Pathogen Saprolegnia parasitica
Taxonomy Oomycota, Oomycotea, Saprolegniaceae
Hosts Salmonid fishes, ayu (Plecoglossus altivelis), Japanese eel (Anguilla japonica), common carp (Cyprinus carpio)
Infection site Skin, fin
Clinical signs Filamentous mycelia are observed on the head and fin (Figs. 1 and 2).
Fungiology The asexual spore or sporangium is formed at the end of hyphal cells on the body surface of a host and can release motile zoospores. These primary zoospores swim before they encyst and release a secondary zoospore which infects a host. In rare cases, the fungus forms an oval oogonium followed by sexual reproduction (Hatai, 2004).
Pathology Diseased fish exhibits epidermal loss and Saprolegnia parasitica penetrates deeply into the muscle. As a result, the host is led to the death due to osmoregulatory failure (Hatai, 2004).
Health hazard Since this fungus is not infectious to human, it is harmless in food hygiene.
Diagnosis Check the mycelium in the lesion (Fig. 3).

Hatai, K (2004): Fungal diseases. Infectious and parasitic diseases of fish and shellfish. (ed. by Wakabayashi, H. and K. Muroga), Koseisha koseikaku, pp. 263-284. (In Japanese)

(Photos by T. Awakura)

Fig. 3. Mycelia of Saprolegnia shikotsuensis

Fig. 2. Damage of the caudal fin caused by the fungus

Fig. 1. Sockeye salmon fry infected with the fungus