Necropsy findings
The cause of death and relevant pathological findings in the twenty-five brown bears studied is shown in Table 1. In four (4/25, 16%) cases it was not possible to determine the cause of death due to bad preservation of the remains found or insufficient tissue availability.
Table 1
Available data, cause of death and pathological findings of twenty-five free-ranging Eurasian brown bears (Ursus arctos arctos) studied from 1998 to 2018 in Asturias and Castilla y León (northwestern Spain).
Bear | Date | Age | Sex | Cause of death |
1 | 08/05/1998 | 7 years | Male | Snare/exertional myopathy/gangrenous myositis (Clostridium sordellii and C. bifermentans) |
2 | 12/06/1998 | Cub | Female | Infanticide |
3 | 10/06/2000 | Adult | N.d. | N.d. |
4 | 06/06/2005 | Subadult | Male | N.d. |
5 | 26/09/2005 | Adult | Male | Shooting |
6 | 19/11/2005 | Adult | N.d. | Poisoning: strychnine |
7 | 14/06/2008 | 1 year | Male | Infanticide |
8 | 27/08/2012 | Adult | Male | Snare/gangrenous myositis (Clostridium sordellii)* |
9 | 29/10/2012 | Cub (9 months) | Female | Died after handling and transport/exertional myopathy |
10 | 12/06/2014 | 3 years | Male | Fighting/gangrenous myositis (Clostridium sordellii and C. septicum) |
11 | 15/06/2014 | 5 years | Male | Infectious disease: CAdV-1** |
12 | 12/12/2014 | 9 years | Male | Fighting/septicemia |
13 | 29/04/2015 | 20 years | Female | Neoplasia: cholangiocarcinoma*** |
14 | 23/05/2015 | Cub (4 months) | Male | Infectious disease: CAdV-1** |
15 | 16/10/2015 | Adult | Male | Traumatic lesions/gangrenous myositis |
16 | 05/03/2016 | Adult | Male | Traumatic lesions due to fall |
17 | 08/10/2016 | Subadult | Male | Shooting |
18 | 27/11/2016 | 6 years | Female | Snare/strangled |
19 | 07/01/2017 | 6 years | Male | Mushroom poisoning; hepatic an renal necrosis |
20 | 03/04/2017 | Cub (3 months) | Female | Infectious disease: CAdV-1** |
21 | 21/04/2017 | 19 years | Male | Fighting and cliff fall |
22 | 21/04/2017 | 20 years | Male | Fighting and cliff fall |
23 | 29/09/2018 | 4 years | Female | Traumatic lesions/gangrenous myositis (Clostridium sordellii) |
24 | 27/10/2018 | 5 years | N.d. | N.d. |
25 | 08/11/2018 | 7 years | Male | N.d. |
* [5]. ** [6]. *** [8]. Grey shadow: deaths by human intervention. N.d.: not determined. CAdV-1: Canine adenovirus type 1. Bears 5, 7, 9, 11, 12, 15, 18, 24 and 25 are from Castilla y León. The remaining bears are from Asturias. |
Usually traumatic death causes such as shooting, wire snares, fighting or infanticide were easier to determine not only based on necropsy and histopathological studies but also using complementary diagnostic techniques (i.e. radiography) or knowledge of the behavior of this species. However other causes were more difficult to discern and are reported below. Some animals (i.e. bear number 1) showed several pathological findings that could lead to death.
Infectious diseases
The five bears (bears number 1, 8, 10, 15 and 23) with gangrenous myositis (Table 1), usually a consequence of wire snare hunting, infanticide, fights or traumas, showed serohemorrhagic edema in the abdominal cavity, thorax, pericardium, and skeletal muscle, and hemorrhages in heart, skeletal muscles, stomach, intestine, liver, spleen, and kidney. Microscopically, vascular damage and hyperacute myodegeneration consisted of myonecrosis, edema, gas, extravasation of fibrin into the interstitial spaces, and lacunar dissolution of myofibers in skeletal muscles were observed in those animals. Clostridium sordellii was identified as the etiological agent of the lesions in four of those brown bears [5]. Clostridium bifermentans was also isolated in one of the former four bears, as well as C. septicum in another one. The presence of Clostridium sordellii was always associated with previous muscle damage (i.e. traumas) that triggered its proliferation.
Gross lesions in the three bears (bears number 11, 14 and 20) with infectious canine hepatitis caused by canine adenovirus type 1 (CAdV-1) consisted of hemorrhages in thymus (in cubs), lungs, heart and mesentery; hemorraghic fluid in thoracic and abdominal cavities; friable and yellowish liver; hepatomegaly; thickening of the gall bladder due to edema and congestion of spleen, kidney and meninges [6]. Microscopically the main pathological findings appeared in liver and gall bladder. Liver showed mild centrolobular multifocal degeneration and necrosis of hepatocytes, with the presence of intranuclear inclusions bodies and low inflammatory infiltration mainly of lymphocytes. The gall bladder showed edema of the wall. Additionally, the brain showed edema, congestion, perivascular cuffing, foci of gliosis, and degeneration of neurons, mainly located in the brainstem. CAdV-1 was confirmed by quantitative polymerase chain reaction (qPCR) and immunohistochemistry.
Exertional (degenerative) myopathy
In two animals exertional myopathy was diagnosed. One of them was a brown bear that died after capture by snare for one week which also showed clostridiosis (bear number 1). An additional female cub died due to exertional myopathy after handling (bear number 9). That cub was found alone in the wild when it was two months old and it was bred in captivity until it was nine months old. Then the cub was transported in order to reintroduce it back into the wild dying during the trip. Both animals showed gross lesions consisted of dry and pale cardiac and some skeletal (mainly intercostals and femoral) muscles (Figs. 1a and 1b). Microscopically severe segmental degeneration of muscles was observed consisted of hypercontracted fibers, extensive Zenker´s hyaline degeneration and coagulative necrosis of myofibers (Figs. 1c to 1 h). Bear number 1 also showed an intensive infiltrate mainly consisting of lymphocytes and macrophages, as well as mineralization in the affected muscles (Fig. 1f). In that animal necrotic myofibers with surviving satellite cells, invading macrophages and elongating myoblasts, all indicative of events of regeneration, were also observed (Fig. 1d). The cub also showed hypoplasia of adrenal glands (1.7 grams, 0.004% relative weight; physiological relative weight 0.03%) [7].
Strychnine poisoning
In the bear affected by strychnine poisoning (bear number 6) general hemorrhages were found in several organs (heart, lungs, liver, kidney, spleen, stomach and intestine) often showing hemotorax, hemopericardium and hemoperitoneum. Microscopically vascular damage and diffuse necrosis in those organs were the most common findings. Strychnine was identified by chromatography from hair samples.
Neoplasia
Cholangiocarcinoma was observed in the liver of an old female (bear number 13) [8]. Microscopically liver tumor tissue showed tubular, acinar or pseudoglandular structures in the area facing a large cavity of necrosis with a thick trabecular growth pattern. Multiple small nodules were also present in the gall bladder. Metastatic encapsulated foci of cholangiocarcinoma were located in lung parenchyma, adrenal glands and articulation of the left elbow.
Mushroom poisoning
Bear number19 showed hemorrhagic gastritis and diffuse hepatic and renal necrosis compatible with mushroom poisoning, likely due to ingestion of poisonous Amanita spp, although that could not be confirmed by toxicological analysis.
Causes of death were classified both based on (i) pathological findings and (ii) caused by “human intervention” or “non human intervention”.
Based on pathological findings seven out of the 21 (33.3%) brown bears in which the cause of death could be determined died due to infectious diseases (clostridiosis n = 4 or infectious canine hepatitis n = 3), two (9.5%) due to exertional myopathy (one of those also showed clostridiosis) and one due to strychnine poisoning (4.8%), neoplasia (4.8%) or mushroom poisoning (4.8%). The remaining animals died due to traumatic lesions (including fights and infanticide), shooting or snare.
Mortality data was also stratified by deaths caused by “human intervention” or “non human intervention”. The death of bear number 9 showing exertional myopathy after handling was not ascribed to any of the former classifications. Six out of the 20 (30%) brown bears died as a consequence of “human intervention” due to illegal hunting (wire snare hunting n = 3 or shooting n = 2) and, strychnine poisoning (n = 1). In contrast, fourteen (14/20, 70%) brown bears died by “non-human intervention”; nine of them (9/20, 45%) due to traumatic lesions (fights n = 4, traumas n = 3 or infanticide n = 2), three (3/20, 15%) due to infectious canine hepatitis caused by CAdV-1 infection, one (1/20, 5%) due to cholangiocarcinoma and another one (5%) due to mushroom poisoning. One of the animals that died due to wire snare hunting and three of those that died due to traumatic lesions showed clostridiosis.