Saprolegnia spp. is an oomycete fungus found in most freshwater sources worldwide. Saprolegnia spp. can cause the disease saprolegniosis - growth of fungal mycelium on the skin and gills - in fish both in farming and in the wild. As the name suggests, Saprolegnia are primarily saprophytes (decomposers of dead organic matter), but some species have also developed the ability to infect living organisms, such as fish. The disease can affect all stages in the freshwater phase, and saprolegniosis has been found in salmonids on every continent. Significant economic losses due to saprolegniosis have been reported.

Saprolegnia species infect hatcheries via the intake water, as “spores”. These spores are robust and are inactivated only to a limited extent by UV filters and ozone treatment. Additionally, Saprolegnia can form a biofilm in pipelines and tank systems, producing spores that increase the infection pressure. For Saprolegnia spores to establish an infection on fish, they depend on damage to the fish’s mucus layer and skin, or an impaired immune system. Regarding infection on eggs, the spores cannot establish themselves on live eggs but depend on the presence of dead eggs for infection during the incubation of the roe. However, Saprolegnia mycelium spreading from dead eggs will also kill live eggs. In Norway, the main problem is the latter, which can cause large numbers of eggs to be lost due to infection.

Infection before hatching can be prevented by removing white (dead) eggs and treating with formalin if necessary. After hatching, the most important measures are to avoid stress and mechanical damage to the fish, and to keep the infection pressure (spore count) as low as possible.

Diagnosis

Saprolegniosis is easily recognisable and is usually diagnosed in the field by observing it macroscopically, as “cotton” on the surface of the fish and on eggs. In fry, infection can be difficult to see macroscopically as the fish often die before visible mycelium is formed. PatoGen now offers RT-PCR analysis on salmonid fish (trout and salmon) that detects Saprolegnia spp. The analysis is available for all geographical areas.

In connection with the launch of our new assay, we are excited to invite you all to a free webinar on saprolegniosis, with PatoGen’s own Even Thoen (Senior Advisor Histopathology, PhD, DVM) and Elisabeth Faureng (Senior Fish Health Advisor, DVM) on Wednesday 18.09 at 13:00.

Registration in link here: Webinar

Want to know more? Please contact our sales team.