Reacting to reactions in general practice

When a patient appears with an unspecified reaction, it’s crucial to gather as much data as possible from the pet owner, the physical exam, and the lab to recognize a serious or even deadly  reaction and respond accordingly.

By Kate Boatright, VMD

Dogs can experience type 1 hypersensitivity reactions to a number of stimuli, including insect bites, stings, vaccinations, and drugs. The severity of these reactions varies from mild facial swelling and pruritus to life-threatening anaphylactic shock and even death.

Recognizing that a reaction is occurring and responding appropriately is essential to patient outcomes.

While vaccines are the most common cause for both anaphylaxis and uncomplicated allergic reactions,1 vaccine reactions occur in only 0.19% of vaccine visits.2 These reactions are more common when multiple vaccines are administered simultaneously and in smaller dogs.2

Managing uncomplicated allergic reactions

The most common presentation of type 1 hypersensitivity reactions in general practice are mild, often localized reactions that present with some combination of:

  • Facial swelling (angioedema)
  • Hives (urticaria)
  • Erythema
  • Pruritus

The overall incidence of uncomplicated allergic reactions in dogs is reported between 0.15% and 1.2%.1,3

Some patients may also exhibit gastrointestinal signs such as vomiting or diarrhea. These patients are hemodynamically stable and usually bright, alert, and responsive at presentation.

These stable patients are often treated on an outpatient basis with some combination of antihistamines and glucocorticoids.

Diphenhydramine (H1-antihistamine) can help to control cutaneous changes while famotidine (H2-antihistamine) can help control gastrointestinal signs.

Glucocorticoids, generally used at anti-inflammatory doses, are thought to help decrease the ongoing inflammatory response in the body, though a recent study found no difference in patient outcomes for dogs treated with diphenhydramine and dexamethasone-SP versus diphenhydramine alone.1,3

Most veterinarians use a combination of injectable medications followed by oral treatment after discharge.3

Recognizing anaphylaxis

Anaphylaxis is a systemic type 1 hypersensitivity reaction that results in a combination of distributive and hypovolemic shock. Patients with anaphylaxis can have a variety of presentations ranging from lethargy and acute vomiting to collapse and sudden death.

  • Anaphylaxis is less common than mild hypersensitivity reactions with an incidence of 0.04%.2
  • Sometimes there is a history of recent exposure to a possible stimulus (vaccination, drugs, bee sting, snake bite, etc.), so obtaining a thorough history can be helpful. However, anaphylaxis should not be ruled out just because an exposure is not identified. In a recent study, no cause was identified in 26% of cases.1
  • The primary source of histamine is the mast cells in the gastrointestinal tract and liver. The release of histamine from these cells results in marked changes in the hepatic vasculature, leading to many of the cardiovascular effects and clinical signs seen with anaphylaxis in dogs.
  • Hypotension is the primary cardiovascular sign and is seen along with tachycardia or bradycardia and signs of poor perfusion (pale mucus membranes, poor pulse quality, and hypothermia).
  • Nausea, vomiting, and diarrhea are also commonly seen and may be bloody.
  • Respiratory signs and cutaneous signs seen with milder reactions are less common in dogs. Cats are more likely to develop respiratory distress as the lungs are the species-specific shock organ.

Laboratory tests and diagnostics for anaphylaxis

There is no clinical sign, laboratory abnormality, or diagnostic imaging finding that is pathognomonic for anaphylaxis. Instead, clinical suspicion and diagnosis is based on the presence of a combination of the abnormalities discussed below with or without a known exposure to an inciting agent.

  • Laboratory testing often reveals elevations in ALT and AST, which rise quickly and become significantly elevated within the first 12 hours after exposure.
  • Additional laboratory abnormalities may include:
    • Hemoconcentration
    • Thrombocytopenia
    • Azotemia
    • Hyperphosphatemic
    • Hyperglycemia
    • Hyperlactatemia
    • Electrolyte abnormalities
  • Coagulation times (PT and PTT) are often prolonged and should be checked if possible.
  • In cases where anaphylaxis is suspected, ultrasound can be a useful diagnostic tool.
    • Many patients will have changes in the appearance of their gall bladder known as the halo effect. This is due to edema in the gall bladder wall secondary to hepatic congestion which develops rapidly after the onset of anaphylaxis.
  • Additionally, patients may have peritoneal effusion, which can be hemorrhagic.

Treatment of anaphylaxis

  • Epinephrine and fluid therapy are first-line treatments for anaphylaxis to provide cardiovascular support.
  • Some patients will need repeated doses of epinephrine and may require a CRI of epinephrine or norepinephrine for blood pressure support.
  • Adjunctive treatments include antihistamines and glucocorticoids. Because they do not address signs of shock, they are not first-line treatments in anaphylaxis.
  • Other treatments are based on clinical signs and may include:
    • Oxygen support
    • Antiemetics
    • Fresh frozen plasma
    • Prophylactic antibiotics in the presence of hematochezia
  • Hospitalization is often required, but general practitioners can start initial stabilization efforts with epinephrine and fluids before transferring to a 24-hour facility for continued care.
  • A survival of 85% is reported in the literature.4 However, successful treatment often requires intensive care and potentially multiple days of hospitalization, which is not feasible for all pet owners.

Preventing future reactions

For patients that have experienced an acute allergic or anaphylactic reaction, clients may often want to know how to prevent a future reaction. If the inciting cause is identified, avoidance of the stimuli can be attempted.

For patients that have experienced reactions to vaccinations, pretreatment with diphenhydramine can be considered prior to future vaccinations. Additionally, splitting vaccinations between multiple visits to reduce antigenic stimulation is recommended in the AAHA 2022 Canine Vaccination Guidelines. These vaccines should be separated by at least two weeks.


  1. Fosset FTJ, Lucas BEG, Wolsic CL, et al. Retrospective evaluation of hypersensitivity reactions and anaphylaxis in dogs (2003–2014): 86 cases. J Vet Emerg Crit Care 2023;33:577–586.  
  2. Moore GE, Morrison J, Saito EK, et al. Breed, smaller weight, and multiple injections are associated with increased adverse event reports within three days following canine vaccine administration. J Am Vet Med Assoc 2023;261(11):1653-1659.  
  3. Helgeson ME, Pigott AM, Kierski KR. Retrospective review of diphenhydramine versus diphenhydramine plus glucocorticoid for treatment of uncomplicated allergic reaction in dogs. J Vet Emerg Crit Care 2021;31:380–386.  
  4. Smith MR, Wurlod V.A, Ralph AG, et al. Mortality rate and prognostic factors for dogs with severe anaphylaxis: 67 cases (2016–2018). J Am Vet Med Assoc 2020; 56(10):1137-1144.  

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