Brevetoxicosis in manatees (Trichechus manatus latirostris) from the 1996 epizootic: Gross, histologic, and immunohistochemical features

In 1996, at least 149 manatees (Trichechus manatus latirostris) died in an unprecedented epizootic along the southwest coast of Florida. At about the same time, a bloom of the brevetoxin-producing dinoflagellates, Gymnodinium breve, was present in the same area. Grossly, severe nasopharyngeal, pulmonary, hepatic, renal, and cerebral congestion was present in all cases. Nasopharyngeal and pulmonary edema and hemorrhage were also seen. Consistent microscopic lesions consisted of catarrhal rhinitis, pulmonary hemorrhage and edema, multiorgan hemosiderosis, and nonsuppurative leptomeningitis. Immunohistochemical staining using a polyclonal primary antibody to brevetoxin (GAB) showed intense positive staining of lymphocytes and macrophages in the lung, liver, and secondary lymphoid tissues. Additionally, lymphocytes and macrophages associated with the inflammatory lesions of the nasal mucosa and meninges were also positive for brevetoxin. These findings implicate brevetoxicosis as a component of and the likely primary etiology for the epizootic. The data suggest that mortality resulting from brevetoxicosis may not necessarily be acute but may occur after chronic inhalation and/or ingestion. Immunohistochemical staining with interleukin-1-beta-converting enzyme showed positive staining with a cellular tropism similar to GAB. This suggests that brevetoxicosis may initiate apoptosis and/or the release of inflammatory mediators that culminate in fatal toxic shock.