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Marine White Spot (Cryptocaryon irritans)

Marine white spot is caused by a ciliate parasite called Cryptocaryon irritans and was only identified as a distinct parasite in its own right in 1951. Although the disease’s symptoms appear similar to freshwater white spot (Ichthyophthirius multifiliis) the two parasites are not very closely related and both need different husbandry and/or treatment regimes to prevent and resolve an outbreak.

One of the most common misunderstandings with this parasite is to assume that it is the same as freshwater white spot and treat it in the same manner - this can lead to disaster. Indeed, marine white spot is often described as marine “Ich” or “Itch” which is confusing as the abbreviated name refers to a shortening of the scientific name of freshwater white spot (Ichthyophthirius multifiliis). One thing is for certain, virtually all treatments for freshwater white spot will lead to disaster if used in a reef tank.
This parasite has a life cycle that has four stages (figure 1). When the parasite is embedded in the skin of a fish (figure 2) it is called the trophont stage. Here it appears as a distinct white spot about 1-2mm across (around the size of a grain of salt). The parasite has bored inside the tissues of the fish (it is not on the skin’s surface) and is hemispherical in shape and covered with minute cilia (figure 3). The parasite rotates inside the capsule it makes in the fishes skin tissues and, it is said, this movement contributes to irritation the parasite causes and the fish often flick against hard surfaces in an attempt to knock this parasite off. In severe infections this flicking can cause additional damage to the fish’s skin. During this stage the parasite is feeding on the host building up its nutritional reserves ready to break out of the skin and start the next stage of the life cycle as a free living trophont. The free living trophont always exits the host at night and has one goal in life - to find a suitable substrate to encyst into the theront. A suitable substrate can be the sand at the bottom of the tank, detritus in a filter or even the surface of living rock. If it does not find a suitable place to form the cyst within 3-4 hours it usually dies.

After about a week as the tomont about 200 infective theronts will have formed within the tomont. The tomont then ruptures releasing the theronts back into the water column where they seek a host. The theronts are attracted by light and move up the water column looking for a susceptible host to infect. After release the theronts are infective for about 24 hours.

Figure 1 - Life cycle of C. irritans showing the 4 developmental stages in the life cycle and the average time the parasite spends in each phase at 25oC. (Modified From Colorni, 1987 by Burgess)

Figure 2 - Tang with severe C. irritans infection. (Images used with permission)

Figure 3 - C. irritans trophont in the fish skin
It is said prevention is better than a cure and this is very true of this parasite. By far the most effective way to keep this parasite from infecting your fish is to avoid introducing it into your display system in the first place. This is particularly true when you consider that the parasite is an obligatory pathogen (it needs a host to survive). So no susceptible animals in a system or successful treatments mean that the parasite can be successful eradicated from a display or quarantine system. This is unlike freshwater white spot where the cyst can survive for a long period off-fish. So the question is how do we avoid introducing the parasite in the first place? Well the following list may seem obvious but should always be followed:
  1. Buy from an LFS with a good reputation that you trust
  2. Quarantine your fish AND invertebrates
  3. Practice good bio-security between tanks, live feeds, visits to the LFS or helping with a friend’s system
  4. Use of dietary immunostimulants
  5. Ozone and UV
Let us consider each one in more detail:

1. Buy from an LFS with a good reputation that you trust
That statement may seem obvious but many a wipe-out has occurred from someone buying a “sympathy” fish, which is doing badly at a poor LFS, and adding it to their valuable display system without quarantine. As a rule of thumb you should get to know your LFS; ask about their health management regime (if they are OATA members they have access to OATA’s excellent document on bio-security and fish heath, which they would be wise to follow). They should be happy to tell you if they quarantine fish before sale, how long they do this for and if they use copper as a routine treatment in their fish systems (which is an excellent way of reducing the risk of this parasite).
The adage should be “if in doubt over a fish’s health leave it where it is”. Only if you are sure that there is little risk to it and your fish from your quarantine system should you buy it. But in reality, with a good LFS it has a good chance of recovery; with a bad LFS vote with your feet and hope they close down due a lack of trade from discerning reef keepers.

2. Quarantine your fish AND invertebrates
This is the most common way that parasites get into a system. Fish with a low level of infection or water from an invertebrate system that has infected fish in it are the major sources of bringing this parasite into a reef tank. Quarantine was designed for ships and refers to a period of 40 days and nights (very biblical). Usually this is sufficient for most mammalian diseases to appear and action to be taken. Fish being cold blooded 40 days may not be long enough. For example it has been calculated with a 50% safety value that to be 95% certain that any fish is marine white spot free then the quarantine period is 71 days (just over 10 weeks). So there’s the figure for those of you who want a definitive answer to the “how long do I quarantine my fish for?” question. However, a confidence interval of 90% is achieved at 6 weeks and for many reef keepers 6 weeks plus a prophylactic treatment is considered a gold standard. Why quarantine my invertebrates? Well you can introduce trophonts, theronts or tomonts either with the water or on living rock or a coral’s skeleton. So a reef tank really needs two quarantine tanks - one for fish, where copper can be used, and one for invertebrates. In the invertebrate tank time is really the only guarantee of eradicating the parasite so we would recommend a quarantine period of 8-10 weeks or the use of a sentinel fish in the invert quarantine tank, which can then be treated in the fish quarantine tank should the need arise. Quarantining inverts also prevents the introduction of coral eating nudibranchs or Zooanthid eating pycnogonids which can be a huge advantage to a good reef tank where the invertebrates are as spectacular as the fish.

What should be in my quarantine tank?
Ideally the tank should be as large as possible have a simple filter (sponge filters are particularly good in this role) have minimal décor (bare bottom, no living rock etc) instead hiding places can be provided by plastic piping. For a fish system lighting should be subdued and it should have its own set of equipment (nets etc) which never go near the main tank. Most importantly you must know the volume of water very accurately to allow for the correct dose of medications to be administered.

3. Practice good bio-security between tanks, live feeds and visits to the LFS or helping with a friend’s system
Bio-security is the terminology that describes methods for preventing the spread of an organism from one system to another. Everyone, hopefully, washes their hands after going to the loo. Well reef keepers should do that before going into their tank. However, there it is a lot more to it than that; each tank should have its own nets, pipe-work, syphons etc. and they shouldn’t be swapped between tanks without disinfection. One of the best disinfectants I have used for fish tank equipment is Milton’s fluid and to use it just follow the instructions and soak all your equipment in it after use and wash thoroughly before drying. So our advice would be always wash and disinfect hands and equipment before using it on your tank. This is true if you are an angler, have loaned kit to a friend or have simply been helping them out and tinkering with their tank.

4. Use of dietary immunostimulants
One of the biggest recent advances in marine reef keeping is the use development of superb dry diets and the use of immunomodulating dry diets and sprays for enriching frozen foods. Unlike garlic and other crude terrestrial plant matter, where there is no evidence in the scientific literature that there is a benefit in fish, there is a huge amount of scientific evidence out there that immunostimulants help reduce the infection rate with fish pathogens (not just white spot). It is excellent to see that some manufacturers have decided to run with this and produce a range of excellent products aimed at the reef keeper. Immunostimulants work by up regulating the non-specific defence mechanisms of an animal thus preventing a pathogen establishing an infection or allowing an already infected animal to be in a position to rid itself of the pathogen more rapidly. However they can’t be fed all the time otherwise the animals become tolerant to their effect and most commercial regimes recommend two weeks on one type of immunostimulant followed by two weeks on a second immunostimulant then 4 weeks on the standard diet before returning to the immunostimulant diet.

5. Ozone and UV
Both ozone and UV can be very useful tools in the fight against this parasite. UV kills the parasite by damaging its genetic material while Ozone disrupts cell membranes. Both methods require pumping of water out of the aquarium through either a UV lamp system or into a protein skimmer that has ozone injected into it. Used correctly and at sufficiently high enough dose both ozone and UV are very effective in removing trophonts and theronts from the water column and hence reducing the infective pressure during an infection or preventing an infection establishing itself.
Once white spot has broken out in a system all the susceptible fish will have the parasite and, even if they have no signs, all the fish in that system will need treating. Otherwise, on re-introduction of the most susceptible species, the parasite will breakout again from the asymptomatic carrier fish that have remained in the main tank.

Treatments for marine white spot generally fall into three categories: chemical, environmental and unproven.

There is insufficient time and space to discuss the unproved group here but suffice to say this covers garlic, tea tree oil and the whole grey literature area of white spot cures.

Chemical cures
By far the most effective white spot cures are copper based and prescription drugs such as fuscidic acid and quinine.

However, the following words of warning should be heeded before chemical treatment of a reef tank is attempted. (Source: Simon Garratt, posted on UltimateReef)
“Modern reef keeping methods have now moved way beyond the days when LR was considered a simple source of bacteria, and filtration. In modern systems the 'critter' population regularly feature as a major player in the functioning of the tank. As such, any treatment should consider the impact it will have on the tanks ecosystem. 'Reef safe' chemicals have often only been tested with the most commonly kept 'show' inverts. Many have not been tested nor claim to be safe with the multitude of background life that 'modern' reef keeping deems as beneficial, and form a substantial part of a modern reef eco-system, especially those containing sand beds etc. The use of 'chemicals' designed to kill parasites can have disastrous consequences in more diverse systems as they will kill off these beneficial animals as well as the parasite. As a rule of thumb Keep chemicals out of the reeftank and carry out any treatments in a specialised quarantine tank. Whilst it’s perfectly acceptable and correct to deem the survival of the fish as a priority, it shouldn’t be at the detriment of the rest of the system, and certainly not to the degree it jeopardises the systems stability.”
That said, copper based treatments are easy to use and readily available from LFS in either ionic or chelated forms (chelated forms have a longer ½ life in the aquarium, otherwise there is no difference between them as the active ingredient, Copper ions, is the same). The two important things to remember about copper are that it will kill all invertebrates and cannot be used in a reef tank and that you must get the dose right. If the dose is too low then the treatment will be ineffective, too high and you will poison your fish. So I recommend that all copper treatments are carried out in a quarantine tank away from your invertebrates and that the levels of copper are monitored daily with a copper test kit to make sure the dose of copper falls into the therapeutic range of 0.2-0.3ppm. Once a tank has been treated with copper it can be problematic reintroducing invertebrates back as copper forms complexes with carbonates in the aquarium and these can leach back into the water with time causing copper toxicity to the invertebrates.

Fuscidic acid and quinine can be used in reef tanks (although ideally they should be used in a quarantine tank situation) as they are relatively safe for invertebrates, although quinine is not tolerated well by clams and anemones. However, it may be easier to remove these animals from a reef rather than catching all the fish if the need arises. These two drugs can also be used on fish species that are intolerant of copper such as sharks and rays and some scale-less species. Both of these compounds need to be prescribed by a vet and the doses of fuscidic acid should be 5mg/l for 10 days and quinine 30mg/l for 14 days. However, fuscidic acid will kill off some filter bacterial while quinine will not. After treatment both of these compounds can be stripped out of the water with activated charcoal.

The final group of chemicals are dye based and these are very popular as they are often sold as reef safe. These dyes are designed to bind to the parasites genetic material and prevent replication. They can be very effective for some reef systems and completely ineffective for others. To some extent this can be attributed to variation in biomass (these dyes readily bind to organic materials and can be rapidly stripped out of reef system in skimmers and in systems with lots of animals in them before they have time to inhibit the parasite) and errors in calculating the system’s volume can cause the medications to seem ineffective. Often they require 2 or 3 consecutive doses to affect a cure.

Environmental conditions
Unlike freshwater white spot, marine white spot cannot be cured with a rise in temperature. This is because the thermal death point of this parasite is higher (around 32-35oC) than the reef fish we are trying to cure (and this temperature would seriously damage most corals etc). Raising the temperature simply speeds up the life cycle allowing more parasites to develop in a given time period. For example at 25oC a tomite completes its part of the life cycle in 6.5 days (fig 1) at 20oC it would take 9.75 days, giving you much more time to intervene before the infection becomes “nuclear”.

Freshwater baths are ineffective against marine white spot as the parasite is embedded in the host’s skin (figure 3) rather than on the hosts’ skin. Here the parasite is protected from the external environment and freshwater cannot damage it.

Hyposalinity treatment can be a useful strategy in systems where it is impossible to get the fish out and treat them by a chemical method. You drop the SG in the tank to below 1.017 (some say as low as 1.009 but IMHE 1.015 is about as far as I would take it) in small steps around 0.002-0.003 units per day. The infectious stages of marine white spot, the tomites, can't survive the low salinity and die off. It can take a long time to cure a tank like this, at least 4 to 6 weeks often 12-16. In addition this treatment is very stressful for shrimps and urchins/starfish/brittlestars and some fish species that are sternihaline and do not osmoregulate well at low salinities.

It also plays havoc with your water chemistry and we make no apologies for repeating a comment Simon Garratt made concerning hyposalinity and the changes it causes to dKH and pH in a post on UltimateReef some time ago.
“Hyposalinity treatment is not a good idea in a modern reef tank using the Berlin method. It has drastic effects on dKH and pH stability.

SG       dKH
1.025  7.04
1.020  5.36
1.015  4.16
1.010  2.96
1.005  1.44

At a dKH of 4, your pH becomes very unstable over a 24hr period with large dips at night, even more so in immature systems that commonly has higher degrees of algae present than older more mature and heavily grazed systems. Whilst severely detrimental to corals (especially hard corals) these levels can also cause a multitude of other problems as well.

Low pH levels are dangerous to all crustaceans especially during moulting which usually happens at night, this isn’t just your shrimps, it’s also your entire population of critter life in the tank that comes under this group, including copepods etc.

It also plays havoc with sand layers and the LR, causing big shifts in boundary layer function, possible dissolution of bound phosphates, and an overall destabilising of the entire nutrient cycling abilities of the tank.”
In conclusion, we hope you have found this article accessible and readable, and hopefully it dispels some of the myths around this common yet serious disease. By far the best strategy is to avoid introducing the disease in the first place and the most effective method here is the use of quarantine and prophylactic treatments to keep the parasite out of your reef tank. However, if you do suffer an outbreak in your tank all is not lost and there are several strategies that can be used to resolve the problem. However, none of these methods are fool proof and can lead onto other problems and, at least for the foreseeable future, marine white spot remains one of the most serious disease issues facing reef keeping.

One of a series of articles written for UltimateReef by fish health professionals.
Published on February 1st, 2007 at

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