Journal Scan: Anaphylaxis in dogs and cats: Are you prepared?

What they did

In this article, the authors review the pathophysiology and clinical signs of anaphylaxis in dogs and cats and review the diagnosis and treatment.

Overview

Currently, anaphylactic reactions can be classified as follows:

1. Immunologic IgE-mediated: Causes may include insect stings or bites, food, reptile venom, or medications.

2. Immunologic non-IgE-mediated: Causes may include transfusion reactions and autoimmune diseases.

3. Nonimmunologic: Causes may include physical factors such as heat and exercise or reactions to certain medications such as opioids or chemotherapeutic drugs.

Immunologic anaphylaxis. This response involves the IgE-mediated Type-I hypersensitivity reaction that we are most familiar with and requires a sensitization phase (first exposure to an antigen [allergen]) followed by re-exposure and subsequent immunologic response. This response involves binding of IgE antibodies to mast cells and basophils, leading to the release of inflammatory and vasoactive mediators such as histamine and platelet-activating factor (PAF).

Anaphylactic responses not mediated by IgE are induced by binding of antigen to IgG molecules that cross-link macrophages and result in the release of PAF. This pathway does not lead to histamine release.

Nonimmunologic anaphylaxis. This pathway can result in the degranulation of mast cells and basophils without the presence of immunoglobulin. These reactions can occur in response to physical stressors such as heat or cold or drugs such as NSAIDs or radiocontrast agents. These triggers may also act through more than one mechanism.

Contributing factors. Other pathways such as the complement system, the coagulation cascade, and the fibrinolytic system may also be activated during anaphylaxis and result in clinical complications with systemic blood pressure and disseminated intravascular coagulation (DIC). In addition, the authors discuss that beyond histamine, other mediators such as prostaglandins, leukotrienes, and tumor necrosis factor, may all be involved in the development of an anaphylactic response.

Different species will also respond differently to systemic anaphylactic processes. Whereas the liver and gastrointestinal tract are the body systems primarily affected in dogs, the respiratory tract is the main organ affected in cats. In general, however, clinical signs are divided into cutaneous, respiratory, cardiovascular, and gastrointestinal.

Diagnosis. Diagnosis will depend mainly on the patient’s history and clinical findings and recognition of a characteristic pattern. The association between serologic or intradermal skin testing and the risk of an anaphylactic reaction is poor but may be useful in documenting sensitivity to a specific trigger if performed within four to six weeks after an event.

Treatment. Treatment of severe acute reactions may require basic life support including intubation and fluid resuscitation. Epinephrine is indicated as a first-line drug in most instances (0.01 mg/kg of a 1:1000 [1 mg/ml] solution given intramuscularly). A maximum dose of 0.3 mg for patients 40 kg is recommended.

In non-life-threatening cases of anaphylaxis, treatment will depend on the severity of the clinical signs. Antihistamines may alleviate minor signs such as urticaria and itching but have not been shown to be consistently effective at preventing an anaphylactic reaction. Glucocorticoids do not treat the initial signs of anaphylaxis, so they are not indicated as first-line drugs in this setting. They may be of use, however, in down-regulating pro-inflammatory mediators several hours after the acute event. As with antihistamines, pretreatment with glucocorticoids will not prevent an anaphylactic reaction. Inhaled bronchodilators such as albuterol may be of benefit in patients with respiratory signs.

Patients should be monitored for at least three days after an allergic response because of the risk of a biphasic anaphylactic reaction.

Take-home message

Acute anaphylactic reactions often present as medical emergencies. Prompt recognition and treatment will be imperative to patient survival.

Shmuel DL, Cortes Y. Anaphylaxis in dogs and cats. J Vet Emerg Crit Care 2013;23:377-394.

Link to abstract: http://onlinelibrary.wiley.com/doi/10.1111/vec.12066/abstract