Lymphocytosis and Lymphopenia Induced by Imported Infectious Diseases: A Controlled Cross-Sectional Study of 17,229 Diseased German Travelers Returning from the Tropics and Subtropics.

The present controlled cross-sectional study aimed to assess relative and absolute lymphocytosis and lymphopenia induced by imported infectious diseases (IDs) seen among patients consulting the Division of Infectious Diseases and Tropical Medicine, Medical Center of the University of Munich (1999-2014) after being in the tropics and subtropics. The analysis investigated data sets from 17,229 diseased German travelers returning from Latin America (3,238), Africa (5,467), and Asia (8,524), and from 1,774 healthy controls who had not recently traveled. Among the cases, the proportion of those with relative lymphopenia (10.5%) and absolute lymphopenia (8.0%) was significantly higher than among controls (3.2% and 3.6%, respectively), whereas relative lymphocytosis was significantly lower among cases (6.1%) than among controls (8.0%). The study identified IDs with significantly larger proportions of relative lymphocytosis (cytomegalovirus [CMV] infection [56%], infectious mononucleosis [51%], and dengue fever [11%]); absolute lymphocytosis (infectious mononucleosis [70%] and CMV infection [63%]); relative lymphopenia (streptococcal pharyngitis [56%], malaria [34%], Campylobacter infection [19%], salmonellosis [18%], and shigellosis [17%]); and of absolute lymphopenia (human immunodeficiency virus infection [53%], malaria [45%], dengue fever [40%], salmonellosis [16%], and Campylobacter infection [11%]). This study demonstrates that relative and absolute lymphocytosis and lymphopenia are useful laboratory findings for travelers returning from the tropics and subtropics, as they are typically caused by imported viral, bacterial, and protozoan IDs.