Ultrastructural Aspects of the Host-Parasite Relationship in Ichthyophthiriasis

Abstract

The parasite, Ichthyophthirius multifiliis, is bordered on its outer surface by one to several layers of host (Ictalurus punctatus) epidermal cells. With reference to the position of the parasite relative to the dermis, there intervenes between parasite and dermis: (1) several layers of epidermal cells and an intact basal lamina; (2) a single layer of epidermal cells and an intact basal lamina; (3) a single layer of epidermal cells and a discontinuous basal lamina; (4) a discontinuous layer of epidermal cells and an intact basal lamina; and (5) a discontinuous basal lamina. In some instances, the epidermal cells are extremely attenuated. Very little or considerable space intervenes between the host cells and the parasite. In no instance did a parasite penetrate into the dermis. Parasites did not feed on host red blood cells. The possibility is noted that host defense mechanisms destroy infecting parasites. Ichthyophthirius multifiliis (Fouquet, 1876) is a holotrichous ciliate protozoon that belongs to the suborder Ophryoglenina of the order Hymenostomatida (see Corliss, 1979). In the epidermal phase (trophont stage) of its life cycle, I. multifiliis is an obligate parasite (Amlacher, 1970), infecting the gills, skin, eyes, and fins of fish (Nigrelli et al., 1976). Nigrelli et al. (1976) mention susceptibility only in fresh-water fish, but van Duijn (1967) and Corliss (1979) state that both marine and fresh-water fish are attacked by the trophonts. The pathological condition produced, viz., ichthyophthiriasis, ich or disease, is characterized by the appearance on the affected parts of the fish of numerous white spots, each having a diameter up to 1.0 mm. Each white spot represents the locus of from 1-6 parasites, more or less surrounded by epidermal cells, which may have undergone a proliferation about the parasites (van Duijn, 1967). The literature contains several long and short reviews of the lengthy history of the causative organism (Canella & Rocchi-Canella, 1976; Chapman & Kern, 1983; Corliss, 1979; Hoffman, 1967; Nigrelli et al., 1976). In spite of this lengthy history, the numerous reports pertaining to the organism, and the recent revival of interest in the organism brought about by world-wide efforts at fish farming and the fact that the organism is one of the most important pathogenic parasites of cultured fish (Goven et al., 1980; Hines & Spira, 1973; Nigrelli et al., 1976), a literature search revealed only seven reports concerned with the ultrastructure of I. multifiliis. These reports discussed: the contractile vacuole system (Mosevitch, 1965); the oral structure, contractile vacuole, and somatic cortex (Roque et al., 1967); the kinetid (Lom & Corliss, 1971); the nuclear changes occurring throughout the cell cycle (Hauser, 1972, 1973); the The author thanks Mr. G. Patrick Riordan, Mr. Robert C. Kern, and Miss Andrea S. Blake for their technical assistance. TRANS. AM. MICROSC. SOC., 103(4): 364-375. 1984. ? Copyright, 1984, by the American Microscopical Society, Inc. This content downloaded from 157.55.39.153 on Mon, 19 Sep 2016 04:45:05 UTC All use subject to http://about.jstor.org/terms VOL. 103, NO. 4, OCTOBER 1984 contractile vacuole and cortex (Chapman & Kern, 1983); and the morphology of the cyst wall (Ewing et al., 1983). The literature includes several contradictory and ambiguous statements about the host-parasite relationship but no electron micrographs illustrating that relationship. Therefore, we examined, by light and electron microscopy, this facet of the biology of ichthyophthiriasis. Additional motivation was provided by the perception that evidence is increasing to suggest that a knowledge of fine structural features can help to explain disease processes, and may even open avenues of treatment or mechanisms of prevention (Fawcett, 1977). MATERIALS AND METHODS A 5.0-cm long channel catfish (Ictalurus punctatus), which had become heavily infected with white spot disease, was decapitated and its soft dorsal fin was removed. The fin was fixed in 2.5% glutaraldehyde and 2.0% paraformaldehyde (Karnovsky, 1965), buffered at pH 7.3 with 0.1 M phosphate (Millonig, 1961), washed three times in buffer, post-fixed in 2% OsO4, rinsed twice in buffer, dehydrated in ethanol followed by propylene oxide, and embedded in Epon 812 (Luft, 1961). Ultrathin sections (40-60 nm) were cut with a diamond knife (E. I. Dupont, Wilmington, Delaware) and a Porter-Blum MT2-B ultramicrotome. The ultrathin sections were stained with 5.0% uranyl acetate in 50% ethanol for 11 min (Gibbons and Grimstone, 1960), washed in distilled water and stained again in 0.2% lead citrate for 2 min (Venable & Coggeshall, 1965). Following washing in distilled water, the ultrathin sections were studied with an RCA EMU-2D electron microscope. Thin sections (0.5-1.0 ,um) also were cut, with the same instrumentation, transferred to histological slides, and stained with methylene blue and Azur II according to the procedure of Richardson et al. (1960). The thin sections were studied with a monocular Bausch & Lomb research microscope, equipped with Bausch & Lomb or Nikon lenses, and illuminated with bright-field transmitted