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Also known as: Branchiomyces infection — Gill Rot


Branchiomycosis is an acute infection of the gills that can cause high mortality and respiratory distress in many species of ornamental and freshwater fish such as the Koi, the eel, the bass and the perch.

It has been reported primarily in Europe and Taiwan but also in the southeast USA.

The two most commonly isolated species are Branchiomyces sanguinis and B. denigrans which have different host specificities and distributions.

The organism attaches to the gill surface and forms hyphae which penetrate the tissue and damage the blood supply to the area. Spores can be released and develop on the floor of the tank in favourable conditions (25-32°C, low oxygen, low pH).

Clinical Signs

Infected fish exhibit respiratory symptoms and a loss of equilibrium. The gill appears necrotic, eroded and pale.

Mortalities can occur in less than 48 hours and can reach 50% of the herd.


Gross evaluation of the gills reveals a patchy marbled appearance due to the haemorrhages and necrosis.

Wet mounts or histopathology of the lesions will enable a diagnosis to be made. Characteristic hyphae within the vessels of the gills or penetrating the gill tissue will be visible. These hyphae are light brown, slightly refractile, branching and non-septate.

Special stains such as the Gomori methenamine silver (GMS) can be used to identify the fungal elements.

B. denigrans appears to affect the entire gill whilst B. sanguinis is restricted to gill blood vessels.

Treatment and Control

There is no known treatment for the disease but some protocols may be beneficial with malachite green, formalin baths, copper sulphate and benzalkonium chloride dips, and oral methylene blue.

Factors that help control the condition include reducing overcrowding, levels of ammonia, algal blooms, levels of organic material, water temperature and improving hygiene.

Infected or dead fish should be promptly removed from the tank, pH in the tank can be increased by adding quick lime, and ponds can be drained and limed.

Fish should be adequately quarantined before being introduced to the tank.

Branchiomycosis Learning Resources
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Test your knowledge using flashcard type questions
Ornamental Fish Q&A 09


Noga, E. (2010) Fish Disease: Diagnosis and Treatment Elsevier Health Sciences

Roberts, H. (2009) Fundamentals of Ornamental Fish Health John Wiley and Sons

Lewbart, G. (1998) Self-assessment colour review of Ornamental Fish Manson Publishing

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