Natural Concurrent Infection by Water Moulds of the Genera <i>Achlya</i> and <i>Protoachlya</i> in a Red Tilapia ( <i>Oreochromis</i> sp.) Hatchery in Malaysia

This paper reports a natural concurrent infection by water moulds of the genera Achlya and Protoachlya in a red tilapia hatchery in Malaysia. In December 2024, a mass mortality of cultured juvenile red tilapia was noticed at 7 days after transportation and introduction into a hatchery. The affected fish showed lethargy at the bottom of the tanks, loss of appetite, pale colouration, and were isolated from the schooling group. Further examination found brownish cotton wool-like growth, particularly at the operculum, gills, and at the base of the fins. Several fish exhibited clinical signs including fin rot, gill inflammation, and cloudy eyes. Histopathological examination of the gills revealed secondary lamella necrosis, fusion, hyperplasia, and infiltration of leucocytes. Morphological and molecular identifications revealed the presence of Achlya bisexualis, A. prolifera, and Protoachlya paradoxa in the diseased fish. These organisms belong to the Oomycetes, a group of fungus-like protists rather than true fungi. Achlya bisexualis and A. prolifera were more sensitive to salinity, with a minimum inhibitory concentration of 15 ppt, compared to P. paradoxa, which was more halotolerant until 25 ppt. The isolated oomycetes demonstrated the ability to grow at temperatures ranging from 5°C to 30°C, with growth rates increasing with temperature. A cumulative mortality rate of 68% with estimated losses of RM 1000 was recorded from the outbreak, while gradual increases in water salinity up to 20 ppt helped to reduce the mortality. Saprolegniasis poses a significant threat to tilapia's egg and fingerling productions in hatchery operations. However, a synergistic multispecies infection of the pathogens may exacerbate the problem.