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Anaphylaxis is an acute disease of often life threatening severity caused by an antigen-antibody (IgE) reaction. If severe it can result in anaphylactic shock.
Aetiology
Most commonly, severe anaphylactic reactions are seen in farm animals following the parenteral administration of a drug or biological product. Other routes of entry of the allergen, such as via the respiratory or gastrointestinal tract, can also result in anaphylactic reactions. The reaction can occur at the site of exposure or in other areas.
The disease occurs because the animal is sensitized to the inciting allergen by previous exposure. The initial exposure usually does not result in any immediate clinical abnormalities, but subsequent exposure of the animal to the antigen results in rapid degranulation of mast cells and, subsequently, eosinophils, with widespread release of vasoactive and inflammatory mediators resulting in anaphylactic shock.
Although severe anaphylactic reactions occur usually after a second exposure to a sensitizing agent, reactions of similar severity can occur with no known prior exposure.
Anaphylactic reactions can occur in the following circumstances:
- Repeated injection of biological preparations
- Repeated blood transfusions from the same incompatible donor or donors.
- Injection of vaccines (e.g. those against foot-and-mouth disease and rabies)
- In some cases after injection of penicillin.
- A systemic reaction after Hypoderma spp. larvae are killed in their subcutaneous sites might be anaphylactic, but is more likely to be a toxic effect from breakdown products of the larvae.
- After inadvertent intravenous administration of mare’s milk to a foal.
Clinical Findings
Cattle
In cattle, initially there is a sudden onset of severe dyspnea, muscle tremors, and anxiety. In some cases, there is profuse salivation, in others moderate bloat, and yet others diarrhea. After an incompatible blood transfusion, the first sign is often hiccough. Additional signs are urticaria, angioneurotic edema, and rhinitis. Muscle tremor can be severe, and a rise in temperature to 40.5°C (105° F) is often observed. On auscultation of the chest there can be increased breath sounds, crackles if edema is present, and emphysema in the later stages if dyspnea has been severe. In most surviving cases the signs have usually subsided within 24 hours, although dyspnea may persist if emphysema has occurred.
In natural cases the time delay after injection of the reagin intravenously is about 15 to 20 minutes.
Sheep and Goats
In sheep, goats, and pigs, acute dyspnea is common. Respiratory distress is evidenced as increased respiratory rate, irregular respiration, coughing, abnormal lung sounds, reluctance to move, shivering or muscle tremors, paddling, and kicking.
Horses
In the horse, naturally occurring anaphylactic shock is manifested by severe dyspnea, distress, recumbency, and convulsions. Death can occur within less than 5 minutes, but it usually takes about an hour. Laminitis and angioneurotic edema are also common signs in the horse.
Pigs
In pigs, anaphylactic shock can be fatal within a few minutes, with systemic shock being severe within 2 minutes and death occurring in 5 to 10 minutes. Labored respiration, severe cyanosis, vomiting, and edema of the larynx, stomach, and gallbladder are the usual outcome.
Treatment
Treatment should be administered immediately; a few minutes’ delay can result in the death of the animal. Epinephrine is the most effective treatment for anaphylaxis and anaphylactic shock.
Epinephrine administered intramuscularly (or one-fifth of the dose given intravenously) is often immediately effective, with the signs abating while the injection is being made. Corticosteroids potentiate the effect of epinephrine and can be given immediately following epinephrine.
