Cryptocaryon irritans, a ciliated protozoan parasite, is one of the most devastating parasites of marine fish cultured in temperate and tropical seas.
C. irritans is an obligate ectoparasite that causes cryptocaryonosis, also known as white spot disease, in marine fish. Although C. irritans is commonly found in tropical, subtropical and warm temperate waters at low infection intensity , infection by this parasite has emerged as a major problem in confined surroundings such as in mariculture and aquariums due to the buildup of the parasite and high population density of fish in these systems.
Based on the remarkable affinities with its “freshwater counterpart” Ichthyophthirius multifiliis, C. irritans has traditionally been included in the Ichthyophthiriidae, but belongs to a distinct family, Cryptocaryonidae.
The C. irritans life cycle involves four stages that require a mean time of 1-2 weeks for completion independent of an intermediate host. The parasitic stage trophont burrows itself within the host epithelium and feeds on both tissue debris and body fluids. During this period, the whitish nodules are observed on the body and fins, depending on the severity of the infection. The mature trophonts leave the host as protomonts after 3-7 days. The protomonts sink and adhere to the substratum following which they encyst and enter the reproductive stage. These newly formed tomonts undergo a sequence of asymmetric binary fissions to become daughter tomites inside the cyst wall. Between days 3-72, cyst rupture leads to the asynchronous release of differentiated tomites into the environment as theronts. A tomont produces approximately 200 theronts, and this infective stage parasite swims freely to find a host and rapidly penetrates the host epidermal layer. The infectivity of theronts decreases 6-8 h post-excystment.
C. irritans has demonstrated a very low level of host specificity, infect just about any teleost fish in a tropical marine environment. Cartilaginous fishes (sharks and rays) appear resistant, but everything else is susceptible to infection. It has even been proven to infect various species of freshwater fish that were acclimated to saltwater, as well as temperate marine fish that were kept at the upper limit of their thermal range.
Although different species of fish showed different susceptibility to Cryptocaryon irritans infections, this parasite is not very fastidious in its host selection.
C. irritans penetrates the skin, gills and eyes of the fish and impairs the functioning of these organs. The key signs of cryptocaryonosis are the formation of pinhead-sized whitish nodules, mucus hyperproduction, skin discoloration, anorexia and respiratory difficulties. C. irritans has low host specificity and can infect a taxonomically broad host range, including both temperate marine fish and saltwater-adapted fresh-water fish that do not encounter the disease naturally.
To date, no commercial vaccines, drugs or diagnostic kits have been developed for white spot disease. Control of this parasite is hindered by factors such as the embedment of the parasite in the host epithelium, which renders many chemicals ineffective; asynchrony in theront release and trophont exit; and ineffectiveness of chemical treatment in large-volume systems. In addition, lack of parasite genomic data has hampered the use of molecular tools in developing control strategies for C. irritans.
In aquarium fish, Cryptocaryon spp infections can be controlled by maintaining copper concentrations of 0.2 mg/L for 3 wk. Because copper is so toxic and difficult to use, formalin is an alternative for less experienced owners. A protocol similar to that recommended for freshwater fish is appropriate (ie, apply the chemical every third day if the tank is maintained at 24-26°C). If treating with formalin in marine tanks, the total duration of treatment should be extended. A minimum treatment time in marine systems is 3 wk. In a marine system, fish must be separated from valued invertebrates before beginning treatment as either copper or formalin will kill them.
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- Merck Veterinary Manual