Summary Report – One Health Harmful Algal Bloom System (OHHABS), United States, 2020

Harmful algal blooms (HABs) that result from the rapid growth of algae or cyanobacteria (sometimes 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 called 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.

HABs are a One Health issue—they affect the health of people, animals, and our shared environment. One Health is a collaborative and multi-sectoral 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.

Within the context of OHHABS, the term HAB event describes the identification of a bloom or the detection of HAB toxins in water or food (i.e., absent 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. The reporting system can link HAB event data with human or animal illness data. OHHABS uses standard definitions [PDF – 3 pages] 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, scum), and laboratory testing. 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.

This summary describes data from OHHABS for events occurring between January 1, 2020 and December 31, 2020. Reports were received by January 3, 2022 and reviewed by CDC using standardized data quality checks. CDC downloaded the final dataset on March 21, 2022. CDC used SAS (version 9.4; SAS Institute) to conduct descriptive analyses to characterize HAB events and associated human or animal cases of illness.

Reports that included environmental testing laboratory results were considered ‘positive detections’ even if they did not include a specific concentration detected (e.g., if a toxin was reported with no concentration). These results were included in the summarized environmental testing data. Reported concentrations of 0 were considered non-detects and not included.

If group animal reports were missing the number of individuals affected, the number was set to two for inclusion in descriptive analyses. If group animal reports indicated deaths occurred but did not provide the number of individuals that died, the number of deaths was set to missing as this could not be extrapolated from available information.

Public health agencies classified HAB events as suspected or confirmed and cases of illness as suspected, probable, or confirmed (2018 OHHABS case and event definitions [PDF – 3 pages]). Animals within group animal reports were classified according to the overall classification (e.g., if a group of 10 animals was classified as “probable,” then each animal was subsequently defined as a probable case of illness).

Overview

During 2020, 227 HAB events were reported by 13 state jurisdictions: Alaska, California, Connecticut, Kansas, Michigan, Ohio, Oregon, Pennsylvania, Tennessee, Utah, Virginia, Washington, and Wisconsin (Figure 1). These HAB events resulted in 95 human cases of illness and at least 1,170 animal cases of illness. Twenty-one groups of animals were reported, ranging in size from 2-1,000 individuals (Figure 2).

Reported HAB events occurred predominantly in summer months, peaking in July (59; 26%) (Figure 3) and were most often classified as confirmed (79%) (Figure 3). Human cases of illness occurred primarily in July and August (73%). A single fish kill (i.e., localized die off of a fish population) in September included at least 1,000 carp. The remaining animal cases of illness (15%) occurred primarily in May and June (43%). Case classification differed between human and animal cases of illness; the majority (61; 64%) of human cases of illness were classified as suspected and 1,134 (97%) animal cases of illness as probable (Figure 4).

HAB Events

Most (173; 76%) of the 227 HAB events occurred in fresh water (Figure 5a), primarily in lakes and reservoirs (88%) (Figure 5b). Within the freshwater HAB event subset, observers most frequently noted green water color (23%), however, one or more observations of clear water were reported during 20% of these HAB events. (Figure 6). Scum or algal matter was observed in 116 of 120 (97%) HAB events that included information on scum presence or absence.

Environmental testing for algal toxins or species was conducted for 187 HAB events (82%), with the most frequently reported reasons being monitoring (72%) and citizen complaints (26%) (Figure 7). Overall, toxins were detected in 51% of all HAB events (115/227), 97% of which were freshwater events (Figure 8). Toxin testing primarily identified microcystins (107; 93%) (Figure 9a); multiple toxins were reported for 11 HAB events. Pseudo-nitzschia was the most identified genus during environmental testing, detected in 22 HAB events. Microcystis was the most commonly identified cyanobacterial genus, detected in 10 of 23 (43%) HAB events caused by cyanobacteria (Figure 9b). Cyanobacterial genera (Anabaena and Nodularia) were identified in marine coastal waters during one HAB event.

Human Illnesses

Of 95 human cases of illness reported to OHHABS for 2020, 41 (43%) illnesses occurred in adults aged 18-45 years and 24 (25%) illnesses occurred in people under age 18 (Figure 10). People who became ill were exposed to HABs predominately at public outdoor areas and parks (72%). National parks were associated with 22 (23%) human cases of illness, with 21 illnesses attributed to a single HAB event. Nearly all (95%) of the 95 people who became ill reported water and 9% reported air as a source of exposure (multiple sources were reported in 7% of illnesses). People sought care in 73 (77%) instances; over half (71%) called a poison control center, and 2% of illnesses resulted in an emergency department visit. One death associated with paralytic shellfish poisoning was reported following a non-commercial shellfish harvest (Table 1).

Numerous signs and symptoms were reported; the most commonly reported types were dermatologic (42%), gastrointestinal (41%), and generalized (36%) (Figure 11). Overall, rash (23%) and nausea (20%) were most commonly reported signs and symptoms (Table 2). Median time to illness onset was 3.75 hours (0.02-96; n=73) and median illness duration was 24 hours (0.17-504; n=47); 64% of ill people still exhibited signs or symptoms at the time of interview (Table 1).

Animal Illnesses

At least 1,170 animal cases of illness occurred and were reported to OHHABS for 2020, including a large fish kill involving at least 1,000 carp. Most cases were classified as probable (97%), regardless of animal category (Figure 12). Among 37 domestic pet cases and 1,133 wildlife cases, dogs (100%) and fish (90%), respectively, were the most common animals affected (Figure 13a). No livestock cases were reported. Veterinary care was provided to 89 animals, including 66 marine mammals, 19 dogs, and 4 birds (Figure 13b).

Data describing signs of illness were available for 37 animals, all of which were dogs. Generalized (76%) and gastrointestinal (68%) signs were most frequently reported overall (Figure 14). Individually, vomiting (46%) was most commonly reported (Table 3). Median time to illness onset was 3.5 hours (0.02-72) for the 19 animals with available information, and median duration of illness was 6 hours (0.02-168) for 12 animals with available information. In total, 1,094 (94%) of the animals died.

Data reported in OHHABS for 2020 are not representative of HAB event or illness occurrence in the United States because reporting is voluntary and not all states are currently reporting to this system. States may not report information for all HAB events or illnesses due to variability in surveillance capacity, surveillance program scope, or limitations to the environmental or health data available for HAB events. Environmental testing might have been limited by availability of testing for one or more toxins. Additionally, the total number of individual animals described in this report underrepresents the total affected or that died during these events because some group animal reports did not provide these numbers or indicated that the number was an underestimate. Summarized data therefore underrepresent the total number of HAB events and illnesses that occurred within or across states. Similarly, relative contributions of HAB events in salt water, toxins identified in fresh water, and types of illnesses (e.g., domestic pets versus wildlife species), may also be affected. The effects of the COVID-19 pandemic on surveillance of HAB events or illnesses (e.g., exposure to HABs or healthcare seeking behavior) are not known.

Thirteen states reported to OHHABS for 2020. The findings from this annual data summary indicate that HAB events and associated illnesses occur throughout the United States. HAB event and illness reporting primarily characterized environmental data, exposures, and outcomes associated with freshwater cyanobacterial blooms. Affected people primarily reported dermatologic and gastrointestinal signs and symptoms, while affected animals—specifically dogs—presented with generalized and gastrointestinal signs. The first report of a HAB-associated death to OHHABS demonstrates the health risks of consuming shellfish contaminated with algal toxins, along with potential for OHHABS to complement state/territorial surveillance (Newell, 2022) by aggregating case data about foodborne illnesses nationally. Data from environmental sampling, human illnesses, and animal illnesses highlight the diversity of HAB species, toxins, and health effects associated with HAB events. Human health, animal health, and environmental health are interconnected; the continued use of a One Health approach to surveillance will inform illness prevention by increasing scientific understanding and data available to characterize HAB events and associated illnesses.

Newell KG. Paralytic Shellfish Poisoning Update ― Alaska, 1993–2021. State of Alaska. Epidemiology Bulletin. No. 5, April 21, 2022. Available at: http://www.epi.alaska.gov/bulletins/docs/b2022_05.pdf [PDF – 1 page]

CDC would like to thank state and local waterborne disease coordinators, epidemiologists, environmental health practitioners, laboratorians, toxicologists, and animal health practitioners, as well as other partners who were involved in investigating or reporting these data to OHHABS.

Centers for Disease Control and Prevention (CDC).  Summary Report – One Health Harmful Algal Bloom System (OHHABS), United States, 2020. Atlanta, Georgia: U.S. Department of Health and Human Services, CDC, 2022.

Marissa Vigar, Muhammad Thuneibat, Megan E. Gerdes, Amy Jacobi, Virginia A. Roberts