pathogens Brief Report Invaders as Diluents of the Cercarial Dermatitis Etiological Agent Anna Stanicka 1, * , Lukasz Migdalski 1 , Katarzyna Szopieray 1 , Anna Cichy 1 , Lukasz Jermacz 2 , Paola Lombardo 3 and El ˙ zbieta ˙ Zbikowska 1   Citation: Stanicka, A.; Migdalski, L.; Szopieray, K.; Cichy, A.; Jermacz, L.; Lombardo, P.; ˙ Zbikowska, E. Invaders as Diluents of the Cercarial Dermatitis Etiological Agent. Pathogens 2021, 10, 740. https://doi.org/10.3390/ pathogens10060740 Academic Editor: Sara V. Brant Received: 15 April 2021 Accepted: 8 June 2021 Published: 11 June 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Invertebrate Zoology and Parasitology, Faculty of Biological and Veterinary Sciences, Nicolaus Copernicus University in Torun, 87-100 Torun, Poland; 293588@stud.umk.pl (L.M.); 285385@stud.umk.pl (K.S.); annacichy@umk.pl (A.C.); ezbikow@biol.umk.pl (E. ˙ Z.) 2 Department of Ecology and Biogeography, Faculty of Biological and Veterinary Sciences, Nicolaus Copernicus University in Torun, 87-100 Torun, Poland; jermacz@umk.pl 3 Limno Consulting, I-00124 Rome, Italy; p.lombardo@limnoconsulting.com * Correspondence: anna.marszewska@umk.pl Abstract: Research on alien and invasive species focuses on the direct effects of invasion on native ecosystems, and the possible positive effects of their presence are most often overlooked. Our aim was to check the suitability of selected alien species (the snail Physa acuta, the bivalve Dreissena polymorpha, and the gammarid Dikerogammarus villosus) as diluents for infectious bird schistosome cercariae—the etiological factor of swimmer’s itch. It has been hypothesized that alien species with different feeding habits (scrapers, filterers and predators) that cohabit the aquatic environment with intermediate hosts of the schistosomatid trematodes are capable of feeding on their free-swimming stages—cercariae. In the laboratory conditions used, all experimental animals diluted the cercariae of bird schistosome. The most effective diluents were P. acuta and D. villosus. However, a wide discrepancy in the dilution of the cercariae between replicates was found for gammarids. The obtained results confirm the hypothesis that increased biodiversity, even when alien species are involved, creates the dilution effect of the free-living stages of parasites. Determining the best diluent for bird schistosome cercariae could greatly assist in the development of current bathing areas protection measures against swimmer’s itch. Keywords: dilution effect; Trichobilharzia; swimmer’s itch; alien species; feeding habits 1. Introduction Widespread pulmonate gastropods are the first intermediate hosts of bird schistosomes (Trematoda: Schistosomatidae) and release cercariae infective for vertebrates that actively move in the water in search of the definitive host—waterfowl [1]. Moreover, cercariae can also attack accidental hosts, including humans, causing cercarial dermatitis ("swimmer’s itch") [2]. Swimmer’s itch is an emerging disease involving an intensely itchy rash but also general symptoms such as catarrh, diarrhoea, fever, insomnia [3–5] and in extraordinary cases disorders of the respiratory system and even anaphylactic shock [6]. Bird schistosome larvae may overcome the barrier of the mammalian skin and reach the internal organs [7]; more specifically, schistosomulae have been found in the lungs, heart, liver, kidney or intestine [7–9]. Some species of bird schistosomes can be extremely dangerous because they show high affinity to the central nervous system [10]. Cercarial dermatitis has been reported from nearly all continents, including Eu- rope [11]. There are currently plentiful reports of outbreaks of swimmer’s itch from recreational water bodies [5,12–16]. Scientists are constantly looking for an effective method of protecting bathing areas from rashes of aquatic origin [17–21]. Methods that require human intervention related to the final host include (i.) reducing the population of bird hosts in recreational water areas by translocating these final hosts to other places, scaring Pathogens 2021, 10, 740. https://doi.org/10.3390/pathogens10060740 https://www.mdpi.com/journal/pathogens