Comprehensive Veterinary Toxicology Notes: Selenium, Molybdenum, Zinc, Arsenic, and Mercury

Curriculum Context and Nutritional Considerations

• Large animal nutrition is critical for clinical evaluation and formulating differential diagnoses, whereas small animal nutrition is generally standard and consistent.

• Educational emphasis should be placed on nutritional toxicology and deficiency states in livestock to aid in recognizing metabolic and environmental diseases.

Selenium (Se) Overview and Functions

• Biological Function:

  • Selenium is an essential trace mineral required for cellular antioxidant defense mechanisms.

  • It functions synergistically with Vitamin E to protect cell membranes against oxidative damage.

• Selenium Deficiency Disorders:

  • White Muscle Disease: Causes degeneration and loss of normal striation and structural organization in skeletal and cardiac muscle tissues, primarily in calves, lambs, and foals.

  • Exudative Diathesis: Seen in poultry; characterized by severe subcutaneous swelling and fluid accumulation due to increased capillary permeability.

• Geographic and Soil Distribution:

  • Seleniferous Soils: Common in the Western United States, leading to elevated selenium content in local flora and crops. Studies in human populations in these areas have suggested potential correlations with localized resistance to certain carcinomas.

  • Deficient Soils: Common in the Eastern and Southeastern United States, predisposing local livestock to deficiency disorders unless supplemented.

• Chemical Characteristics:

  • Selenium exists in multiple valence states, allowing it to form various chemical compounds that differ significantly in their toxicological potential.

  • It is chemically similar to sulfur and tellurium, allowing it to substitute for sulfur in amino acids (e.g., replacing sulfur to form selenocysteine or selenomethionine).

  • In marine organisms, selenium bound to cysteine is frequently accumulated without causing acute toxicity, though marine mammals remain susceptible to high exposure levels.

• Commercial Supplementation:

  • Common injectable selenium and Vitamin E preparations include Bo-Se, E-Se, and L-Se, utilized in bovine and equine medicine to prevent nutritional muscular dystrophy and maintain skin and muscle tissue integrity.

Selenium Accumulator Plants

• Obligate Selenium Accumulators:

  • Require high concentrations of soil selenium for normal growth and development.

  • Accumulate selenium levels exceeding $1000\,\text{ppm}$.

  • Represent the primary toxic threat to livestock grazing on seleniferous soils.

  • Examples include:

    • Locoweed (Astragalus species)

    • Goldenweed (Oonopsis species)

    • Stanleya species

    • Xylorhiza species

• Facultative Indicator Plants:

  • Absorb and tolerate substantial amounts of soil selenium but do not require it for growth.

  • Accumulate selenium levels up to $100\,\text{ppm}$.

  • Examples include:

    • Atriplex species

    • Machaeranthera species

    • Sphaeralcea species

    • Castilleja species

    • Comandra species

• Non-Indicator Plants:

  • Includes standard forage plants, pasture grasses, and crops.

  • May accumulate up to $25\,\text{ppm}$ selenium when grown in seleniferous soils, but generally do not concentrate toxic levels.

Selenium Toxicity Mechanisms

• Depletion of cellular glutathione ($GSH$) reserves.

• Induction of secondary lipid peroxidation.

• Substitution for sulfur in sulfur-containing amino acids, leading to dysfunctional proteins and structural keratins.

• Depression of cellular ATP synthesis.

• Alteration of tissue Vitamin C (ascorbic acid) concentrations.

• Generalized capillary and vascular damage.

Acute Selenium Poisoning

• Susceptibility & Exposure:

  • All species are susceptible, but acute cases are most frequently documented in cattle.

  • Typically results from feed formulation errors, accidental administration of species-inappropriate premixes, or acute parenteral/oral overdose.

• Therapeutic Window:

  • Selenium possesses a very narrow margin of safety (narrow therapeutic window); toxic doses are only slightly higher than therapeutic requirements.

• Clinical Presentation:

  • Rapid onset of severe gastrointestinal signs, including profuse diarrhea.

  • Respiratory distress and dyspnea.

  • Profound weakness, collapse, and sudden death within a few hours to several days.

• Pathological Lesions:

  • Major pathology involves the cardiovascular, pulmonary, and renal systems.

  • Severe pulmonary edema, hydrothorax, organ congestion, widespread petechial/ecchymotic hemorrhages, gastroenteritis, and myocardial necrosis.

  • In swine, acute toxicosis can manifest as polioencephalomalacia (focal symmetrical poliomalacia/poliomyelomalacia).

• Diagnostic Findings:

  • Confirmation requires blood or tissue (liver and kidney) analysis showing selenium concentrations exceeding $2\,\text{ppm}$.

• Treatment Protocols:

  • Termination of exposure.

  • Symptomatic and supportive care, including fluid therapy for gastroenteritis/shock and diuretics for pulmonary edema.

  • Administration of $N$-acetylcysteine to protect mucosal surfaces and support glutathione restoration.

  • Dietary supplementation with copper and methionine (or other sulfur-containing amino acids).

Chronic Selenium Poisoning (Alkali Disease)

• Etiology:

  • Results from long-term ingestion (months) of selenium-accumulating plants or grains grown on seleniferous soils. Most commonly observed in horses and cattle.

• Clinical Features & Pathological Lesions:

  • Affects tissues high in keratin (hair and hooves).

  • Marked emaciation, lethargy, and muscular weakness.

  • Dermatological signs: Loss of hair from the mane and tail in horses ("bobtail" appearance), dry/rough coat, and abnormal hair follicle growth leading to a "roached" mane appearance.

  • Hoof pathology: Severe hoof wall deformation, circumferential irregular ridges/cracks, horizontal hooves cracking, structural failure, and sloughing of the hoof wall. Affected animals exhibit severe lameness, reluctance to walk, prolonged recumbency, and reduced feed intake due to pain.

  • Reproductive signs: Reduced fertility, embryonic death, loss of reproductive efficiency, and teratogenic malformations in offspring.

  • Circular cutaneous necrotic lesions may occur.

• Diagnostic Protocols:

  • Blood/tissue concentrations typically exceed $5\,\text{ppm}$.

  • Sample Preparation Caution: Hair and hoof samples must be thoroughly washed prior to laboratory extraction to eliminate external soil or dust contamination.

  • Coat Color Variation: Baseline hair selenium concentrations are naturally higher in dark-pigmented cattle breeds (e.g., Angus).

• Management & Prevention:

  • Immediate removal of livestock from seleniferous pastures/feeds.

  • Testing of local soils and forage crops to map toxic areas.

  • Dietary inclusion of inorganic arsenic compounds has been utilized historically to promote biliary selenium excretion, though this treatment remains controversial.

  • Dietary supplementation with sulfur-containing amino acids and copper.

Blind Staggers Syndrome

• Historically attributed to selenium toxicity from grazing obligate accumulator plants, though modern evidence indicates the syndrome is primarily caused by high dietary sulfate content rather than selenium itself.

• Historically referenced in anecdotes such as the Battle of the Little Bighorn, where Custer's horses allegedly consumed locoweed and developed severe neurological impairment.

• Clinical Presentation:

  • Rapid onset over $1$ to $2$ days.

  • Impaired vision/blindness, anorexia, and aimless wandering or circling.

  • Progression to paralysis, severe recumbency, and death.

Management and Treatment of Selenium Toxicosis

• Immediate Action: Complete termination of exposure to suspect feedstuffs, pastures, or supplements.

• Symptomatic Therapy:

  • Administration of diuretics for fluid overload or edema.

  • Use of $N$-acetylcysteine to protect mucosal membranes.

  • Dietary supplementation with methionine, copper, and essential amino acids.

• Environmental Management: Routine analytical screening of soil and forage samples to determine background selenium concentrations.

• Case Example (Marine Toxicology): Chronic/acute mortalities in captive sea lions shipped from California to Oklahoma were traced back to localized batches of commercial fish containing lethal levels of accumulated selenium, demonstrating rapid toxicity and sudden death in high-exposure events.

Molybdenum (Mo) Toxicosis and Induced Copper Deficiency

• Sources of Exposure:

  • Industrial waste emissions, environmental contamination near brick manufacturing plants, and molybdenum-rich soils.

• Susceptibility:

  • Cattle are exceptionally sensitive; toxicity is characterized by high herd morbidity but variable mortality.

• Pathophysiology (Mineral Interaction):

  • Molybdenum directly binds to copper in the gastrointestinal tract and blood to form an insoluble copper-molybdenum complex (thiomolybdates).

  • This complex facilitates rapid urinary and biliary excretion of copper, inducing a secondary copper deficiency.

  • Impairment of copper-dependent enzymes, such as tyrosinase (required for melanin synthesis), leads to characteristic clinical signs.

• Clinical Signs:

  • Severe, persistent, foul-smelling diarrhea (often called "teart pastures" diarrhea).

  • Emaciation, joint pain, lameness, osteoporosis, and spontaneous bone fractures.

  • Achromotrichia: Loss of hair coat pigmentation, particularly around the eyes ("spectacled" appearance in cattle).

  • Pica (Depraved Appetite): Affected animals ingest non-nutritive materials such as dirt (geophagia), clay, ice, cornstarch, or plastics.

  • Small Ruminant Effects: Ewes develop depigmentation and loss of normal wool structure ("steely wool" due to loss of crimp). Lambs exhibit enzootic ataxia, severe weight loss, and cortical blindness.

• Treatment:

  • Oral or parenteral copper supplementation (e.g., copper sulfate or copper glycinate) to restore the balanced copper-to-molybdenum ratio.

Zinc (Zn) Toxicosis

• Common Sources:

  • United States pennies minted after $1982$ (contain a pure zinc core coated with a thin copper layer; pre-$1982$ pennies are predominantly copper).

  • Galvanized metal hardware, wire transport cages, plumbing pipes, and stock water tanks.

  • Topical zinc oxide preparations (e.g., ointments licked by dogs suffering from acral lick dermatitis).

  • Excessive dietary supplementation or historical denture adhesives (e.g., Polydent).

• Susceptibility:

  • Documented across all species, with the highest clinical incidence occurring in dogs and exotic/zoo animals (e.g., animals ingesting coins thrown into enclosures).

  • Ruminants exhibit high biological sensitivity to elevated zinc concentrations.

• Pathophysiology & Target Organs:

  • Direct mucosal irritation of the gastrointestinal tract.

  • Disruption of cell membrane stability.

  • Primary target organ systems: Gastrointestinal tract, hematopoietic system, liver, and pancreas.

• Clinical Presentation:

  • Early Signs: Anorexia, severe vomiting, diarrhea, lethargy, and weight loss.

  • Progressive Signs: Intravascular hemolysis leading to severe anemia, pale/icteric mucous membranes, hemoglobinuria, polyuria, polydipsia, polyphagia, central nervous system depression, convulsions, coma, and death.

• Diagnostic Findings & Clinical Pathology:

  • Regenerative intravascular hemolytic anemia with abnormal red blood cell morphology (e.g., spherocytes, Heinz bodies, target cells).

  • Elevated serum bilirubin, elevated alkaline phosphatase ($ALP$), elevated blood urea nitrogen ($BUN$), and elevated serum creatinine reflecting hepatic and renal compromise.

  • Radiology: Diagnostic imaging reveals radiopaque metallic foreign bodies within the stomach or gastrointestinal tract.

• Treatment Protocols:

  • Immediate surgical (exploratory laparotomy) or endoscopic removal of retained metallic foreign bodies to halt ongoing systemic absorption.

  • Chelation therapy: D-penicillamine is the chelator of choice. Calcium disodium EDTA ($CaNa_2EDTA$) serves as an alternative.

  • Supportive care including intravenous fluids, blood transfusions, and gastrointestinal protectants.

• Case Example: A $3$-year-old male dog presenting with severe pica selectively ingested $38$ post-$1982$ pennies, resulting in severe hemolytic anemia, marked elevations in bilirubin, $ALP$, $BUN$, and creatinine. The pennies were identified on abdominal radiography and removed surgically.

Arsenic (As) Toxicosis

• Chemical Forms and Toxicity Gradient:

  • Trivalent arsenic ($As^{3+}$) is significantly more toxic than pentavalent arsenic ($As^{5+}$).

  • Inorganic arsenic formulations are far more toxic than organic arsenical compounds.

• Mechanisms of Action:

  • Inorganic / Trivalent Arsenic: Inhibits essential cellular respiration enzymes by binding to sulfhydryl groups and disrupting the tricarboxylic acid ($TCA$) cycle. Acts as a potent capillary poison, inducing endothelial dilatation, hyperpermeability, fluid transudation, severe edema, and tissue congestion. Tissues with high oxygen demands (GI mucosa, kidney, liver) are most vulnerable.

  • Pentavalent Arsenic: Directly uncouples oxidative phosphorylation, creating an acute cellular energy ($ATP$) deficit.

• Veterinary Pharmaceuticals:

  • Caparsolate (Thiacetarsamide sodium): Historical trivalent organic arsenical used as a canine heartworm adulticide. Exhibited high systemic toxicity and required strict intravenous administration due to severe tissue necrosis upon perivascular extravasation.

  • Melarsomine dihydrochloride (Immiticide): Modern organic arsenical adulticide with a significantly higher safety profile than Caparsolate. Administered via deep intramuscular injection into the lumbar epaxial musculature.

• Clinical Presentations:

  • Peracute / Acute Toxicity: High morbidity. Causes sudden collapse, severe hemorrhagic/watery diarrhea, shock, acute renal necrosis, anuria, and death.

  • Subacute Toxicity: Most common form in cattle due to agricultural runoff, fertilized field access, or environmental contamination. Clinical signs manifest over several days: severe depression, anorexia, persistent watery diarrhea, chronic respiratory distress, renal failure, hypothermia, weakness, hindlimb ataxia/paralysis, and central nervous system depression. Cats are frequently exposed via grooming contaminated paws/fur.

  • Chronic Toxicity: More common in human populations exposed to unmonitored well water; results in hyperkeratosis, hyperpigmentation, cutaneous lesions, and potential metabolic endocrine disruption.

• Diagnostic Pathology:

  • Severe mucosal hyperemic congestion, edema, ulceration, and necrosis throughout the gastrointestinal tract, accompanied by renal tubular necrosis.

Mercury (Hg) Toxicosis

• Environmental Dynamics & Biomagnification:

  • Inorganic mercury released into aquatic environments undergoes biotransformation by microorganisms into monomethylmercury or dimethylmercury (organic mercury).

  • Organic mercury undergoes bioconcentration and biomagnification across ecological food chains, accumulating to highest levels in apex predators (e.g., large pelagic fish such as tuna).

  • Minamata Disease: Historical industrial discharge of methylmercury into Minamata Bay, Japan, caused severe neurological disease, congenital malformations, and extreme behavioral manifestations in local animals ("dancing cat disease") and humans.

• Target Organ Systems:

  • Inorganic Mercury: Targets the gastrointestinal tract (causing severe ulcerative/necrotic gastroenteritis) and kidneys (causing acute tubular necrosis and oliguric/anuric renal failure).

  • Organic Mercury (Methylmercury): Primary neurotoxin. Readily crosses the blood-brain barrier and placenta, causing cerebral cortical necrosis, laminar neuronal necrosis, and developmental cerebellar hypoplasia in kittens born to exposed queens.

• Historical Equine Practice:

  • Mercury compounds were historically used as topical counterirritants or "blistering agents" applied to the limbs of horses to alter gait or manage lameness, presenting toxic risks upon ingestion or systemic absorption.