Comprehensive Notes on Heavy Metals Toxicity: Arsenic, Lead, Mercury, and Selenium

Arsenic Toxicity

Definition and Forms:
- Arsenic is a chemical element that exists in several distinct forms.
- It is found in both organic and inorganic forms.
- Valency numbers for arsenic range from $+3$ to $+5$ .
- Toxicity comparison: $As^{+3}$ (arsenite) is significantly more toxic than arsenate or $As^{+5}$ .
- The overall toxicity of arsenic is primarily determined by its specific chemical form.
Arsenic Sources:
- Environmental: Widely distributed in soil and ores, often combined with sulfur in the form of pyrites.
- Industrial: Produced as arsenic trioxide ( $As_2O_3$ ) during the metal refining process.
- Pesticides and Herbicides:
  - Used in pesticide formulations as arsenic trioxide (known as white arsenic).
  - Sodium arsenite is utilized as an herbicide and a defoliant.
  - Insecticides include Copper acetoarsenite (Paris green), arsenic trioxide, and lead arsenate.
- Miscellaneous: Used as a paint pigment (emerald green), in certain detergents, and in medicaments such as Fowler’s solution.

Mode of Action of Arsenicals

Comparative Toxicity and Excretion:
- Trivalent arsenicals are consistently more toxic than pentavalent forms.
- Excretion pathways: Once absorbed, pentavalent arsenic is readily excreted by the kidneys. In contrast, trivalent arsenic is predominantly excreted into the intestine via the bile.
- Attribution of effects: Nearly all major toxic actions of arsenic are attributed to the trivalent form.
Biochemical Mechanism:
- All arsenicals exert their effects by reacting with sulfhydryl ( $SH$ or thiol) groups within cellular environments.
- The reaction is represented as: $R-As=O + 2RSH \rightarrow R-As(SR)_2 + H_2O$ .
- Net Physiological Effect: This reaction blocks fat and carbohydrate metabolism and inhibits cellular respiration.
- Enzymatic Inhibition:
  - Arsenite ( $As^{+3}$ ) reacts with the sulfhydryl groups of proteins, blocking active enzymatic groups.
  - Specifically inhibits alpha-keto oxidases, which contain dithiol groups necessary for the oxidation of pyruvate.
  - Inhibits Lipoic acid, an essential co-enzyme for pyruvic acid oxidase and alpha-oxyglutaric acid oxidase, which are vital precursors in the tricarboxylic acid (TCA) cycle.
- Susceptibility: Cells that are actively dividing and have high oxidative energy requirements are the most susceptible to arsenical interference.
Pentavalent Arsenic and Uncoupling:
- The arsenate (pentavalent) ion is capable of uncoupling oxidative phosphorylation.
- While the clinical importance of this is not fully established, it may account for reported effects on the spinal cord and peripheral nerves in humans.
Vascular and Renal Damage:
- Capillary Poisoning: Arsenic is a potent capillary poison that causes damage and dilatation. This results in the transudation of plasma into the intestinal tract, leading to sharply reduced blood volume and pressure, which often falls to shock levels followed by circulatory failure.
- Arsenic Nephrosis: Commonly observed in humans and small animals.
  - Glomerular capillaries dilate, allowing plasma to escape.
  - Results in swelling and varying degrees of tubular degeneration.
  - Progresses to oligourea due to low blood pressure and fluid loss (anhydremia).
  - Urinary findings include protein, RBCs, and casts.
Arsine Gas:
- A highly toxic industrial gas that combines with hemoglobin.
- It is oxidized into hemolytic compounds, causing severe blood cell destruction.

Clinical Signs and Classifications of Arsenic Poisoning

Peracute Toxicity:
- Often results in sudden death within minutes to a few hours.
- Occurs if a high dose of dissolved arsenic is ingested.
Acute Poisoning:
- Symptoms: Abdominal pain (colic), vomiting, staggering gait, weakness, incoordination, rapid and weak pulse, and shock.
- Gastrointestinal: Profuse diarrhea, followed by collapse and death.
- Dermal Contact: Results in blisters, edema, cracking, and bleeding of the skin, making it prone to secondary infections.
Subacute Poisoning:
- Results from lower doses over a longer period; animals may survive for several days.
- Clinical manifestions: Depression, anorexia, stiff and incoordinated movements.
- Gastrointestinal: "White rice Diarrhea," which is dark, possibly hemorrhagic, and very fluid.
- Renal: Hematuria, proteinuria, and the presence of urinary casts.
Chronic Poisoning:
- Signs: Easy fatigue and dyspnea during movement.
- Animals exhibit intense thirst, a rough dry hair coat, and dry, brick-red mucous membranes.
- Cattle specific sign: Enlarged joints.

Lesions and Postmortem Findings in Arsenic Toxicity

Gross Lesions:
- Generally dependent on dose and survival time (except in peracute cases where lesions may be absent).
- Most animals exhibit excess fluid in the Gastrointestinal (GI) tract.
- Cattle: Hyperemia of the abomasum may be the only finding. In more severe cases, necrosis of the rumen mucosal epithelium occurs.
- Ruminants: Gelatinous serosal edema in the rumen, reticulum, omasum, and abomasum.
- Liver: Characteristically soft and yellow.
Microscopic Lesions:
- Evidence of demyelination in the optic nerve and the posterior spinal cord.

Treatment and Management of Arsenic Toxicity

Initial Intervention:
- Early diagnosis is critical.
- In non-vomiting species (ruminants and horses), large doses of saline purgatives are used to remove unabsorbed material from the GI tract.
- Demulcents can be administered to coat and protect irritated GI membranes.
General Antidotes:
- Sodium thiosulphate: Administered orally or intravenously.
  - Adult Horse/Cattle Dose: $20-30\,g$ orally in $300\,ml$ water; $8-10\,g$ intravenously in a $10-20\%$ solution.
  - Sheep/Goat Dose: Approximately one-quarter of the adult bovine dose.
British Anti-Lewisite (BAL) / Dimercaprol:
- A sulfhydryl-containing specific antidote for trivalent arsenic.
- Mechanism: Arsenic's affinity for the $SH$ radical allows BAL to form a thioarsenite complex, which is rapidly removed from tissues and excreted by the kidney.
- Dosage Protocol: $3\,mg/kg$ body weight intramuscularly via $5\%$ solution.
  - Days 1-2: Every 4 hours.
  - Day 3: Four times daily.
  - Next 10 days: Twice daily until recovery.
Small Animal Protocol:
- Stomach emptying via gastric lavage with warm water or $1\%$ sodium bicarbonate (preferred before absorption occurs).
- Emetics like apomorphine can be used early; however, lavage and emetics are contraindicated once clinical signs are already present.
- BAL Dosage: $3\,mg/kg$ three times daily.
Supportive Therapy:
- Parenteral fluids (electrolytes) for rehydration.
- Uremia management: Lactate Ringer’s solution with Vitamin B complex and $10\%$ dextrose at $20\,ml/kg$ body weight.
- Acidosis management: $50\%$ sodium lactate added to Ringer’s at $2-5\,ml/1000\,ml$ .
- Other: Daily Vitamin B complex injections, blood transfusions for shock/anemia, antibiotics for secondary infection, meperidine for abdominal pain, and high protein diets.

Lead Toxicity Overview

Properties and Source:
- Bluish-white to gray heavy metal; historically the first recognized toxic element.
- Symbol: $Pb$ , from the Latin "plumbum" (liquid silver).
- Primary source: Galena ore (lead sulfide).
Uses:
- Current: Lead-acid storage batteries.
- Historical: Paint pigments (white, yellow, red) and gasoline additives (tetraethyl lead).

Mechanism of Lead Action

Enzymatic and Chemical Interference:
- Binds to sulfhydryl and other nucleophile functional groups.
- Inhibits enzymes and alters Calcium and Vitamin D metabolism.
- Contributes to oxidative stress.
Hematological Impact:
- Inhibits hemoglobin synthesis by interfering with the heme pathway.
- Specifically inhibits delta-aminolevulinic acid dehydratase and ferrochelatase activity.
- Leads to increased fragility of red blood cells and chronic anemia.
Neurotoxicity:
- Disrupts brain structures by mimicking or inhibiting Calcium as a cell regulator.
- Alters neurotransmitter release (Dopamine, Acetylcholine, and GABA).
- Enhances spontaneous release while inhibiting evoked release. This is attributed to protein kinase activation and blockade of voltage-dependent calcium channels.
- Breakdown of the Blood-Brain Barrier (BBB): High levels of lead alter endothelial cell properties and damage astrocytes that maintain BBB integrity.
- Neuronal Changes: Causes necrosis, shrunken cytoplasm, pyknotic nuclei, and increased perineuronal space.

Clinical Signs and Diagnosis of Lead Poisoning

Major Systems Affected:
1. Gastrointestinal system.
2. Central nervous system.
3. Hematological system.
General Signs:
- Abdominal pain, diarrhea, anorexia, and vomiting.
- Neurological: Depression, weakness, ataxia, muscle tremors/fasciculations, head pressing (ruminants), blindness, and seizure-type activity.
Species-Specific Manifestations:
- Horses: May be more susceptible to chronic toxicosis. Show clinical signs of laryngeal paralysis (known as "roaring"), abdominal colic, and seizures.
- Avians: Primarily chronic wasting and peripheral neuropathy. Psittacines show both GI and neurological abnormalities.
Diagnostic Findings:
- Hematological: Basophilic stippling of erythrocytes and inhibition of hemoglobin synthesis.
- Blood Concentrations:
  - Normal: Below $0.1\,ppm$ .
  - Clinical Toxicity: Concentrations above $0.35\,ppm$ .
- Postmortem Samples: Kidney and liver with concentrations above $10\,ppm$ (wet weight basis) are diagnostic.
- Gross Lesions: Often minimal; lead-containing objects might be visible in the GI tract.
- Histology: Renal tubular epithelium necrosis or acid-fast inclusion bodies. Cerebral cortex may show shrunken, angular/triangular neurons with pyknosis and rhexis.

Treatment of Lead Toxicity

Chelation Therapy:
- Livestock: Calcium disodium edetate ( $Ca-EDTA$ ) at $110\,mg/kg/day$ IV or SC (split into 2 doses for 3 days; repeat after 2 days).
- Dogs: $Ca-EDTA$ at $110\,mg/kg/day$ (split into 4 doses SC in $5\%$ dextrose for 2-5 days).
- D-Penicillamine: Used in dogs ( $110\,mg/kg/day$ PO for 2 weeks), though emesis and anorexia are side effects. Not recommended for livestock.
- Succimer (DMSA): Effective in dogs ( $10\,mg/kg$ PO for 10 days) and birds; fewer side effects than $Ca-EDTA$ .
Specific and Supportive Treatments:
- Thiamine: $2-4\,mg/kg/day$ SC to reduce tissue deposition and clinical signs.
- Cathartics: Magnesium sulfate ( $400\,mg/kg$ PO) or rumenotomy to remove lead from the gut.
- Control of convulsions: Barbiturates or tranquilizers.
- Antioxidants: N-acetylcysteine ( $50\,mg/kg$ PO) used with DMSA to limit oxidative damage.

Mercury Toxicity

Forms and Environment:
- Three forms: Elemental (metallic), inorganic, and organic.
- $80\%$ of environmental mercury is metallic, originating from human activities like mining and fossil fuel combustion.
- Uses: Thermometers, batteries, dental amalgams, electric switches, and medical tools.
Compound Classifications:
- Inorganic: Salts formed with Chlorine, Sulfur, or Oxygen (e.g., fungicides, laxatives, antiseptics).
- Organic (Organomercurials): Mercury combined with Carbon (e.g., Methylmercury, Ethylmercury, Phenylmercury).
- Interchangeability: In environment and mammalian systems, forms are interchangeable (e.g., inorganic can be methylated by bacteria).
Bioaccumulation and Minamata Disease:
- Methylmercury bioaccumulates at the top of the food chain.
- Minamata Disaster (Japan, 1950s): Caused by methylmercury release into the bay; resulted in severe brain damage in thousands of people.

Toxicokinetics and Mechanism of Mercury

Absorption and Distribution:
- Metallic Mercury: $80\%$ absorbed via lungs; minimal GI absorption. Highly lipophilic; crosses the Blood-Brain Barrier (BBB) and placenta. Converted to divalent mercury in the brain and trapped.
- Organic Mercury: $95\%$ absorbed from the GI tract. Readily accumulates in the brain and fetus.
- Excretion: Urine, feces, milk, and trace amounts in exhaled air.
Toxic Mechanism:
- Primary target organs: Kidney and Brain.
- High-affinity binding of divalent cationic mercury to thiol or sulfhydryl ( $SH$ ) groups.
- Consequences: Inactivation of enzymes, structural proteins, transport proteins, and formation of mercaptides which alter membrane permeability.
- Includes: Increased oxidative stress, mitochondrial dysfunction, glutathione depletion, BBB permeability increase, and disruption of DNA replication.
Clinical Toxic Effects:
- Horses: Acute gastroenteritis and nephritis. Chronic: Neurological dysfunction, laminitis, and renal disease (glycosuria, proteinuria, azotemia).
- Sheep: Severe neurological symptoms and tetraplegia.
- Pigs: Incoordination and recumbency.
- Sensitivity: Cats and monkeys are more sensitive to neurotoxic effects than rodents.

Diagnosis and Treatment of Mercury Toxicity

Diagnostic Samples:
- Recent exposure: Blood analysis.
- Metallic/Inorganic: Urine testing.
- Methylmercury: Whole blood or hair.
- Postmortem: Kidney is the ideal specimen.
Treatments:
- Activated Charcoal: Effective at $1-3\,g/kg$ body weight PO.
- Chelators:
  - Dimercaprol (BAL): $3\,mg/kg$ IM is most effective, though it may redistribute mercury to the brain.
  - Sodium thiosulfate: $1\,g/kg$ orally.
- Antioxidants: Vitamin E may decrease toxicity.

Selenium Poisoning (Selenosis)

Occurence and Naming:
- Occurs in regions with high soil selenium content.
- Known as "blind staggers" or "alkali disease" in America/Canada and "dog murrain" in Ireland.
Plant Categories:
1. Obligate accumulators (Indicator plants): Need selenium for growth ( $1000-1500\,ppm$ ), e.g., Astragals spp..
2. Facultative accumulators (Secondary absorbers): Can accumulate large quantities if present, e.g., Aster spp..
3. Non-accumulators: May take up enough to become dangerous in rich soil.
- Other source plants: Acacia cana, Atriplex spp..
Absorption and Toxicity:
- Organic forms in plants and grains are twice as toxic as selenide or selenite forms.
- Distribution: High in liver, spleen, and kidney; lowest in brain and muscle. Stored in hair and hoofs over time.
- Placenta: Causes congenital malformations in foals and lambs.
- Oral Lethal Doses: Horse ( $3\,mg/kg$ ), Cow ( $10\,mg/kg$ ), Pig ( $17\,mg/kg$ ).
- Milk: Concentrations up to $3\,ppm$ can toxicize nursing calves.
Biochemical Mechanism:
- Inhibits oxidation-reduction systems and succinic dehydrogenase.
- Replacement of Sulfur by Selenium in proteins.
- Decreases ATP, Methionine, and Ascorbic acid in the liver.

Forms of Selenium Toxicity

Acute Form:
- Causes: Ingestion of indicator plants or accidental high doses during treatment for muscular dystrophy.
- Signs: Weak pulse, dyspnea, bloating, polyurea, death by respiratory failure.
- Postmortem: Generalized hemorrhage and ascites.
Sub-acute Form (Blind Staggers):
- Causes: Feeding on plants with $10-20\,ppm$ selenium for 7-8 weeks.
- Signs: Staggering gait, impaired vision (circling), salivation, paralysis, and abdominal pain.
- Postmortem: Liver cirrhosis and necrosis; brain hemorrhage and softening.
Chronic Form (Alkali Disease):
- Signs: Loss of mane/tail hair in horses; rough coat and emaciation in cattle.
- Hoofs: Rings appear below coronary band; hoofs may crack or slough (horse) or overgrow (cattle). Lameness due to erosion of articular surfaces.
- Avian: Decreased hatchability and chick deformity.
- Postmortem: Heart atrophy and liver cirrhosis.

Diagnosis, Treatment, and Control of Selenosis

Diagnosis: Clinical signs, analysis of selenium in tissues/plants, and history of indicator plants in pasture.
Treatment Limitations:
- BAL is NOT used as it increases renal toxicity.
- $EDTA$ produces a toxic compound.
- No effective treatment for acute selenosis.
Management Strategies:
- Arsenic Protection: Arsenic binds selenium in the liver into a non-toxic form. Add $5\,ppm$ sodium arsenite to drinking water.
- Bromobenzene: Increases urinary elimination of selenium.
- Dietary Protections: High protein content and addition of linseed oil to rations offer protection.
- Control: Manual removal of indicator plants or use of weed killers; soil testing; adding arsenilic acid ( $0.01\%$ ) to cattle rations.