Parkinson's disease or Parkinson symptoms following seasonal influenza

Background Influenza may cause neurological sequelae and has been associated with encephalitis lethargica, an entity displaying Parkinson's disease (PD) signs and symptoms that followed the 1918 influenza pandemic. We studied the association between diagnosed influenza and idiopathic PD or Parkinson symptoms (PS) not followed by a firm PD diagnosis. Methods We used the UK-based General Practice Research Database to perform a case-control analysis. We identified cases who developed an incident diagnosis of PD or PS between 1994 and March 2007, and we matched four controls on age, gender, general practice, calendar time, and history in the database to each case. We calculated odds ratios (OR) with 95% confidence intervals (CI) using conditional logistic regression to assess the relative risk of developing PD or PS in association with previous influenza diagnoses. Results We identified 3976 PD cases and 18 336 PS cases. The risk of developing PD was not associated with previous influenza infections. However, PS was associated with recent influenza (last infection 0-29 days: OR 3 03, 95% CI 1.94-4.74; 30-364 days: OR 1.36, 95% CI 1.14-1.63), number of influenza episodes (1 attack: OR 1.20, 95% CI 1.12-1.28; 2 attacks: OR 1.52, 95% CI 1.28-1.81; >= 3 attacks: OR 2.00, 95% CI 1.45-2.75), and severity of preceding influenza infections (>= 1 severe attack: OR 1.45, 95% CI 1.25-1.68). Conclusions Influenza is associated with PD-like symptoms such as tremor, particularly in the month after an infection, but not with an increased risk of developing idiopathic PD.