Carcass examinations in the Atlantic Unusual Mortality Event
Updated January 5, 2023
An Unusual Mortality Event (UME) caused by starvation due to lack of forage in the Indian River Lagoon (IRL) started in December 2020 and is ongoing on the Atlantic coast. The IRL provides vital habitat for manatees in all seasons and is central in manatee migration on this coast. Consequently, the effects of malnutrition have been documented in manatees all along the Atlantic coast. Mortality has been high during the past two winters because colder temperatures add extra health stressors to manatees that are already compromised by chronic malnutrition. The negative health impacts of malnutrition are long-lasting and have been documented in Atlantic coast carcasses during the warmer months as well.
Necropsy findings of chronic starvation in manatee UME carcasses
External features of emaciation range from flattening of the body, loose skin, folds in the belly, distinction of head from the rest of the body with a dip in the neckline (‘peanut-head’), to visible outline of skeletal structures. On necropsy, the transformation of fat, muscle and connective tissue into a watery gelatinous substance (‘serous atrophy’) suggests the manatees in this UME experienced prolonged starvation and malnutrition, depleting their bodies. Lower than normal liver weights are consistent with chronic starvation. This state of extreme wasting is further evidenced by atrophy of vital organs such as liver, heart, and diaphragm on microscopic examination.
All manatee carcasses, regardless of disposition, are documented in the manatee mortality database after verification by FWC manatee biologists. Manatee carcass numbers have increased over time (with added peaks during UMEs) and it is no longer feasible nor necessary to necropsy nearly every carcass in Florida. To meet essential information needs, necropsies are performed under surveillance protocols in order to track key threats and additionally certain carcasses are selected for more detailed necropsies and specific health investigations. These detailed necropsies are specifically needed for the investigation of starvation which requires extra data and analyses (e.g., weights, measurements, microscopic examination). For example, more than 100 carcasses have been sampled for investigation of tissue atrophy in this UME. This is done from formalin-fixed tissues, which are made into microscopic slides for examination by a veterinary pathologist (histopathology). In this UME, portions of a carcass may still be examined for findings consistent with starvation. Such carcasses are listed as ‘Verified; Not Necropsied’ because a full necropsy was not performed and a primary cause of death was not determined, but starvation findings are still reported in the mortality database to aid in documenting the extent of starvation among carcasses.
Review of 2022 mortality data (preliminary)
This past winter, between 1 December 2021 and 31 March 2022, 457 manatee deaths were verified on the Atlantic coast. Primary starvation was the leading cause of death in necropsied carcasses on the Atlantic coast (these deaths are included in the ‘Natural; Other’ category in FWRI mortality web tables).
Out of the 313 carcasses in the ‘Verified; Not Necropsied’ category, 231 received a fat, muscle, and stomach inspection; 204 of these (88%) had findings consistent with malnutrition. Necropsy data indicate that manatees had experienced starvation and malnutrition before entering winter. Overall, the total number of verified carcasses on the Atlantic coast was lower than the previous winter when 582 carcasses were documented during this timeframe. This does not mean that manatee health improved from the winter before. To the contrary, necropsies demonstrated severe, extensive tissue atrophy, which is likely the result of experiencing malnutrition through multiple seasons. It is possible that a relatively warm December and less winter cold contributed to the lower total mortality number this past winter compared to the previous. Carcass numbers remained unprecedented and far above what can be expected in normal winters on this coast.
Since late April, numbers of manatee deaths were near the weekly baseline compared to mortality in the five years before the onset of this UME. Over the summer, at least half of the necropsied carcasses died from watercraft-related injuries, which highlights the pressure of consistent and cumulative threats to manatees on the Atlantic coast. Hot summer temperatures expedite carcass decomposition significantly, and most cases are too decomposed to assess for findings of chronic malnutrition during this time of the year. Regardless of lower mortality numbers during the summer, concern remained for the health of Atlantic coast manatees that continue to experience shortages in proper nutrition. Since October, at least five mortalities have been attributed to primary starvation. The total- and starvation-related mortality in 2022 was lower compared to the year before, but mortality in this UME is still expected to be elevated this winter.