Aquaculture Research. 2019;00:1–4. wileyonlinelibrary.com/journal/are | 1 © 2019 John Wiley & Sons Ltd Received: 13 February 2019 | Revised: 18 March 2019 | Accepted: 7 April 2019 DOI: 10.1111/are.14087 SHORT COMMUNICATION A massive Chilodonella hexasticha infestation associated with yellowtail tetra Astyanax lacustris mortality in aquaculture: Identification and pathology Gustavo Moraes Ramos Valladão 1 | Manuela Martins Pereira 2 | Sílvia Umeda Gallani 3 | Fabiana Pilarski 3 1 Postgraduate Program in Aquaculture, Nilton Lins University, Manaus, AM, Brazil 2 Ministry of Agriculture, Livestock and Supply in Brazil, Ipumirim, Brazil 3 Laboratory of Microbiology and Parasitology of Aquatic Organisms, São Paulo State University (UNESP), Jaboticabal, Brazil Correspondence Gustavo Moraes Ramos Valladão, Postgraduate Program in Aquaculture, Nilton Lins University, Manaus, AM 69058‐030, Brazil. Email: gmrvalladao@gmail.com KEYWORDS: chilodonellosis, gill, histopathology, lambari, parasite Microorganisms of the genus Chilodonella are free‐living protozoa in aquatic environments, with two species, that is Chilodonella hex- asticha and C. piscicola, already well known to cause disease in fish globally, leading to economic losses (Bastos Gomes et al., 2017c). In a recent review by Bastos Gomes, Jerry, Miller, and Hutson (2017b), the authors showed that Chilodonella spp. have caused productivity losses in over 16 species of farmed freshwater fishes in more than 14 countries. However, records of this group of parasites in South American fish production are scarce, and to our knowledge there has been only one identification of C. hexasticha on the continent, in farmed pacu Piaractus mesopotamicus (Pádua et al., 2013). The disease caused by both species of the genus Chilodonella is called chilodonellosis and has been associated with mild to severe tissue lesions, ranging from epithelial gill hypertrophy, hyperplasia, lamellar fusion and oedema to severe necrosis and tissue structure loss (Bastos Gomes et al., 2017b; Noga, 2010; Pádua et al., 2013). According to Bastos Gomes et al. (2017b), these parasites have a vo‐ racious appetite for living cells (their cytostome is used to graze bac‐ teria, diatoms and algae present on the surface of fish gills and skin). The yellowtail tetra Astyanax lacustris (syn: Astyanax jacuhien- sis, A. asuncionensis and A. altiparanae) (Lucena & Soares, 2016) is a small South American fish that has excellent characteristics for cap‐ tive cultivation, for use as live bait (especially for commercial tuna fishing) and as a gourmet snack (Valladão, Gallani, & Pilarski, 2018). Yellowtail tetra production has been developed over the past two decades as an alternative financial source for small rural producers (Fonseca, Costa‐Pierce, & Valenti, 2017). However, because of these promising characteristics, cultivation has been growing among large producers, illustrating its social and economic importance in South America. Currently, research on diseases affecting this important fish is scarce, but it is likely to increase as, with the intensification of production, fish health problems will become more frequent. A better understanding and description of the pathogens and parasites that can negatively affect this production is essential. As part of this effort, this study reports on the diagnosis and pathological description of one of the main parasitic diseases world‐ wide, chilodonellosis, during an outbreak of mortality in a new host. Yellowtail tetra adults (approximately 10 cm in length), at the final stage of cultivation, were brought to the laboratory. This re‐ sulted from a history of mortality post‐management, when they were removed from the earth pond and maintained in plastic tanks. The water quality was not controlled by the fish farmer. The fish had microhaemorrhages on their skin, loss of some scales and whitish lesions on their gill tissue (Figure 1a). No other changes were noteworthy. A total of 12 moribund animals were anaesthe‐ tized with benzocaine (0.1 g/L) and euthanized via medullary section for necropsy with a parasitological and microbiological analysis. To investigate the ectoparasites, mucus scrapings of the skin and gills were deposited on glass slides and analysed by optical micros‐ copy (40, 100 and 400x). Slides containing parasites were air‐dried, impregnated with 2% silver nitrate for 8 min (Klein, 1958), and ex‐ posed to ultraviolet light for 15 min for further morphological and morphometric evaluation. For the measurement of nuclear struc‐ tures (macronucleus and micronucleus), slides were stained using Giemsa. Photomicrographs of the stained parasites (1000x) were taken using an optical microscope equipped with a Nikon E200 camera, and were used for measurements with Image ProPlus® software. Character parameters were measured and the parasite was identified according to Fan, Ma, Al‐Farraj, and Gu (2014). For endoparasite research, the stomach and intestine of the fish were