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| Ichthyophthirius multifiliius: Freshwater "Ich" |
Dr. Nicholas R. Silvaggi
Freshwater-Aquarium-Fish.com |
Introduction
"Ich" or "white spot disease" is probably the most common pathogen of freshwater aquarium fish. The disease is most often characterized by small white dots on the fish's skin, as if someone had salted them. Ich has long been the scourge of aquariums and aquaculture facilities alike, because it is very often fatal and it spreads very, very easily. As feared as ich is, it need not be a death sentence for your fish. Armed with a basic understanding of the biology of the causative organism, you will be better able prevent and, if necessary, treat outbreaks in your system. Ich is caused by the protozoal parasite Ichthyophthirius multifiliius. The Latin name translates roughly to "fish louse with many sons." The species name is a reference to the prodigious reproductive ability of the protozoan. While I. multifiliius has been responsible for countless fish deaths, not to mention knocking many an aquarist out of the hobby, outbreaks are not inevitable. With good husbandry practices you can prevent this scourge from ever entering you system. |
Life Cycle
I. multifiliius goes through a direct life cycle, meaning that it does not need any intermediate host, such as snails or birds. This organism is also an obligate parasite. This means that the protozoan cannot live outside of a fish for more than a few days at 75-78°F. The timing of the life cycle is profoundly affected by temperature. As the temperature increases, the length of time the free-swimming parasites can survive outside of a host decreases. Conversely, the free-swimming infective stages can live much longer at cooler temperatures.
The stage of the infection that aquarists see is the trophont stage. Trophonts can be large (up to 1mm in diameter) and are the only certain way to diagnose an ich infection. The trophonts grow in a fish's epithelium (skin) where they are protected from the environment, including chemical treatment, and they can feed off the host's body. Their presence causes the formation of the white pustules that are normally the only visible sign of infection.
Once the trophont has matured, the pustule bursts and the trophont enters the next stage of the cycle, the tomont stage. This is the reproductive stage. Upon exiting the host, the tomonts fall to the bottom of the tank where it attaches to gravel, décor, aquarium glass, or any other surface it comes in contact with. Tomonts then quickly encase themselves in a tough outer coating. This cyst protects the tomont from chemical treatments. Inside the cyst, the tomont divides repeatedly, making up to 2000 new organisms. This is the reason that even one confirmed I. multifiliius trophont is cause for treating an entire tank. Even one trophont can quickly lead to a massive infestation.
When the reproduction is complete the cyst breaks releasing hundreds to thousands of free-swimming tomites (theronts) that constitute the infective stage of the cycle. Tomites are teardrop-shaped organisms with a penetrating apparatus in the pointed end, which they use to burrow under the scales of fish where they become trophonts. Since I. multifiliius is an obligate parasite, the tomites have a limited life span outside of a fish host. At temperatures above 90°F the tomites die very quickly. At lower temperatures (65-75°F) the infective tomites can live for 96 hours or more. This is why ich infections can be devastating for aquaculture facilities raising cold-water fish like salmon and trout. The tomite stage is also important because this is the only stage that is vulnerable to chemical treatments. |
The life cycle of I. multifiliius. Mature trophonts are shed from infected fish into the water column (A). In this free-living stage of the life cycle, the organisms are known as tomonts. The tomonts settle on the substrate, aquarium glass, or other surfaces they come in contact with and attach themselves. Protected by a thick casing, the tomont undergoes repeated divisions, producing potentially thousands of new organisms (B-D). Eventually, the case bursts (E), releasing thousands of free-swimming theronts. The theronts then find a new host and begin the cycle again (F). |
Pathology
I. multifiliius negatively impacts its host in a number of ways. First, the very presence of trophonts in the skin is irritating to the fish. Indeed, one of the symptoms of ich is called "flashing" and refers to the way infected fish will swim erratically and rub themselves on the gravel or aquarium décor. Flashing is a direct response to irritation of the skin. In response to the embedded trophonts, fishes' skin secretes excess slime, and the epithelium actually thickens.
Ich can kill fish in several ways. First, fish use their gills to take oxygen from the water and rid their blood of carbon dioxide. Unfortunately for fish, the trophonts often target the gill tissue, perhaps because it is relatively exposed and richly supplied with blood and nutrients. At any rate, the thickening of the gill epithelium, together with the excess production of mucous and the physical blockage of the gills by the trophonts, limits their ability to supply oxygen to the blood. If left unchecked, I. multifiliius can damage gill tissue to the point where the host suffocates. I. multifiliius also impairs the osmoregulatory processes of fish. The salts, proteins, and other components in the cells of freshwater aquarium fish are more concentrated than in the surrounding water, with the net result that water is constantly entering fish cells by osmosis. The skin and mucous layer work together to maintain the proper level of hydration in the body, but because I. multifiliius trophonts disturb the structure and function of both the skin and the mucous layer, infected fish are forced to spend a great deal of extra energy on osmoregulation. Finally, in addition to their osmoregulatory function, the skin and mucous layer are also a physical barrier to infection. The disruption of this barrier coupled with the stress and energy demands placed on the fish by the infection, make the fish vulnerable to a wide variety of secondary bacterial infections. Pseudomonas infections are especially common. |
Micrograph showing a trophont maturing in the gill tissue of an infected fish. Notice the characteristic "U-shaped" nucleus of the protozoan. |
Diagnosis and Treatment
The surest treatment for ich, or any other aquarium disease, is to prevent infections altogether. But this is easier said than done, right? Wrong. By simply quarantining all new arrivals for a minimum of four weeks, keeping water quality high and stable, and feeding highly nutritious food, you can be assured of years of disease free fish-keeping. The problem is that most hobbyists either do not know they need to quarantine new arrivals, or do not have the patients to do it properly. This is truly a shame because quarantine is the single most important preventive measure an aquarist can take, and it is simple and inexpensive to implement. All that is needed is a small (5-10gallon) aquarium, a heater, an inexpensive filter, and ample patients. All of these components should be strictly for quarantine use. As for procedure, just acclimate new arrivals to the quarantine system, and then wait for four weeks. At that point, any sick fish will have manifested disease, and weak or injured fish will have healed or succumbed to their wounds. As for maintaining water quality, even though fish can tolerate less-than-ideal water conditions, it costs them energy to do so. If they are forced to live under sub optimal conditions for long periods, they gradually become weak and exhausted and can fall prey to opportunistic infections. For this reason, maintaining proper water quality is critical to the long-term health of your fish. Similarly, malnourished fish will not have the energy required to fight off the constant barrage of microbes living in the aquarium water. Feeding healthy, nutritious food is as important for a fish's well being as it is for a human's.
While white spots on a fish's skin are almost always diagnosed as ich, the spots alone are not actually diagnostic. In reality, there are a number of conditions that can cause the appearance of spots on the skin. The same is true for the other symptoms of ich, like flashing and gasping for air. Flashing is merely indicative of an irritation to the skin that can just as easily be caused by ammonia in the water. Similarly, gasping and similar behaviors signal respiratory distress, and may be caused by anything from low dissolved oxygen to gill flukes. The only truly reliable way to diagnose ich is to look for the trophonts on the skin. This test is not nearly as difficult as it sounds. If the fish can be caught, simply rub a glass microscope slide along its body from head to tail, to avoid damaging the scales. Put a small drop of water on the smear and then place a coverslip over that. Since I. multifiliius is the largest parasite affecting fish, with individual trophonts measuring up to 1mm in diameter, only modest magnification is needed in order to identify them. The trophonts appear as large, slowly tumbling spheres covered with cilia, which look like tiny hairs. The u-shaped nucleus is also a distinguishing characteristic of I. multifiliius.
Treatment of ich is complicated by the fact that only the tomites, the free-swimming stage of the ich life cycle, are vulnerable to treatment. Happily, the tomites are vulnerable to a number of chemical agents, including copper, potassium permanganate, formalin, and malachite green. Also, manipulating environmental factors such as temperature and salinity can increase the effectiveness of chemical treatments.
Copper is, unfortunately, the most common treatment for ich. I say "unfortunately," because, while copper is an effective killer of tomites, it is also toxic to just about everything else in the aquarium. It is especially damaging to the biological filter. When treating with copper one must keep a close eye on the ammonia and nitrite levels in the tank in case the biological filtration crashes. It is notoriously difficult to keep a stable level of copper in the water, and the concentration ends up swinging between elevated levels that are toxic to fish and depleted levels that are not toxic, even to the ich organisms. Furthermore, the toxicity of copper sulfate is affected by the alkalinity of the water. Before treating, test the alkalinity and then consult the instructions included with the medication to determine the exact dose to use. When treating with copper or any other medication, discontinue the use of any chemical filter media like carbon (charcoal) and ion exchange pads or resins. These will pull the copper out of solution, where it is obviously unavailable to kill tomites.
Potassium permanganate, KMnSO4, is a strong oxidizer. As such, it oxidizes (damages) dissolved organic matter in the water, as well as free-living bacteria and protozoans. Potassium permanganate is safer to use than copper. However, it is more expensive, and since most aquariums have high concentrations of dissolved organic compounds, treatment requires a large amount of the chemical to reach a therapeutic dose.
Formalin and malachite green are often used in tandem. Even individually they are very effective medications. Unfortunately, they are dangerously toxic to fish and plants. Great care must be taken if one chooses to treat with either (or both) of these compounds.
Salt (NaCl), in the form of aquarium salt mix or kosher salt, is a moderately effective treatment that is reasonably safe to use. Avoid the use of table salt or other "iodized" salts, since the iodine released into solution can become toxic. The I. multifiliius organisms cannot tolerate salt concentrations above 1ppt, but fish and plants can. Maintaining the correct salt concentration is straightforward, since one can test this directly using a hydrometer. The only danger with NaCl treatment is that the bacteria comprising the biological filter may also be killed. As with the chemical treatments, monitor the ammonia and nitrite concentrations in the aquarium.
Raising the water temperature above 86ºF for at least two weeks is also an effective treatment of ich, and can be used to increase the effectiveness of the treatments described above. High temperature coupled with NaCl is a rather effective and still fairly low-risk treatment plan. Increasing the water temperature does kill some tomites directly, but its real advantage lies in the fact that the I. multifiliius life cycle is accelerated at high temperatures. This means that one can force all of the ich organisms to go through the vulnerable tomite stage in a shorter period of time and end the treatment sooner. |
Conclusions
The solution to ich is simple: Don't let it become a problem in the first place. I multifiliius is an opportunistic pathogen. It causes disease when fish are stressed, or when the protozoans are present in such large numbers that they overwhelm the fish's immune system. Proper husbandry can prevent most outbreaks. Keep the water clean and maintain the proper chemistry. Provide enough space and cover for all of your fish. Provide good nutrition. All of these things will keep your fish healthy and ready to fish any opportunistic infection. Observing a strict two week quarantine for ALL new arrivals (including snails and other invertebrates) will prevent a massive number of ich organisms from entering your tank. Finally, when all else fails, ich is one of the more treatable aquarium ailments out there. |
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