9.2 Envenomation Syndromes & Acute Anaphylaxis Management

Envenomation Syndromes & Acute Anaphylaxis Management

VTS Critical Concept: In toxicological and immunological emergencies, minutes dictate survival. Pit viper envenomation triggers rapid coagulopathy (VICC) and tissue necrosis where antivenom dose is governed by the venom load rather than the patient's weight. In acute anaphylaxis, intramuscular Epinephrine (1:1,000) is the sole first-line drug capable of halting mast cell degranulation, reversing bronchospasm, and restoring systemic vascular resistance.

1. Crotalid (Pit Viper) Envenomation Syndromes

North American Pit Vipers (Crotalinae) include Rattlesnakes (Crotalus and Sistrurus spp.), Copperheads (Agkistrodon contortrix), and Cottonmouths/Water Moccasins (Agkistrodon piscivorus). Pit vipers possess bilateral heat-sensing facial pits between the eye and nostril, retractable hinged front fangs, and elliptical pupils.

[ Crotalid Venom Inoculation ]
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Metalloproteinases & Hyaluronidase     Thrombin-like Enzymes & Phospholipase A2
• Breakdown of extracellular matrix    • Rapid fibrinogen consumption (VICC)
• Capillary basement membrane lysis    • Platelet destruction & dysfunction
• Massive hemorrhage, edema, necrosis  • RBC membrane lipid hydrolysis (Echinocytes)
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[ Profound Hypotensive Shock + Severe Coagulopathy + Compartment Syndrome ]

Venom Biochemistry & Pathophysiology

• Zinc Metalloproteinases: Hydrolyze capillary basement membranes, endothelial cell adhesion proteins, and extracellular matrix, causing massive extravasation of blood, severe localized edema, blister formation, and deep tissue necrosis.
• Phospholipase A2 (PLA2): Hydrolyzes cell membrane phospholipids, inducing direct skeletal muscle necrosis (myotoxicity), neurotoxicity, hemolysis, and platelet dysfunction.
• Thrombin-Like Enzymes (Serine Proteases): Cleave fibrinopeptides from fibrinogen to form weak, non-crosslinked, unstable fibrin polymers that are rapidly degraded by plasmin. This results in complete fibrinogen depletion.
• Venom-Induced Consumptive Coagulopathy (VICC): Characterized by severe hypofibrinogenemia, dramatically prolonged Prothrombin Time (PT) and activated Partial Thromboplastin Time (aPTT), marked thrombocytopenia, and elevated D-dimers. Unlike classical DIC, VICC is non-thrombotic and results directly from enzymatic consumption.

Clinical Presentation & Diagnostic Hallmarks

1. Severe Local Reaction: Puncture wounds with rapid, progressive edema, severe ecchymosis, bleb/bullae formation, hemorrhagic exudate, and excruciating pain.
2. Echinocytes on Blood Smear: The appearance of echinocytes (burr cells)—erythrocytes with uniform, sharp, spicular projections—is a classic diagnostic hallmark. PLA2 alters red blood cell outer membrane lipids within 15-30 minutes of envenomation.
3. Cardiovascular Collapse: Severe distributive, hypovolemic, and hemorrhagic shock caused by widespread capillary leak, third-spacing, and systemic vasodilation.

Antivenom Therapy: The Cardinal Dosing Principle

• Antivenom Formulations: Crotalidae Polyvalent Immune Fab (CroFab, ovine-derived Fab fragments) or Antivenin (Crotalidae) Polyvalent (equine whole IgG).
• THE GOLDEN RULE OF ANTIVENOM DOSING: Dosing is guided entirely by the severity of envenomation, the rate of progression of swelling, and coagulation derangements—NEVER BY THE PATIENT'S BODY WEIGHT.
• A 4 kg Dachshund injected with 50 mg of venom has a much higher circulating venom concentration per milliliter of blood than a 40 kg Labrador receiving the same venom dose; therefore, smaller patients require the same or greater number of vials of antivenom.
• Administration Protocol: Reconstitute the initial vial(s) (typically 1-2 vials for mild/moderate, 3-5+ vials for severe/rattlesnake envenomations) in 100-250 mL of isotonic saline. Infuse slowly over the first 10-15 minutes while monitoring for acute anaphylactoid reactions (facial swelling, urticaria, hypotension). If no adverse reaction occurs, administer the remainder over 60 minutes. Repeat dosing if swelling progresses past major joints or PT/aPTT remain unmeasurable.

2. Elapid & Arachnid Envenomation Syndromes

Coral Snake Envenomation (Micrurus fulvius)

• Identification: Eastern and Texas Coral snakes exhibit alternating red, yellow, and black bands where "Red on yellow kills a fellow; red on black venom lack" (referring to non-venomous milk/king snakes).
• Neurotoxic Mechanism: Coral snake venom contains non-enzymatic neurotoxins that cause irreversible post-synaptic competitive blockade of nicotinic acetylcholine (ACh) receptors at the skeletal neuromuscular junction.
• Clinical Presentation: Unlike pit vipers, there is minimal to no local swelling, pain, or necrosis at the bite site. Clinical signs are purely neurological and frequently delayed by 1-18 hours:
  • Generalized Lower Motor Neuron (LMN) flaccid tetraparesis progressing to complete tetraplegia.
  • Cranial nerve deficits: Loss of palpebral reflexes, dysphagia, ptosis, pupillary dilation (mydriasis), and jaw drop.
  • Fatal Respiratory Arrest: Diaphragmatic and intercostal muscle paralysis leading to hypoventilation, hypercapnia, and asphyxiation.
  • Central intravascular hemolysis and severe hemoglobinuria (especially in cats).
• ICU Management: Immediate endotracheal intubation and prolonged positive pressure mechanical ventilation (IPPV) for 24-72+ hours until new acetylcholine receptors are synthesized. Equine Micrurus fulvius antivenom if obtainable.

Black Widow Spider Envenomation (Latrodectus mactans)

• Toxin: alpha-Latrotoxin, a potent presynaptic neurotoxin that binds to latrophilin and neurexin receptors on presynaptic membranes, forming massive non-selective cation pores.
• Pathophysiology: Massive, uncontrolled influx of extracellular calcium (Ca2+) into nerve terminals triggers catastrophic, explosive exocytosis of acetylcholine and catecholamines (norepinephrine, epinephrine).
• Clinical Signs:
  • Severe, excruciating muscle cramping and rigid "board-like" abdomen without palpable visceral pain.
  • Severe systemic arterial hypertension (MAP > 160-180 mmHg) and tachyarrhythmias.
  • Severe muscle fasciculations, vocalization, motor restlessness, tremors, and respiratory compromise.
  • Cats are extraordinarily sensitive to latrodectism, developing flaccid ascending paralysis, hypersalivation, hypothermia, and high mortality.
• Therapy: Potent pure mu-opioids (Fentanyl, Methadone) for excruciating somatic pain; Methocarbamol (55-110 mg/kg IV slow) for muscle spasms; antihypertensives (Nitroprusside or Amlodipine) for severe hypertension; equine Latrodectus antivenom in refractory cases.

3. Acute Anaphylaxis: Species-Specific Target Organs & Emergency Resuscitation

Anaphylaxis is an acute, life-threatening, multi-system Type I IgE-mediated hypersensitivity reaction. Exposure to an allergen (hymenoptera venoms, vaccines, biological blood products, antibiotics, chemotherapeutics) cross-links pre-existing allergen-specific IgE antibodies bound to high-affinity FceRI receptors on mast cells and circulating basophils. This triggers explosive degranulation of preformed vasoactive mediators (histamine, heparin, proteases, TNF-alpha) and rapid synthesis of lipid mediators (leukotrienes LTC4/LTD4, prostaglandin PGD2, Platelet-Activating Factor [PAF]).

Species Shock Organs: Canine vs. Feline Anaphylaxis

Emergency Resuscitation Protocol

[ ACUTE ANAPHYLACTIC COLLAPSE ]
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1. FIRST-LINE DRUG: EPINEPHRINE 1:1,000 (1 mg/mL)
   • Dose: 0.01 mg/kg (0.01 mL/kg) IM in Anterolateral Thigh / Quadriceps
   • Actions: α1 (vasoconstriction/MAP), β1 (inotropy/HR), β2 (bronchodilation & halts mast cell degranulation)
   • Repeat q5-15 min if hypotensive shock persists
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2. AGGRESSIVE VOLUME RESUSCITATION (Distributive Shock)
   • Dogs: Isotonic Crystalloids 10-20 mL/kg bolus over 15 min (repeat up to 60-90 mL/kg)
   • Cats: Isotonic Crystalloids 5-10 mL/kg bolus over 15 min
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3. SECONDARY ADJUNCTIVE THERAPIES (Do NOT replace Epinephrine!)
   • Diphenhydramine: 2 mg/kg IM (H1 antihistamine; avoid rapid IV bolus)
   • Famotidine: 0.5 mg/kg slow IV (H2 antihistamine)
   • Dexamethasone SP: 0.1-0.2 mg/kg IV (blunts late-phase 4-6h biphasic inflammatory rebound)
   • Terbutaline / Inhaled Albuterol for refractory feline bronchospasm

The Route of Epinephrine Administration: Why IM Over SC or IV

• Intramuscular (IM) Injection into the Anterolateral Thigh (Quadriceps): Pharmacokinetic studies prove that IM injection into vascular thigh muscle produces significantly faster absorption and higher peak plasma concentrations than subcutaneous (SC) administration, which is severely delayed due to shock-induced peripheral vasoconstriction.
• Intravenous (IV) Epinephrine: Reserved strictly for patients in full cardiopulmonary arrest or refractory, decompensated shock with continuous ECG monitoring (0.01-0.02 mg/kg of diluted 1:10,000 solution or CRI titrated at 0.05-0.1 mcg/kg/min). Rapid IV boluses in a beating heart can trigger lethal ventricular arrhythmias, severe hypertension, and myocardial infarction.