Prevalence and genetic analyses of Saprolegnia strains isolated from Nile tilapia farms at northern Egypt

Abstract

This study was undertaken to investigate saprolegniasis outbreaks in Nile tilapia farms at northern Egypt and its linkage to the water quality parameters, besides, their genetic diversity across different farms and the induced ultrastructural lesions. Water quality parameters were recorded and a total of 200 Nile tilapia showing saprolegniasis like signs were collected from December 2017–March 2018 from three different regions at El-Manzala, Gamasa, and El-Sirw, located at northern Egypt. A significant increase was observed in salinity, where El-Manzala showed the lowest level lowest 0.25 g/L followed by Gamasa and El-Sirw at salinity levels of 3.5 g/L, and 2.15 g/L, respectively. The prevalence of saprolegniasis showed significant variations between different regions (P < 0.05) and across different months (P < 0.0001). El-Manzala showed the higher prevalence (90.91%, 100/10) compared to El-Sirw (77.5%, 31/40), but no significant prevalence changes were observed between Gamasa (80%, 40/50) compared to both El-Manzala and El-Sirw regions (P > 0.05). Similarly, monthly prevalence where significantly increased during all months, but March; and Saprolegnia isolates were highly recovered from skin and fins followed by gills. All strains isolated were morphologically classified in the genus Saprolegnia, and they were identified presumptively as S. parasitica/diclina, S. ferax, and S. litoralis according to their sexual characteristics. The genetic variation among the 23 Saprolegnia isolates were investigated using RAPD-PCR, an approach has not been done yet for Nile tilapia saprolegniasis, and results revealed isolates split into two distinct clades using based on genetic similarities, demonstrating that there is genetic diversity among the isolates. Clade I contains seven isolates out of 23 (30.4%), whereas clade II contains sixteen isolates out of 23 (69,5%). Ultrastructural examination revealed sexual and asexual stage of Saprolegnia with numerous mycelia, besides skin necrosis and ulceration, however the attachment components, such as appressoria and hooked hairs were not observed. Our results provide a better overview of the Saprolegnia outbreaks in different regions and their association to the risk factors contributing to Saprolegnia pathogenicity, and the ultrastructure alterations induced in fish skin confirming the disease. Further, for the first time a genetic diversity of Saprolegnia isolates among different localities has been explored in Nile tilapia.