We describe the seasonal variation of acute toxoplasmosis in the United States. Acute toxoplasmic lymphadenopathy (ATL) can be a surrogate of acute toxoplasmosis in patients in whom the date of onset of lymphadenopathy matches the window of acute infection predicted by serological tests performed at a reference laboratory. We used the electronic database of the Palo Alto Medical Foundation Toxoplasma Serology Laboratory (PAMF-TSL) (1997-2011) to identify cases of ATL. We tested the uniformity of distribution of ATL cases per month, across the 12 calendar months, using circular statistics uniformity tests. We identified 112 consecutive cases of ATL. The distribution of cases was not uniform across the 12 calendar months. We observed the highest peak of cases in December and a second highest peak in September. Similar months were identified in patients with acute toxoplasmosis in rural areas in France. The results were similar when we performed weighted analyses, weighting for the total number of Toxoplasma gondii IgG tests performed per month in the PAMF-TSL laboratory. This is the largest study to date of the seasonal variation of ATL in the United States. Physicians should advise high-risk individuals to avoid risk factors associated with T. gondii infections especially around those months.
Keywords: Acute toxoplasmic lymphadenopathy; United States; seasonal variation.