There are a number of causes of decreased platelets in dogs and cats, but whenever platelet counts fall acutely an immune-mediated
disorder should be suspected. Bleeding disorders that occur as platelets drop can cause rapid deterioration and death. Early
recognition and aggressive management is important with immune-mediated thrombocytopenia (IMT). Immune-mediated arthritis
also has various underlying causes, but usually has a slower onset and is less likely to be severe or fatal. The following
discussion briefly reviews etiology, clinical signs, diagnosis, and management of IMT and arthritis associated with immune
system disorders. During the presentation, cases will be presented as time permits.
A similar mechanism to IMHA occurs in IMT, as IgG antibodies are directed against platelet membranes resulting in destruction
(see previous Proceedings article on IMHA). Primary (idiopathic) disease is more common in dogs than secondary IMT. There
is likely to be a genetic predisposition, as certain breeds are over-represented in studies including Cocker Spaniels, Old
English Sheepdogs, Poodles, and Cavalier King Charles Spaniels. Young to middle age adults are most often affected, and females
are slightly more likely than males to be diagnosed with primary IMT.
Thrombocytopenia and thrombocytopathia (abnormal platelet function) with an immune-mediated component also occurs secondary
to other diseases. Among the reported causes are:
o Rocky Mountain Spotted Fever
o Heartworm disease
o Feline leukemia virus
o Feline immunodeficiency virus
o Feline infectious peritonitis
- Drugs (hundreds of examples in human medicine)
o Cardiovascular drugs
o Oral hypoglycemics
o Methimazole, propylthiouracil
o Diabetes mellitus
- Vasculitis, DIC
- Neoplasia (e.g. lymphoma)
- Chronic infectious or inflammatory disease (e.g. IBD)
- Other immune-mediated disorders (e.g. IMHA, SLE)
As with IMHA, the owner may only notice lethargy, weakness, and anorexia. On physical exam, pale mucous membranes and petechiae
or ecchymoses are common. A careful inspection of the mucous membranes and skin may be needed (hair should be clipped if thick)
to detect pinpoint hemorrhages. Bleeding such as epistaxis, melena, hematuria, hyphema, and neurologic signs is less common.
The blood loss associated with platelet destruction may mimic IMHA or other causes of anemia. Splenomegaly and fever may be
noted. Hematomas or bleeding into body cavities suggests rodenticide toxicosis or other causes or coagulopathy, not IMT.