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Definition

Anaphylactoid reactions produce a similar clinical picture as true anaphylactic reactions, but are not mediated by IgE and occur through a direct non‐immune mediated release of histamine and other mediators from mast cells and/or basophils.

Risk factors

Some opioid drugs (morphine and meperidine [pethidine])

 Route, dose and rate of drug administration. The administration of a high dose of a rapid intravenous bolus of the drug leads to a greater histamine release compared with lower doses administered as a constant rate infusion [9].

Pathogenesis

Morphine and meperidine (pethidine), when injected intravenously, induce histamine release due to mast‐cell degranulation by a non‐immunological mechanism (non‐IgE mediated) [10]. The most potent at causing this effect is meperidine (pethidine) [11].

The clinical consequences of an anaphylactoid reaction are the same as those of a true anaphylactic reaction, most commonly hypotension and tachycardia, but other effects such as bronchoconstriction, pruritus, urticaria or cardiovascular collapse may also occur (non‐allergic anaphylaxis).

A retrospective study of intraoperative administration of morphine at doses of 0.1–0.17 mg/kg in horses found no significant increase in problems during or immediately after anesthesia, which included no cardiovascular side effects when compared with a similar protocol without the opioid [12]. However, a case report of two horses who received intravenous meperidine, one sedated and one anesthetized, describes the occurrence of tachycardia and profuse sweating, which may have been due to an anaphylactoid reaction [13]. Unfortunately, no blood pressure was measured in these horses. Both horses recovered uneventfully from this reaction within 10 minutes.

Prevention

An alternative route of administration such as intramuscular should be considered when drugs known to cause histamine release are administered, especially meperidine. When morphine is administered intravenously, it should be injected slowly while monitoring the horse for any possible side effects (especially heart rate and blood pressure).

Previous administration of an anti‐histaminic drug such as chlorphenamine or diphenhydramine, may be considered when these drugs are used in debilitated animals with a reduced cardiovascular reserve. However, no problems associated with the use of intravenous morphine at clinical doses are usually observed in healthy horses [12] or horses with colic (personal observation) and therefore the routine use of anti‐histaminic drugs is not recommended as they may produce other unwanted effects (e.g. sedation).

Diagnosis

Histamine release occurs within minutes of drug administration and the consequences appear quickly. The first clinical signs of histamine release are hypotension and tachycardia, which may be mild and short lasting or severe, even causing cardiovascular collapse. These signs can be easily observed if the heart rate and blood pressure are being monitored (e.g. under general anesthesia), but in conscious horses they may go unnoticed.

Treatment

Usually, treatment is not necessary. Supportive treatment of hypotension includes the administration of intravenous fluids and/or vasoconstrictors (e.g. phenylephrine). If the reaction is severe, causing bronchoconstriction and cardiovascular collapse, epinephrine (adrenaline) should be immediately administered as well as oxygen supplementation, while blood pressure, heart rate and oxygenation (pulse oximetry and/or blood gases) are being monitored.

Expected outcome

Outcome should be good if supportive treatment is instituted rapidly.