Trichodiniasis

Trichodiniasis is the disease caused by infection of the gills and/or integument by peritrichous ciliated protozoa of the family Trichodinidae. Trichodinidae are collectively known as trichodinids. They are recorded from freshwater fish species worldwide. The majority are believed to be non-pathogenic, whereas a few species are known to be primary pathogens of fishes causing considerable mortality particularly among juveniles. Infection is common in native and introduced species, including aquarium species and occurs in both captive and wild populations. Although trichodinids are commonly found in low numbers on healthy fish in their natural environments, the presence of any trichodinids on aquarium fish warrants treatment.

In general, trichodinids are ectocommensal; however, some species are certainly primary pathogens and are parasitic on fishes. They are characterised by the presence of a ring of interlocking cytoskeletal denitcles, which provide support for the cell and allow for adhesion to the surfaces of the fish.

Trichodinids have a simple direct life cycle. That is, they have a single host and do not use alternation of generations or mass asexual replication off the host. The reproduce by binary fission, literally cell-splitting. This produces daughter cells with half the number of denticles of the parent cell. The full complement of denticles is restored by synthesis of new denticles from the outer edge of the cell, working inwards.

Trichodinids are typically found on the gills, skin and fins of fishes, though some species parasitise the urogenital system. A range of invertebrates is also host to trichodinid infections, including the surfaces of copepods and the mantle cavity of molluscs. Transmission occurs by direct contact of infected and uninfected hosts, and also by active swimming of trichodinids from one host to another. Trichodina cells swim with the adoral surface facing forwards. On surfaces, they move laterally, with the adoral surface facing the substrate. Few trichodinids are host specific and have been reported to survive up to 1-2 days off their host.

Trichodiniasis is usually a relatively mild disease with a low morbidity and mortality, however, under certain conditions, especially confinement of infected fish in aquaria, clinical disease and high mortalities may occur. The length of incubation (the time from infection to the appearance of the first signs of the disease) depends on susceptibility, temperature, and severity of exposure. Infectious signs can appear within 2 hours of initial exposure and attain 100% prevalence after 10 days. Larvae and juveniles are particularly susceptible to infection.

In heavy infections, trichodinids attach firmly to the epithelium causing irritation and cell damage resulting in hyperplasia of the epithelium and mucous secretion. Infected fish are generally darker in coloration and may exhibit a whitish or bluish-grey colouration of the skin. Fish may exhibited listlessness and lack of appetite. At advanced stages of the infection, they adopt a vertical hanging position near the water surface with continuous lethargic swimming motions. A “tattered” appearance of the skin and fins may be apparent, with strands of mucous apparent. In mild infections, trichodinids graze over the epithelial surface feeding on particulate matter and detritus and cause insignificant damage.

In combination with inappropriate aquarium conditions, secondary bacterial infection and intercurrent disease, high mortalities may occur. Because of the lack of information on protozoan parasites of rainbowfishes, most cases of trichodiniasis are not identified or more often, are simply misdiagnosed.

Although trichodinid protozoa commonly occur on native fish in Australia, most genera and species remain to be described. In a research project (Dove & O'Donoghue, 2005) freshwater fishes were surveyed across eastern Australia. Trichodinid ciliates were collected on an opportunistic basis during the survey for other parasites that encompassed 58 sites throughout Victoria, New South Wales, the Australian Capital Territory and Queensland.

Twenty-one putative trichodinid species were recovered from 33 fish species examined. Trichodina heterodentata, T. mutabilis and T. reticulata were the exotic species recovered regularly; a single specimen matched a fourth exotic species, T. acuta.

Two new native species were also described: Trichodina cribbi from Hypseleotris galii, H. klunzingeri, and Hypseleotris sp. (Lake's Carp Gudgeon); and T. bassonae from Selenotoca multifasciata. The native fishes of Australia harbour a speciesrich fauna of native trichodinids, with at least 17 species of undescribed trichodinids detected from 33 species of native fishes examined from survey sources. It is estimated that the Australian trichodinid fauna may include up to 150 as yet undescribed species.

Treatment Trichodina can be controlled with any of the treatments used for ichthyophthiriasis. Correction of environmental problems is necessary for complete control. It is a good idea to do a 50-75% waterchange and a thorough cleaning of the aquarium to reduce the level of organic wastes before starting any treatment. This includes the removal of particulate matter (faeces, uneaten food, detritus, etc.); the removal of algae from tank walls and the removal of particulate matter from the filter (change filter wool or wash sponge etc.). Trichodina infections are often associated with monogenean trematode infections and treatment has to be applied for both.

Control of trichodiniasis has been successful using a long term bath of malachite green added to aquaria at 0.08 mg/L. A Formalin bath at a concentration of 250 ppm for one hour, or 40-50 ppm for a longer period has also been used with success. Common salt (NaCl) at 2-3% concentration is another successful treatment for infected fish. However, a malachite green/formalin combination is probably the best treatment for aquaria. This combination exerts a mild antibacterial effect as well, but in most circumstances will not destroy biological filtration bacteria, although they may have a slight effect for a short period. WS3, a commercial medication containing malachite green, acriflavine and quinine sulphate has also been used as a bath treatment combined with a salinity of 3‰. One should follow the manufacturer's instructions for treatment, as different manufacturers use different concentrations of the active ingredients.

Garlic (Allium sativum) and Indian almond (Terminalia catappa) have been used as an alternative to chemicals to treat trichodiniasis. There is a fast growing interest in screening antiparasitic substances from plants to replace chemical and antibiotic alternatives. In laboratory trials, the use of crude extracts of garlic and Indian almond at 800 mg/ L have been reported to significantly eliminated Trichodina infections in tilapia. However, the amount of activity varies widely among the different varieties of garlic and its use may prove to be unrealistic; the same may be said of Indian almond.