Chapter 146 - Hypersensitivity and Anaphylaxis

Question 1

In human medicine, what is the difference between Mild generalized hypersensitivity reactions and Moderate/Severe generalized hypersensitivity reactions

Answer 1

Mild generalized hypersensitivity reactions
causing cutaneous reaction only, such as generalized erythema, urticaria, and angioedema

Moderate generalized hypersensitivity reactions comprise manifestations of severe illness with respiratory, cardiovascular, and gastrointestinal involvement. Symptoms: nausea, vomiting, and dizziness, along with processes known to lead to hypotension or hypoxemia such as dyspnea and stridorous respiration.

Severe generalized hypersensitivity reactions are documented hypotension (systolic blood pressure <90 mmHg) or hypoxemia (SpO2<92%), as well as clinical signs of neurological compromise resulting from hypotension or hypoxia, such as collapse and altered mental status.

Moderate and severe generalized hypersensitivity reactions are defined by features associated with hypoxemia and hypotension and therefore can be classified as anaphylactic reactions.

Question 2

Anaphylactic reactions?

Answer 2

• immediate (type 1) hypersensitivity reactions
• Mast cells and basophils express Fc-epsilon-RI receptors on their cell surface that have a high affinity for the Fc portion of immunoglobulin E (IgE) antibodies.
• Initial exposure to an antigen leads to increased production of IgE antibodies, which are subsequently bound to mast cell Fc-epsilon-RI receptors in sensitized individuals
• After re-exposure to the same antigen, the antigen induces cross-linking of two adjacent surface IgE antibodies, leading to mast cell degranulation
• Multiple vasoactive mediators are released during mast cell degranulation, including histamine, tryptase, heparin, and cytokines.
• Histamine effects are exerted via histamine receptors and receptor stimulation can lead to pruritus, urticaria, angioedema, conjunctivitis, and rhinitis in allergic reactions as well as the vasodilation and increased vascular permeability responsible for cardiovascular collapse in anaphylactic reactions.
• Platelet-activating factor (PAF) has also been shown to play a major role in anaphylactic reactions. PAF is a potent bronchoconstrictor, increases vascular permeability, and enhances platelet aggregation, histamine release, and vasodilation .

Question 3

Common substances of hypersensitivity reactions and anaphylaxis reported in veterinary medicine?

Answer 3

1. vaccine proteins
2. insect and reptile venoms and their antivenoms
3. blood products
4. antimicrobial agents
5. non-steroidal anti-inflammatory agents (NSAIDs)
6. opioids
7. radiographic contrast agents
8. food
9. physical factors such as heat or cold
   * Single cases of fatal anaphylaxis associated with a dexamethasone suppression test and an intraoperative anaphylactic reaction to accidental dissection of an adult heartworm during lung lobectomy have also been reported in dogs

Question 4

True or False?

The time to onset of clinical signs DOES CORRELATE WITH the severity of systemic anaphylactic reactions.

Answer 4

TRUE
The more rapidly the symptoms manifest, the more severe a reaction will be and the more likely it is to progress to be life-threatening.

Question 5

In dogs with hypersensitivity reactions, which organ systems are most likely affected?

Answer 5

In dogs with hypersensitivity reactions, cutaneous and gastrointestinal signs predominate. Specifically, in vaccine-associated adverse events, dermal signs including facial edema, pruritus, erythema, and urticaria are the most common clinical manifestations in 68% of patients, followed by gastrointestinal signs such as vomiting, diarrhea, and anorexia in 45% of patients.

Question 6

In dogs with anaphylactic reactions, which organ systems are most likely affected?

Answer 6

In anaphylactic reactions, the liver and gastrointestinal tract are most commonly affected in the dog.

Histamine released from the gastrointestinal tract leads to significant sinusoidal and hepatic venous congestion, which in some cases may progress to hepatic hemorrhage and hepatocellular necrosis. This can lead to distributive shock.

Increased vascular permeability can contribute to cardiovascular collapse and massive fluid shifts of up to 35% of the intravascular volume to the interstitial space can occur. Rapid hemodynamic compromise can occur without preceding cutaneous signs. A study of canine hypersensitivity reactions reported that only 57% of dogs showed cutaneous signs during anaphylactic reactions, and that their manifestations were more subtle than in milder allergic reactions.

Respiratory manifestations of anaphylaxis in the dog are rare but can consist of pulmonary congestion or hemorrhage clinically manifesting as tachypnea and increased respiratory effort

Question 7

True or False?

Anaphylactic reactions following vaccines are very common in DOGS.

Answer 7

FALSE!
Anaphylactic reactions following vaccine administration are less commonly observed and appear to rarely be fatal in DOGS.

Question 8

The most common clinical signs with vaccine- associated allergic reactions in cats?

Answer 8

lethargy with or without fever, localized vaccination site reactions such as soreness or inflammation, vomiting, facial edema, or generalized pruritus.

Question 9

Common signs of anaphylactic reactions in cats?

Answer 9

Pulmonary and gastrointestinal signs such as open-mouth breathing, tachypnea, respiratory distress, salivation, vomiting, and diarrhea. Respiratory signs are thought to result from bronchoconstriction, laryngeal edema, and increased mucus production.

Question 10

Treatment with antihistamines in alleviating cardiovascular signs of anaphylaxis?

Answer 10

Antihistamines should only be considered in the treatment of mild to moderate hypersensitivity reactions or as an ancillary treatment in anaphylactic shock to relieve cutaneous, ocular, and nasal signs.

Question 11

Some believe the use of epi as a first-line treatment can be controversial for canine anaphylaxis. WHY?

Answer 11

Distributive shock in canine anaphylaxis may be due to pooling of venous blood in the hepatic circulation and the use of epinephrine as a first-line treatment is controversial. In experimental models of canine anaphylactic shock, low-rate constant-rate infusions of epinephrine were considered more effective than a single bolus dose. Subcutaneous administration is not recommended due to potent vasoconstriction and unpredictable absorption in states of anaphylactic shock.