Investigation and systematic review of temporal associations between vaccination and onset of immune-mediated hemolytic anemia or thrombocytopenia in dogs.

Vaccination shows temporal association with IMHA onset in dogs but does not establish causation.

This multicenter retrospective case-control study with meta-analysis investigated temporal associations between vaccination and immune-mediated hemolytic anemia (IMHA) or thrombocytopenia (ITP) in dogs. The study included 295 IMHA cases, 163 ITP cases, and 1,180-1,652 control dogs across four referral hospitals. Results demonstrated a significant temporal association between vaccination and IMHA onset, with 11.9% of IMHA dogs vaccinated within 30 days of disease onset compared to 6.1% of controls (P=0.0015). Dogs with IMHA vaccinated within 30 days were more likely to have received L2 leptospira vaccines. Meta-analysis across multiple studies showed IMHA dogs were 2.48 times more likely to have been vaccinated in the past month than controls, though heterogeneity was high. No significant temporal association was found between vaccination and ITP onset. Importantly, the authors emphasize these findings establish a temporal relationship but not causation, suggesting vaccination may be a trigger in susceptible dogs but not a definitive causative agent.

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