Carcass examinations in the Atlantic Unusual Mortality Event
Updated November 8, 2023
Background
Emaciated carcass. The body is flattened.
An Unusual Mortality Event (UME) caused by starvation due to lack of forage in the Indian River Lagoon (IRL) started in December 2020 on the Atlantic coast and its investigation is ongoing. 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. Colder temperatures during the winter 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.
(A) Emaciated carcass. There is a distinct dip in the neck region (‘peanut-head’) and sunken eyes; (B) Tissue section of fat and muscle from two emaciated manatees. The manatee on the left died from cold stress disease and still had grossly normal fat despite it being thin and depleted. The fat and muscle of the manatee on the right that died from primary starvation was depleted but also very watery (‘serous atrophy’), further emphasizing the prolonged state of wasting; (C) Microscopic view of liver atrophy. The liver cells (light pink with purple nucleus) are smaller than normal. The brown pigment in the cells is from the breakdown of the components that make up the cell. Image credit Dr. Dave Rotstein.
Carcass sampling
Histopathology sample collection has expanded to meet the needs of specific health investigations. Before this UME, histopathology cases averaged about 30 cases per year.
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 185 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 mortality data (preliminary)
During the first two winters of this UME (1 December 2020 through 31 March 2021 and 1 December 2021 through 31 March 2022), 582 and 457 manatee deaths were verified on the Atlantic coast, respectively. Primary starvation was the leading cause of death in necropsied carcasses on the Atlantic coast during these winters (these deaths are included in the ‘Natural; Other’ category in FWRI mortality web tables).
Starting in December 2021, the procedure of partial necropsy was implemented to inspect carcasses in the ‘Verified; Not Necropsied’ category for gross findings of starvation. Out of the 313 carcasses in the ‘Verified; Not Necropsied’ category during winter 2021-2022, 231 received a fat, muscle, and stomach inspection; 204 of these (88%) had findings consistent with malnutrition. Necropsy data indicated that manatees had experienced starvation and malnutrition before entering winter, but it is possible that a relatively warm December 2021 and less winter cold contributed to the lower total mortality number compared to the first UME winter. During both winters, carcass numbers were unprecedented and far above what can be expected for this time of the year on this coast.
During summer, numbers of manatee deaths in this event have been near the weekly baseline compared to mortality in the five years before the onset of this UME. 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. However, over the past two summers, a significant portion of the necropsied carcasses died from watercraft-related injuries (50% in 2022 and 30% in 2023), which highlights the pressure of consistent and cumulative threats to manatees on the Atlantic coast. Regardless of lower mortality numbers during the warmer months, shortages in proper nutrition during this time of the year lead to a suboptimal condition entering winter, which subsequently lessens the ability to survive winter stressors.
This past winter (1 December 2022 through 31 March 2023, the third winter since the onset of the UME), the number of verified carcasses on the Atlantic coast and cases of primary starvation were much lower than the previous two winters, despite significant cold weather stressors occurring in late December and early January. Preliminary data indicate that overall manatee body condition had improved compared to the previous two years, which may be explained by manatees finding more forage options over the summer giving them a better condition going into winter. So far, observations from this past summer do not show a worsening of IRL habitat condition or alarming trends in manatee rescue and mortality. Manatee health depends on continuing recovery of this important ecosystem, which still has a long way to go. Concern remains for the long-term effect of these lean UME years on manatee health and the population.
Carcass Findings Atlantic Coast December 2021 - October 2023
Carcass examinations show that starvation was the leading cause of death among fully necropsied carcasses during winter 2021-2022 and to a lesser extent in April of that year. Starvation cases were again documented during winter 2022-2023, but in much lower numbers than the winter before. The striped portion of carcasses that were verified but not fully necropsied (210 of 374 carcasses, VNN in blue) shows the minimum number of VNN cases that had findings of starvation. Over summers 2022 and 2023, watercraft-related trauma was the most common cause of death among known causes.
Carcass numbers and causes of death by month for the past three winters
Monthly carcass numbers for the past three winters (2020 – 2023) on the Atlantic coast show high mortality in the first two winters (582 and 457 carcasses, respectively, for December through March) compared to lower numbers in the past winter (51 total for December through March). Carcass data for the first winter only show totals. The surveillance of starvation findings in ‘Verified; Not Necropsied’ carcasses started in the second winter. Because of differences in sampling approach between the first winter and the second and third winter, the specific cause of death trends cannot simply be compared between the first and more recent winters.