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1

Facts about Cyanobacteria (Blue-green Algae)

and Cyanobacterial Harmful Algal Blooms 

(CyanoHABs)

Cyanobacteria (blue-green algae)

Cyanobacteria are bacteria that grow in water and are photosynthetic (use sunlight to create 

food and support life). Cyanobacteria live in terrestrial, fresh, brackish, or marine water. They 

usually are too small to be seen, but sometimes can form visible colonies. Cyanobacteria have 

been found among the oldest fossils on earth and are one of the largest groups of bacteria. 

Cyanobacteria have been linked to human and animal illnesses around the world, including 

North and South America, Africa, Australia, Europe, Scandinavia, and China.

Cyanobacterial blooms and how they form

Cyanobacterial blooms occur when algae that are normally present grow exuberantly. Within 

a few days, a bloom can cause clear water to become cloudy. The blooms usually fl oat to the 

surface and can be many inches thick, especially near the shoreline. Cyanobacterial blooms can 

form in warm, slow-moving waters that are rich in nutrients such as fertilizer runoff or septic 

tank overfl ows. Blooms can occur at any time, but most often occur in late summer or early fall.

They can occur in marine, estuarine, and fresh waters, but the blooms of greatest concern are the 

ones that occur in fresh water, such as drinking water reservoirs or recreational waters.

What a cyanobacterial bloom looks like

Some cyanobacterial blooms can look like foam, scum, or mats on the surface of fresh water 

lakes and ponds. The blooms can be blue, bright green, brown, or red and may look like paint 

fl oating on the water. Some blooms may not affect the appearance of the water. As algae in a 

cyanobacterial bloom die, the water may smell bad.
2

Cyanobacterial harmful algal blooms 

(CyanoHABs) 

CyanoHABs are algae blooms that threaten 

people, animals, or the environment. They are 

dangerous for many reasons: 

• Dense CyanoHABs can block sunlight and 

use up all the oxygen in the water, killing 

other plants and animals.

• Some cyanobacteria that can form 

CyanoHABs produce toxins that are among 

the most powerful natural poisons known. 

These toxins have no known antidotes.

• CyanoHABs can make people, their pets, 

and other animals sick. Often, the fi rst sign 

that an HAB exists is a sick dog that has 

been swimming in an algae-fi lled pond.

• Children are at higher risk than adults for 

illness from CyanoHABs because they 

weigh less and can get a relatively larger 

dose of toxin.

Other effects of fresh-water 

CyanoHABs

• CyanoHABs can make drinking water smell 

and taste bad.

• They can make recreational areas unpleasant.

Species of cyanobacteria that 

form CyanoHABs in fresh water

• Microcystis aeruginosa

• Anabaena circinalis

• Anabaena fl os-aquae

• Aphanizomenon fl os-aquae

• Cylindrospermopsis raciborskii

Cyanotoxins

Cyanotoxins are a diverse group of chemical 

substances that are categorized by their 

specifi c toxic effects as follows:

• Neurotoxins affect the nervous system.

 ° Anatoxin-a

 ° Anatoxin-a(s)

 ° Saxitoxin

 ° Neosaxitoxin

• Hepatotoxins affect the liver.

 ° Microcystins

 ° Nodularins

 ° Cylindrospermopsin

• Tumor promoters are chemicals that can 

increase tumor growth.

 ° Microcystins

• Lipopolysaccharides are chemicals that can 

affect the gastrointestinal system.

See the table below for a list of cyanotoxins 

and their specifi c toxic mechanisms, their 

effects, the symptoms they cause, and 

treatments for poisoning.

How you could be exposed to 

CyanoHABs and cyanotoxins

• Drinking water that comes from a lake or 

reservoir with a CyanoHAB.

• Drinking untreated water.

• Engaging in recreational activities in waters 

with CyanoHABs.

• Inhaling aerosols from water-related 

activities such as jet-skiing or boating.

• Inhaling aerosols when watering lawns, 

irrigating golf-courses, etc. with pond water.

• Using cyanobacteria-based dietary 

supplements that are contaminated with 

microcystins.

• Receiving dialysis (this has been 

documented only in Brazil).
3

Types of illnesses people and animals 

can get from exposure to CyanoHABs

• Getting it on the skin may give people a 

rash, hives, or skin blisters (especially on 

the lips and under swimsuits).

• Inhaling water droplets from irrigation 

or water-related recreational activities can 

cause runny eyes and nose, a sore throat, 

asthma-like symptoms, or allergic reactions.

• Swallowing water that has cyanobacterial 

toxins in it can cause

 ° Acute, severe gastroenteritis (including 

diarrhea and vomiting).

 ° Liver toxicity (i.e., increased serum 

levels of liver enzymes). Symptoms 

of liver poisoning may takes hours or 

days to show up in people or animals. 

Symptoms include abdominal pain, 

diarrhea, and vomiting.

 ° Kidney toxicity. 

 ° Neurotoxicity. These symptoms can 

appear within 15 to 20 minutes after 

exposure. In dogs, the neurotoxins can 

cause salivation and other neurologic 

symptoms, including weakness, 

staggering, diffi culty breathing, 

convulsions, and death. People may 

have numb lips, tingling fi ngers and 

toes, or they may feel dizzy.

Testing for cyanobacterial toxins

• Most of the toxins require specialized 

testing that can take weeks.

• Some kits are available to test for 

microcystins on site.

How to protect yourself, your family, 

and your pets from exposure to 

CyanoHABs

• Donʼt swim, water ski, or boat in areas where 

the water is discolored or where you see 

foam, scum, or mats of algae on the water.

• If you do swim in water that might have a 

CyanoHAB, rinse off with fresh water as 

soon as possible.

• Donʼt let pets or livestock swim in or drink 

from areas where the water is discolored or 

where you see foam, scum, or mats of algae 

on the water.

• If pets (especially dogs) swim in scummy 

water, rinse them off immediately—do not let 

them lick the algae (and toxins) off their fur.

• Donʼt irrigate lawns or golf courses with 

pond water that looks scummy or smells bad.

• Report any “musty” smell or taste in your 

drinking water to your local water utility.

• Respect any water-body closures announced 

by local public health authorities.

How to treat people or animals 

that have been exposed to 

cyanobacterial toxins

• Get medical treatment right away if you 

think you, your pet, or your livestock might 

have been poisoned by cyanobacterial toxins.

• Remove people from exposure and give 

them supportive treatment.
4

How to help reduce the occurrence 

of CyanoHABs

• Reduce nutrient loading of local ponds and 

lakes by using only the recommended amounts 

of fertilizers and pesticides on your yard.

• Properly maintain your household septic 

system.

• Maintain a buffer of natural vegetation 

around ponds and lakes to fi lter incoming water.

How to get more information about 

cyanobacteria:

Federal

Centers for Disease Control and 

Prevention (CDC)

Harmful Algal Blooms (HABs) site

http://www.cdc.gov/habs

This site defi nes HABs; describes CDCʼs 

HABs-related activities; and provides links to 

data, publications, and other HABs resources.

Cyanobacteria site 

http://www.cdc.gov/hab/cyanobacteria/

This site defi nes cyanobacteria; describes 

CDCʼs cyanobacteria-related activities; and 

provides links to data, publications, and other 

cyanobacteria resources.

Environmental Protection Agency (EPA)

Drinking Water Contaminant Candidate 

List Site

http://www.epa.gov/safewater/ccl/cclfs.html

This site provides information about EPAʼs 

list of contaminants that are not regulated, 

occur in public water systems, and may 

require regulation under the Safe Drinking 

Water Act. Algae that can be harmful are on 

this list.

International

State of Queensland Australia

HAB site

http://www.nrm.qld.gov.au/water/blue_green/

index.html

This site describes the stateʼs plans and 

procedures for agency responding to HABs.

World Health Organization

Water Site

http://www.who.int/topics/water/en/

This site provides links to drinking and 

recreational water quality, including the 

impacts of cyanobacteria and cyanobacterial 

toxins.

States

North Carolina Department of Health and 

Human Services

Occupational and Environmental 

Epidemiology program, HABs Site

http://www.epi.state.nc.us/epi/hab/

This site gives an overview of North 

Carolinaʼs HAB program and provides links 

to the stateʼs HAB-related surveillance, 

research, and education activities.
5

Cyanobacterial toxins, effects, 

signs and symptoms of poisoning, and therapy

Toxin Acute Effect Signs and Symptoms Therapy

Anatoxin-a Neurotoxicity Humans: not documented.

Animals: progression of muscle fasciculations, 

decreased movement, abdominal breathing, 

cyanosis, convulsions, death.

Birds: opisthotonos (“s”-shaped neck).

Supportive care.

Respiratory support 

may allow time for 

detoxifi cation and 

respiratory recovery.

Anatoxin-a (s) Neurotoxicity Humans: not documented.

Pigs: hypersalivation, mucoid nasal 

discharge, tremors, fasciculations, ataxia, 

diarrhea, recumbency.

Ducks: regurgitation, paresis, opisthotonos, 

clonic seizures.

Mice: lacrimation, hypersalivation, urination, 

defecation, death from respiratory arrest.

Rats: red-pigmented ears.

Supportive care.

Cylindrospermopsin Hepatotoxicity, renal 

toxicity, chromosome 

breakage, aneuploidy

Humans: enlarged liver, malaise, anorexia, 

vomiting, headache.

Mice: huddling, anorexia, slight diarrhea, 

gasping respiration.

Supportive care.

Microcystins Hepatotoxicity Humans: elevated gamma-glutamyl transpeptidase.

Humans, mice: elevated alanine aminotransferase. 

Rats: embryo lethality, teratogenicity.

Mammals: weakness, reluctance to move, 

anorexia, pallor of extremities and mucous 

membranes, mental derangement, survivors 

may be photosensitized. 

Humans: powdered 

charcoal, supportive 

care.

Animals: 

cholestyramine.

Nodularin Hepatotoxicity Humans: Skin and eye irritation from 

skin contact.

Experimental systems: inhibition of protein 

phosphatases, tumor-promoter.

Supportive care.

Saxitoxin, 

neosaxitoxin

Neurotoxicity Humans: paresthesia and numbness of lips 

and mouth within 1⁄2 to 3 hours after exposure, 

extending to face, neck, extremities; motor 

weakness; incoordin-ation; respiratory and 

muscular paralysis.

Animals: incoordination, death by 

respiratory failure.

Activated charcoal, 

artifi cial respiration.

Supportive care.
6

References:

Carmichael WW, Beasley V, Bunner DL, Eloff JN, Falconer IR, Gorham PR, et al. Naming of 

cyclic heptapeptide toxins of cyanobacteria (blue-green algae). Toxicon. 1988;26:971-3.

Carmichael, WW, Falconer IR. Diseases related to freshwater blue-green algal toxins, and 

control measures. P. 187-209. In: Falconer I R (ed.). Algal Toxins in Seafood and Drinking 

Water. London: Academic Press. 1993: pp187-209.

Dahlem AM, Hassan AS, Swanson SP, Carmichael WW, Beasley VR. A model system for 

studying the bioavailability of intestinally administered microcystin-LR, a hepatotoxic peptide 

from the cyanobacterium Microcystis aeruginosa. Pharmacol. Toxicol. 1989;64:177-81.

Falconer, IR, Beresford AM, Runnegar MT. 1983. Evidence of liver damage by toxin from a 

bloom of blue-green algae, Microcystis aeruginosa. Med J Aust. 1983;1: 511-4.

Hawkins PR, Chandrasena NR, Jones GJ, Humpage AR, Falconer IR. Isolation and toxicity of 

Cylindrospermopsis raciborskii from an ornamental lake. Toxicon. 1997;35:341-346.

Humpage A R, Hardy SJ, Moore EJ, Froscio SM, Falconer IR. Microcystins (cyanobacterial 

toxins) in drinking water enhance the growth of aberrant crypt foci in the mouse colon. J Toxicol 

Environ Health, Part A. 2000;61:155-65.

Kirpenko YA, Sirenko LA, KirpenkoNI. Some aspects concerning remote after-effects of 

blue-green algal toxin impact on warm-blooded animals. In: Carmichael WW (ed.). The Water 

Environment, Algal Toxins and Health. New York: Plenum, 1981: pp257-69.

Soong F S, Maynard E, Kirke K, Luke C. Illness associated with blue-green algae. Med J Aust. 

1992;156:67.

Turner PC, Gammie AJ, Hollinrake K, Codd GA (1990). Pneumonia associated with contact with 

cyanobacteria. Br Med J. 1990;300:1440-1.