In vitro cultivation model for Heterosporis saurida (Microsporidia) isolated from lizardfish, Saurida undosquamis G Kumar 1 , M Saleh 1 , A-A S Abdel-Baki 2,3 , S Al-Quraishy 2 and M El-Matbouli 1 1 Clinical Division of Fish Medicine, University of Veterinary Medicine, Vienna, Austria 2 Zoology Department, College of Science, King Saud University, Riyadh, Saudi Arabia 3 Zoology Department, Faculty of Science, Beni-Suef University, Beni-Suef, Egypt Abstract Heterosporis saurida is a microsporidian that infects lizardfish, Saurida undosquamis (Richardson, 1848), in the Arabian Sea. Spores were isolated from infected lizardfish and used to infect derived fish cell lines: common carp brain (CCB), epithe- lioma papulosum cyprinid (EPC), fathead min- now epithelial (FHM), rainbow trout gonad (RTG), bluegill fry (BF-2) and chinook salmon embryo (CHSE). Non-fish cell lines were also tested that include: insect (SF-9), rabbit (RK-13) and African green monkey (Vero E6). No growth of H. saurida was observed in any fish cell line, SF-9 or Vero E6 cell lines. H. saurida spores grew only in RK-13 cell line and were detected by immunofluorescence. Developmental stages of H. saurida were seen in RK-13 cells by light and transmission electron microscopy, and species identification was confirmed by sequencing. This study demonstrated that H. saurida was able to proliferate in the mammalian RK-13 cell line, which thus represents an in vitro model for con- ducting molecular genetics and cell–pathogen interaction studies of Heterosporis. Keywords: cell lines, Heterosporis saurida, lizardfish, microsporidia, rabbit kidney cell line. Introduction Microsporidia are obligate intracellular spore- forming eukaryotic parasites that infect a wide range of invertebrates and vertebrates (Shaw & Kent 1999; Lom & Nilsen 2003). The parasites intracellular life cycle has two phases: proliferation (merogony), which is responsible for a massive increase in parasite numbers, and spore production (sporogony), in which sporonts produce sporoblasts that mature into spores (Cali & Takvorian 1999). Fish microsporidia of genera Glugea, Loma, Nuce- lospora and Heterosporis are widely distributed and cause high mortality rates in economically impor- tant fish species (Shaw & Kent 1999). Heterosporis spp. infect muscle tissues diffusely and do not produce xenomas, but they may ulti- mately be bounded by host connective tissue and form cysts (Lom et al. 2000). Lizardfish, Saurida undosquamis (Richardson, 1848), is one of the most economically important fish in the Arabian Sea and Red Sea regions (Peyghan et al. 2009). Recently, a novel microsporidian, H. saurida, was described from lizardfish (Al-Quraishy et al. 2012). The parasite infects skeletal muscle, body cavity and mesenteric tissues and forms white, cyst-like structures, which contain numerous spores. Cell cultures are an essential tool for studying viral and microbial pathogens of humans and other animals and have recently been applied to the study of microsporidia in insects and mam- mals (Monaghan et al. 2009). Development of tools for studying microsporidia is important, as these parasites cause several animal diseases of considerable economic importance, including diseases that hamper aquaculture of fish and which have been a factor in the attenuation or collapse of several fisheries (Shaw & Kent 1999; Becker & Speare 2007). Correspondence M El-Matbouli, Clinical Division of Fish Medi- cine, University of Veterinary Medicine, Veterina ¨rplatz 1, Vienna 1210, Austria (e-mail: Mansour.El-Matbouli@vetmeduni.ac.at) 1 Ó 2013 John Wiley & Sons Ltd Journal of Fish Diseases 2013 doi:10.1111/jfd.12123