Freshwater HAB-related Illness Tracking in California

August 4, 2025

What are freshwater harmful algal bloom (HAB)-related illnesses?

When cyanobacteria (also known as blue-green algae) and algae in freshwater and estuarine waterbodies occur at levels that pose a risk to humans, animals, and the environment, they are referred to as freshwater harmful algal blooms (FHABs). Humans and animals can become sick after ingesting or contacting cyanobacteria, water contaminated with cyanotoxins, or algal mats. Cyanotoxins may also accumulate in fish and shellfish. Signs and symptoms can vary widely, may occur within minutes or days following HAB exposure, and may include:

irritation of skin, ears, eyes, nose, or throat
abnormal breathing (coughing, wheezing, asthma-like symptoms)
vomiting, diarrhea, abdominal pain
headaches, agitation, weakness
seizures and death (more common in animals).

How can I report an illness that may be related to freshwater HABs?

Submitting a HAB-related incident report, including the illness information section
Emailing CyanoHAB.Reports@waterboards.ca.gov, or
Calling (844) 729-6466 (toll free)

How can I protect myself, my family, and my pets from freshwater HAB-related illness?

Check if a waterbody has a reported bloom by checking the HAB Reports Map, contacting the waterbody manager, and looking for posted advisory signs.
Check to see if the water is discolored or has a scum or algal mats.
Practice healthy water habits at your local lake, river, or stream.

Who is tracking freshwater HAB-related illnesses in California?

Potential freshwater HAB-related illnesses in humans and animals throughout California are tracked and investigated by the Interagency HAB-related Illness Workgroup, consisting of staff from:

Office of Environmental Health Hazard Assessment (OEHHA)
State Water Resources Control Board (SWRCB)
California Department of Public Health (CDPH)
California Department of Fish and Wildlife (CDFW).

All potentially HAB-related illness reports submitted to the workgroup are evaluated based on the available environmental and health-related information. California implements the Voluntary Guidance for Response to HABs in Recreational Inland Waters for freshwater and estuarine HABs and may include sample analysis for the most commonly occurring cyanotoxins (anatoxins, cylindrospermopsin, microcystins, and saxitoxin), when appropriate.

The number of illnesses considered HAB-related are included in the final summaries (as noted below). Much of the information presented on this web page is also available as a one-page freshwater HAB illness tracking factsheet. This workgroup also investigates and tracks marine HAB-related illnesses in California. For more information on marine HABs and how to report a potentially related human or animal illness, see OEHHA’s webpage on marine HAB-related illnesses.

How many freshwater HAB-related illnesses have been reported for California?

From 2018 to 2024, the Interagency HAB-related Illness Workgroup investigated all illness reports received and determined that 200 met the Centers for Disease Control and Prevention (CDC) case definition criteria for a HAB-associated illness. All freshwater HAB-related human and animal illnesses for California were reported to the CDC One Health Harmful Algal Bloom System (OHHABS). A summary of HAB-related illnesses reported by year is provided in the figure below (Figure 1). No livestock incidents have been reported to date.

Graph shows freshwater HAB-related illnesses by year (2018-2024) and by category (human, fish, wildlife, and dog).

Figure 1. Freshwater illnesses reported as HAB-related to OHHABS from 2018 – 2024. Number represents individuals, except when submitted as a group, such as for fish or wildlife illness events.

In which counties did these reported freshwater HAB-related illnesses occur?

The maps below (Figure 2) display the counties in which freshwater HAB-related illnesses for California (reported to OHHABS) have occurred for previously reported years combined (2018-2023) and for 2024. Illnesses are grouped by human-only, human and animal (domestic or wild), or animal only. A more detailed table with cumulative HAB-related illnesses in humans and animals, by county, is also available.

Map of California counties where HAB-related illness incidents were reported to CDC One Health Harmful Algal Bloom System (OHHABs)in 2024.

Figure 2. Maps of freshwater HAB-related illnesses (reported to OHHABS) by county for 2018-2023 combined (left) and 2024 (right).

During what times of year have freshwater HAB-related illnesses been reported?

HAB-related illnesses have been reported during most months of the year, with higher numbers throughout the summer months (Figure 3). These seasonal trends may represent a combination of an increased likelihood of HAB events and higher levels of recreational activities in or near water bodies during the summer. Practicing healthy water habits is recommended year-round since some water bodies have reported HABs during all times of the year.

Graph shows freshwater HAB-related illness totals from 2018-2024 by month and category (human, domestic pet, and fish and wildlife).

Figure 3. Reported human and animal HAB-related illnesses by exposure month (2018-2024). Number represents individuals, except when submitted as a group, such as for fish or wildlife illness events.

How are HAB-related illnesses being tracked across the United States?

The CDC’s OHHABS is a voluntary reporting system available to state and territorial public health departments and their designated environmental and animal health partners. It collects data on individual human and animal illness cases from HAB-associated exposures, as well as environmental data about HABs. OHHABS is an example of One Health surveillance. One Health is an approach that recognizes that human, animal, and environmental health are interconnected, and that human health, animal health, and environmental health communities can more effectively address many linked health challenges by working together.