Abstract

In August of 1999 until February of 2000, an epizootic of approximately 120 bottlenose dolphins stranded dead across five counties in the Florida panhandle including Okaloosa, Walton, Bay, Gulf and Franklin. This increased stranding rate is four times greater than the historic annual stranding rate average for this species in that region. This increased mortality/stranding rate occurred concurrently with a harmful alga bloom of Gymnodinium brev. Examination of minimally to mildly decomposed animals showed significant upper respiratory changes which consisted of a mostly lymphoplasmacytic oropharyngitis and tracheitis. Immunoperoxidase for brevetoxin showed immunoreactivity in tissues from two fresh-dead animals examined. Reverse polyermerase chain reaction for morbillivirus was negative in the animals tested. Brevetoxin was quantified by receptor assay and HPLC-mass spectroscopy in the livers (9 of 18 examined), kidney (6 of 10 examined), and stomach contents from three stranded animals. The highest level of brevetoxin was detected in stomach contents, which principally consisted of fish. There is moderate circumstantial evidence indicating a brevetoxin epizootic; however, rate of exposure during the event is known. The impacts or effects of acute brevetoxin exposure with respect to repeated or cumulative chronic brevetoxin intoxication are unknown in marine mammals and warrants further research to better understand the impacts of these harmful algal bloom events.

Introduction

Unusual marine mammal mortality events are monitored and investigated in the United States by the Marine Mammal Health and Stranding Response Program of the National Marine Fisheries Service. The detection of, response to, and investigation of unusual marine mammal mortality events depends on the marine mammal stranding network and operates in consultation with the Working Group on Marine Mammal Unusual Mortality Events. The monitoring and investigation of unusual events was initiated in the early 1990s and has continued until present day with investigations ranging from one to four per year. In the last decade there have been several associations between epizootics with mortalities and harmful algal blooms. The following is a description of the most recent investigation of the mortality of bottlenose dolphins in the Florida panhandle associated with brevetoxin and a long lasting harmful algal bloom of Gymnodinium breve.

Epidemiology

From August 1999 until February 2000 over 120 bottlenose dolphins stranded in the Florida panhandle in a five county area (Fig. 1). This number represents more than a 4-fold increase from historic annual averages of bottlenose dolphin stranding for this area. The strandings were evenly distributed among males and females and all age classes were represented. All of the animals were found dead and most were moderately to severely decomposed, particularly during the earlier part of the event. From August through October 1999, the strandings were principally localized in the eastern counties, with a hiatus of strandings in November. However, strandings again increased during December through February 2000 and were localized in the western counties, primarily in the Choctawhatchee Bay area.

A late summer bloom of Gymnodinium breve was present from August 1999 until February 2000. Using ocean color satellite imagery the bloom was monitored and this imagery allowed ships to be directed to the bloom for actual analyses of the bloom characteristics. The investigation comprised a multi-disciplinary approach, thereby the research team was able to monitor the bloom, determine Gymnodinium breve cell counts in water, evaluate toxin levels in dolphin tissues and stomach contents, analyze fish and marine mammal specimens, and define the exposure pathway characteristics.

Results

Although most of the animals were too decomposed to examine histologically or chemically, 10% of the animals (most of which stranded in Choctawhatchee Bay) were examined in detail. Examination of minimally to mildly decomposed animals showed significant upper respiratory changes in some animals which consisted of a mostly lymphoplasmacytic oropharyngitis and tracheitis. Other nonspecific changes observed included varying degrees of lymphoplasmacytic interstitial pneumonia and lymphoid tissue depletion. These lesions are similar to those reported in manatees during the manatee brevetoxin epizootic in 1996.¹Using the immunoperoxidase staining technique developed by Dr. Bossart and colleagues, brevetoxin was demonstrated in the tissues of two fresh-dead animals examined. PCR for morbillivirus was negative in the animals tested. In addition, brevetoxin was quantified by receptor assay² and HPLC-mass spectroscopy in the livers (9 of 18 examined), kidney (6 of 10 examined), and stomach contents from three stranded animals. Animals that stranded in South Carolina were used as negative controls for these comparisons. The highest level of brevetoxin was detected in stomach contents, which principally consisted of fish. Concentrations found in liver and kidney were similar to those found in manatees during the 1996 epizootic.

Other Species Involved

During this stranding period there were also numerous sea bird, sea turtle, fish and squid mortalities in this area. Fish and sea turtle tissues are being analyzed to detect the presence or impacts of brevetoxin. In addition, numerous reports from beach goers indicated that humans were also affected complaining of symptoms of upper respiratory irritation and burning of eyes.

Discussion

Brevetoxicosis has been implicated in previous strandings in the Gulf of Mexico and along the eastern seaboard. These events include: 1946-47-bottlenose dolphins in southwest Florida, 1982-manatees in southwest Florida, 1987-88-bottlenose dolphins along the Atlantic seaboard, 1996-149 manatees in southwest Florida, and 2000-manatees in southwest Florida. The cause of the mortalities has been linked to brevetoxicosis using the epidemiology of the event, the spatial and temporal overlap between the bloom and the mortalities, the lesions noted in the animals, the presence of brevetoxin in tissues, fish, and associated with the lesions, and the lack of other evidence of infectious diseases. The route of exposure in this case was most likely through both inhalation and ingestion, which has been previously observed and documented in other manatee epizootics.¹

This event was most likely the result of brevetoxin intoxication; however, neither the chronicity nor the cumulative rate of exposure during the event is known. The impacts or effects of acute brevetoxin exposure with respect to repeated or cumulative chronic brevetoxin intoxication are unknown in marine mammals and further research is warranted. Through the development of better diagnostic tools for the detection and localization of brevetoxin, we have been able to detect and respond more efficiently to marine animal stranding events during suspected "red-tide" events. In fact these methods are now being used to detect human exposures to brevetoxin during harmful algal bloom events. These tools will become increasingly important as we investigate the potential cumulative effects of exposures to brevetoxin in marine animals and humans.

Figure 1. Bottlenose dolphin strandings by month in the Florida Panhandle.

References

1.  Bossart GD, DG Baden, RY Ewing, B Roberts, SD Wright. 1998. Brevetoxicosis in manatees (Trichechus manatus latirostris) from the 1996 epizootic: gross histologic, and immunohistochemical features. Toxicologic Pathology 26(2):276-282.

2.  Van Dolah FM, EL Finley, BL Haynes, GJ Doucette, PD Moeller, JS Ramsdell. 1994. Development of rapid and sensitive pharmacologic assays for marine phycotoxins. Natural Toxins 2:189-196.