Harmful Algal Blooms

Overview

A harmful algal bloom (often shortened to HAB) is when tiny water plants called algae, or bacteria that act like algae (cyanobacteria, sometimes called blue-green algae), grow out of control in a lake, river, pond, or the sea. Some of these blooms make poisons called toxins (harmful substances) that can make people and animals sick. The water may look like spilled green paint, pea soup, or a foamy scum, and it may smell bad — but sometimes the water looks normal even when toxins are present. Harmful algal blooms are not the same as the harmless seaweed or algae you might see on a beach.

Harmful algal blooms are becoming more common worldwide, partly because of warmer weather and nutrient pollution (extra nutrients from fertilizers and waste washing into water). Not every bloom makes toxins, and most people who come into contact with one do not become seriously ill. Still, blooms are frequent enough in many coastal and lake areas that public health warnings are routine in some regions.

Anyone can be affected, but some people are more at risk: young children, because they tend to swallow more water while playing; people who swim, boat, or fish regularly; pets and livestock, which may drink affected water; people with asthma or other lung problems who breathe in droplets near a bloom; and people with liver or kidney conditions. Shellfish eat the algae and can store toxins, so anyone eating shellfish from a bloom area can be affected.

Symptoms

Common symptoms

• Skin rash, itching, or irritation after contact with the water
• Red, sore, or watery eyes
• Irritated nose and throat, sneezing, or a cough, especially if you breathed in spray or droplets
• Stomach upset, nausea, vomiting, or diarrhea after swallowing water
• Headache or feeling generally unwell
• Flu-like feelings such as tiredness or mild fever

Symptoms in children

• More stomach upset and vomiting, because children swallow more water while swimming
• Skin rash in areas that stayed wet, such as under swimsuits
• Fussiness, poor feeding, or unusual sleepiness — young children may not be able to describe how they feel
• Cough or wheezing after playing near spray or shoreline scum

Symptoms in older adults

• More noticeable weakness, dizziness, or dehydration after vomiting or diarrhea
• Breathing problems more likely if there is existing lung or heart disease
• Confusion or unusual drowsiness should always be checked promptly
• Symptoms may last a little longer and recover more slowly

Causes

Main causes

• Overgrowth of certain algae or cyanobacteria (blue-green algae) that produce toxins
• Extra nutrients in the water, especially from farm fertilizers, septic systems, and stormwater runoff
• Warm water temperatures and long sunny days, which help algae grow quickly
• Slow-moving or still water, such as ponds, lakes, and calm bays, where growth builds up

Risk factors

• Swimming, wading, or water-skiing in lakes, ponds, or rivers during a bloom warning
• Children playing in shallow water or near shoreline scum
• Breathing in spray from boats, jet skis, or hoses near affected water
• Eating shellfish or fish caught in an area with a bloom alert, or drinking untreated surface water
• Having asthma or other lung conditions, or a weakened immune system
• Pets drinking or licking affected water off their fur

When to see a doctor

See a doctor urgently if:

• Vomiting, diarrhea, or stomach pain that is severe or not settling
• A skin rash that is spreading, painful, or blistering
• Cough, wheezing, or chest tightness after being near affected water
• Symptoms after eating shellfish from a bloom area
• You are pregnant, have a long-term illness, or are caring for a young child or older adult with any symptoms

Book a routine appointment if:

• Mild skin irritation or eye redness that clears with washing and a few days of rest
• Mild stomach upset that is improving
• You want advice about whether it is safe to swim, fish, or eat fish from a local waterway

Diagnosis

There is no simple test that says 'you have a harmful algal bloom illness'. A doctor usually works it out from your story — where you swam, fished, or ate — plus your symptoms and a check-up. They will also rule out other common causes of stomach upset, rash, or breathing problems. Local health or environment agencies test the water and issue alerts, so checking your area's beach or waterway advice can help confirm whether a bloom was present.

Tests that may be done

• A conversation about recent contact with lakes, rivers, ponds, sea water, or shellfish
• A physical check, including skin, eyes, breathing, and hydration
• Sometimes blood or urine tests if you are very unwell, to check salts, liver, and kidney function
• Stool tests if diarrhea is a main symptom, to rule out infection
• Water testing or bloom reports from your local environment or health agency

What to expect at your appointment

For most people the visit is short and simple. Your doctor will ask about the water you were in and when symptoms started, examine you, and usually give self-care advice rather than tests. If you are more unwell, or in a higher-risk group, they may arrange blood tests and ask you to come back if things do not improve. Bringing the name of the beach, lake, or shellfish supplier with you is genuinely helpful.

Treatment

There is no single medicine that reverses these toxins. Care focuses on easing symptoms, replacing lost fluids, and watching for anything serious. Most people recover fully at home within a few days with simple self-care. People who are very unwell may need to be seen in hospital for fluids, oxygen, or close monitoring.

Self-care at home

• Rinse your skin, hair, and eyes with clean, fresh water as soon as you can after contact
• Take off wet swimwear and wash it, and shower with clean water
• Drink plenty of fluids — water or an oral rehydration drink — if you have vomiting or diarrhea
• Rest and avoid going back into the affected water until local warnings are lifted
• Use gentle, plain moisturizer on irritated skin and avoid scratching
• Keep pets away from the water, and rinse their fur with clean water if they went in
• Note down when symptoms started and what water you were in, in case you need to see a doctor

Medical treatments

A doctor will assess how unwell you are and treat the symptoms. This may include fluids given by mouth or, if needed, through a drip to correct dehydration; oxygen or breathing support for people with breathing problems; medicine to settle nausea or stomach cramps; and creams or cooling treatments for skin irritation. In more serious cases — for example with seizures, severe breathing difficulty, or confusion — hospital care with close monitoring is needed. Your care team will also check for other causes, because many illnesses look similar. Always tell your doctor about any recent contact with water or shellfish.

When is surgery considered?

Surgery is not used to treat illnesses caused by harmful algal blooms.

Living with this condition

Recovery is usually quick, and most people can go back to normal activities within a few days. In the meantime, avoid the affected waterway and any shellfish from it until your local health agency lifts the warning. If you live or work near water that blooms often, it helps to know where the official advisories are posted and to make checking them part of your routine before swimming, boating, or fishing.

Lifestyle tips

• Check local beach, lake, and shellfish warnings before water activities
• Avoid water that looks like spilled paint, pea soup, or has a thick scum or foam — and keep children and pets away too
• Shower with clean water after swimming outdoors, and wash hands before eating
• Never drink untreated water from lakes, rivers, or ponds, and do not use it for cooking or brushing teeth
• Follow local advice about eating fish and shellfish from affected areas
• Support cleaner waterways where you can — reducing fertilizer runoff and reporting blooms to local authorities helps everyone

Diet and exercise

Drink plenty of fluids while you recover, and eat light, simple meals if your stomach is unsettled. There is no special diet needed. Rest until you feel better, then ease back into normal activity. Once the local warning is lifted, swimming and other water sports are fine to enjoy again — being active outdoors is good for your health, and checking advisories first keeps it safe.

Mental health and emotional wellbeing

Being unwell after a swim or a family day out can be unsettling, and if a child or pet was affected it can be worrying. Symptoms are usually mild and short-lived, and that is worth holding on to. If anxiety about water safety, or stress from being ill, is affecting your sleep or mood, talk to your doctor. If you ever feel overwhelmed or hopeless, please reach out for crisis support in your country straight away — you do not have to cope alone.

Prevention

You cannot control when a bloom happens, but you can greatly lower your risk. The key steps are checking local water warnings before you swim or fish, staying out of water that looks or smells unusual, and never drinking untreated surface water. Remember that boiling does not remove these toxins, and home filters are not a reliable way to make affected water safe. Pets should be kept away from scummy water and rinsed with clean water if they go in.

Vaccines

There is no vaccine for harmful algal blooms or the illnesses they cause.

Screening programmes

There is no screening test for people. Screening happens at the water itself — health and environment agencies regularly test popular beaches, lakes, and shellfisheries and issue alerts when toxins are found.

Complications

If left untreated

• Ongoing vomiting or diarrhea leading to dehydration, especially in young children and older adults
• Breathing problems for people with asthma or lung conditions
• In rare cases, serious effects on the liver, kidneys, or nervous system after heavy or repeated exposure
• Shellfish poisoning, which can cause numbness, tingling, or paralysis and needs emergency care
• Illness in pets, which can be severe and fast — contact a vet straight away if a pet drinks affected water

Long-term outlook

The outlook is good for most people. Typical symptoms are mild and clear up within a few days with rest and fluids, and there are no lasting effects. Serious illness is uncommon and is usually linked to large exposures or to eating contaminated shellfish. Knowing the local warnings, rinsing off after contact, and getting medical help early if symptoms are severe all make a full recovery very likely.

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