Red Streaks on Fish: Septicemia vs. Ammonia Burn

Red streaks, reddened skin or hemorrhages on a fish's body and fins can signal bacterial septicemia or a chemical 'burn' from ammonia or nitrite. Learn how to tell them apart, when it is an emergency, and how to respond.

What red streaks mean

Red streaks or blotches, reddened skin and pinpoint hemorrhages on the body and fins usually mean one of two things: a bacterial infection of the bloodstream (septicemia) or irritation and tissue damage from toxic water (an ammonia or nitrite 'burn'). Telling them apart guides the response.

Common causes

• Bacterial septicemia (motile Aeromonas, Pseudomonas, Vibrio). Causes external reddening and hemorrhage that can progress to ulcers and rising mortality.
• Aeromonas salmonicida (goldfish ulcer disease). The acute form shows hemorrhages in fins, tail, muscle and gills; the chronic form produces crater-like skin ulcers.
• Ammonia or nitrite poisoning. Ammonia reddens and damages gills and tissue; nitrite turns gills brown (methemoglobin).
• Poor water quality generally, which predisposes fish to septicemia.

Septicemia vs. ammonia burn

• Septicemia: red streaks and hemorrhages across the body and fins, often with ulcers or open lesions; systemic and progressing even when the water test is clean. Red streaks that persist with normal water parameters favor bacterial septicemia.
• Ammonia or nitrite 'burn': reddened (ammonia) or brown (nitrite) gills and generalized irritation that correlates with an elevated ammonia or nitrite reading and improves with water changes. The water test is the key differentiator.

Approach

Remove predisposing factors first: improve water quality, reduce organic load, and cut handling, temperature stress and low oxygen. Do a water change and reduce feeding. When fish are dying from bacterial septicemia, antimicrobial therapy - ideally guided by sensitivity testing and an aquatic veterinarian - is warranted.

Sources: Merck Veterinary Manual, Bacterial Diseases of Fish (www.merckvetmanual.com); Merck Veterinary Manual, Environmental Diseases of Aquatic Animals (www.merckvetmanual.com); The Fish Site, Ammonia in Aquatic Systems (thefishsite.com).