Immune-Mediated Conditions in Dogs and Cats

Andrew Mackin, BVMS, MVSc, DVSc, FANZCVSc, DACVIM (SAIM) | PacVet 2022 Speaker By

Veterinarians commonly encounter a range of immune-mediated diseases, such as immune-mediated hemolytic anemia (IMHA), immune-mediated thrombocytopenia (IMT or ITP), and immune-mediated polyarthritis (IMPA), and are generally adept at recognizing these conditions and instituting appropriate therapy. The more straightforward versions of these conditions tend to be easy to diagnose and respond well to standard therapy.

Veterinarians can get into trouble, in my experience, when confronted with the case-by-case nuances that make management of these conditions more of an art than a science.

Conditions like IMHA, ITP, and IMPA can present in a wide variety of ways, can commonly deviate from the “typical presentation” clues that are usually used to diagnose them, and can respond very differently to standard treatment strategies. Such “atypical” cases are, in my experience (admittedly, I have the bias of a specialist that gets referred difficult cases), actually more common than “typical” presentations. The art, when dealing with such cases, is to develop an individualized approach to every patient that considers variables like species, breed, age, severity of disease, likelihood of underlying disease, patient tolerance of medication, and owner budget.

Furthermore, when practitioners encounter an atypical presentation, diagnostic result, or response to therapy, they should be prepared to change course and resort to individualized therapy on a trial-anderror basis. There is no “cookbook” when it comes to dealing with these conditions.

One challenge that clinicians commonly encounter is that immune-mediated diseases can present in both very mild forms and very severe forms. Mild IMPA, for example, can be very hard to recognize and may present solely as a subtle fever of unknown origin. Mild ITP can present with platelet counts that are not low enough to cause bleeding; thus, clinicians can struggle to distinguish laboratory artifact from genuine disease in order to decide if treatment is even needed.

At the opposite end of the spectrum, patients with severe cases of transfusion-dependent IMHA or ITP can be at such an immediate risk of death that the usual time to respond to standard immunosuppressive therapy—approximately one week or more—can seem interminably long. In such circumstances, clinicians will be forced to explore a wide range of more aggressive (and often more expensive) treatment options that will get the patient through the first week of therapy, including but not limited to vincristine, high-dose intravenous human immunoglobulins, therapeutic plasma exchange, and splenectomy. Both subtle disease and severe lifethreatening presentations will force clinicians to deviate from standard cookbook approaches.

Immune-mediated diseases are often debilitating and even lifethreatening but are also typically treatment-responsive with appropriate therapy. Development of a thorough understanding of the range of diagnostic and therapeutic management options that are available for these conditions can therefore lead to highly rewarding outcomes for clinicians.