Algal Bloom–Associated Disease Outbreaks Among Users of Freshwater Lakes

Editorial Note

Outbreaks associated with freshwater HABs previously were reported infrequently; only three were reported to WBDOSS for 1978–2008.^†† Freshwater cyanobacteria blooms are most likely to form on warm, stable bodies of water that are rich in nutrients.^[1] All outbreaks reported for 2009–2010 occurred in northern states during June–August. The World Health Organization (WHO) has proposed cell density guidelines for cyanobacteria bloom risk levels for recreational waters: 20,000 cells/mL is associated with risk for short-term adverse health outcomes; at 100,000 cells/mL, additional risk for long-term illness exists; and cyanobacterial scum formation in bathing areas is associated with the additional risk for "potentially severe health outcomes".^[2]

This report highlights the challenges of recognizing HAB-associated illness. During recreation in or alongside water with HABs, persons might experience multiple routes of exposure and multiple health effects. Reported exposure routes might reflect reported health effects rather than the true exposure route, given the difficulty of determining if exposure occurred via ingestion, inhalation, or contact. Previous reports of health effects associated with recreational water exposure to HABs include gastrointestinal, respiratory, eye, ear, and dermatologic effects; fever and neurologic effects are reported less frequently.^[3] The nonspecific nature of these effects might make it difficult for health-care providers to identify HAB-associated illness. Health-care providers should be aware that HAB-associated illness might present differently from other recreational water–associated illnesses and onset might occur soon after exposure. For example, four of the HAB-associated outbreaks were notable for associated neurologic symptoms or confusion/visual disturbance and HAB-associated illness might occur soon after exposure (Table 2). Among the 11 reported outbreaks, most persons reporting illness were aged ≤19 years. Children might be at higher risk for HAB-associated health effects because of more frequent exposure to and greater ingestion of recreational water.^{[4] §§}

Animal deaths associated with HABs are sentinel events signaling potential risk for human illness associated with exposure to recreational waters. In two outbreaks, dead fish were reported by beach attendees and there were possibly associated dog deaths at each site. Biologic samples from affected persons and animals can be analyzed for cyanotoxins to improve exposure assessment; however, these assays are performed only by research or specialty laboratories at this time.^[5–7]

It is important to maintain surveillance for HAB-associated illnesses to understand their public health impact. In the United States, cyanobacteria and their toxins are unregulated drinking water contaminants.^[8] No federal regulation of cyanobacteria nor cyanotoxins in drinking water or any recommended guideline values for recreational waters, no standardized methods for cyanotoxin detection in water or in biologic specimens, and no national monitoring programs for occurrence currently exist. However, many states rely on guidelines published by WHO;^[2] others have derived their own risk assessments and developed guidelines to support public health decision-making, such as posting advisories or closing bodies of water to any use.^[3,9] A summary of guidelines from different countries for exposure to cyanobacteria and their toxins was recently published.^[10]These guidelines and reports include action levels that might be applied by local, regional, state, or tribal entities to reduce exposure of humans and animals to cyanobacteria and their toxins. Although HABISS has been discontinued, NORS will continue to provide a mechanism for national reporting of HAB-associated outbreaks.

Testing water samples for cyanotoxins can contribute to the investigation of HAB-associated illness. Microcystins were detected during all eight outbreak investigations in which cyanotoxin testing was performed. Microcystin concentrations of ≥20 μg/mL exceeded the WHO guideline for moderate health risks in four outbreaks (Table 3).^[2] During investigations of these outbreaks, saxitoxin, cylindrospermopsin, and anatoxin-a also were detected. To date, there are few reports of documented human exposure to these cyanotoxins in recreational waters and there are no United States or international public health guidelines on their concentrations. Notably, neurologic effects were reported in 75% of outbreaks that included detection of known neurotoxins (anatoxin-a and saxitoxin).

The findings in this report are subject to at least three limitations. First, outbreak detection varies among localities. Second, reporting is voluntary. Finally, the reports described here likely represent underreporting of freshwater HAB-associated outbreaks.

This report represents a first attempt to summarize a group of freshwater HAB-associated recreational waterborne disease outbreaks. More resources are needed for improvements in risk characterization of cyanobacteria and cyanotoxins exposure, water monitoring for potentially toxic cyanobacteria, cyanotoxin analysis of water samples and biologic specimens, and case-finding for human illnesses associated with exposure to HABs in recreational waters. HAB-associated outbreaks will likely increase as warm eutrophic bodies of water become more common over time as predicted by development and climate projections. Better characterization of the occurrence of blooms, bloom-associated environmental conditions, and of human illness associated with exposure to algal blooms is needed to develop evidence-based prevention strategies (e.g., optimized control of nitrogen and phosphorus nutrient pollution). EPA, which regulates recreational water quality, and CDC can support local and state health jurisdictions to optimize national outbreak surveillance, and thus better inform prevention and control efforts by providing guidance on what epidemiological, clinical, and environmental data are needed to support detection and investigation.

^††One outbreak in New Hampshire (August 2001) in which Oscillatoria was suspected and isolated from a lake in a state park, and gastrointestinal symptoms were reported in 42 children; and two outbreaks in Nebraska (July 2004) involving microcystin from a lake, in which respiratory and dermatologic symptoms were reported in 22 persons. Additional information available at http://www.cdc.gov/healthywater/surveillance/rec-water-surveillance-reports.html.
^§§Data available at http://www.census.gov/compendia/statab/2012/tables/12s1249.pdf.

Morbidity and Mortality Weekly Report. 2014;63(1):11-15. © 2014  Centers for Disease Control and Prevention (CDC)