According to the CDC, harmful algal blooms (HABs) that result from the rapid growth of algae or cyanobacteria (also referred to as blue-green algae) in natural waterbodies can harm people, animals, or the environment. HAB events of public health concern are primarily caused by microalgae (e.g., diatoms and dinoflagellates), cyanobacteria, and the toxins they can produce. HAB events, which can be intensified by factors such as nutrient pollution and warmer water temperature, can have public health, environmental, and economic impacts.

During 2021, the CDC reports that 368 HAB events were reported by 16 state jurisdictions: Arizona, California, Illinois, Kansas, Michigan, Minnesota, North Carolina, Ohio, Oregon, Pennsylvania, Tennessee, Utah, Virginia, Washington, Wisconsin, and Wyoming. These HAB events resulted in 117 human cases of illness and at least 2,715 animal cases of illness. Twenty-one groups of animals were reported, ranging in size from 2-2,000 individuals.

Reported HAB events occurred predominantly in summer months, peaking in August (92; 25%) and were most often classified as confirmed (311; 85%). Human cases of illness occurred primarily in June (38; 33%). Animal cases of illness occurred primarily in August (2,328; 86%). Case classification differed between human and animal cases of illness; the majority (104; 89%) of human cases of illness were classified as probable and 2,018 (74%) animal illnesses as confirmed.

Sixteen states voluntarily reported 368 harmful algal blooms (HABs) that resulted in a total of 117 human illnesses and at least 2,715 animal illnesses.
HAB events predominantly occurred in summer months, peaking in August (92; 25%), and most (90%) of the 368 HAB events occurred in freshwater bodies such as lakes and reservoirs.
Human illnesses occurred primarily in June (38; 33%), and the most commonly reported signs and symptoms were gastrointestinal, generalized (e.g., headache, fever), and dermatologic.
Animal illnesses occurred primarily in August (2,328; 86%) and mostly involved wildlife, including a mortality event that killed at least 2,000 bats.
The most commonly reported signs in animals were genitourinary (e.g., dark urine) in wildlife, followed by gastrointestinal (e.g., vomiting) and generalized (e.g., lethargy) in domestic pets.
Most HAB events were classified as confirmed (85%), human illnesses as probable (89%), and animal illnesses as confirmed (74%).

HABs are a One Health issue--they affect the health of people, animals, and our shared environment. One Health is a collaborative and multisectoral approach that involves engagement across disciplines including public health, animal health, and environmental health. Using a One Health approach, CDC collects data about HAB events and associated human or animal illnesses through the One Health Harmful Algal Bloom System (OHHABS) to inform public health prevention efforts.

In OHHABS, the term HAB event means either identification of a bloom or the detection of HAB toxins in water or food without a visual bloom. Human illnesses are reported individually. Animal illnesses are reported as single cases of illness or in groups, such as flocks of birds or schools of fish. The reporting system can link HAB event data with human or animal illness data. OHHABS uses standard definitions to classify HAB events as suspected or confirmed and human or animal illness as suspected, probable, or confirmed.

OHHABS is available for voluntary reporting by public health agencies and their designated environmental health or animal health partners in the United States, District of Columbia, Federated States of Micronesia, Guam, Marshall Islands, Northern Mariana Islands, Palau, Puerto Rico, and U.S. Virgin Islands. Public health agencies use standard forms to report HAB events, human cases of illness, and animal cases of illness to OHHABS. Public health agencies do not need to submit all three types of forms to participate.

Data collected for HAB events include general information (e.g., observation date), geographic information, water body characteristics (e.g., salinity), observational characteristics (e.g., water color, presence or absence of scum), and laboratory testing results. Data collected for cases of illness include general demographic characteristics, exposure information, signs and symptoms, medical care, and health outcomes. OHHABS is a dynamic electronic reporting system; data within individual reports are subject to change over time. Data included in this report are from a specific point in time.