This article originally appeared in SeaScopeTM, Volume 11, Summer 1994, Aquarium Systems, copyright 1994. All rights reserved by Aquarium Systems, Mentor, Ohio. Permission to reproduce is granted by the publisher to The Breeder's Registry and is not transferrable.
By: Dr. Harry W. Dickerson
Dr. Harry W. Dickerson, B.V.Sc. Ph.D., is Associate Professor at the University of Georgia College of Veterinary Medicine, Athens, Georgia.
Cryptocaryon irritans (white spot disease) is a common ciliated protozoan parasite that can be inadvertently introduced into marine aquaria through infected fish or contaminated inorganic material. Most marine fish are susceptible to infection, visible as white spots on the skin, and will succumb to infection unless the parasites are aggressively treated.
Life Cycle of Cryptocaryon Irritans (Diagram by Becci Velasquez) |
Life Cycle
The life cycle of Cryptocaryon is relatively simple. Cryptocaryon is an obligate parasite, one that cannot survive without a period of growth on a fish. It spends five to seven days (at 24 to 27°C) feeding and growing in the skin and gills. The parasite at this stage is called a trophont, and ranges from 60 to 370 microns in size. When the parasite reaches maturity it leaves the fish and enters the water as a large single cell called a tomont. The parasite swims for 12 to 18 hours until a sticky, opaque cyst wall is secreted which allows it to attach to substrates such as rock, coral, or glass.
These cysts measure from 200 to 400 microns in diameter and can sometimes be seen on surfaces inside the aquarium. The cells divide within each cyst to produce up to 200 daughter parasites called tomites. This multiplication process takes anywhere from 3 to 28 days (Colorni 1985). The tomites are much smaller (25 to 60 microns) than the original cell because they result from its cell division. They produce cilia which allow them to bore through the cyst wall and emerge into the water. These free-swimming cells are the infective form of the parasite called theronts. They are very small and not easily seen by the naked eye at this stage, and survive for only a few hours unless they find a host. When the theronts encounter a fish, they burrow into skin and gill tissue where they begin to grow, thus completing the life cycle.
The metabolic activity of Cryptocaryon is affected by the ambient temperature of the water. The warmer the water, the faster the parasite grows, thus speeding up all stages of the life cycle.
Treatment
The objective of treatment is to break the cycle of infection, and treatment must be started as soon as parasites are detected. Heavily infected fish will often die from severe gill damage despite treatment. Cryptocaryon is inaccessible inside the skin and gills, becoming vulnerable to treatment only after it leaves the fish. The parasite is difficult to treat because its rate of development is extremely variable, even in the same population. This means that when you treat the aquarium some parasites will inevitably remain on the fish and survive.
The key to success, then, is multiple treatments. In my opinion, this is the most important aspect of controlling Cryptocaryon, even more than the method used. Multiple treatments ensure that Cryptocaryon is killed or removed as it comes off the fish and before it gets a chance to reinfect. Heavily parasitized fish may not survive the stress of infection and treatment. They should be removed as soon as they die to prevent the release of parasites into the aquarium.
Water Changes
Free-swimming parasites are eliminated from an aquarium each time the water is changed, but water changes alone are not completely effective because some parasites will inevitably attach to fish before they can be removed. Once parasites make contact with a fish they penetrate into the skin in five minutes.
Cysts attached to sand, gravel, and other material will also remain in the aquarium. Complete water changes diligently repeated every day for three to four weeks can eliminate the parasite in bare aquariums. One could also move fish to clean aquaria every other day and clean and dry the empty aquaria between use. This should be repeated over a period of at least three weeks.
A simpler, although initially more expensive, alternative is to recirculate aquarium water through an ultraviolet (UV) sterilizer. Free-swimming parasites are killed as they pass through the UV light until the entire population is eliminated. Success depends on using an adequate UV dosage and the correct water flow rate. Gratzek et al. (1983) reported success in treating Ichthyophthirius multifilis (Ich) using a sterilizer rated at 91,900 microwatts/ second/ centimeter2. The advantage of this method is that it effectively eliminates the establishment of future Cryptocaryon infections.
Chemical Treatment
Aquaria can usually be freed of Cryptocaryon by using a combination of water changes and chemical treatment. Multiple application is the key to success. Formalin used at a concentration of 25 parts per million (ppm) every other day for two weeks with a complete water change on alternate days has been successful for us. To treat at this level, use 1 milliliter (ml) formalin in approximately 10 gallons of water. Formalin should be handled carefully. If the aquarium contains invertebrates that could be injured by treatment, the fish should be removed and treated in a conditioned hospital aquarium for the duration of the medication schedule.
Repeated water changes in the original infected aquarium will remove parasites that emerge from cysts. A study recently completed in my laboratory at the University of Georgia found that Cryptocaryon develops inside cysts at different rates, but theronts are always released in the morning, regardless of the day they emerge. Assuming that morning release is a common phenomenon in all ocurrences of Cryptocaryon, one should be able to exploit this for treatment by adding formalin to the water at the time infective parasites are released.
Parasite development can also be speeded up by raising the water temperature as high as permissible without stressing the fish. This will shorten the time that the parasite remains on the fish and ensure that it will be in the water during the period of treatment.
Ionic copper is also commonly used to treat Cryptocaryon. It can be effective if it is properly administered at a constant level of 0.15 to 0.2 ppm. Because of the long incubation period of some tomonts, treatment may need to be continued for as long as 4 weeks to be totally effective. There are a few negative aspects to the use of copper. It cannot be used when invertebrates are present. It has also been claimed to reduce the immune response of fish, making them more susceptible to secondary infections.
Prevention
The best way to eliminate problems with Cryptocaryon is to prevent its introduction into your aquarium in the first place. Steps to avoid outbreaks include buying fish from a reputable supplier; placing fish, coral, and any other live material that could act as a source of contamination into quarantine for two to three weeks before introduction into the aquarium; disinfecting inanimate material such as gravel or sand before adding it to the aquarium; and disinfecting nets.
Editor's Note: Low salinity is another approach to controlling Cryptocaryon. Colorni (1985, 1987) reported various methods for treating fish using hyposalinity. One approach was to lower salinity in fish aquariums to 10 parts per thousand (ppt), or a specific gravity of 1.007, for 3 hours every third day to kill tomonts in the aquarium before they could hatch. He also found that tomonts exposed to salinity of 20 to 25 ppt (specific gravity of 1.0145 to 1.018) produced tomites, but their hatching was delayed up to 28 days. Thus reducing salinity to 23 ppt (specific gravity of 1.017) may not be consistently effective. Cysts kept at 15 ppt (specific gravity of 1.011) for 48 hours or more did not produce any live tomites.
We have used low salinity to treat fish with Cryptocaryon here in our lab. We remove half the water in the aquarium and replace it over a 1 hour period with dechlorinated fresh water. The resulting salinity (approximately 15 ppt or specific gravity of 1.011) is maintained for 7 to 10 days. This treatment should not be used for invertebrates or especially sensitive fish, but most marine fish will tolerate it well.