Veterinary session

Water moulds are fungus-like filamentous organisms belonging to Oomycota, which include saprotroph, plant and animal pathogenic species. They cause disease called saprolegniosis in fish, mainly as secondary infections in immunodeficient condition. The most dangerous fish pathogens of water moulds belong to the order Saprolegniales, such as Saprolegnia parasitica. Water moulds can infect and kill all stages of fish, amphibians, and crustaceans. Moreover, the aquatic saprotroph and plant pathogenic oomycetes also have importance for animal health, as some of them have been proven to be fish pathogenic in recent years. Thus, the aim of the present study was to investigate the occurrence and diversity of water moulds in the natural waters in Hungary to obtain a comprehensive picture of the current infestation.

Water moulds were isolated on antibiotic-containing culture media from water or fish skin swabs collected from 63 sampling sites in natural waters throughout the country. First, DNA was extracted from the isolates, then a fungus-specific ITS–rDNA-specific PCR was performed, followed by DNA sequencing. Species-level identification of isolates was based on DNA sequences. The phylogenetic relationship of the isolates was analyzed using maximum likelihood method. Of the 138 samples, 88 isolates were successfully obtained, from which 61 were oomycete species.

Among the isolates, 11 different species of water moulds were identified. In addition to the egg-damaging Saprolegnia australis and Saprolegnia ferax, the known fish pathogens Saprolegnia delica and Saprolegnia parasitica were detected in 12 fish samples. Of the 41 water samples positive for oomycetes, 3 Pythium species, 1 Achlya, and 6 Saprolegnia spp. were identified. Of these, two new Pythium and Saprolegnia species were detected in addition to the pathogenic species. The two most common species were S. ferax and S. parasitica.

This was the first large-scale, comprehensive investigation of water moulds in the natural waters in Hungary. The identified water moulds may pose a threat to fish populations in our natural waters, as well as to fish stocks in fish farms connected to rivers and streams. Consequently, these results have both ecological and animal health importance. Further studies are also planned to investigate larger areas and obtain a complete picture of the species composition and genetic diversity of oomycetes in our natural waters.