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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Anaphylactic shock is an immediate (type 1) hypersensitivity reaction that is systemic and life-threatening
  • Anaphylactoid reactions, which are not a hypersensitivity, may induce similar clinical signs and outcomes

Pathophysiology!!navigator!!

  • Exposure to antigens (allergens) can result in synthesis of antigen-specific IgE
  • IgE binds to, and sensitizes, tissue mast cells or circulating basophils
  • Reexposure to antigen results in release of substances from sensitized cells that mediate type 1 hypersensitivities and, in severe cases, anaphylaxis
  • Sensitization occurs ~10 days after first exposure, but can persist for years
  • Anaphylactoid reactions usually involve activation of the complement system and degranulation of mast cells or basophils. These reactions do not require previous exposure and host sensitization with IgE

Systems Affected!!navigator!!

  • Shock organs of the horse are the respiratory and gastrointestinal tracts
  • The skin and hemic systems may also be involved

Genetics!!navigator!!

High levels of IgE are associated with certain ELA-DRB haplotypes.

Incidence/Prevalence!!navigator!!

Prevalence unknown, but 16% of blood transfusions reported allergic reactions in one study, and 6–10% of adverse drug reactions are reported to be allergic.

Signalment!!navigator!!

There is no breed, sex, or age predilection.

Signs!!navigator!!

General Comments

Mild, moderate, or severe clinical signs may occur depending on the dose and route of antigen challenge, organ system involved, and individual inflammatory responses of the patient.

Historical Findings

Possible exposure to antigens and/or agents associated with anaphylaxis.

Physical Examination Findings

  • Mild cases may present with urticaria and rhinitis; moderate cases with angioedema, diarrhea, sweating, and colic; severe reactions with dyspnea and respiratory distress, tachypnea, pulmonary emphysema, coughing, hypotension, and collapse
  • Pale mucous membranes, poor peripheral pulses, and cold extremities may be associated with cardiovascular collapse

Causes!!navigator!!

  • A wide range of antigens or agents may induce anaphylaxis or anaphylactoid reactions
  • Reactions can occur at any time, including, rarely, after the first exposure to highly charged or osmotically active agents (e.g. iodinated radiocontrast medium, dextran)
  • Implicated agents include insect venom, vaccines, whole blood or blood products, antimicrobials, anthelmintics, thiamine, vitamins (B, K, E/selenium), iron, copper, halothane, thiamylal, and guaifenesin, or inadvertent injection of products, e.g. milk
  • The sensitizing agent is often not identified

Risk Factors!!navigator!!

  • Prior exposure to potential allergens
  • Previous anaphylactic reactions
  • Repeated parenteral administration of the same biologic preparation at high doses

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

  • Diagnosis largely based on history and classical clinical presentation
  • A rule of twos states that reactions begin from 2 min to 2 h following injection, infusion, ingestion, contact, or inhalation
  • Response to treatment may support the diagnosis

Differential Diagnosis!!navigator!!

  • Acute pneumonia may resemble anaphylaxis, but horses are usually more toxemic and lung changes are prominent in ventral lobes compared with widespread involvement with anaphylaxis
  • An inappropriate dose or route of drug administration (such as intracarotid injection) may result in collapse associated with neurologic deficits (e.g. blindness, seizures)

CBC/Biochemistry/Urinalysis!!navigator!!

Hemoconcentration, leukopenia, thrombocytopenia, hyperkalemia, increases in hepatic and myocardial enzyme activities, and coagulation deficits are reported, although diagnostic relevance is uncertain.

Other Laboratory Tests!!navigator!!

N/A

Imaging!!navigator!!

N/A

Other Diagnostic Procedures!!navigator!!

Provocative intradermal/conjunctival challenge testing with the suspected antigen may help confirm diagnosis, but value is questionable due to high rate of false negatives and risk of inducing anaphylaxis.

Pathologic Findings!!navigator!!

  • Severe, diffuse pulmonary emphysema, and peribronchiolar edema at necropsy
  • Widespread petechiae, edema, and extravasation of blood in the wall of the large bowel, subcutaneous edema, congestion of the kidney, spleen, and liver, and evidence of laminitis

Treatment ⬆ ⬇

Outline

TREATMENT

Aims!!navigator!!

Immediate steps include (1) stop exposure to trigger if possible; (2) rapid assessment of patient status; (3) secure airway and check vital parameters; and (4) perform tracheotomy if necessary (e.g. respiratory distress).

Appropriate Health Care!!navigator!!

In severe cases, early recognition of clinical signs and immediate steps are critical for patient survival.

Nursing Care!!navigator!!

Oxygen Therapy

  • High-flow oxygen therapy is indicated during systemic anaphylaxis to help improve tissue oxygenation
  • Recommended flow rates are 5–10 L/min for foals and 10–15 L/min for adults directly into the nasal passage

Fluid Therapy

  • Horses that remain hypotensive following epinephrine treatment should receive fluid therapy
  • Crystalloids and colloid fluids may help restore intravascular fluid volume, cardiac output, and aerobic metabolism. Careful monitoring is required to evaluate for worsening of tissue edema that accompanies anaphylaxis, especially the upper airways, lungs, and neural tissues
  • Volumes of crystalloids recommended in foals is 50–80 mL/kg divided into separate bolus doses; for adults 10–20 mL/kg/h
  • Hypertonic (7.2%) saline solution may also be used in adult horses exhibiting shock at 2–4 mL/kg simultaneously with crystalloid fluids
  • As a general rule ~10 L of crystalloids should be provided over time for each liter of hypertonic saline

Diet!!navigator!!

  • Oral intake should be discontinued until recovery from an anaphylactic event
  • Diet should be evaluated to determine if components are potential cause of anaphylactic reaction

Surgical Considerations!!navigator!!

Nasotracheal intubation or tracheotomy if severe respiratory distress to secure the airways prior to epinephrine treatments.

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Epinephrine is the most effective treatment for systemic anaphylaxis and shock
  • Epinephrine should be administered immediately upon recognition of signs, even if there is doubt, and simultaneously during patient assessment
  • In adult horses, recommended dose is 0.01 mg/kg of a 1:1000 dilution slowly IV, or 0.02 mg/kg IM when dyspnea and hypotension are mild
  • When IV access is not possible, a higher dose may be administered via the intratracheal route (20 ml/450 kg horse)
  • In foals 0.01–0.02 mg/kg given slowly IV is recommended
  • Additional medications may be considered for severe cases, or for localized reactions

Vasopressors

  • Vasopressors should be considered for horses with persistent hypotension refractory to epinephrine and volume replacement
  • Dobutamine (50 mg diluted in 500 mL of 5% dextrose solution; 100 μg/mL; using 5–10 μg/kg/min over 10–20 min), dopamine (titrated as a continuous infusion at 1–20 μg/kg/min), norepinephrine (0.1–1.5 μg/kg/min), and vasopressin (0.01–0.04 μg/kg/min as a constant infusion for refractory hypotension or 0.3–0.6 μg/kg as a single dose for cardiovascular collapse)
  • Drugs should be titrated to maintain a mean arterial blood pressure > 60–70 mmHg
  • Cardiac monitoring is recommended as arrhythmias may develop

Bronchodilators

  • May be used in horses exhibiting bronchospasm resistant to epinephrine
  • Inhalation therapy with β2-agonists (e.g. albuterol) (every 3–4 h via an inhaler; 720 μg or 8 puffs at 90 μg/puff) or nebulization (2–5 mL of a 0.5% solution diluted in sterile saline)

Furosemide

Furosemide (1 mg/kg IV) is indicated in horses with pulmonary edema.

Antihistamines

  • Antihistamines may be used to control clinical signs such as urticaria and pruritus
  • If cardiovascular status of horse is stable doxylamine succinate (0.5 mg/kg slowly IV or IM), pyrilamine maleate (1.0 mg/kg slowly IV, IM, or SC), tripelennamine hydrochloride (1 mg/kg IM not IV), or hydroxyzine hydrochloride (1–1.5 mg/kg PO) may be administered every 6-12 h

Glucocorticoids

  • Systemic glucocorticoids are not useful in the acute phase, but are indicated to halt progressive inflammation, prevent recurrence of anaphylaxis, and prevent late-phase or protracted reactions
  • Dexamethasone (0.2–0.5 mg/kg IV) is used for treating rapidly progressing edema
  • Prednisolone sodium succinate (0.25–10 mg/kg IV) is preferred for systemic reactions
  • Prednisolone (0.4–1.6 mg/kg PO SID) may be used for managing persistent urticaria

Contraindications!!navigator!!

The use of equine plasma and dextrans is contraindicated in anaphylaxis treatment owing to concerns with induction of subsequent anaphylactic events.

Precautions!!navigator!!

  • Epinephrine may increase the risk of arrhythmia
  • Dobutamine potentiates hypoxemia-induced cardiac arrhythmias
  • Glucocorticoid administration has been associated with laminitis
  • If antihistamines are given, slow IV administration is recommended to avoid adverse effects

Possible Interactions!!navigator!!

Antihistamines enhance the effects of epinephrine on vascular resistance and, therefore, should be used separately.

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW UP

Patient Monitoring!!navigator!!

Biphasic, multiphasic, or protracted anaphylactic episodes may occur after the primary event despite therapy.

Prevention/Avoidance!!navigator!!

  • Avoidance of known allergens/antigens
  • If avoidance is not possible, pretreatment with flunixin meglumine and antihistamines or corticosteroids and antihistamines may be warranted
  • Allergen-specific IgE testing may aid in detecting sensitivity to agents and facilitate avoidance
  • Desensitization procedures have not been well documented in horses
  • In horses with suspected drug reactions avoid drugs with immunologic or biochemical similarity
  • Oral therapy versus parenteral should be considered if drug is required
  • Horses with known history of anaphylaxis should be monitored for 20–30 min after giving agents of unknown sensitivity
  • Cross-matching prior to transfusions may decrease risk of anaphylaxis
  • Adverse reactions to procaine penicillin G are less likely with avoidance of repeated injected sites, slow drug administration, and proper drug storage. Reaction is more likely due to the excitatory effects of unbound procaine than anaphylaxis

Possible Complications!!navigator!!

Complications can include laminitis, purpura haemorrhagica, subcutaneous edema, hemolytic anemia, and hemorrhagic colitis.

Expected Course and Prognosis!!navigator!!

  • Prognosis depends on type and severity of the event; speed of disease onset and recognition by owners/veterinarians; response to treatment; and development of complications
  • Local reactions are rarely life-threatening and the prognosis will largely reflect response to treatment
  • Severe systemic reactions carry a considerable risk of death
  • The risk of death increases when recognition of a systemic response, or subsequent emergency treatment, is delayed

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

None

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

None

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Synonyms!!navigator!!

None

Abbreviations!!navigator!!

Ig = immunoglobulin

Suggested Reading ⬆ ⬇

Radcliffe RM.Anaphylaxis . In: Felippe MJB, ed. Equine Clinical Immunology. Ames, IA: Wiley Blackwell, 2016:31–38.

Author(s) ⬆

Author: Jennifer L. Hodgson

Consulting Editors: David Hodgson, Harold C. McKenzie, and Jennifer L. Hodgson