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
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,