Nitrite Poisoning (Brown Blood Disease)
Nitrite Toxicity, Methemoglobinemia or Brown Blood Disease is a disease caused by high nitrite concentrations in the water. The source of the nitrite is the metabolic wastes produced by fish when they metabolize the protein in their diet. The primary nitrogenous waste product of fish is ammonia. When this is excreted into the water it is oxidized by the Nitrosomonas bacteria to form nitrite. The nitrite is subsequently oxidized by the Nitrobacter bacteria to form nitrate.
Most circumstances causing ammonia poisoning can also lead to nitrite Poisoning. Nitrite build up usually occurs after ammonia has peaked .This is because the nitrobacter bacteria that convert nitrite (NO2) to nitrate (NO3) require time to become active. Nitrite nitrifyers (nitrobacter) are also inhibited by ammonia and other chemicals causing sudden rise in nitrite level.
If fish are heavily fed and/or if the Nitrobacter do not efficiently oxidize the nitrite to nitrate, the nitrite concentration can increase to problematic levels.
Nitrite poisoning is common in fall because the temperature is not the optimum for nitrobacter bacteria or during the spring as the water temperature increases, Fish activity can often increase faster following a temperature increase than the bacterial action does.
This also can happen following the addition of a large number of new fish to a pond Or when the biological filter becomes partially obstructed with waste materials reducing its effectiveness that can cause the nitrite levels to increase.
Nitrite is not a problem in flow-through systems because there is no significant conversion of ammonia to nitrite during the short time that the water is present in the system.
Nitrite toxicity is affected by many factors including chloride level in the water, PH, fish size, previous exposure, nutritional status and dissolved oxygen level. Nitrite can become toxic to fish at concentrations as low as 0.5 mg/L (= ppm).
Nitrite, NO2, measured in parts per million (ppm), is the second chemical measurement (after ammonia) made to determine the "health" of the biological filter. Nitrite should not be detectable in a pond with a properly functioning biological filter. Thus the ideal and normal measurement of nitrite is zero.
At all times Levels of nitrite should be kept below 0.1 mg/L.
Pathogenesis and Clinical signs of nitrite poisoning :
1. Nitrite has been termed the invisible killer as the pond water may look great, but nitrite cannot be seen.
2. Nitrite damages the nervous system, liver, spleen, and kidneys of the fish. Even lower concentrations over extended periods.
3. Nitrite is actively transported across the gills where it enters the blood stream and oxidizes hemoglobin to methemoglobin.
4. Methemoglobin cannot transport oxygen efficiently so tissues are deprived of oxygen.
5. Oxygenated hemoglobin is red, while methemoglobin is brown.
a. Fish with nitrite poisoning have pale brown colored gills.
b. Methemoglobin concentration of about 40% causes the blood to become chocolate brown and pale brown gills.
6. Anemic fish have pale gills but with red tinge
7. Fish with methemoglobin are dyspneic even with adequate oxygen.
8. Behavioral changes associated with nitrite poisoning are similar to those of hypoxia including lethargy, gasp at the water surface or crowded near the water inlets.
9. A common indication of a fish that has a severe nitrite spike in the past is that the gill covers may be slightly rolled outward at the edges. They do not close flat against fish's body.
Diagnosis of nitrite poisoning :
1. Case history: over-crowding, recent medication or other chemicals added, failure of the biological filter and fall season.
2. Clinical signs:- mentioned before.
3. Measurment of nitrite concentration in water (colorimetric kits can be used).
4. Measurment of methemoglobin concentration in the blood of fish.
5. Exposure of fish to very high nitrite concentration is associated with presence of foci of accumulated iron-containing macrophages caused by increased erythrocyte destruction.
Treatment of nitrite poisoning:
1. Increase aeration to maximum.
2. It is best to keep nitrite level as low as possible.
3. Nitrite is much less toxic when chloride is present [chloride inhibits nitrite uptake across the gills competitively] so addition of chloride is recommended (usually as sodium chloride) to the affected water system as soon as possible. In most cases, the addition of 1.0 part per thousand (ppt) salt will yield approximately 500 ppm chloride (more than enough to do the job). One part per thousand is the same as 1 gram per liter (about 4 grams per gallon).
4. 25-50% water change depending on nitrite concentration.
5. Addition of nitrifying bacteria (nitrobacter).
6. Increase the biological filtration.
7. Decrease stocking density of fish.
8. Reduce the feeding rate.
9. Reduce the water temperature.