Epidemiology, Histopathology and Ultra-structure of an Ectoparasitic Ciliate Associated with Mortality of Coral Reef Fish at the National Aquarium in Baltimore
Sarah L. Poynton1, PhD; Renate Reimscheussel2, DVM, PhD;
Brent Whitaker3, MS, DVM; Julie Jackson4, BS; Chris Coco4,
Introduction and Background
Protozoans infecting marine fishes have a variety of different relationships with their hosts. Some of them are true parasites, depending completely on nutrition derived from their hosts. Parasites may be well adapted to their hosts, and except for depriving them of nutrients, cause practically no harm. In other cases, the dynamic interaction of the parasites virulence and the hosts defence mechanisms results in a pathogenic infection Certain protozoan such as the flagellate Amyloodinium and the ciliate Cryptocaryon cause detectable and problematic infections only under conditions debilitating the host, such as those found in aquaculture or aquarium environments (Lom, 1984).
Among the ecological factors affecting protozoan infections in fish, temperature is one of the most important, since it inhibits or enhances the proliferation of parasites, and controls the immune response of the fish host. Other important abiotic factors include water currents, dissolved oxygen concentration, illumination, water depth and salinity. The artificial abiotic and biotic factors in aquariums frequently combine to permit significant outbreaks of parasitic protozoa among the stressed fish.
The best known ciliate that is an obligatory parasite on the body surface and gills of captive marine fish, and one of the most problematic (Sindermann, 1990), is Cryptocaryon irritans Brown 1951. This parasite is commonly considered to be the marine counterpart of Ichthyophthirius multifiliis, causative organism of the "ich "or "white spot" diseases of freshwater fish. The surface of fish affected with cryptocaryoniasis has numerous macroscopic white - grey spots, which contain the feeding parasites, which have invaded beneath the epithelium. The ciliates feed on host cells, damage the epithelium, and cause irritation resulting in excess mucus production, epithelium hyperplasia, inflammation and hemorrhage. In the gills, the ciliates destroy the secondary lamellae. Heavy infections are fatal.
Cryptocaryoniasis has been reported from many aquaria and aquaculture facilities worldwide including those in England (Brown, 1963), Israel (Colorni, 1985), Japan (Sikama,1961), Kuwait (Tareen,1980) and the United States (Huff & Burns,1981; Nigrelli & Ruggieri,1966; Wilkie & Gordin, 1969). The ciliate attacks many species of fish, especially those from warm waters (Brown, 1963; Cheung et al., 1979; Nigrelli & Ruggieri 1966).
The life cycle of C. irritans is similar to that of I. multifiliis namely (1) the trophont or parasitic feeding stage, which is oval to spherical, 70-400 um long, (2) the tomont or encysted stage which develops from the trophont after leaving the host and (3) the tomites, infective free swimming, non-feeding ciliates, pear-shaped, 40-50 um long (Brown, 1951; Cheung et al., 1979).
In July 1990 the National Aquarium in Baltimore experienced an outbreak of a pathogenic ectoparasitic ciliate believed to be C. irritans. The outbreak occurred in the 335,000 gallon Atlantic Coral Reef exhibit tank, holding approximately 50 species of fish, principally grunts and tangs collected from the reefs in the Florida Keys. The water temperature in the tank was maintained at 77°F. and salinity varied from 28 - 34 ppt. During the outbreak, which was treated with copper sulfate, numerous fish died.
Data collected and work in progress. In order to document the outbreak, the following data was collected:
1. observations on the gross appearance of the infected fish
2. mortality (in numbers and percent) for the different families of fish
3. treatment regime
4. physical (abiotic conditions) in the tank
In order to extend our knowledge of the organism, particularly its identity and relationship with the host, the following investigations are in progress:
1. morphology of the trophons and tomites , using protargol (silver protein impregnation) (Montagnes & Lynn, 1987)
2. histopathology of infected gills and skin
3. transmission electron microscopy of trophonts and tomites, (scanning electron microscopy was done by Cheung et al, 1981)
The studies we are undertaking at the National Aquarium will extend our knowledge of this organism, which we believe to be C. irritans, beyond simply describing the outbreak and treatment. Our novel contributions will be protargol impregnations of the organisms, and ultra structure of the parasite and the interface with the host as revealed by transmission electron microscopy.
These results should clarify and assist our understanding of the taxonomy, treatment and host-parasite relationship as follows:
1. The confusion over the taxonomic position of C. irritans.
It is presently in the order Hymenostomatida, along with I. multifiliis, but this is questionable (Cheung et al., 1981). An understanding of its taxonomic affinities is in itself interesting, and may help to explain the host-parasite relationship. Ancient host-parasite relationships are well evolved to coexistence. However, more recently evolved parasites may be more damaging to their hosts. Perhaps C. irritans is more closely related to free-living marine ciliates than to the parasitic I. multifiliis.
2. The internal structure of C. irritans and treatment options.
For example if the parasite is shown to ingest red blood cells, it may be possible to treat fish with oral anti-malarial drugs, such as chloroquine. This would eliminate the potential toxicities associated with copper sulphate, and other external treatments. Systems such as living reefs would benefit greatly from chloroquine use, as therapeutic levels of copper sulfate are deadly to invertebrates.
3. The nature of the host-parasite interface in C. irritans
It is not known beyond that discernable with the light microscope. Our transmission electron microscopy studies will show what the interface is like, and this information will be useful for assessing the efficacy of treatments, and may suggest approaches for vaccine development.
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