Anaphylaxis

Introduction

Anaphylaxis generally refers to the systemic anaphylactic reaction, which is a severe type 1 hypersensitivity reaction after the animal has become sensitised to a substance.

It is mediated by IgE and characterised by the release of vasoactive substances from mast cells and basophils.

Signs occur within seconds to minutes after exposure to the allergen.

A multitude of agents can trigger anaphylaxis, including: foods, insect venoms from bites and stings, and any drug or blood product.

The primary organs affected differ between cats and dogs.

In dogs, the major shock organ is the liver and mast cell degranulation in the portosystemic vasculature causes venous dilatation and pooling of blood.

In cats, the major shock organs are the lungs and mast cell degranulation in the pulmonary vasculature causes constriction of bronchial airways or pulmonary veins and pooling of blood in the pulmonary vascular bed, which results in severe respiratory distress.

All other organs are affected due to the resultant hypovolaemia and hypoxaemia.

Disseminated intravascular coagulation (DIC) can lead to haemorrhages.

Clinical Signs

In dogs, these include: restlessness, urticaria, angioedema, nausea, vomiting, diarrhoea, weakness, dyspnoea, collapse and possibly death.

In cats, these include: restlessness, pruritus, vocalisation, vomiting, defecation, weakness, tachypnoea, bronchoconstriction, laryngeal dysfunction, pulmonary oedema and possibly death.

Diagnosis

There may be a known exposure to a substance, such as the administration of a drug or envenomation by an insect or snake.

The timing of the clinical signs following exposure is often a valuable diagnostic clue.

At post-mortem: dogs will show: hepatic and splanchnic congestion and gastrointestinal haemorrhage.

Cats will show: laryngeal oedema, bronchoconstriction, emphysema and pulmonary haemorrhage.

Treatment

The agent causing the anaphylaxis should be eliminated if possible, or the suspect drug should be discontinued.

For acute and severe anaphylaxis, the treatment is intravenous epinephrine to counteract bronchoconstriction and vasodilation and increase blood pressure. It can also be given intramuscularly if necessary.

Antihistamines are of no benefit in acute anaphylaxis, but may be used if there are only mild signs and urticaria. Drugs such as ranitidine or famotidine can be given.

Glucocorticoids such as prednisolone or dexamethasone can antagonise the delayed mechanisms of the allergic reaction, but take several hours to act.

Ancillary support for blood pressure include: fluids such as crystalloids or colloids to expand the intravascular volume.

Vasopressors such as dopamine

Bronchodilators for cats such as terbutaline or albuterol.

Oxygen is always indicated and can be given in a flow-by system.

The prognosis is very good if treatment is rapid and the response is immediate.

Anaphylaxis Learning Resources
Flashcards Test your knowledge using flashcard type questions	Small Animal Emergency and Critical Care Medicine Q&A 17

References

Merck and Co (2008) The Merck Veterinary Manual Merial

Wingfield, W. (2002) The veterinary ICU book Teton NewMedia

Mazzaferro, E. (2011) Blackwell's 5-minute consult clinical companion Wiley-Blackwell

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