Camallanus Worms
Camallanus is a genus of parasitic roundworms that can affect any fresh water fish, some amphibians, and even aquatic reptiles such as turtles. They typically are transmitted via a crustacean intermediate host, but Camallanus cotti does not require an intermediate host to infect other hosts. An infection of camallanus worms can be fatal if left untreated. These nematodes will burrow into the gut of other animals and feed off of their blood. In less serious cases, the worms will coexist with their hosts for a long time, and can generally go undetected. It's usually when something stresses out a fish that problems will arise. Most of the complications of camallanus worms come from secondary infections from a weakened immune system and torn up insides, however when the infections get bad enough, the worms can overwhelm the fish, and will prove fatal.
C. cotti eggs are typically spread through the feces of infected fish, though the eggs remain viable on other things like plants and substrate from infected tanks, and can spread through home aquariums on equipment like nets and siphons. The fish that pick at the plants and substrate where the microscopic eggs reside will ingest them causing the cycle to continue. Some fish may never show symptoms before dying, while others will show obvious symptoms. Camallanus worms are known to be very hard to get rid of, and will often pop back up a few weeks after purging them. Treatment will be hard on most living things, but can be effective if done right.
Symptoms
As mentioned above, some less severe cases can be asymptomatic and not overly dangerous. The tell tale sign of camallanus worms doesn't occur until late in an infection when the worms are laying eggs. The female worms will move towards the anus of the fish and will stick out about ¼ of an inch to spread eggs into the water. They will stick out further when the fish isn't moving and will retract when the fish does move. They are easily identified by their vibrant red color and thread like appearance.
Between the asymptomatic phase and the red thread phase, there are some more subtle signs of infection that progress as the infection does. At first, juvenile fish won't develop very quickly due to a lack of nutrients from the worms feeding off of them. You might lose some fish here and there every few weeks to months, but nothing too out of the ordinary. Some report a loss of appetite in the fish, though this might not happen every time. As the infection progresses, a change in the fish's belly will be noticeable as one of two things can happen. Either the fish will start to get a sunken belly from a lack of nutrients or the fish will get a swollen belly from all the worms building up in the gut. Following this is when you will notice the red threads sticking out from the fish's anal vent.
Treatment
The good news is, there is an effective treatment for camallanus worms. It's not all doom and gloom for your fish tank. Unfortunately, the solution is a combination of medication that not many people have in the medicine cabinet. Camallanus worms are resistant to pretty much any antiparasitic medication, including the typical praziquantel used for other parasitic infections. except for levamisole and fenbendazole, no other treatment will work. On their own, each is somewhat effective for treating the worms, but when reasonably mixed, they will do the trick.
Levamisole will only paralyze the worms, which will allow them to be expelled from the fish. It does not kill them. If you're treating it with levamisole alone, you will need to continuously vacuum the substrate, which in planted tanks is not very feasible, and the worms and eggs can land on the plants, and get missed. Fenbendazole will actually kill the worms without harming the fish. It can take a while for fenbendazole to work on killing the worms since they are so hardy. When used in tandem, they can provide fast and lasting relief from the worms. These two medications are quite potent and they will take their toll on your ecosystem. Overdosing with either or both will crash a cycle, as they are both harmful to the beneficial bacteria that oxidize ammonia. Daily water testing is crucial during treatment. If your ammonia or nitrites rise, discontinue treatment and immediately do a water change. It is recommended to take filter media out of the tank during treatment and put it back in following treatment and a water change to not permanently cripple your bacteria culture.
Fenbendazole and levamisole are both deadly to most invertebrates. Treatment will kill snails of any kind, but can spare shrimp. Unless you plan to house the snails in a different tank for a month or two without any fish, they are going to be necessary casualties of war. They can spread the eggs just like plants and aquarium equipment. Once a single fish shows symptoms, assume the whole tank is infected. Sanitizing equipment after using it, washing your hands if you need to do anything in the infected tank, and not using a hospital tank for treatment will minimize the risk of spreading the worms to other tanks. They do not affect humans or other mammals, fortunately.
Sourcing these medications can be a challenge for those outside of the US, and even in the US. Fenbendazole is commonly used as a dog dewormer and is sold under the brand name Pancur C. Just because it's marketed for dogs doesn't mean it won't work for fish. The chemical is the same, either way. Levamisole is a little more tricky to get a hold of. Fritz Expel-P is a brand name levamisole with dosing instructions for aquariums. One or both of these medications might not be available in some countries without a veterinarian prescription.
Treatment should proceed as follows; firstly, turn off any UV sterilizers running, as they will break down the medication rendering it useless. Secondly, dose the water with both of the medications. A little will go a long way. The water will need to be dosed with both to keep expelled worms paralyzed while killing them. The proper Pancur C dosage (22% fenbendazole) is only 10 mg per gallon, or roughly 2.5 mg per liter. Levamisole needs to get to 2 ppm to be effective, or 75 mg per gallon (roughly 20 mg per liter). This dosage will ensure that expelled worms will stay paralyzed and die, not reinfect your fish. Lastly, a little of each mixed in with their food will catalyze the whole thing. You should not be putting a full dose for the water in the food. Only a pinch is needed for a whole batch. Once the chemicals make it into the fish, the worms will not last long. Wait 3 days after the initial dosage and do a large water change. Vacuuming the substrate is not necessary, but can be done if desired. The worms will stay paralyzed and die, due to the medicated water. Remedicate the water, and continue to feed medicated food. Do another water change after 3 days, and discontinue all treatment. Repeat after 2-3 weeks to ensure that the eggs laid by the now dead worms will hatch and die before they can lay eggs themselves. If this step is not taken, it will put you right back to where you started. Following this second dosing, the tank should be free of any worms. Wait another month before adding any fish to the tank so that they don't bring some other disease that will finish off your fish.
Following each round of medication, and after a water change, the filter media taken from the tank can be put back in. This should allow the bacteria to build back up in the tank to effectively handle ammonia and nitrites. The eggs are made to sink, and any that somehow end up in the filter will be kept away from the fish. Filter media is about the only thing that can be safely transferred between tanks, assuming it's housed in a HOB or canister. Sponge filters can be a little trickier, since they are generally kept in the tank with the fish.