Mercury (Hg) is a toxic heavy metal that naturally exists as an inorganic or organic compound in the environment (Ebinghaus et al. 1999). Concerns over mercury in the environment developed during the 1950s and 1960s with the poisoning of humans and wildlife in Japan and Sweden. No matter how clean an area is, if there is human activity such as local development, the ecosystems could be contaminated even at very low concentrations when mercury is released (Gworek et al. 2020; Verma et al. 2018), because released mercury is methylated by microorganisms in sediments and converted into highly toxic compounds called methylmercury (MeHg) (Boening 2000; Mergler et al. 2007). MeHg has a much higher bioavailability than normal inorganic mercury and can penetrate cells where there are normally barriers to toxins (Mergler et al. 2007; Ullrich et al. 2001). In other words, it can cross the blood-brain barrier and the placental barrier and directly damage brain and fetal cells. In addition, since bioaccumulation in the food chain occurs as a result of its long half-life and high bioavailability, mercury accumulates in higher concentrations as it moves toward upper predators (Driscoll et al. 2013; Lavoie et al. 2013).

Mercury poisoning causes coma, ataxia, spinal cord degeneration, limb paralysis, tremors, and convulsions and can permanently damage the kidneys, liver, and brain (Boening 2000; Chan et al. 2003). It can also affect the dopamine and cholinergic systems that promote learning and memory, motor function, thermoregulation, and cognition (Basu et al. 2006). In the case of fetuses, spontaneous abortion or fetal cerebral palsy may occur (Chan et al. 2003).

Recently, the anthropogenic emission of mercury has been increasing more rapidly than natural emission, and this is considered a big problem. For example, the amount of mercury measured in Arctic indicator species has increased significantly compared to pre-industrial times (Dietz et al. 2009; Dietz et al. 2011). The international community adopted the Minamata Convention in 2013 to reduce anthropogenic emissions of mercury and thereby prevent damage to people and ecosystems caused by mercury, and the convention came into force in 2017 (Kessler 2013; Science for Environment Policy 2017).

Sentinel animals are widely used as an environmental indicator species when monitoring pollutants. Predatory apex predators in river such as otters and minks are commonly used as sentinel species for risk assessment in freshwater ecosystems (Baos et al. 2022; Basu et al. 2007). Unlike birds, they have a small range and do not hibernate or migrate, so they are exposed to pollutants year-round (Hung and Law 2016). Biomarkers of exposure, such as tissue concentrations, can be used to determine environmental concentrations using sentinel species and enable assessment of the risk posed to humans (Peterson and Schulte 2016).

Hwacheon-gun, South Korea, is nicknamed the city of water for its clean environment. The freshwater ecosystem is well preserved and inhabited by many Eurasian river otters, and it is a place with varied natural scenery. One Eurasian river otter found in this clean area was unable to control its body, and it was rescued but died soon after. Various examinations, including necropsy, were performed. The cause of death was revealed as mercury poisoning.

Case history

An adult male Eurasian river otter (Lutra lutra) with ataxia and lethargy was rescued at Hwacheon-gun, Gangwon-do (38°04’34.0”N, 127°41’14.7”E) in South Korea. The Eurasian river otter, 41 cm in body length, presented with dehydration, emaciation, and serious lethargy. When it was taken up from the water to observe its movement, the Eurasian river otter exhibited a unilaterally spinning movement. Its body temperature was 38.8°C.

As a routine treatment for this kind of case, 10 mg/kg cefazolin sodium (Cefazolin Inj.; Chong Kun Dang Pharmaceutical Corp., Seoul, Korea), 0.5 mg/kg famotidine (Gaster Inj.; Dong-A ST, Seoul, Korea), 2 mg/kg ornipural solution (Vetoquinol, Magny-Vernois, France), 6 mg/kg aminoethyl sulfonic acid (Taurine-F Inj.; Samyang Anipharm Co., Ltd., Seoul, Korea) and 1 mg/kg L-Ornithine-L-Aspartate (Hepa-Merz Infusion; Hanwha Pharma Co., Ltd., Seoul, Korea) were intravenously injected.

There was no recovery from this condition, and the otter died at 4 days post rescue and was then subjected to postmortem examination. During necropsy, representative tissue samples were collected in 10% neutral buffered formalin for histopathological examination or placed in a deep freezer (–80°C) for further additional examination, if necessary.

Serological and radiological examination

Blood tests and radiological examination for neurological signs, ataxia, and spinning movement were performed for detailed diagnosis. The Eurasian river otter was anesthetized in sternal recumbency with 2.5 mg/kg alfaxalone (Alfaxan^®; Jurox Inc., North Kansas City, MO, USA), maintained with isoflurane inhalation, followed by the computed tomography (CT) scanning using a Somatom Emotion 6 (Siemens, Erlangen, Germany) and magnetic resonance imaging (MRI) using a Vantage Elan (Toshiba, Tokyo, Japan).

Histopathological analysis

Tissue samples were fixed in 10% (w/v) neutral formalin for 24–48 hours and routinely processed as described elsewhere (Cho et al. 2023). Briefly, formalin-fixed tissue samples were trimmed, dehydrated in a series of graded ethanol solutions, cleared in xylene, and embedded in paraffin blocks. Sections of 4 μm thickness were prepared from formalin-fixed paraffin-embedded tissues and stained in hematoxylin and eosin (H&E).

Mercury analysis

The total mercury levels were measured in the liver, kidneys and fur using a DMA-80 Direct Hg Analyzer (Milestone, Sorisole, Italy). The analytical quality was controlled using procedural blanks, triplicates and BCR^®-277R Estuarine Sediment (Institute for Reference Materials and Measurements [IRMM], Geel, Belgium) as certified reference material (CRM).

Serological analysis

Complete blood counts showed elevations in white blood cells, neutrophils, monocytes (MON), mean corpuscular volume, mean corpuscular hemoglobin and a decrease in lymphocyte (Table 1). Serum chemistry analysis revealed elevations in alkaline phosphatase, alanine aminotransferase and a decrease in amylase (Table 1). The results are suggestive of anemia, chronic hepatitis, pancreatitis, and inflammation-like immunomodulation.

Table 1 . Findings on complete blood counts and serum chemistry analysis of the Eurasian river otter.

Parameter	Value	Reference range
Complete blood counts
WBC (109/L)	14.01 (high)	2.00–10.00
LYM (109/L)	0.14 (low)	0.40–6.50
MON (109/L)	0.96 (high)	0.10–0.70
NEU (109/L)	12.91 (high)	0.80–4.50
RBC (1012/L)	8.01	7.80–13.00
HGB (g/dL)	18.8	12.4–18.7
HCT (%)	55.54	36.00–56.00
MCV (fL)	69 (high)	40–48
MCH (pg)	23.5 (high)	13.5–16.5
MCHC (g/dL)	33.9	32.1–35.5
PLT (109/L)	453	96–776
MPV (fL)	8.1	6.1–10.1
Serum chemistry analysis
ALB (g/dL)	3.5	2.8–4.2
ALP (U/L)	285 (high)	3–120
ALT (U/L)	405 (high)	54–289
AMY (U/L)	< 5 (low)	-
TBIL (mg/dL)	0.4	0.0–1.0
BUN (mg/dL)	47	10–45
CA (mg/dL)	9.9	8.0–11.8
PHOS (mg/dL)	8.3	4.0–10.1
CRE (mg/dL)	0.6	0.2–1.0
GLU (mg/dL)	118	62–207
NA+ (mmol/L)	164	137–162
K+ (mmol/L)	4.4	4.1–7.7
TP (g/dL)	8.6	-
GLOB (g/dL)	5.1	2.0–4.0

WBC: white blood cells; LYM: lymphocytes; MON: monocytes; NEU: neutrophils; RBC: red blood cells; HGB: hemoglobin; HCT: hematocrit; MCV: mean corpuscular volume; MCH: mean cell hemoglobin; MCHC: mean cell hemoglobin concentration; PLT: platelet; MPV: mean platelet volume; ALB: albumin; ALP: alkaline phosphatase; ALT: alanine aminotransferase; AMY: amylase; TBIL: total bilirubin; BUN: blood urea nitrogen; CA: calcium; PHOS: phosphate; CRE: creatinine; GLU: glucose; TP: total protein; GLOB: globulin; -: not applicable.

Radiological analysis

In CT and MRI, chronic hemorrhage was observed in the meninges and brain parenchyma (Fig. 1).

Figure 1. Radiological findings of the brain. Adult male Eurasian river otter. (A) Transverse computed tomography scan of the head. (B) Transverse magnetic resonance imaging (MRI) T1w scan of the head. (C) Transverse MRI T2w scan of the head. Chronic hemorrhage in meninges and brain parenchyma was observed.

Gross pathological analysis

After conducting a gross pathological examination, no abnormalities were observed in the external appearance except for severe emaciation (Fig. 2).

Figure 2. Gross appearance before necropsy. Adult male Eurasian river otter. Grossly, no specific trauma or abnormal findings were observed.

At postmortem examination, severe meningeal hemorrhage and edema was observed in the brain (Fig. 3). Diffuse hemorrhage of lungs, left ventricular hypertrophy of heart, and enlarged auxiliary lymph nodes were observed. Hyperemia, erosion, ulceration, and bleeding were observed in the stomach.

Figure 3. Gross appearance of the brain. Adult male Eurasian river otter. (A) Hemorrhage was observed in the arachnoid membrane. (B) Congestion was observed in the entire cerebral sulcus.

Histopathological analysis

Microscopically, diffuse sinusoidal dilation and hepatocellular necrosis in the liver (Fig. 4A), and myocardial focal necrosis in heart (Fig. 4B) were found. Diffuse congestion and/or hemorrhage and edema were observed in lungs. In particular, infiltration of inflammatory cells around the capillaries and vascular edema was seen in the right cerebrum and multifocal edema was observed in the cornu ammonis 2 region and molecular layer (Fig. 4C).

Figure 4. Histopathological findings. Adult male Eurasian river otter, H&E staining. (A) Liver. Scale bar = 200 μm (inset). High magnification. Scale bar = 50 μm. Diffuse sinusoidal dilation and hepatocellular necrosis are visible. (B) Heart. Scale bar = 100 μm. Myocardial focal necrosis is visible. (C) Brain. Scale bar = 500 μm (inset). High magnification. Scale bar = 200 μm. Multifocal edema and infiltration of inflammatory cells around the capillaries in cornu ammonis 2 region are observable.

Mercury analysis

The mercury concentrations in the liver, kidney and fur were 0.878 ± 0.027, 1.807 ± 0.049, and 5.712 ± 0.102 μg/g, respectively, which are 6.7 times, 13.7 times, and 43.3 times higher than mercury CRM, respectively (Table 2).

Table 2 . Total Hg measurement value of each sample.

Sample	Value (μg/g)
Blank	0.112
CRM	0.132
Kidney	0.878 ± 0.027
Liver	1.807 ± 0.049
Fur	5.712 ± 0.102

Values are mean ± standard error.

Hg: mercury; CRM: certified reference material.

Mercury poisoning can cause various gross pathological changes in the affected individual. One of the common findings is central nervous system (CNS) damage, as seen in our case. Mercury has a strong affinity for nervous tissue, and chronic exposure can cause damage to the CNS. Gross pathological changes in the brain may include atrophy, focal necrosis, and edema (Takahashi and Shimohata 2019). Other lesions include interstitial nephritis, tubular necrosis and glomerular damage in the kidney; hyperemia, erosions, and ulcers in the stomach and intestines; interstitial congestion and fibrosis, emphysema, and bronchitis in the lungs. In the case of pulmonary damage, this may be due to inhalation of mercury vapors, which can cause damage to the lungs (Raffee et al. 2021). Mercury toxicity can also affect the cardiovascular system, causing damage to the heart muscle and blood vessels. Myocardial necrosis, endocarditis, and vasculitis were seen in our case (Omanwar and Fahim 2015).

Initially, the attribution of the Eurasian river otter’s demise to mercury poisoning did not appear tenable, given the location of its discovery in a protected otter habitat that was deemed environmentally pristine. However, the surrounding environs had undergone significant developmental activity, including the recent establishment of a waste disposal site and a golf course, prompting doubts about the ecological integrity of the area.

Lesions consistent with mercury poisoning include CNS damage, renal necrosis and fibrosis, vascular damage, and liver damage. Renal necrosis and fibrosis have been reported in several experimentally administered species, including otters, mink, and human (Farina et al. 2011; Heinz 1996). The cause of these pathological changes in the kidney is thought to be autoimmune glomerulopathy due to chronic inorganic mercury filtration (Eto et al. 1997; Gao et al. 2022). In the case of liver damage, mercury oxidative stress is known to induce liver necrosis and sinusoid dilation (Balali-Mood et al. 2021; Yahyazedeh et al. 2017). In neurological symptoms, MeHg-induced excitotoxicity and demyelination due to oxidative stress are the main causes (Fernandes Azevedo et al. 2012). Moreover, vascular damage is also caused by MeHg-induced oxidative stress, which further accelerates damage to other organs (Takahashi and Shimohata 2019).

Humans, wildlife, and the environment are organically linked as part of the “One Health” concept. Obtaining information on mercury exposure in wildlife, especially sentinel animals such as river otters can provide insight into risks to the environment and humans. Various countries, such as Denmark and the United States, are already conducting monitoring of pollutants using sentinel species, and research on models is also being actively conducted (Dibbern et al 2021; Fusillo et al. 2022; Yates et al. 2005).

The mercury concentration was recorded in as 1.807 ± 0.049 μg/g in the liver in this study, which does not exceed the lowest observed effect level of 3.4 μg/g (Basu et al. 2006; Klenavic et al. 2008). However, even at low concentrations, long-term accumulation can cause symptoms, and the possibility that these heavy metals have accumulated in other wild animals cannot be ruled out. Therefore, it seems that continuous monitoring using sentinel animals is necessary.

Through the necropsy of an adult male Eurasian river otter (Lutra lutra) with neurological symptoms, we confirmed a broad range of vascular damage caused by mercury toxicity in several organs, hepatocellular necrosis and vacuolation in the brain. In addition, it was confirmed that the mercury concentration was high throughout measurement. Therefore, the cause of death was concluded to be mercury poisoning (Fig. 5).

Figure 5. Summary of diagnostic findings in necropsy of the Eurasian river otter (Lutra lutra). CT: computed tomography; MRI: magnetic resonance imaging; CRM: certified reference material; Hg: mercury.

This suggests that even in clean areas with low concentrations of environmental pollutants, toxic symptoms can be caused by the continuous accumulation of pollutants. Therefore, it cannot be ruled out that other wild animals may also accumulate pollutants such as heavy metals. It is important to measure concentrations of pollutants in the environment, but it will also be important to evaluate the extent to which pollutants have accumulated in the body using sentinel animals.

Not applicable.

CRM: Certified reference material.

Hg: Mercury.

MeHg: Methylmercury.

CT: Computed tomography.

MRI: Magnetic resonance imaging.

H&E: Hematoxylin and eosin.

IRMM: Institute for Reference Materials and Measurements.

CNS: Central nervous system.

KGR and SJA collected and analyzed data, wrote original draft. SJL, SYC, and BRDK collected data. HSC conceptualized and supervised the research, and reviewed the draft. YO acquired the research grant and conceptualized and supervised the research, and reviewed and edited the draft. The authors read and approved the final manuscript.

This work was supported by Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through Animal Disease Management Technology Advancement Support Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (Project No. 122013-2).

All data generated or analyzed during this study are included in this published article.

Not applicable.

Not applicable.

The authors declare that they have no competing interests.