Real life case: facial swelling
The one-year-old Labrador in the photo presented with acute onset facial swelling having been camping with his owners. The owners had also noticed some raised bumps around the base of his ears and in his groin. Clinical examination was unremarkable except for cutaneous symptoms: facial angioedema, generalised urticaria and pruritus. This is the typical pattern of signs for an immune-mediated hypersensitivity reaction, in this case probably caused by an insect sting. Other common triggers of hypersensitivity reactions include drugs (particularly antibiotics), vaccines, and food. This dog displayed the most common cutaneous signs of hypersensitivity: pruritus, angioedema (which involves the deep dermis and subcutaneous tissues and tends to be confined to the head) and urticaria (which involves the superficial dermis and tends to be generalised). Erythma is also a common cutaneous sign but may go unnoticed in our patients due to their fur coat.
The clinical signs of a type-1 hypersensitivity reaction are due to mast-cell degranulation, with histamine being the major mediator. Therefore the symptoms are seen in body systems with highest numbers of mast cells:
- Cutaneous system: urticaria, angiodema, erythema, and pruritus. These are the most common signs we see in dogs
- Gastrointestinal system: nausea, vomiting and diarrhoea (sometimes haemorrhagic) with hepatic venous congestion and portal hypertension. Gastrointestinal symptoms are more common in dogs than cats
- Respiratory system: dyspnoea, stridor and coughing due to pharyngeal and laryngeal oedema, bronchoconstriction and excessive mucus production. Respiratory signs of hypersensitivity appear to be more common in cats. Respiratory signs of hypersensitivity appear to be more common with inhaled antigens
- Cardiovascular system: hypotension, tachycardia, fluid extravasation and vasodilation
The majority of hypersensitivity reactions we see in practice do not meet the criteria for anaphylaxis, which is defined as "a severe systemic life-threatening hypersensitivity reaction that occurs within minutes to hours of a potential provoking agent". Most cases where symptoms are solely cutaneous do not meet this definition and resolve quickly with treatment. An injectable anti-histamine, such as promethazine or chlorphenamine, is the first-line treatment and is all that is needed in most cases of cutaneous hypersensitivity. More severe cases may also benefit from a one-off injection of a short-acting corticosteroid such as dexamethasone.
However, it is important to recognise if patients are displaying the life-threatening manifestations of hypersensitivity, such as the respiratory and cardiovascular signs described above, and to consider anaphylaxis in these cases. In cases of anaphylaxis, anti-histamines and corticosteroids are adjunctive treatments: adrenaline is the most important first-line treatment.