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Thread: Fish diseases: A Primer
04-04-2009, 08:42 AM #1
Fish diseases: A Primer
I got a PM recently from a user who wanted to know if his sister's fish had Ich. I told him yes, and what to do about it. And that PM got me thinking about Ich and other diseases that affect freshwater and marine fishes. So, I thought it time to write up a post about the most common fish diseases you're likely to run into, and how to cure a fish infected by them.
Surprisingly, several of the same diseases are both found in freshwater and marine fish, and the life cycle of the parasites are very similar. For example, both marine and freshwater fish contract Ich, and though the protozoa that causes the disease is different, with the freshwater parasite Ichthyophthirius multifilius and the marine Cryptocaryon irritans, the symptoms of the disease are exactly the same, as are the reproductive strategies.
In this post I'll list several diseases as to what they are, how they operate, and how to cure them. Correct diagnosis is essential to a cure, so I encourage you to make yourself familiar with what the parasites look like. Google images is handy in that respect.
I should say that this primer shouldn't make you afraid that around every corner there's a parasite there to kill your fish; on the contrary, it's unlikely you'll run into the majority of them, if you're a conscientious aquarium keeper. So, I wrote this primer on what to do if you ever run into one of them.
Now, on to the diseases.
Ich (Ichthyophthirius multifilius) is perhaps the most common disease that plagues freshwater fishes, whether in the aquarium or in commercial fish farming. In this post I'll tell you what it is, how to prevent it, and how to cure it.
Ich is simply a protozoan, that is, a tiny unicellular organism. Though it's among the largest protozoa that infects freshwater fishes, it is so tiny you'd need a microscope capable of 100X magnification to see it, similar to what you'd need to see things like amoebae or paramecium, clearly. The adult Ich protozoan is round to slightly oval, and covered with thousands of cilia (hair-like threads used for propulsion). Its nucleus is shaped roughly like a horseshoe, making the parasite easy to pick out through a microscope. The much smaller motile protozoans are teardrop-shaped, and it takes a very practiced eye and high magnification to identify them. The natural life cycle of Ich is a week to 10 days, depending on the temperature.
And it's a killer. Ich is an obligate parasite, that is, it has to have a live fish to during it's life cycle to perpetuate its species. When a fish is compromised by a chill (rapid lowering of tank temperature) or stress, or an infected fish is introduced, the parasite can penetrate a fish's protective slime coat, and burrow into the fish's epidermal layer where it encysts itself and is referred to at that stage as a trophont (a reproductive body). Thus encysted, the parasite grows up to 1 mm in size by feeding on fish cells. The parasite divides into many hundreds of copies of itself, the cyst drops off the fish within a week, anchors to the substrate, and in a day or so it bursts, and the tear-drop shaped copies surge out looking for a fish to encyst upon. Without fish, the motile parasites die within 48 hours at 80 degrees.
The motile parasites can encyst themselves on the fish they came from, which is why Ich can spread so rapidly on a fish. A fish literally covered by cysts is quite usually doomed. Unchecked, Ich can easily kill every fish in a tank. It is one of the few fish parasites that is 100 percent fatal if not promptly addressed.
Thankfully, when identified early enough, when there are just a very few spots of Ich, the prognosis is excellent.
Hundreds if not thousands of times I've recommended founding a quarantine/hospital tank to prevent introducing disease into your main tank. The benefit vies a vise Ich and other diseases is quarantine tanks are most usually small, like a 10-gallon tank. Thus, an observant person can pick out a fish with an Ich spot or two quite easily, and take prompt steps to eliminate the disease.
The most common excuse not to have a quarantine tank is financial, but replacing dead fish routinely will cost you more than the money for a 10-gallon quarantine aquarium set-up in a short time.
There are several methods of eliminating the parasite, and some are more effective and safe then others. The best cures are those that stress the host fish as little as possible.
The most popular cure is marine salt and heat. The former, because most fish can tolerate the salt, the latter, to speed up the life cycle of the parasite. Average temperature for the process is 82 degrees, and salt is added at a prescribed amount for the types of fish you keep. It's best to use real sea salt in this process.
It works because the motile parasites, which are those that burst out of those cysts, do not adapt to the osmotic change in the water due to the salt, making it difficult for the motile parasites to move, killing some, and more difficult for them to find a host fish in time. As you remember, if the motile parasites can't find a host fish in 48 hours, they'll die.
I say more difficult to move because at the size protozoans are, water is thick like pancake syrup. Protozoans are mostly water. They have a nucleus where the DNA (Deoxyribonucleic acid) and some of the RNA (Ribonucleic acid) are, with those making the blueprint of what the creature is. The nucleus is surrounded by cytoplasm, which is thick enough to maintain just enough pressure against the water pressure for the protozoan to maintain it's shape. In that cytoplasm there are a few to several small ovals of most of the RNA in communication with the DNA and RNA in the nucleus.
When salt is added, it makes life difficult for the motile protozoans. I. multifilius is a freshwater creature. Protozoans very much are what they are, and thus are such simple organisms with just a single cell they can't adapt to change like higher organisms can. You and I can walk out of a warm house to say, a 30 degree day. People can do it easily; walk between changing temperatures and air pressure. Fighter pilots go through maneuverers of many times the force of gravity. Higher animals can easily adapt to change. Simple organisms can not, at least not in any kind of hurry.
The heat is reputed to speed up the life cycle (encyst to motile protozoan). The motile protozoans must find a fish at 80 degrees within two days. The thought is that high heat - say in the low to mid-80's, would speed up the life cycle so the salt can take care of the motile protozoans before they have time to infect another fish.
The downside of this process is it's very, very hard on small, and scaless, fish. Those that developed in soft, acidic waters like the majority of tetras and Corydoras catfish, also don't do well in the long period (usually a week to 10 days) of salt and heat. Also, large partial water changes are needed to remove the salt from the aquarium system.
The astute among you have picked up that if the motile protozoans can't find a fish to encyst upon within two days, it dies. And that, my friends, is why I recommend a quarantine tank.
A quarantine tank is designed to give new fish or fishes the once-over, and also to get them eating. Personally, I quarantine every fish at least a month. A fish that comes in with a spot or two of Ich is isolated, treated, then later returned to quarantine. In over 30 years no disease of any kind has gotten into my show aquariums because I use quarantine.
Another method of curing Ich, and the one I favor, is a marine dip.
A marine dip is a container of marine water of the same temperature as the tank the fish came from. When I say marine water, it's tank water that has commercial marine salts added to make it like sea water, that is, 1.026 specific gravity at 77 degrees. I use a clear glass beaker for the container, and run an air stone during the process for the fish's benefit and to help evenly mix the salt. You can use whatever container you like, as long as you can see the fish clearly, and you'll need a refractometer or hydrometer to measure the salinity as you add the salt..
An infected fish is identified, and removed to another container of quarantined aquarium water, since it does healthy fish no good to be treated for a disease they don't have.
The affected fish is then placed in the container of salt water, and removed when it shows distress. Different species of fish take different amounts of time before they show distress. Larger Cichlid species can take it up to 10 or 15 minutes fairly easily, small tetras and similar well under five. When the fish shows distress (rapid breathing is a sign) it is removed and returned to isolation.
It works because the fish can stand the osmotic change from fresh to marine water for a short time. The encysted protozoans can not, and die almost immediately; within seconds. Some recommend following the dip with a formalin* bath, but I've not found it necessary with freshwater Ich.
The cysts darken and fall of the fish within a day if there are just a few spots, perhaps two if there is a heavier infestation. The marine water does cause the fish to produce more slime coat, and is perfectly healthy and ready to be returned to quarantine usually within a week.
It is vitally important to maintain the same temperatures in this process, as you want to stress the affected fish as little as possible. All my tanks run at 77 degrees, so all my quarantine tanks are the same.
The downside of the marine dip is you have to sit and watch the fish very closely to detect distress. Also, you have to purchase marine salt, refractometer, airstone and an air pump, and have a container for the process, and another to isolate the fish after it.
Last edited by Fishalicious; 04-05-2009 at 07:08 PM.