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Illness Associated with Red Tide --- Nassau County, Florida, 2007

A "red tide" is a harmful algal bloom that occurs when toxic, microscopic algae in seawater proliferate to a higher-than-normal concentration (i.e., bloom), often discoloring the water red, brown, green, or yellow. Red tides can kill fish, birds, and marine mammals and cause illness in humans (1). Florida red tide is caused by the dinoflagellate Karenia brevis, which produces toxins called brevetoxins and is most commonly found in the Gulf of Mexico; however, K. brevis blooms also can occur along the Atlantic coast. On September 25, 2007, a cluster of respiratory illnesses was reported to the Nassau County Health Department (NCHD) in northeastern Florida. All of the ill persons were employed at a beach restoration worksite by a dredging company operating at Fernandina Beach; they reported symptoms of eye or respiratory irritation (e.g., coughing, sneezing, sniffling, and throat irritation). NCHD and the Florida Department of Health promptly conducted epidemiologic and environmental investigations and determined the illnesses likely were associated with exposure to a red tide along the Atlantic coast. These actions highlight the importance of rapid investigation of health concerns with potential environmental causes to enable timely notification of the public and prevent further illness.

Epidemiologic Investigation

The dredging company had been contracted by the U.S. Army Corps of Engineers to clear a channel for military submarines to navigate the Amelia River. During September 25--29, as part of this operation, the company was dredging material off the ocean floor from a ship located 3 miles offshore, near the mouth of the river. The dredged material was pumped through a pipe from the ship to the beach worksite. Approximately 50 dredging company workers were stationed aboard the ship and 13 at the beach worksite, where they redistributed the piped mix of sediment on the beach. All of the dredging company employees worked 12-hour shifts. Ship workers spent a greater portion of their shifts working indoors than did beach workers and had varying levels of exposure to outdoor elements.

On September 25, after receiving the initial reports of respiratory illness among the dredging company workers, NCHD staff members suspected the cause might be exposure to a chemical toxin. However, when staff members visited the Fernandina Beach worksite on the same day, they observed dead fish and detected the characteristic odor of brevetoxin, the toxin produced naturally by K. brevis. During September 25--26, NCHD conducted interviews with workers in two groups: those working at the beach worksite and those working aboard the company ship. The interviews used a standard questionnaire for outbreaks to assess exposure to dredging materials, occupational and recreational water exposure, travel history, medical history, and current health status. Ten of the 13 beach workers with daytime exposure history (the other three worked only at night) were interviewed, followed by the first 10 workers who were available on the ship. Because of logistical difficulties, additional workers on the ship could not be interviewed.

Mean age of the 20 dredging company workers was 45 years (range: 23--66 years); 90% were male. Six workers reported preexisting health conditions, including two with asthma. Nine of the 20 reported a recent history of smoking. The 20 workers reported experiencing symptoms of respiratory or eye irritation beginning September 16, when the dredging operation began. Predominant symptoms were coughing (12 workers), throat irritation (12), eye irritation (11), sneezing (11), and sniffling (10) (Table 1). None of the workers required medical care or experienced impairment of their ability to do their jobs. Several reported abrupt onset and resolution of their symptoms upon arrival and departure each day from the beach worksite.

During September 25--29, additional reports of respiratory irritation were received by public health agencies from persons along Florida's Atlantic coast, up to 200 miles south of Fernandina Beach. Also during this period, approximately 15--20 reports were received daily by NCHD from beachgoers with symptoms of respiratory illness.

Environmental Assessment

On September 25, water samples were collected from the Atlantic Ocean near the Fernandina Beach shoreline for evaluation by the Fish and Wildlife Research Institute of the Florida Fish and Wildlife Conservation Commission. Light microscopy was performed to assess algal species composition and abundance.

The water samples from near the Fernandina Beach worksite first revealed K. brevis on September 25. Within 2 weeks, samples with K. brevis had been collected from additional locations up to 200 miles to the south (2). The initial water samples had "medium" levels of K. brevis (100,000 to <1,000,000 cells/L), which can cause respiratory irritation and fish kills (Table 2). However, September 26, water samples collected in Jacksonville, 35 miles south of Fernandina Beach, had "high" levels (>1,000,000 cells/L), which can cause seawater discoloration in addition to respiratory irritation and probable fish kills. Onshore wind patterns likely facilitated the transport of aerosolized brevetoxins, resulting in exposure to beachgoers.

On September 29, a storm with prolonged wind, rain, and flooding struck northeast Florida, and public reports of respiratory symptoms began to decline. Water samples collected after September 29 detected "low a" levels of K. brevis (>1,000 to <5,000 cells/L) and "present" levels (<1,000 cells/L), indicating that the storm likely contributed to dissipation of the red tide (Table 2). On November 8, all five water samples collected in Nassau County had cell counts of zero (3).

Public Health Actions

During the red tide event, NCHD issued several beach advisories, beginning September 25, alerting the public to the health risks of exposure to brevetoxins, especially for persons with preexisting respiratory conditions. Advisories were disseminated using Nassau County Emergency Management (NCEM) and NCHD communications systems and "blast faxes" to local physicians, veterinarians, schools, governmental organizations, hotels, and restaurants. In addition, advisories were posted at beach locations, in local newspapers, and on NCHD and NCEM websites. Persons who experienced respiratory irritation or sought additional red tide information were instructed to contact NCHD or the Florida Poison Control Center's Aquatic Toxins Hotline.

Reported by: A Reich, MS, MSPH, Aquatic Toxins Program, Bur of Community Environmental Health; C Blackmore, PhD, DVM, Div of Environmental Health; R Hopkins, MD, Bur of Epidemiology; R Lazensky, MPH, Florida Dept of Health. K Geib, MSN, E Ngo-Seidel, MD, Nassau County Health Dept, Fernandina Beach, Florida.

Editorial Note:

The initial detection of the 2007 northeast Florida red tide described in this report was unusual because public health authorities were first alerted by a cluster of reported symptoms of human respiratory illness among dredging workers rather than by more common means (e.g., observation of dead fish or birds, detection of contaminated seafood, or use of satellite imagery or routine beach water sampling). Upon initial investigation of the human illnesses, NCHD observed dead fish and detected the odor of brevetoxin, both indications of red tide. Water sampling confirmed that an ongoing red tide bloom was in the proximity. Because only a small convenience sample of workers could be interviewed on the dredging ship, no conclusions can be drawn about the relative prevalence of red tide symptoms at the two worksites. However, the results suggest that symptoms occurred more frequently among beach workers. During red tides, symptoms are frequently more intense in persons exposed on beaches, because of aerosolization of brevetoxins in beach surf (4).

Wildlife species have been particularly valuable sentinels for human brevetoxin illness. In the past, the Florida Department of Health has used reports of dead fish or birds (which eat contaminated fish) as an early warning mechanism for red tide blooms (5). During the red tide event described in this report, dead sea turtles were observed on Nassau County beaches. Brevetoxin also accumulates in molluscan shellfish and is associated with human neurotoxic shellfish poisoning when contaminated seafood is ingested (6). Shellfish beds in Florida coastal waters are sampled routinely for brevetoxin.

Studies attempting to assess the human health effects of red tide blooms have been reported. One study, in Sarasota, Florida, found a 19% increase in the rate of pneumonia cases diagnosed during a 3-month onshore red tide event and, among coastal residents, a 54% higher rate of diagnoses of respiratory illness (pneumonia, bronchitis, asthma, and upper airway disease) (7). Other studies have found significant measureable adverse changes in the lung function of asthma patients after exposure to brevetoxins (6,8).

Red tide blooms have been uncommon in northeastern Florida, occurring with much greater frequency in the Gulf of Mexico. Florida red tide was first documented on the Atlantic coast in 1972, south of Fernandina Beach, and further south in Jacksonville in 1980 and 1999 (9). Florida records indicate that, before the 2007 bloom, K. brevis had not been detected in Nassau County since 1953; that detection was not associated with a red tide event.

In addition to the limited number of interviews with the ship workers, the findings in this report are subject to at least two other limitations. First, assessment of symptom onset dates was not possible because symptom-specific onset dates were not collected. Second, systematic collection of data on symptoms of other persons in the area of the bloom was not possible; therefore, the effects of the red tide event among populations other than the dredging company workers (e.g., beachgoers) could not be assessed.

During this red tide event, prompt investigation of a small cluster of symptoms led to quick identification of the K. brevis bloom. This public health vigilance enabled authorities to take immediate action to issue advisories and otherwise alert the public to an illness of environmental etiology.


  1. Fish and Wildlife Research Institute. Red tides in Florida. St. Petersburg, FL: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute; 2008. Available at
  2. Fish and Wildlife Research Institute. Karenia brevis counts, September 24--28, 2007. St. Petersburg, FL: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute; 2007. Available at
  3. Fish and Wildlife Research Institute. Karenia brevis counts, November 3--9, 2007. St. Petersburg, FL: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute; 2007. Available at
  4. Backer LC, Kirkpatrick B, Fleming LE, et al. Occupational exposure to aerosolized brevetoxins during Florida red tide events: effects on a healthy worker population. Environ Health Perspect 2005;113:644--9.
  5. Fish and Wildlife Research Institute. Fish kill database search. St. Petersburg, FL: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute; 2007. Available at
  6. Fleming LE, Kirkpatrick B, Backer LC, et al. Initial evaluation of the effects of aerosolized Florida red tide toxins (brevetoxins) in persons with asthma. Environ Health Perspect 2005;113:650--7.
  7. Kirkpatrick B, Fleming LE, Backer LC, et al. Environmental exposures to Florida red tides: effects on emergency room respiratory diagnoses admissions. Harmful Algae 2006;5:526--33.
  8. Fleming LE, Kirkpatrick B, Backer LC, et al. Aerosolized red-tide toxins (brevetoxins) and asthma. Chest 2007;131:187--94.
  9. Fish and Wildlife Research Institute. East Coast red tide history. St. Petersburg, FL: Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute; 2007. Available at

Table 1

TABLE 1. Number of interviewed dredging company workers
who reported eye or respiratory symptoms during red tide
(Karenia brevis) algal bloom, by worksite  Nassau County,
Florida, 2007
Total Beach Aboard ship
Symptom (N = 20) (n = 10) (n = 10)
Coughing 12 10 2
Throat irritation 12 9 3
Eye irritation 11 10 1
Sneezing 11 9 2
Sniffling 10 9 1
Mucous with cough 9 7 2
Breathing difficulty 5 4 1
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Table 2

TABLE 2. Laboratory classifications and possible effects of Karenia brevis, by cell count  Fish and Wildlife Research Institute,
Florida Fish and Wildlife Conservation Commission
Classification K. brevis (cells/L) Possible effects (K. brevis only)
Present background levels of <1,000 cells None
Very low a >1,000 to <5,000 Possible respiratory irritation
Very low b 5,000 to 10,000 Possible respiratory irritation and requisite shellfish harvesting closures
Low a >10,000 to <50,000 Respiratory irritation, but chlorophyll levels too low to be detected by satellites
Low b 50,000 to <100,000 Respiratory irritation, possible fish kills, and bloom chlorophyll probably detected by satellites
Medium 100,000 to <1,000,000 Respiratory irritation and probable fish kills
High >1,000,000 As above, plus discoloration
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Date last reviewed: 7/3/2008


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