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Manatee mortality in Mud River during winter 2024

A small clump of dark green algae.

Green alga from Mud River recovered with manatee carcass MNW24028. Ingestion of a large amount of such macroalgae can disrupt the gut flora which then leads to intestinal infection.

Six manatee carcasses were reported in the Mud River (Spring Hill, Hernando County) between 5 January and 21 March, which is an unusually high number for this location. In the past 15 years, the highest number of carcass reports in Mud River during this time period in a single year was one carcass. Researchers from the Fish and Wildlife Research Institute’s (FWRI) Marine Mammal Pathobiology Lab worked together with colleagues from the University of Florida, Marine Mammal Pathology Services, and FWRI’s Ecosystem Assessment and Restoration section to investigate these deaths. The cause of the mortality was attributed to an acute bacterial infection in the gastrointestinal tract.

The mortalities included five adults (3 males, 2 females) and one subadult (male). These manatees were in good body condition and did not have any chronic disease. The gastrointestinal tracts were filled with vegetation but had bloat and watery contents that included macroalgae. The carcasses tested negative for chemicals and algal biotoxins.

Microscope images with purple staining. Two yellow circles are around oval shaped black spots.

Microscopic image of intestinal cells from manatee carcass MNW24028. The disruption of gut flora causes Clostridium bacteria to sporulate, which can initiate infection and production of bacterial toxin. The yellow circles in this image show Clostridium spores. Image credit Dr. Nicole Stacy from the University of Florida.

Microscopic examination of cells and tissues showed an inflammatory reaction in the small intestine with bacteria and spores suggestive of an infection with Clostridium bacteria. Molecular testing of the gut contents confirmed the presence of these bacteria. Clostridial toxin was observed in the gut tissue through a special stain that was developed by FWRI researchers for the investigation of an Unusual Mortality Event (UME) in 2013. This 2013 UME in the Indian River Lagoon was attributed to a lethal clostridial infection which was initiated by a dietary shift to a predominantly macroalgal diet. Clostridium bacteria may be part of the normal manatee gut flora, but a significant disruption of the flora, for example from a dietary change to macroalgae with less fiber than seagrass, can cause Clostridium bacteria to multiply and produce toxins in the intestine. These toxins cause a rapid inflammatory reaction and can lead to sudden death without options for treatment. The findings of the investigation into the 2013 UME are published in Frontiers in Marine Science.

Manatees using Mud River this winter had an abundance of macroalgae available as their food source, and the microscopic findings plus bacterial analyses indicate that they experienced a similar gastrointestinal infection with Clostridium bacteria as was published for the 2013 UME in the Indian River Lagoon. This disease has since been recognized in other parts of Florida when animals undergo a dietary shift to macroalgae. There are possibly many factors that determine whether a manatee experiences negative effects of such a dietary shift. The mortalities from gastrointestinal disease in Mud River likely ceased because manatees moved into areas with more seagrass with the spring migration.