Cumulative mortalities in white leg shrimp, Litopenaeus vannamei Boone 1931, cultured in biofloc system in Egypt reflected new record of Fusarium verticillioides infection

Abstract

The Egyptian aquaculture sector is still struggling to establish L. vannamei culture. In an effort to stimulate shrimp production with minimal environmental impact and less water loss, biofloc technology (BFT) has been employed. Chronic mortality was recently observed in L. vannamei which was raised in the BFT system. Examining samples of moribund shrimp revealed sporadic melanization on the musculature and gills, suggesting the potential existence of black gill disease. The present work was carried out to isolate and identify potentially implicated fungus species based on mycological properties of interest. To isolate and serve as initial identification tools, wet mount preparations and colonial features, along with slide culture techniques, were used. PCR assays, gene sequencing, and phylogenetic analyses were used in the molecular identification process to clarify the confirmation of the fungal isolates that were acquired. The pathogenicity tests were designed to fulfil Koch’s postulates in addition to clarifying the infectivity of isolated fungus. Moreover, histopathological examination was employed to notice tissues’ alterations associated with experimentally infected shrimp. The results revealed the isolation of three fungal isolates, namely, BNS 1117, BNS 2117, and BNS 3117, that were confirmatively identified as Fusarium verticillioides, and their selected sequences were submitted to Genbank. Moreover, experimental infection using F. verticillioides selected isolate (BNS 31,117) at both high (5 × 105 CFU/mL, group A) and low (3 × 105 CFU/mL, group B) doses of its conidial suspensions experienced cumulative mortality of 72.2% and 21.1%, respectively. Besides, the selected isolate was re-isolated from the experimentally infected shrimp in a pure form, which satisfied Koch’s postulates. Histopathological alterations demonstrated several degrees of either degenerative, oedema, or proliferative changes in the hepatopancreas, musculatures, and gills, respectively. This study sets a new record for F. verticillioides to be included in Fusarium species linked to shrimp mortality.