Animal Exposure to Lead, Mechanism of Toxicity and Treatment Strategy-A Review

Lead is a metal discovered earlier by humans. It has several unique properties such as softness, high malleability, ductility, low melting point, resistance to corrosion and low cost. This unique property has made lead’s widespread usage in different industrial sectors, which in turn has led to its occurrence in free form in biological systems and the inert environment. Over the last few decades, with the adverse effects of lead coming to the forefront, countries all over the world have started recognizing lead toxicity. It has caused several debilitating effects on animals. This review covers the history behind the usage of lead, sources of lead exposure in animals, absorption, distribution and excretion of lead, toxic signs of lead toxicity, diagnosis, lesions of lead toxicity in animals and the current treatment regimen in animals. This also covers the details of current research work going on in the area of herbal remedies against lead induced liver damage.

Heavy metals contamination in the environment is a major global challenge, because of toxicity and threat to animals, human life and ecosystem. Metals and its toxicity are increasing in all areas of the environments, this includes, air, water and soil. Metal contaminated environments pose serious threat to animal health and ecosystems [1]. One of the earliest metals discovered by the human population is Lead (Pb) [1]. It has a number of unique properties such as low melting point, ductility, high malleability, resistance to corrosion and low cost. This unique property has made its widespread usage in different industrial sectors, which in turn has led to occurrence in free form in the environment. During those periods, hazardous effects of lead was not considered but, in recent years the side effects of lead toxicity become pronounce. At the same time, because of lead’s non-biodegradable nature, it is considered potent hazardous toxins and could cause serious health issues to animals. There are a number of reports evidencing this aspect. One of such report is the incidence of childhood lead poisoning; it is mainly due to drinking contaminated water in West Bengal and Bangladesh [2]. The use of lead and its toxicity in the urban and rural areas has become a national calamity when compared to its occurrence in Himalayan population with no industrial lead exposure.

History Behind Usage of Lead

The use of lead by humans’ dates back to thousands of years to the times of Romans, Egyptians and Babylonians for making statues, coins, water pipes and weights. It was not used for ornamental usage because of its soft nature [1]. They used lead compounds to glaze containers used for food and water, boil and condense grape juice in lead pots for pre serving and sweetening of wine [3]. One of the major sources of lead exposure are from lead acid batteries, cosmetics, leaded gasoline and paints. The usage of leaded gasoline was banned in US from 1970, followed by 1975 many countries including Western Europe, Korea, Thailand, Australia, China, Vietnam, the Philippines, Japan, Canada, Mexico, Central and South America stopped using this of leaded gasoline [4]. India banned the use of leaded petrol by 2000.

Sources of Lead Exposure

Domestic Environment

Lead has a widespread occurrence in the environment; the contributing sources includes both natural and human activities. Human activity which includes smelting, refining and mining, this may result in lead concentration in the environment.

Food: Lead presence in the environment may get deposited in growing plant or food processing and results in lead contamination, few reasons are given below:

• Lead present in pesticide, fertilizer or soil may take up into root of plant and gets deposited in leaf.
• Lead from industrial origin may get deposited in the plant.
• Canned food which is leached from the solder in the seams of can serve as the source of lead.
• Leaching of lead from vessels like lead glazed ceramic or porcelain pots

Drinking water: Usage of water pipes which are made up of lead may contaminate the drinking water and increases the blood lead level of those who consumes it.

Air: Lead in the air comes from various sources. The largest contribution comes through leaded gasoline. Highest concentration is observed near smelters.

Lead based paint: Lead has been used as a pigment and drying agent in primers, paints and resins. Its usage was banned in 1970’s. The usage of lead paints was banned for residential purpose, but still older building that were already painted with leaded paint may cause lead exposure in young children or animals [5].

Occupational Environment

Occupational and environmental exposure [Table 1] to lead may result in serious health issues in developing countries [6].

Table 1: Industries and occupation associated with lead exposure.

Absorption, Distribution and Excretion of Lead

Absorption 

Routes of absorption of Lead (Pb) is through ingestion, inhalation or through skin. Exposure to lead occurs mainly through gastrointestinal (GI) tracts and respiratory system [7]. Absorption is through respiratory system mainly, depending on size of the particle. It is estimated that between 30-40% of inhaled lead reaches the bloodstream [8]. Absorption rate through GI tract also, depends on the age and nutritional status of the exposed individual. The average percentage of lead absorption in adult monogastric animals is 15% of the ingested quantity whereas it decreases to 5% in ruminants, and young animals can absorb up to 95%. The percentage of lead absorption increases in fasting state and in the deficiency of calcium, iron, phosphorus or zinc [9]. Iron leads to defective absorption of lead, hence, Iron deficiency leads to increased concentration of blood lead in young animals [10]. Researchers demonstrated that increased intake of intraluminal calcium supplementation results in decreased absorption of lead [11]. High intake of magnesium, phosphate and dietary fat leads to decrease gastrointestinal absorption of lead [12]. Lead absorption occurs through both passive and facilitated diffusion [13]. Some studies have evidenced the hypothesis that, divalent metal transporter 1 (DMT1) is responsible for lead transport [14]. Inorganic form of Pb from food, water, paint, vinyl products and tetraethyl lead from leaded gasoline are absorbed through the skin [15]. In case of lead absorption through skin, it is first transported into the plasma and rapidly concentrated into the extra cellular fluid pool like sweat and saliva without significant uptake by erythrocytes.

Distribution of Lead 

After absorption lead gets accumulated in blood, soft tissues and bone. Approximately 99% of the absorbed lead accumulates in the erythrocytes, 1% is left in plasma and serum. The kinetics of lead transfer from blood to soft tissues which tends to be low and takes approximately 4 to 6 weeks. Because of the short half-life of 35 days in the blood, the blood lead level cannot be used to diagnose the lead exposure [16]. Higher percentage of lead is taken up by the kidney followed by liver and other soft tissues [17]. Half-life of lead in various organs varies, in blood the half-life is 35 days [18]. In soft tissues half-life was found to be 40 days and in bones it was 20 to 30 years [19]. Lead distribution in various organs depends on the blood flow to the tissues. It can cross the blood brain barrier [20]. Various studies revealed that oral intake of inorganic lead affect the immune system.

Excretion of Lead 

Inorganic lead does not metabolize in our body, which is excreted unchanged in urine. The mechanisms by which the absorbed lead excreted appear unclear. Lead is Excreted by many ways which includes secretion into the bile, gastric fluid, and saliva [2]. Alkyl lead like tetraethyl and tetramethyl lead on oxidative dealkylation, form a highly neurotoxic compound [22]. This reaction is catalyzed by cytochrome p450-dependent monooxygenase enzyme present in liver [23]. Other routes of excretion of lead includes nails and sweat [24,25]. On the whole, lead is excreted very slow and tends to accumulate in the body with biological half-life of 10 years. Lead is also excreted in milk in concentrations of up to 12 µg/L. Animals are more vulnerable to lead toxicity due to the following reasons: 

1. Frequent licking of walls and their bodies. 
2. Nervous systems are rapidly developing in animals. 
3. Lead in kidney interfere with vitamin D 1,2 dihydroxy cholecalciferol. 
4. Lead interferes with the formation of active vitamin D, thereby interfere with calcium absorption. 

The absorption, distribution and excretion of lead is presented in [Figure 1].

Figure 1: Diagrammatic representation showing the source of exposure, absorptions, distribution and excretion of lead [1].

Biochemical indicators of lead toxicity

Generally, Blood Lead Level (BLL), Blood δ Amino Levulinic Acid Dehydratase (ALAD) activity, Urinary Amino Levulinic Acid (ALA), Erythrocyte Protoporphyrin level and creatinine levels are measured to evaluate lead toxicity [1].

Toxic signs of lead toxicity in animals

Blood Led Level (BLL) of 60 μg/dL was considered safe during 1960s. In 1985, the acceptable level was reduced to 25 μg/dL and it was further reduced to 10 μg/dL in 1991 [26,27,28]. The World Health Organization (WHO) lead guidelines recommended tolerable lead intake level based from review of the scientific evidence conducted in 2010, the Joint Food and Agriculture Organization of the United Nations estimated the safer weekly intake of 25 µg/kg body weight. Acute lead poisoning is more common in young animals. The prominent clinical signs are associated with the GI and nervous systems. In cattle, clinical signs that appear within 24–48 hours of exposure include ataxia, blindness, salivation, spastic twitching of eyelids, jaw champing, bruxism, muscle tremors, and convulsions [34].

Subacute lead poisoning, usually seen in sheep or older cattle, is characterized by anorexia, rumen stasis, colic, dullness, and transient constipation, frequently followed by diarrhea, blindness, head pressing, bruxism, hyperesthesia, and incoordination [34]. Chronic lead poisoning, occasionally seen in cattle, may produce a syndrome that has many features in common with acute or subacute lead poisoning. Impairment of the swallowing reflexes frequently contributes to development of aspiration pneumonia. Embryotoxicity and poor semen quality may contribute to infertility [34].

GI abnormalities, including anorexia, colic, emesis, and diarrhea or constipation, are predominant manifestations in dogs. Anxiety, hysterical barking, jaw champing, salivation, blindness, ataxia, muscle spasms, opisthotonos, and convulsions may develop. CNS depression rather than CNS excitation may be evident in some dogs. In horses, lead poisoning usually produces a chronic syndrome characterized by weight loss, depression, weakness, colic, diarrhea, laryngeal or pharyngeal paralysis (roaring), and dysphagia that frequently results in aspiration pneumonia [34].

In birds, anorexia, ataxia, loss of condition, wing and leg weakness, and anemia are the most notable signs.

Lesions

Animals that die from acute lead poisoning may have few observable gross lesions. Oil or flakes of paint or battery may be evident in the GI tract. The caustic action of lead salts causes gastroenteritis. In the nervous system, oedema, congestion of the cerebral cortex, and flattening of the cortical gyri are present. With pathologic analysis, endothelial swelling, laminar cortical necrosis, and oedema of the white matter may be evident. Tubular necrosis and degeneration and intranuclear acid-fast inclusion bodies may be seen in the kidneys. Osteoporosis has been reported in lambs. Placentates and accumulation of lead in the fetus may result in abortion [34].

Mechanism of lead-induced toxicity 

Though various mechanisms were postulated about lead induced toxicity, the mechanism represented in [Figure 2] was considered to be most important mechanism which involves oxidative stress. Enormous number of evidences have shown that lead induced generation of reactive oxygen species resulted in oxidative stress and weakens the cells defense mechanism [29].

Figure 2: Schematic representation of various mechanisms of lead toxicity.

There is an important indirect mechanism which involves the depletion of cells’ major sulfhydryl resulting in oxidative stress [30]. When Glutathione (GSH) is depleted in the body by lead, the body starts making more GSH from cysteine. This antioxidant defense mechanism may be protected by many enzymes. The cofactors like selenium, zinc, copper of many enzymes may be replaced by lead, and thereby, resulting in enzyme inactivation. Various studies in lead exposed animals were reported to have, either elevated lipid peroxidation or decreased intrinsic antioxidant defense in various tissues. 

Oxidative Stress 

Superoxide Dismutase (SOD), a free radical scavenger and metalloenzyme (zinc/copper) [31]. Various research revealed that lead exposure significantly decreases the level of SOD. This may be due to an increase in lead concentration in these tissues and their possible reaction with this enzyme thereby, reducing the disposal of superoxide radicals [31]. Catalase is an efficient decomposer of H2O2 and known to be susceptible to lead toxicity. Lead induced decrease in brain Glutathione Peroxidase (GPx) activity may arise as a consequence of impaired functional groups such as GSH and Nicotinamide Adenine Dinucleotide Phosphate (NADPH) or selenium mediated detoxification of toxic metals. While antioxidant enzyme Glutathione S-transferase (GST) is known to provide protection against oxidative stress [32]. Lead reaction with oxyhemoglobin results in superoxide radical formation. 5-ALAD is involved in the formation of heme precursor (porphobilinogen) by the condensation of two S-aminolaevulinic acid (ALA). Hence, the ALAD inhibition results in the impairment of heme synthesis, and resulting in accumulation of ALA [Table/Fig-4]. Accumulation of ALA undergo metal catalyzed autooxidation, resulting in the conversion of oxyhemoglobin to methemoglobin. This conversion results in the formation of ROS like superoxide and hydroperoxides. 

Diagnosis of Lead Poisoning in Animals

• Tentative diagnosis is based on neurologic and gastrointestinal manifestations
• Confirmed by whole-blood analysis antemortem or by analysis of liver and kidney tissues postmortem

Lead concentrations in various tissues may be useful to evaluate excessive accumulation and to reflect the level or duration of exposure, severity, and prognosis and the success of treatment. Concentrations of lead in the blood at 0.35 ppm, liver at 10 ppm, or kidney cortex at 10 ppm are consistent with a diagnosis of lead poisoning in most species. Many countries have deemed blood lead concentrations >0.05–0.10 ppm to be a notifiable disease in food-producing animals. Inspection or clearance by a regulatory veterinary officer or biosecurity inspector is mandatory before shipment for food consumption is permitted. Hematologic abnormalities, which may be indicative but not confirmatory of lead poisoning, include anemia, anisocytosis, poikilocytosis, polychromasia, basophilic stippling, metarubricytosis, and hypochromia. Blood or urinary delta-aminolevulinic acid and free erythrocyte protoporphyrin levels are sensitive indicators of lead exposure but may not be reliable indicators of clinical disease. Radiologic examination may be useful to determine the extent of lead exposure. Lead poisoning may be misinterpreted with other diseases that causes nervous system or GI abnormalities. In cattle, such diseases may include polio encephalomalacia, nervous coccidiosis, tetanus, hypovitaminosis A, hypomagnesemic tetany, nervous acetonemia, organochlorine insecticide poisoning, arsenic or mercury poisoning, brain abscess or neoplasia, rabies, listeriosis, and Haemophilus infections.

In dogs, rabies, distemper, and hepatitis may appear similar to lead poisoning.

Current Treatment Strategy for Lead Toxicity 

Chelation Treatment [33] 

Some common chelating agents used against lead poisoning are given below: 

1. Calcium disodium ethylene diamine tetra acetic acid (CaNa2 EDTA).
2. D-penicillamine.
3. Meso 2,3-dimercaptosuccinic acid (DMSA).
4. Sodium 2,3-dimercaptopropane-1-sulphonate (DMPS).

Figure 3: Lead effects in haemoglobin synthesis pathway. ALAD: Amino levulinic acid dehydratase.

Limitations of Current Chelating Agents 

Treatment with DMSA and DMPS has got lesser adverse effects given in [Table 2]. Most of the conventional chelators have side effects like, reducing essential element level in the body. The Centers for Disease Control and Prevention (CDC) recommended chelating agent only when the blood lead level goes beyond 45 μg/dL. So, there is always a need for an alternative treatment with no side effects [1].

Table 2: Limitations of current chelating agents.

CaNa2 EDTA: Calcium disodium ethylene diamine tetraacetic acid; ECF: Extracellular fluid

Chelation therapy is for companion animals, while treatment with chelators is not recommended for food -producing animals, though useful. A poor prognosis related to bad response to therapy, extensive supportive care associated with anorexia, permanent tissue damage, lack of approved chelation products, and significant tissue residues create significant economic and public health concerns. Prompt chelation therapy in companion animal species is often successful [34]. If tissue damage is extensive, particularly to the nervous system, treatment may not be successful. In food producing animals, calcium disodium edetate (Ca-EDTA) is administered IV or SC (110 mg/kg per day) divided twice a day for 3 days; this treatment should be repeated 2 days later. In dogs, a similar dose divided 4 times a day is administered SC in 5

Lead is a heavy metal that causes environmental pollution. It gets to the environment through the deterioration from lead-based paints, batteries and business that involves lead. When blood lead level reaches more than 10 microgram/dL in animals, it produces toxicity. The signs and lesions of lead toxicity in animals vary depends upon the dose of exposure. It affects the Nervous system, Renal system, Endocrine glands, Blood, Gastrointestinal tract, Cardiovascular system and Reproductive system of animals. The major mechanism for lead toxicity is due to increased production of reactive oxygen species and inhibition of enzyme action (Lead binds to enzymes with sulfhydryl group). Chelating agents like DMSA-2,3 dimercaprol succinic acid and monois amyl DMSA can be used against lead toxicity. But, the main disadvantage of chelators is that, they are toxic in nature and it cannot completely remove lead from all tissues. Newer trend in treating lead toxicity in animals is by using natural antioxidants. Antioxidant properties of herbal products can have beneficial effect on treating lead induced toxicity. This review will help young researchers to start more works on lead related toxicity study in animals.

Funding Declaration

There was no funding for this research

Data Availability

Data is provided within the manuscript