventilated lab. In 1997, eight Maryland waterman who had direct contact with the Pocomoke fish complained of health problems and skin lesions.

When the fish died this summer hundreds of gulls cleaned up the dead fish from the lower Pocomoke river. Scientists are trying to determine if Pfiesteria could cause illness if it is passed through the food chain by birds or other animals feeding on infected fish.

The nutrient loading in North Carolina and Maryland is a result of non-point source pollution. Non-point source pollution is any pollution that is general in nature, as distinguished from "point source" pollution from factories and waste treatment plants. Although non-point sources include lawn herbicides and fertilizers and oil residues from city streets the heaviest non-point contributor to water pollution is agricultural runoff. Non-point pollution is generally unregulated due to the difficulty of identifying the source. Over the past 25 years the E.P.A. has invested an average of $2.5 billion a year in grants and loans for sewage treatment plants (point sources) around the country, with excellent results. But, by contrast, non-point source pollution control is voluntary and largely ineffective. E.P.A. regulations apply to some agricultural situations such as waste lagoons. When these lagoons overflow during rainstorms or develop cracks in the concrete liners, toxic residues may leak out. These residues regarded as point sources and fines may be levied. When hog slurry and chicken waste are applied to fields or sold as fertilizer they become non-point sources of pollution.

The Clean Water Act was intended to assure citizens a safe, clean, water resource, but due to problems in defining and solving non-point source pollutants very few regulations are actually imposed on farmers and producers, the primary contributors to non-point pollution. A recommendation to strengthen the Clean Water Act would enable it to identify key watersheds, set clear targets for reductions of nitrogen loads, and offer incentives to farmers to adopt sound management practices. These changes could require farmers to use less fertilizer and to take land out of production in order to create buffer zones (filter strips) near streams and lakes.


1. Do you support stricter controls of non-point pollutants?

2. Do you value wildlife and other natural resources for their intrinsic value, or only as indicators of the health of an ecosystem and, therefore, of the ecosystem's instrumental value to humans?

3. Should the government strengthen the Clean Water Act as suggested?


NCSU Aquatic Botany Laboratory, Pfiesteria piscicida Homepage:

Jack Cooper editorial, New York Times, Sept. 22, 1997.

The Washington Post, Todd Shields, staff writer, Friday, August 8, 1997, "Fish Kill Prompts Closing Part of the Pocomoke River."

Updated: July 23, 1998

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