Water mold infections of freshwater fish: Recent advances

Various investigations have identified morphological and physiological differences among water mold isolates. There is some evidence that isolates taken from fish lesions are more infective for live fish and are more resistant to host defenses, compared to environmental isolates. Many other characteristics, including propagule retention on fish, propagule morphology, propagule response to nutrients, fungal isozyme patterns, and antigenic relatedness of fungal isolates, have been examined. However, the significance of these characteristics to pathogenicity is uncertain, either because the putative pathogenic marker does not consistently differentiate pathogenic from saprophytic isolates, or the adaptive significance of the marker is unclear. Water molds grow rapidly in dead tissue, yet do not easily cause disease in healthy, unstressed hosts. They are usually relegated to the skin and superficial muscle. The fact that they are common secondary invaders of wounds implies that in these instances there is a significant compromise of host defenses. Trauma, nutritional deficiencies, endocrine changes, and stressful water quality have been implicated. There may be major differences in how different fish species respond to such stresses. Both humoral and cellular defensive responses to invading water molds have been observed. The type of response may modulate the gross clinical signs of the resulting infection.