Ichthyosporidium Hoferi (Plehn & Mulsow, 1911), An Internal Fungoid Parasite of the Mackerel

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Abstract
A fungus has been found causing a fatal disease among mackerel in British waters, and a very high rate of infection has been maintained for over three years. It is spread throughout the viscera by the circulation, being particularly common in the kidney and spleen: it is very rare in the muscles and has never been found in the gonads or central nervous system of these fish.Growth of the aseptate, sparsely branched hyphae is usually radial, from one of three types of central body. The length and thickness of the hyphae varies according to the density of the infected tissue and the degree of resistance which is set up. Connective tissue capsules are laid down round spores and hyphal bodies, occluding the small vessels or capillaries in which they become lodged. Growth of the parasite stretches the capsule, which may become a tubular gallery containing many growth stages. This may effectively seal the parasites which then degenerate; or, more usually, the strong proteolytic enzymes secreted by the advancing hyphae perforate it, and proceed to convert the whole organ into a necrotic mass in which hyphae grow to great lengths. There is always a tendency for hyphal walls to collapse behind the outgrowing hyphae, giving the appearance of hair-like threads with swollen ends.Any of the following developments may occur, apparently on any hypha, in tissue undergoing necrosis: (1) Bunches of large elongated chlamydospores, with a very hard exospore and a thick (?) gelatinous endospore—a germination pore is present; (2) Single dome-shaped conidia (like those of Entomophthoraceae); (3) Hyphal bodies of various shapes, either growing at once where they become lodged or becoming rounded and encysted—these may subdivide irregularly, each spherule having a separate cyst; (4) Branched conidiophores with rounded tips which contain endo-conidia—liberated as minute amoeboid bodies into the blood vessels; (5) Simple clavate sporangia from the ends of unbranched hyphae, also containing endo-conidia; (6) Hyphal fusions of numerous kinds, immediately proceeding to further outgrowths of similar hyphae; (7) ‘Spores produced by hyphal fusion’: fusion of two hyphae followed by the participation of neighbouring hyphae which also contribute to form the outer, resistant, rugose wall.