Ichthyophonus
Fish species parasitized:
Ichthyophonus is a common parasite of many species of freshwater and marine fishes over a wide geographic range.

Location in host:
Many organs can be infected. In salmonids, heaviest infections are frequently found in the heart, skeletal muscle, liver, kidney, and spleen, and are often characterized by the appearance of whitish nodules in these organs. Severe infections of the brain and central nervous system have also been reported in salmonids and topminnows (Poeciliopsis). Significant mortality has been reported in both marine and freshwater fishes; salmonids, herring, and plaice are particularly susceptible.

Characteristics:
Fish presumably become infected with Ichthyophonus by ingestion of spores. Based on culture and microscopy studies and analysis of previous work, Spaangaard et al. (1995) have proposed a life cycle in which the thick-walled spherical multinucleated spores germinate in the stomach in response to low pH. The branching aseptate hyphae penetrate the digestive tract; when they reach a blood vessel (neutral pH), the hyphae rupture, releasing uni- and binucleate bodies and/or amoeboid cells. These small cells are easily transported in the blood vessels and spread in organs richly supplied with blood (heart, kidney, spleen, liver and muscle) where they develop into multinucleate spores. Soon after the death of the host, the spores germinate.

Appearance in tissue sections:
The stage most commonly observed in tissue sections is the multinucleate "resting spore," which appears roughly circular, measures 10-250 µm in diameter, and has a thick fibrous wall that stains PAS-positive. A severe granulomatous response around the spores is characteristic, although sometimes individual spores may evoke minimal host response. The presence of hyphae protruding through the outer spore wall is a definitive characteristic; these are visible in tissues sampled post-mortem.

Dermocystidium
The genus name Dermocystidium has been applied to a variety of pathogenic organisms that infect aquatic animals including amphibians and fish. They have in common spherical stages designated as spores that have some morphological similarities (see Characteristics). These organisms have often been relegated to unspecified groups of lower fungi, and formation of hyphae in D. koi has been suggested as evidence of the possible fungal nature of this parasite. Based on ultrastructural and molecular evidence, one species, D. salmonis, is currently placed in the new class Ichthyosporea within the subphyllum Choanozoa (phylum Neomonada) in the kingdom Protozoa. The molluscan parasite, formerly D. marinum, has been determined to be an apicomplexan and has been transferred under the generic name Perkinsus. Nevertheless, the taxonomic status of most members of the catch-all genus Dermocystidium remains unclear.

Fish species parasitized:
Parasites identified as Dermocystidium have been described from a variety of freshwater fishes including cyprinids, salmonids, centrarchids, eels, pike, sticklebacks, smelt, and perch.

Location in host:
The species infecting fish locate either in epithelial tissues of the skin, fins and gills or in visceral organs. The infections often appear as small round, oval or elongate white nodules or cysts in the affected tissue.

Characteristics:
Species assigned to the genus Dermocystidium have in common a spherical spore stage with a large central vacuole or refractile body and the cytoplasm with the nucleus restricted to a narrow peripheral layer. Species infecting the skin and gills form spores containing a solid central refractile body (vacuoplast), whereas the viscera-infecting species form spores with a large central vacuole instead of a refractile body. In most species the spores are of relatively uniform size, but in D. koi the spores vary greatly in size. In the developmental cycle of most skin- and gill-infecting species that have been studied, a small multicellular plasmodium grows, becomes confined within a distinct wall, then fragments into uninucleate spores, which divide into two sporoblasts that mature into spores. In some species (e.g., D. cyprini) the fragmentation of plasmodia is associated with the formation of small comparments, whereas in others (e.g., D. branchiale), it is not. The development of the visceral Dermocystidium species does not include the growth of large plasmodia, and the small plasmodia may not be confined within capsule walls. In the skin pathogen D. koi, a web of aseptate hyphae is formed; within the hyphae, multinucleate cytoplasmic contents eventually produce a large number of spores. The formation of flagellated zoospores within spores has been observed in the gill pathogens D. cyprini and D. salmonis; these have been shown to be an infective stage in D. salmonis.

Appearance in tissue sections:
Diagnosis of Dermocystidium in tissue sections of skin or gills is usually made by observing the mature round spores of a relatively uniform size (3-12 µm diameter, depending on the species), with a large, refractile, PAS-negative vacuoplast and a surrounding thin ring of cytoplasm with a peripheral nucleus. Large numbers of spores are enclosed within a hyaline cyst wall; cysts of some species may be compartmentalized. Spores of some Dermocystidium species infecting viscera of salmonids may have a vacuole without a central vacuoplast (inclusion). The spores of visceral species may be located in foci without any limiting capsule walls. In skin and subdermal tissue of fish infected with D. koi, aseptate, multinucleate, branching hyphae may be visible; these segment into multinucleate and uninucleate cells that transform into variable-sized spores (6.5-15 µm). Granulomatous inflammation is often observed in association with Dermocystidium infections. Epithelial hyperplasia and fusion of gill lamellae is common in gill infections.

Rosette Agent
Fish species parasitized:
The rosette agent has been described as a serious pathogen of chinook salmon (Oncorhynchus tshawytscha) and Atlantic salmon (Salmo salar) on the east and west coasts of North America, and some other salmonid species have been shown susceptible to experimental infection. The parasite is ultrastructurally similar to certain "Dermocystidium-like" agents described from salmonids in Europe. Infections have been detected in fish held in freshwater and seawater.

Location in host:
Two forms of infection have been described. In the disseminated form of the disease, the rosette agent is found in many organs and tissues both extracellularly and and intracellularly in a variety of cell types: hematopoietic, epithelial, and mesenchymal, with minimal host inflammatory cell response. In the nodular form of the disease, the parasites are found within and between macrophages in granulomas located predominantly in the kidney, liver, and spleen.

Characteristics:
Two forms or stages of the organism have been identified. The nondividing stage is smaller than the dividing stage. The dividing stage is characterized by partitioning of the mother cell cytoplasm and its organelles to generate two or more daughter cells. Evidence suggests that potential sources for transmission of the organism include shedding in bile or urine, shedding from the gut epithelium and perhaps the gills and skin, or shedding in ovarian and seminal fluids.

Appearance in tissue sections:
In disseminated disease, rosette agent parasites can be found in both intra- and extracellular locations in a variety of tissues, forming aggregates of small numbers of closely apposed organisms ("rosettes") along with associated necrotic cellular debris. In nodular disease, aggregates of the rosette agent are found in the central zones of well-demarcated granulomas; the parasites often appear as clusters ("rosettes") within and between macrophages. In H&E-stained sections the rosette agents are deeply eosinophilic spheres, about 2-4 µm in diameter for the nondividing stages and 4-6 µm in diameter for the dividing stages. The rosette agent stains Gram-positive, strongly PAS-positive, argyrophilic (Warthin-Starry and Grocott’s), and basophilic after Giemsa staining but is not acid-fast. The agent can also be detected in Gram-stained or Giemsa-stained tissue imprints.