AUTHOR=Landsberg Jan H. , Tabuchi Maki , Rotstein David S. , Subramaniam Kuttichantran , Rodrigues Thais C. S. , Waltzek Thomas B. , Stacy Nicole I. , Wilson Patrick W. , Kiryu Yasunari , Uzal Francisco A. , de Wit Martine 

TITLE=Novel Lethal Clostridial Infection in Florida Manatees (Trichechus manatus latirostris): Cause of the 2013 Unusual Mortality Event in the Indian River Lagoon

JOURNAL=Frontiers in Marine Science

VOLUME=Volume 9 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.841857

DOI=10.3389/fmars.2022.841857

ISSN=2296-7745

ABSTRACT=The Indian River Lagoon (IRL) on Florida’s east coast is a biologically diverse estuary and an important habitat to the threatened Florida manatee (Trichechus manatus latirostris). An unusual mortality event (UME) was declared by the Working Group on Marine Mammal Unusual Mortality Events in 2013 after a marked increase in manatee deaths in the IRL of an unknown cause. This UME followed a dramatic reduction of seagrass coverage in the IRL due to chronic nontoxic phytoplankton blooms, with a resultant ecosystem shift to mixed macroalgal dominance. At least 199 manatee deaths were documented in and adjacent to the IRL during 2012–2019; mortality was highest in 2013, when 111 deaths fit the UME case definition. The case definition included carcasses in good nutritional condition, with multiorgan congestion or wet lungs indicating drowning without trauma. The gastrointestinal compartments of manatee carcasses were filled with diverse macroalga species, and the contents were notably more fluid than usual. Gross intestinal findings included vesicular to segmental thickening of the wall. Microscopic lesions were primarily intestinal, indicating necrosis, edema, hemorrhage, mucosa-associated lymphoid changes, and inflammation, sometimes associated with Gram-positive bacterial rods. A multidisciplinary approach of environmental and carcass sampling found no causative evidence through tests for micro- and macroalgal biotoxins, trace metals, general toxin screening, or vitreum biochemistry. Isolation of Clostridiales from intestinal contents, identification of spore-formers by cytology, positive Clostridioides difficile toxin A immunohistochemical staining, detection of C. difficile toxins A/B and toxin A gene, Paeniclostridium sordellii lethal gene, and Clostridium perfringens alpha and epsilon toxin genes in intestinal or fecal content and in clostridial isolates, plus the sequencing of potential virulence factors in a P. sordellii strain, indicate that the cause of death in this manatee UME to be associated with clostridial infection, initiated by a shift to a predominantly macroalgal diet.