Calvert County Health Department

Blue-Green Algae: Microcystis aeruginosa

Microcystis aeruginosa is a phototropic bacterium.  It grows readily in nutrient-rich, slowly moving water. Because its occurrence as a greenish deposit on the surface of ponds or lakes resembles an algal bloom, it is commonly referred to as “blue-green algae”.  Such blue-green algal blooms have been reported for decades throughout the world and are not uncommon in Maryland waters.  Aside from being a nuisance and a danger to fish and wildlife, a bloom of Microcystis aeruginosa may be harmful to humans during recreational water use, either through body contact, inadvertent ingestion, or inhalation of water droplets.  Nevertheless, reported instances of health effects from recreational water use are few and no fatalities have been reported.

Body contact during water recreation may lead to minor skin irritations or allergic reactions of skin, including eye irritation and blistering of the lips.  It is believed that polysaccharides derived from the cell wall of the bacterium cause these external health effects.  On a more serious level, inadvertent ingestion or inhalation of Microcystis aeruginosa may, within several hours after exposure, lead to abdominal cramps, nausea, vomiting, diarrhea, fever, sore throat or hay fever-like symptoms, with recovery within 48 hours; the bacterial substance causing these symptoms has not been specifically identified.  It is noteworthy however, that very young children, splashing vigorously in the water and having a small body mass, may be more likely than adults to experience these internal reactions.

Ill-health effects caused by recreational water exposure to blooms of Microcystis aeruginosa are minor and self-limited.  Moreover, the risk of developing these adverse reactions increases as the intensity of the bloom increases.  It is also characteristic of these blooms to wax and wane, or even vanish, from day to day.  Given these facts, it is reasonable for public health authorities to post an advisory cautioning against swimming in water where a bloom is readily visible rather than outright closure of the beach for all activities.  However, it is important to keep in mind that very young children may be at higher risk than older persons.

Some strains of Microcystis aeruginos, produce a stable, water soluble toxin called microcystin.  Microcsytin is a large cyclic peptide that causes damage to the liver.  Pets and livestock are especially susceptible to the toxic effects of microcsytin.  For example, in Nebraska in May of 2004, three dogs died shortly after swimming and presumably drinking the water in a private, residential lake with an algal bloom; subsequent analysis of the lake water and tissue specimens indicated that microcystin caused those deaths.  Likewise, deaths in livestock have also been reported, particularly where droughts may induce animals to drink algae-contaminated water when fresh water is not readily available.

Human beings are likely to experience serious illness from drinking water containing large concentrations of microcystin.  In 1988 in Brazil, massive contamination of a drinking water reservoir with microcsytin caused over 2,000 cases of severe gastroenteritis with 88 deaths.  Fatalities have also been reported where microcsytin accidentally contaminated water used for kidney dialysis.  However, as noted above, there are no reports of fatalities or even serious illness resulting from recreational water activities related to Microycstis aeruginosa.  Nor is it clear what role, if any, microcsytin might play in causing the minor gastrointestinal or respiratory symptoms that sometimes occur following recreational water use where microcsytin is present in the water.