Ecotoxicology: Lead (Pb) Exposure
in Scandinavian Brown Bears

Bears as sentinels for ecosystem health

A recent study has begun attempting to use bears as sentinels for ecosystem health, from the One Health perspective. The most recent research has focused on assessing the amount of lead (Pb) and lead isotopes in the blood, milk, muscle, and tissues of bears and identify sources and pathway of lead exposure. However, all collected samples have also been analyzed for 72 different elements, which will be included in future studies to assess ecosystem health in a wider context. In the Scandinavian brown bear, so-called reference ranges for blood or serum element concentrations have been established (1). Such reference values can be used for health assessment of individual animals or to compare different populations of the same bear species. Typically, only essential elements are included in such studies and often the number of elements are limited. Blood concentrations of nonessential (toxic) trace elements are often reported separately from individual elements (2) or as a group containing the classical heavy metals, As, Cd, Hg and Pb (3). The same approach is also seen in extensive monitoring programs, such as the Arctic (4).

Background and context

Trace elements: The term ‘trace elements’ is used to indicate elements that are present in living tissues in very small amounts. Some trace elements are known to be nutritionally essential, others are potentially essential (inconclusive evidence) and the remainder are considered to be nonessential (5). Fourteen elements, listed alphabetically (with the chemical symbol in parentheses), are commonly referred to as trace elements: aluminum (Al), arsenic (As), copper (Cu), cadmium (Cd), chromium (Cr), fluorine (F), iodine (I), iron (Fe), lead (Pb), manganese (Mn), mercury (Hg), molybdenum (Mo), selenium (Se), and zinc (Zn).

Essential trace elements: Essential trace elements include copper (Cu), chromium (Cr), fluorine (F), iodine (I), iron (Fe), manganese (Mn), molybdenum (Mo), selenium (Se), and zinc (Zn). These nine elements have different roles in mammalian physiology, e.g., they may function as catalysts in enzyme systems, they participate in oxidation-reduction reactions in energy metabolism, or they are ingredients in vital molecules such as hemoglobin, myoglobin, and thyroxine. Meeting the body’s physiological needs requires an optimal, balanced intake of essential trace elements, and the concentrations of these elements in the blood and tissue are naturally regulated within certain levels to sustain a healthy organism. All essential trace elements are toxic if consumed at sufficiently high doses and/or for long enough periods. On the other hand, if the intake and blood or tissue concentrations are below critical levels, diseases due to deficiencies may develop.

Nonessential (toxic) trace elements: By definition, the nonessential trace elements, aluminum (Al), arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb), have no known biological function in mammals and birds. They are, however, frequently ingested as contaminants in food or water and may be highly toxic even at very low levels of intake. The World Health Organization  has stated that there is no level of exposure to lead that is known to be without harmful effects (6). Several of these nonessential elements accumulate in the body (bioaccumulate) due to long half-lives in the body (e.g. 10-30 years for lead in humans) and may reach toxic tissue levels during the lifetime of an individual. This is especially true for species, such as the brown bear, with a long lifespan, i.e., bears can live for 20-30 years. Mercury is known to ‘biomagnify’, which means that its concentration increases from one trophic level to the next within the food web. Interactions between various nonessential elements are an important aspect. This is known as the ‘cocktail effect’, a term used for the harmful effects from the combination of several toxic substances, even if the concentrations of the individual elements are below their known thresholds for toxicity.

Other essential elements: There are several other nutritionally essential elements, such as calcium (Ca), chloride (Cl), cobalt (Co), magnesium (Mg), phosphorus (P), potassium (K), sodium (Na), and sulfur (S). In addition, mammalian and avian blood and tissues contain trace amounts of numerous other elements because they exist in Earth’s crust and are found in soil, water, or food. Many of these elements have either no known biological function or the evidence for any physiological role is inconclusive. Others may occasionally show up at increased concentrations in wildlife due to contamination, e.g. cesium (Cs) from atmospheric fallout.

Lead (Pb) and other heavy metals in the Scandinavian bear population

Lead levels are high in the Scandinavian brown bear population. The mean blood lead concentration in bears was 96.6 μg/L (range: 38.7–220.5 μg/L) (2). Importantly, both the mean and range are well above established threshold concentrations for developmental neurotoxicity (12 μg/L), increased systolic blood pressure (36 μg/L) and prevalence of chronic kidney disease in humans (15 μg/L) (2). Interestingly, lactating females had higher lead blood concentrations compared to younger, non-lactating females. Blood lead concentrations of dependent cubs were correlated with their mother’s blood lead concentration, which in turn was correlated with the lead concentration in the milk. The mean lead concentration during hibernation (111.5 µg/L) was significantly higher than during the active state (69.5 µg/L), potentially posing a higher risk of health effects in hibernating animals (7).

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Bears in Scandinavia also have other heavy metals in their system, including mercury, arsenic, and cadmium (3). A recent study comparing heavy metals in bears in Scandinavia and Alaska suggests that cadmium and lead blood concentrations were higher in Scandinavian bears than in Alaskan bears (3). However, Alaskan bears consuming salmon and other marine foods had higher mercury and arsenic blood concentrations compared to Scandinavian bears feeding on berries, ants, and moose (3). Blood concentrations of cadmium, lead, and arsenic in female bears increased with age, while arsenic and mercury concentrations in male bears appeared to decrease with age (3).

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Despite the high levels of heavy metals, including lead, in the bear population in Scandinavia, the effects of these toxins on individual and population health remain unknown. Recent research suggests that three blood measurements indicative of chronic kidney disease (hemoglobin, hematocrit and creatinine) were correlated with lead (7). Lead was present in all brown bear tissues analyzed and the results suggest a body distribution similar to humans (three-compartment model) (7). However, no histopathological changes were identified in liver, kidney, or spinal cord tissues, so the definite health effects of lead in brown bears are still undetermined (7).

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So where does the lead come from? The main sources of lead seem to come from human activities; mainly lead-based ammunition, leaded gasoline, and industrial activity. Environmental lead concentration is the main predictor of blood lead levels in bears (8). Furthermore, moose harvest distribution and scavenging on slaughter remains was an additional source of lead exposure in bears (8).

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Caution when consuming

Edible tissues from hunted Scandinavian bears pose a risk to human health if consumed (7). The majority of samples exceeded the maximum allowable lead concentrations in meat and offal from domestic livestock. Again, the mean blood lead concentration in bears was 96.6 μg/L (range: 38.7–220.5 μg/L) (2). Importantly, both the mean and range are well above established threshold concentrations for developmental neurotoxicity (12 μg/L), increased systolic blood pressure (36 μg/L) and prevalence of chronic kidney disease in humans (15 μg/L) (2). Additionally, 98% of the hunted bears were shot with lead-based ammunition, further increasing contamination of bear meat products with lead.

References