Bacterial, Fungal & Viral Diseases in Koi
Causes, Symptoms & PreventionRoot Causes of Infection
Most bacterial, fungal, and viral outbreaks trace back to underlying factors: elevated ammonia, nitrite, or nitrate levels; accumulated organic waste; overfeeding or nutritional deficiency; physical injuries; overcrowded ponds; existing parasitic infections; and significant temperature fluctuations.
Bacterial Infections
The most common bacterial threats include Aeromonas (deep ulcers, “hole in the side disease”), Flexibacter Columnaris (fin rot, tail rot, mouth rot), Pseudomonas (open sores, reddened belly), and Mycobacteriosis (fish tuberculosis). When Aeromonas combines with Mycobacteriosis, it produces dropsy — where scales lift across the entire body like a pinecone. Symptoms include clamped fins, protruding eyes, haemorrhaging, and rigid body posture.
Fungal Infections
Saprolegnia and Branchiomyces (gill fungus) are the primary fungal threats. Fungi are always present in pond environments, thriving on organic material. They become pathogenic when koi tissue is exposed through wounds or parasitic damage. Visible white strands (hyphae) on a wound typically indicate advanced fungal penetration.
Viral Infections
Koi Herpes Virus (KHV) and Spring Viraemia of Carp (SVC) are the most dangerous viral diseases. KHV can destroy an entire collection within 7 to 14 days. Symptoms include downward head posture, lethargy, sunken eyes, necrotic gills, and sandpaper-textured skin. KHV is a reportable disease in South Africa. Carp Pox produces whitish pustules, while Lymphocystis causes abnormally large cell growth. Viral diseases have no direct cure — management focuses on isolation and supportive care.