Nitrite Toxicosis In Freshwater Fish

"Brown Blood Disease"

A common problem in freshwater aquariums and production systems is nitrite poisoning. This typically occurs in newly established tanks systems ("New Tank Syndrome") where the nitrifying bacteria, Nitrobacter, has not become established, in tanks that are over crowded and overfed, and after treating the tank with antibodies or chemicals that kill the bacteria. In all of these situations ammonia from metabolic wastes from the fish and from organic matter (uneaten food, dead plants, etc.) builds up. Ammonia is extremely toxic to fish. The bacteria Nitrosomonas oxidizes the ammonia to nitrite which is usually further oxidized to nitrate which has a low toxicity for fish. The nitrates are then removed through water changes and uptake by plants. If the Nitrobacter bacteria is not established or becomes overwhelmed by the amount of nitrite present, nitrite levels quickly reach toxic levels.

Nitrites are actively transported across the gills and readily oxidize hemoglobin to form methemoglobin. Methemoglobinemia results in hypoxia severe enough to cause sudden death but often the fish will live until they exert themselves. The term "brown blood disease" comes from the appearance of the blood that has high levels of methemoglobin (which is brown). Often, gross lesions are lacking, therefore; the brown appearance of the blood can be a diagnostic tool. Another diagnostic method involves measuring nitrite levels in the water. This may be unrewarding if mortality is high enough to decrease fish density and subsequent nitrite levels. Nitrite levels should not exceed 0.10 mg/1 in channel catfish or 0.50 mg/1 in salmonids. The LC50 for the majority of freshwater fish ranges from 0.60 to 200 mg/1. Saltwater fish have a much higher tolerance for nitrites.

Treatment not only includes decreasing the population to decrease ammonia levels, but also adding a chloride salt (in the form of sodium chloride or calcium chloride) to the water. The level of salt needed to treat (<50 mg/1) is not toxic to freshwater fish. The chloride ion competes with the nitrite ion at the gills. When the chloride ion is present at least three times and not more than six times the level of the nitrite ion, it is preferentially transported across the gills. Thus transport of the nitrite ion is reduced. Keeping the chloride levels in the water at least 20 mg/1 can prevent nitrite toxicosis. Additional treatments can include emergency water changes to dilute the nitrite problem.

- by Melanie Greeley, DVM

- edited by Tim Muench,