Oomycete infections (family Saprolegniaceae) may be commensal or act directly as primary infectious agents and this group, in general terms, is represented by two species, S. parasitica and S. diclina affecting wild and farmed fish at all developmental stages in fresh water (Figs. 7.2, 7.3, 7.4 and 7.5). The presence of superficial cotton-wool-like tufts, particularly on the integument and gills of host fish or eggs, is likely to be the result of Saprolegnia infection. Thin, white or grey threads with circular or crescent-shaped colonies grow by radial extension until adjacent lesions merge (Figs. 7.6 and 7.7). Diseased fish become increasingly lethargic with a loss of equilibrium shortly before death, following lethal haemodilution. Respiratory difficulties may also feature when infection is associated with the gills.

Microscopic examination shows the characteristic filamentous, coenocytic mycelium of non-septate hyphae with many zoosporangia. An early infection comprises rapid degenerative changes in the muscle resulting in a diffuse oedema. As the infection radiates from the focus of infection, more of the epidermis is destroyed, and consequently hyphae can penetrate the basement membrane, with growth continuing into the dermis, hypodermis and musculature (Fig. 7.8). Thrombi are frequently observed in blood-vessels as a result of the penetrating hyphae. There is loss of integrity of the integument and oedema of the hypodermis accompanied by marked myofibrillar degenerative changes. Severe infection causes swelling in the inter-myotomal connective tissue, loss of nuclei and a minor host reaction. More aggressive lesions show deeper myofibrillar and focal cellular necrosis, spongiosis or intracellular oedema and ultimate sloughing of the epidermis.