Toxoplasmic retinochoroiditis is thought to recur randomly. We sought to determine whether there is, instead, a longitudinal pattern of recurrences and to identify risk factors for recurrence.

Longitudinal cohort study.

We collected the following data for 143 patients with toxoplasmic retinochoroiditis in The Netherlands: gender, first affected eye, age at first episode, mode of Toxoplasma gondii infection (congenital vs postnatal), treatment history, and presence of retinal scars at initial examination. For each episode, we determined age, duration since first episode, and interval since previous episode. We estimated the relationship between disease-free interval after an episode and recurrence risk. The influence of host and disease factors on recurrence risk was analyzed using Cox regression with frailty modeling for correlated intrapatient recurrence times. We performed a Monte Carlo test for occurrence of clusters after prolonged disease-free intervals.

Follow-up ranged from 0.3 to 41 years (323 episodes in first-affected eyes). Recurrence risk was highest immediately after an episode, then decreased with increasing disease-free intervals, a pattern consistent with clustering. Relative risk (RR) of recurrence declined 72% (RR, 0.28; 95% confidence interval [CI], 0.22 to 0.36; P < .001) with each 10-year interval since first episode, and declined 15% (RR, 0.85; 95% CI, 0.71 to 1.01; P = .06) for each 10-year increase in age at first episode. Patients more than 40 years of age were at higher risk of recurrence than younger patients (RR, 1.74; 95% CI, 1.06 to 2.86; P = .03). Clusters of episodes occurred after prolonged disease-free intervals.

Toxoplasmic retinochoroiditis occurs in clusters over time. Recurrence risk is influenced by patient age and duration of infection.