The lake looked green and murky, with foam along the shoreline and a faint smell of something unpleasant. A family swam in it anyway. A dog that drank from it died within hours. This scenario has played out with devastating regularity across American summer recreation sites in recent years — and the record heat of summer 2026, which is already driving emergency room visits from heat-related illness in Phoenix, Houston, and Chicago, is simultaneously creating ideal conditions for the worst harmful algal bloom season in years.
Harmful algal blooms (HABs) are rapid overgrowths of algae — primarily cyanobacteria, commonly called blue-green algae — in lakes, ponds, rivers, reservoirs, and coastal waters. They are called "harmful" because many cyanobacteria species produce toxins — collectively called cyanotoxins — that can cause serious illness in humans, pets, and wildlife. They are not always visually alarming: some blooms are invisible. But many produce the characteristic blue-green, pea-soup, or neon green discoloration of the water, surface scums or mats, and foaming along shorelines that should prompt immediate avoidance.
Cyanobacteria thrive when water is warm, sunlight is abundant, and nutrients (particularly phosphorus and nitrogen from agricultural runoff, lawn fertilizers, and septic systems) are elevated. These are precisely the conditions that exist in thousands of American lakes, ponds, and slow-moving rivers during summer 2026 — and that are being intensified by record air temperatures that warm surface waters faster and to higher temperatures than historical averages. The relationship between global temperature rise and HAB expansion is documented in multiple peer-reviewed studies, with researchers projecting continued increases in HAB frequency, duration, and geographic range as temperatures continue to rise.
What Cyanotoxins Do to the Human and Animal Body
The health effects of cyanotoxin exposure vary by the specific toxin and the route and duration of exposure. The most common cyanotoxins are microcystins, which primarily target the liver and can cause acute liver injury, nausea, vomiting, diarrhea, and abdominal pain after ingestion of contaminated water. Chronic low-level microcystin exposure has been linked to increased liver cancer risk in several epidemiological studies.
Anatoxins target the nervous system, blocking neuromuscular transmission and potentially causing muscle paralysis, respiratory failure, and death — particularly in dogs and wildlife, which may drink large amounts of water from contaminated sources without the human capacity to recognize danger. Dogs that swim in or drink from HAB-affected water are at extreme risk: anatoxin poisoning can cause death within minutes to hours of exposure, and cases occur every summer across the United States.
Cylindrospermopsins cause multi-organ damage, including liver, kidney, and lung injury. Saxitoxins — typically associated with paralytic shellfish poisoning in marine environments — have been documented in freshwater cyanobacteria in some U.S. states.
Skin contact with cyanobacteria or toxin-containing water can cause contact dermatitis, rashes, blistering, and eye and nose irritation. Inhalation of aerosols from HAB-affected water during activities like jet skiing, waterskiing, or swimming can cause respiratory irritation, coughing, and asthma exacerbation. Children are particularly vulnerable because they swallow more water per body weight during swimming, they play in contaminated water for longer periods, and their developing organ systems are more susceptible to toxin-induced damage.
What Families and Pet Owners Must Do
The practical guidance from CDC's HABABI surveillance system and state health departments converges on a simple rule: if in doubt, stay out. Do not swim in, wade through, or allow pets to drink from water that appears green, blue-green, pea-soup colored, or has surface scums, mats, or foam along the shoreline. This rule applies even if the water is at a familiar swimming hole where no previous issues have occurred — HAB conditions can develop rapidly within 24 to 48 hours in the right weather conditions.
Check your state health department's website before visiting a lake or swimming area this summer. Most states with active HAB programs post current advisories by water body. Many also maintain hotlines for reporting suspected HABs.
If a dog swims in or drinks water from a potentially HAB-affected area, rinse the dog thoroughly with clean water immediately and monitor closely for drooling, staggering, convulsions, difficulty breathing, or sudden weakness. Any of these signs requires immediate emergency veterinary care with explicit disclosure of potential cyanotoxin exposure — there is no antidote, but supportive care must begin immediately.
Frequently Asked Questions
Q: What are harmful algal blooms, and are they dangerous?
A: Harmful algal blooms are rapid overgrowths of cyanobacteria (blue-green algae) in water bodies. Many cyanobacteria produce toxins that can cause liver damage, neurological injury, skin reactions, and death — especially in dogs. They are triggered by warm temperatures and high nutrient levels.
Q: How can I tell if a lake or pond has a harmful algal bloom?
A: Discolored water (green, blue-green, pea-soup, or brownish), surface scums or mats, foam along shorelines, and unusual odors are common warning signs. However, some blooms are not visible, and only testing can confirm cyanotoxin presence.
Q: Why are dogs at such high risk from harmful algal blooms?
A: Dogs often drink directly from water bodies and may ingest large amounts of water relative to their body weight. Anatoxins — the neurotoxic cyanotoxins most commonly associated with rapid animal deaths — can kill dogs within minutes to hours of exposure.
Q: What should I do if I or my child swam in water that looked like it had a bloom?
A: Rinse off immediately with clean water. Watch for symptoms including skin rash, eye irritation, respiratory irritation, nausea, vomiting, diarrhea, fever, headache, or muscle weakness. Seek medical care if symptoms develop and disclose the potential bloom exposure.
Q: Where can I find advisories for specific lakes in my state?
A: Check your state health department website. Most states with HAB monitoring programs post current advisories by water body name. The EPA also maintains a national HAB resource page. Look for closures before swimming at any natural freshwater site this summer.
