Avian Schistosomes and Cercarial Dermatitis in Denmark
PhD student: Azmi Al-Jubury, azmi@sund.ku.dk
PhD defended: June 2022
Background
During the last decades human infections with avian schistosome larvae have been observed in Denmark with increasing frequency. The infection in human is termed cercarial dermatitis or swimmer’s itch, which is a skin disorder caused by penetrating avian schistosome cercariae. Patients become infected during their contact with infected freshwater containing infective cercariae, and the disease picture is therefore often described after bathing in freshwater lakes in Denmark.
The infectious agents are avian schistosome larvae (cercariae) of the genus Trichobilharzia which usually die in the human skin shortly after penetration. This parasite uses snails as intermediate hosts and birds as final hosts, whereas human is an accidental host, in which full development to the sexual stage does not take place.
Previous studies of snails in a limited number of freshwater lakes in Denmark have shown that bird schistosomes Trichobilharzia spp. and Bilharziella spp. are widespread. However, the increasing number of human cases during recent years called for further mapping of risk areas.
Purpose of the project
The aim of the Ph.D. study was to shed light on cercarial dermatitis in Denmark by
- To describe clinical characteristic cutaneous and possibly associated extra-cutaneous symptoms of cercarial dermatitis in Denmark.
- To identify the causative agents of cercarial dermatitis in Denmark.
- To determine the distribution of infected snails in artificial and natural freshwater lakes in Denmark.
- To develop a risk assessment method for water bodies and elucidate the ecological factors influencing the occurrence of the diseases.
Results
The Project documents the potential risk of contracting swimmers’ itch when bathing or swimming in natural or artificial freshwater lakes in Denmark. The occurrence of intermediate and definitive hosts in lakes, commonly used for recreational activities such as bathing, swimming and fishing, sustains lifecycles of T. franki, T. szidati, T. regenti and T. anseri, all bird schistosomes with a potential to infect humans. The overall prevalence of infections in snails is low but the production of infective cercariae is high.
The occurrence of snails and the abundance and infection in relation to physicochemical parameters in the freshwater lakes showed a significant positive association between temperature and infections, whereas other abiotic parameters showed no significant association. Cercarial shedding, behavior and life span are light and temperature dependent. It is discussed if repeated exposures to bird schistosome cercariae influences the severity of symptoms. Climate changes and increased occurrence of bird schistosomes, along with increased human water contact.
Future perspectives
There are several issues that still need to be addressed in future studies. These include:
- Optimization of Trichobilharzia molecular diagnostics. Based on mitochondrial genome databases new primers and probes for known and cryptic species may improve precision of diagnostics.
- Optimization of eDNA diagnostics for bird schistosomes from water samples may be a future tool for extensive surveys in the future where neither snails nor birds specimens are necessary.
- Optimization of Tricobilharzia spp. diagnostics should be developed for human skin biopsies to correlate human infections with various bird schistosome species in bathing and swimming areas.
- Due to the importance of cercarial dermatitis in Denmark continuous surveys of infections in snails and migratory birds should be included in future studies.
- Design and implementation of effective control methods for bathing sites should be developed. Thus, protocols for snail control and parasite elimination should be developed.