Ichthyophthirius multifiliis Fouquet and Ichthyophthiriosis in Freshwater Teleosts
Introduction
The ciliate Ichthyophthirius multifiliisFouquet, 1876, commonly called ‘ich’ (pronounced ‘ik’), is probably the most widespread parasite of freshwater teleosts with a geographical range extending from the tropics to temperate regions, northwards in Europe to the Arctic Circle. The first records of the characteristic ‘white spots’ in fish, which mark the location of the parasite in the skin (Figure 1), were from China before ad 1126 (Dashu and Lien-Siang, 1960). These observations support the view that I. multifiliis was originally endemic in the Far East, being introduced to Europe in the Middle Ages with the development of carp culture (Hoffman, 1970b) and to many other countries, including the United States, through the importation of goldfish, Carassius auratus (see Hoffman (1970a), Hoffman (1978)). The parasite was probably introduced to South Africa in the 18th century with importation of carp (Huchzermeyer, 1994; Mouton et al., 2001), this fish now being established in temperate waters of the continent (De Moor and Bruton, 1988). Paperna (1972), Paperna (1991) has reviewed the occurrence of I. multifiliis in warmer regions of the world. Key factors in the current worldwide distribution of I. multifiliis in freshwater teleosts, including cold water and tropical species, are wide temperature tolerance together with low degree of host specificity and direct life cycle. In the Far East, I. multifiliis is well adapted to survive the seasonal changes in temperature (4 to 28°C) experienced by its common host, Cyprinus carpio. Nigrelli et al. (1976) considered the possibility of multiple physiological races and even different species of Ichthyophthirius, adapted to different temperatures. Two new species of ‘ich’-like parasites have been described, namely, Ichthyophthirioides browniRoque and de Puytorac, 1968 and Neoichthyophthirius scholtfeldtiBauer and Yunchis, 2001 from tropical fish; however, strains or races of I. multifiliis based upon physiochemical characters have yet to be defined. Dickerson et al. (1993) have demonstrated serotypic variation among isolates of the parasite, which they suggested could find application as biochemical markers in strain identification and in epidemiological studies of the parasite. Recent studies by Aihua and Buchmann (2001) on the development of a Nordic isolate of I. multifiliis provided a valuable approach to defining strains, based on physiological features of the free-living stages in the life cycle.
The disease ichthyophthiriosis, or ‘white spot’, probably accounts for more damage to freshwater fish populations worldwide than any other eukaryote pathogen (Hines and Spira, 1973a; Rogers and Gaines, 1975). Aquarists have been aware of the condition in ornamental species since the turn of the century, associating mortalities with the appearance of the white spots within the skin and gills. Today, ichthyophthiriosis is a significant factor not only in the freshwater ornamental industry but also in the intensive farming of salmonids (Valtonen and Keränen, 1981; Wahli and Meier, 1987), carp (Hines and Spira, 1973a; Körting, 1984), channel catfish (Klesius and Rogers, 1995), eels (Egusa et al., 1970), and Tilapia species (Subasinghe and Sommerville, 1986). Although epizootics have been less frequently observed in wild fish populations, they are usually associated with mass kills (Elser, 1955; Allison and Kelly, 1963; Kozel, 1976; Wurtsbaugh and Tapia, 1988) following recent introduction (Hoffman, 1970b). Nevertheless, that the disease can have a significant impact on commercial fisheries is indicated by a more recent outbreak in pre-spawning and spawning sockeye salmon, when an estimated 153.6 million fewer fry were produced as a result of ichthyophthiriosis (Traxler et al., 1998).
I. multifiliis has been used as a model in fundamental studies of fish behaviour and fish immunology. Milinski and Bakker (1990) used the parasite to investigate the function of secondary sexual ornamentation in teleosts. They found that infections influenced female choice of partner in stickleback populations, the white spots associated with the parasite in the epidermis reducing the intensity of male red breeding coloration. Dickerson et al. (1997) and Dickerson and Clark (1998) have discussed the development of I. multifiliis as an experimental system for the investigation of cutaneous immunity in fish.
MacLennan (1935a) and Butcher (1943) provided interesting accounts of the early studies on the life cycle of I. multifiliis, later work on the subject being reviewed by Matthews, R.A. (1994) and Dickerson and Dawe (1995). I. multifiliis is an endoparasite in which the trophont, or feeding stage, occurs within the epidermis of the fish host. The life cycle is direct (Figure 2). The trophont, following a period of growth and development, transforms to the tomont, which actively leaves the host tissues, encysting within the aquatic environment. The tomont undergoes a rapid phase of division, normally within the cyst, with the production of daughter cells called tomites. Following a set number of divisions each tomite differentiates into a theront, the stage infective to the fish host.
According to Fouquet (1876), Hilgendorf and Paulicki were probably the first to publish (in 1869) details concerning both the morphology and life history of I. multifiliis, noting somatic cilia, macronucleus, contractile vacuoles and granules, and that the trophont stage leaves the fish host to encyst and undergo reproduction as a free-living stage. Although they had suggested that the ciliate might be related to Opantotricha Ehrenberg, Fouquet (1876) proposed the name I. multifiliis for this parasite, the specific name relating to the large number of daughter cells (tomites) produced at encystment. Fouquet (1876) described the parasite from juvenile trout, noting further details of morphology including mucocysts and cytopharynx. The oral ciliature, as described by Roque et al. (1967), forms the basis for the inclusion of I. multifiliis within the class Oligohymenophorea. The relationship of I. multifiliis to other histophagous parasites of fish within the Hymenostomatida is shown in Figure 3. Canella (1964) established the suborder Ophryoglenina to include all species with an organelle of Lieberkühn, this structure being recognized as an important taxonomic feature. Lynn et al. (1991) confirmed the suborder as a monophyletic taxon following ultrastructural investigations of the organelle within the tomont of Ophryoglena sp. and theront of I. multifiliis. Separation of Ophryoglenina from Tetrahymenina is clearly supported by studies of ciliate phylogeny based upon the characterization of the histone H3/H4 gene region (Van den Bussche et al., 2000) and similarities in small subunit rRNA gene sequences (Wright & Lynn (1995), Wright & Lynn (1997); Hammerschmidt et al., 1996). The family Ichthyophthiriidae Kent, 1881 was erected solely for I. multifiliis; the status of Ichthyophthirioides browni has still to be confirmed, as there has been no further record of the species since its description by Roque and de Puytorac (1968). Cryptocaryon irritans, the cause of white spot in marine fish, is no longer included within Hymenostomatidae (Diggles and Adlard, 1995; Wright and Colorni, 2002). Similarities with Ichthyophthirius multifiliis concerning life cycle and course of infection are attributed to parallel evolution. Recently, Wright and Colorni (2002) sequenced the complete 18S rRNA gene of C. irritans and on the basis of phylogenetic analysis have re-assigned this parasite to the class Protostomatea, erecting a new family, Cryptocaryonidae, within the order Prorodontida.
Section snippets
Behaviour and encystment
The tomont culminates the growth phase within the fish host (Figure 2), attaining a size and stage of development sufficient to complete the aquatic phase, including location of substrate, encystment and production of viable theronts (Ewing and Kocan, 1992). This free-swimming stage encysts within 15 min to 6 h of leaving the host fish epidermis (MacLennan, 1937). Nickell and Ewing (1989) demonstrated a marked photoresponse to substrate colour, significantly larger numbers encysting on light
Invasion of the Fish Epidermis
Although there have been many observations concerning the entry of I. multifiliis into the epidermis of host fish, there is little agreement concerning the mechanism involved (Matthews, R.A. 1994). MacLennan (1935b), in the first major work on the parasite, considered that the theront bored into the epidermis using ‘the non-ciliated anterior region’ as a wedge to force the tissues apart, noting that ‘the closely set cilia are more powerful and active than those of the average ciliate’. Roque et
Trophont
Transformation from theront to trophont is characterized by discharge of mucocysts, disappearance of the organelle of Lieberkühn, enlargement of the cytopharynx, and the onset of phagocytosis. These events are completed within 12 h in primary infections of juvenile carp at 20°C (Matthews, R.A., et al., 1996).
i-Antigens
Dickerson and Clark (1996) provided an excellent review of their pioneering studies of immobilization (i-) antigens in I. multifiliis. Lin and Dickerson (1992) were the first to identify and characterize i-antigens in this parasite using an isolate (G1) from tropical fish purchased in Georgia, USA and maintained through serial passage in Ictalurus punctatus (see Dickerson et al., 1989). They isolated polypeptides of 48 and 60 kDa from a single serotype A, designated G1 (serotype A), with the
Ichthyophthiriosis
Hines & Spira (1973a), Hines & Spira (1973b), Hines & Spira (1974a), Hines & Spira (1974b), in their seminal works on I. multifiliis, provided a comprehensive account of the pathogenesis of ichthyophthiriosis in carp covering aspects of immunopathology and pathophysiology. Ventura and Paperna (1985) have undertaken a detailed histopathological investigation of the disease in the skin and gills in a range of fish hosts, including carp. Further studies have also focused on events within the skin
Control and Treatment
Control of ichthyophthiriosis, like that of other parasitic diseases, requires an integrated approach based upon knowledge of aetiology and epidemiology. In temperate regions, disease risk forecasting by monitoring water temperatures in spring and early summer would provide some warning of impending epizootics and ensure a more targeted approach in the application of control measures. The prevalence of I. multifiliis is known to fluctuate with season (Dickerson and Dawe, 1995). In central
Conclusion
Significant advances have been made in our knowledge of I. multifiliis since reviewing the parasite in 1994 (Matthews, R.A., 1994). Notable achievements include the characterization of serotypes, the isolation of genes expressing i-antigens, and the successful insertion of one of these genes into Tetrahymena thermophila, affording a way forward for the development of recombinant vaccines against ichthyophthiriosis. Immunological studies have led to a better understanding of the protective
Acknowledgement
This review would not have been possible without the support of my wife, Dr Ben Matthews. I thank her for her patience and valuable advice throughout its preparation, and for providing the electron micrographs.
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