Understanding Calabar swellings: Assessing subcutaneous loiasis using ultrasound

Loiasis is known for two main clinical manifestations: eyeworm and Calabar swellings. Calabar swellings are non-pitting edemas, often on the upper limbs or presenting as unilateral orbital swelling. They are described as itchy or painful, limit mobility, and cause notable morbidity. Although adult Loa loa filariae (macrofilariae) are known to migrate widely through host tissues, their exact pathways and tissue preference (tropism) remain unclear. Whilst the exact mechanisms remain unclear, Calabar swellings are considered to be localized allergic reactions to worm antigens, resulting in angioedema. Leveraging advances in portable ultrasound, this project aimed to characterize Calabar swellings via ultrasound to better understand the pathophysiology behind them. This study was conducted in the Lékoumou department, Republic of Congo, as an ancillary investigation within the MorLo (Morbidity due to Loiasis) project. During a follow-up visit in September 2023, participants and interested villagers were invited to present if they had Calabar swellings. Participants completed a standardized questionnaire and were examined by a physician who also performed ultrasound imaging. Twenty-two individuals presented with visible swellings; 8 (36.4%) were classified as "not typical", 12 (54.5%) as "typical" Calabar swellings and fourteen (63.6%) had subcutaneous nodules on the forearms or wrists, with 8 (36.4%) having both simultaneous swellings and nodules. Ultrasound videos of typical swellings showed nonspecific angioedema, whereas videos of nodules revealed subcutaneous trans-fascial, possibly fluid filled, retentions extending through fascia and crossing anatomical boundaries with mixed-echogenic content. Within these retentions, hyperechoic thread-like structures were visible, displaying autonomous movement in two videos. Their measured diameters corresponded to L. loa macrofilariae. The ultrasound findings are most consistent with trans-fascial migration of adult L. loa filaria, associated with fascial lesions and fluid retention. The classical Calabar swelling appears closely related to-but not directly over-the underlying lesions. Migration of adult L. loa filaria causes a destructive and disabling process that warrants research and public health responses to ensure appropriate care for affected individuals.