No statistically significant association between recent vaccination and a diagnosis of presumptive primary immune-mediated
thrombocytopenia (IMT) was found in a recent retrospective study that examined 48 dogs presumed to have IMT and 96 control
dogs.1
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As noted in the study, vaccines often are withheld from dogs with a history of IMT (and other immune-mediated conditions)
because of the concern that vaccinations may trigger the immune system and result in recrudescence of that immune-mediated
condition.1 These are valid concerns, as there have been reports in the human literature of recent vaccination being associated with
idiopathic thrombocytopenic purpura. So in lieu of automatic vaccination (especially in patients with a history of more than
one episode of IMT), titers may be submitted to support adequate immunity to those specific infectious diseases.
If vaccination is deemed necessary, vaccine administration can be staggered throughout the year. Medications or supplements
the patient was receiving just before or during the first episode of IMT also should be avoided (when possible) in all future
treatment protocols in that patient. All of these precautions may apply to other known immune-mediated conditions as well.
Although a statistically significant relationship between vaccination and IMT was not found in this study, the authors note
that one cannot rule out the possibility of an infrequent association between the two or a transient, nonclinical, postvaccination
IMT.1 They discuss several important limitations of the study, including a lack of information regarding the number of vaccines
in the subject's lifetime, an inherent difficulty of confirming the diagnosis of idiopathic IMT, a small sample size and the
study's retrospective nature. So it is difficult to conclude whether recent or cumulative vaccinations may or may not increase
the risk of idiopathic IMT.
IMT overview
Table 1: Conditions associated with secondary thrombocytopenia in dogs
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Thrombocytopenia is either primary (idiopathic) or secondary (a potential cause can be identified; Table 1). Idiopathic IMT
is an autoimmune disease usually mediated by immunoglobulins directed against platelet membranes and, theoretically, requires
antiplatelet antibody testing for a definitive diagnosis. The source of these antiplatelet antibodies currently has not been
identified.
Because of the limited availability and variable sensitivity and specificity of antiplatelet antibody testing, IMT is typically
diagnosed based on clinical and laboratory criteria, exclusion of other disease processes and response to treatment. IMT usually
results in a severe thrombocytopenia (< 50,000/μl).
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