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

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

doi:10.3389/fmars.2022.841857

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 non-toxic phytoplankton blooms, with a resultant ecosystem shift to mixed macroalgal dominance. At least 199 manatee deaths fitting the UME case definition were documented in and adjacent to the IRL during 2012–2019; mortality was highest in 2013, when 111 of these deaths were documented. The case definition included carcasses in good nutritional condition, with multiorgan congestion or wet lungs consistent with 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 blebbing to segmental thickening of the wall. Microscopic lesions were primarily intestinal, including 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. Microbiological, cytological, immunohistochemical, and molecular analyses of Clostridiales from intestinal samples identified Clostridioides difficile toxin A, toxins A/B and toxin A gene; Paeniclostridium sordellii lethal gene (and other potential virulence factors from a sequenced strain); and Clostridium perfringens alpha and epsilon toxin genes. The results from this 8 year-long investigation are indicative that the cause of death in this manatee UME was associated with clostridial infection, initiated by a shift to a predominantly macroalgal diet.

Highlights

The Florida manatee (Trichechus manatus latirostris) is a threatened marine mammal inhabiting the southeastern United States, primarily the coastal waters of Florida
Two carcasses were found outside the Indian River Lagoon (IRL) (Tampa Bay and Caloosahatchee River, on Florida’s west coast) but were included for disease investigation purposes because their necropsy findings were consistent with the case definition and provided a rare opportunity for fresh sample collection in this investigation
Because the human consumption of uncooked Rhodophyta Polycavernosa tsudai (= Gracilaria edulis), G. verrucosa, G. chorda, and G. coronopifolia (Gracilariales) has been associated with fatal food-poisoning incidents in the Pacific region (Halstead and Haddock, 1992; Hanne et al, 1995; Yotsu-Yamashita et al, 2004; Kumar and Sharma, 2021), we focused on testing G. tikhaviae, manatee stomach contents, and feces for polycavernoside

Summary

Introduction

The Florida manatee (Trichechus manatus latirostris) is a threatened marine mammal inhabiting the southeastern United States, primarily the coastal waters of Florida. Lethal boatstrike injuries, loss of warmwater habitat, and brevetoxicosis are the largest threats to the sustainability of the manatee population (Martin et al, 2017; Runge et al, 2017). Extreme low temperatures in the winters of 2009–2010 and 2010–2011 led to record numbers of coldrelated manatee deaths and the declaration of unusual mortality events across the state (Hardy et al, 2019). The Marine Mammal Protection Act defines an unusual mortality event (UME) as “a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response.” These previous two UMEs were associated with high manatee mortality primarily in Brevard County, in east-central Florida. An aerial abundance survey conducted in March 2012 estimated that 70% (95% CI 60–80%) of the manatees on the east coast of Florida were present in Brevard County at the time (Martin et al, 2015)

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