Background Loiasis is a parasitic infection highly endemic in West and Central Africa. Previously often considered as a benign infection, recent studies have suggested that high microfilaria loads in loiasis patients may be associated with increased mortality. The true impact of loiasis on affected communities is unknown. Reports on clinical symptoms and changes in laboratory values due to loiasis infection are focused mostly on returning travellers. Assessments in endemic populations have scarcely been performed. Laboratory alterations such as eosinophilia are thought to be present mostly in patients from non-endemic areas.

Methods This cross-sectional study was performed in rural Gabon to investigate the clinical symptoms and laboratory changes caused by loiasis in an endemic population. Inclusion criteria were local residency for at least two years and an age above two years. Participants were interviewed with a questionnaire covering loiasis-specific symptoms, exploratory questions and history of eye worm. Local vocabulary was employed if needed. At the same time L. loa microfilaria diagnostics were performed including thick blood smear preparation and concentration techniques. Additionally, differential blood count and haemoglobin measurements were performed. Peripheral blood withdrawals were done between 10 am and 3 pm.

Results A total of 1030 participants were recruited, of whom 56% were female; they were between 2 and 98 years of age. L. loa microfilariae were detectable in 27% of all participants. Microfilariae densities ranged from 1 to 76 250 Mf/mL. Overall a positive history of eye worm was reported in 56%, with 25% of participants reporting eye worm passage and 36% Calabar swelling within the last year. Also, an analysis of laboratory parameters in comparison to clinical data and infection status was performed.

Conclusion Loiasis infection should receive more attention by the scientific community and further studies are needed, as the disease causes substantial morbidity in endemic populations.