Hikui in koi carp: biopsy before naming the lesion

The term “Hikui” is often applied to any chronic lesion developing over a red area of a koi carp. That habit risks treating a colour pattern or surface appearance rather than establishing a diagnosis. A 2016 case series of nine koi characterised compatible lesions as perivascular wall tumours through histology, immunohistochemistry and transmission electron microscopy. The study provides a pathological basis for the term, but it does not turn every plaque, erosion or skin mass on a koi into Hikui.

What can be observed without overdiagnosing

The reported fish represented several koi varieties and were kept in ponds holding 10,000–50,000 litres. General condition, swimming and appetite were often considered normal. Lesions consisted of multifocal raised skin thickenings on the head, dorsum and lateral body, with a notable association with red-pigmented areas. Some fish also had erosion, ulceration, hyperaemia or haemorrhage.

These features guide examination but are not specific. Repeated trauma, bacterial or fungal infection, inflammation, parasites, a different neoplasm and scar tissue can create similar appearances. Normal red pigmentation must not be mistaken for inflammation, and chronicity does not prove a tumour type.

Teams should record lesion size, elevation, surface, location and progression with standardised photographs. Appetite, swimming, body condition, water quality, previous treatments and lesions in other fish complete the case record. A slow lesion in an otherwise stable animal still deserves a planned assessment rather than indefinite observation.

Why biopsy changes the level of evidence

The published diagnosis depended on microscopic tissue examination followed by complementary methods to define cell origin. The authors found a proliferation arranged around vessels and compatible with a perivascular wall tumour. This separates a clinical description—“a lesion compatible with Hikui”—from a pathological characterisation.

Biopsy should be planned with the aquatic veterinarian and diagnostic laboratory. A representative sample may need to include the transition between altered and apparently normal skin. Tissue that is too superficial, crushed, cauterised or placed in the wrong fixative can become uninterpretable. Fish size, lesion site, bleeding risk and anaesthetic requirements all influence the procedure.

The pathology report must be linked to the exact lesion sampled. Another mass on the same fish or a tank mate does not automatically share the diagnosis. When additional stains or tests are recommended, they address a defined differential diagnosis; they should not be replaced by serial empirical treatments.

Plan examination around fish welfare

A koi should not be removed from water merely to obtain a more dramatic photograph. Prepare the examination tank, appropriate net, wet support, oxygenation, measuring equipment and anaesthetic plan when an invasive procedure is intended. The body must be supported, the gills protected and air exposure kept brief.

After biopsy or excision, recovery should take place in controlled water with documented monitoring. The plan specifies return of coordinated swimming and ventilation, wound checks and criteria for rejoining the collection. A treatment or observation tank can help protect the surgical site, although prolonged social isolation and unstable water quality can create additional welfare problems.

Analgesia, haemostasis and wound management should be tailored to the procedure and fish. A topical product safe in one species or system cannot be assumed safe for koi, biofilters or other organisms. The procedure record should state the agent, dose, route, timings and recovery observations.

What nine cases cannot establish

Nine fish form a descriptive case series, not a prevalence study or therapeutic trial. The authors established the microscopic nature of the lesions examined. They did not demonstrate a single cause, contagion, a decisive effect of sunlight or a universally effective treatment. The work does not show that every red-pigmented koi faces the same risk.

The word “disease” can also be misleading here: the study describes a tumour, not a demonstrated infectious agent. There is no evidence-based reason to expose an entire system to an antiparasitic, antibiotic or disinfectant because a lesion looks like Hikui. Empirical treatment can delay diagnosis, alter tissue and unnecessarily expose animals and filtration biology.

If an ulcerated lesion is secondarily infected, that infection still needs separate evidence and management. Treating a secondary complication does not identify or remove the underlying tumour.

A practical pathway for managed collections

Five steps keep the decision traceable: measure and photograph the lesion; examine the whole fish and environment; define realistic differentials; select representative tissue; and connect the laboratory result with follow-up. Retain dates, images, previous medications, the pathology report and post-procedure progression in one record.

Biosecurity remains relevant even when the tumour itself is not considered contagious, because an open lesion can coexist with secondary infection. Care equipment should be cleaned and disinfected between systems, and medication should follow a benefit–risk assessment rather than visual guesswork.

The central message is restrained: Hikui cannot be confirmed by appearance alone. Visual examination chooses the next investigation; pathology gives the diagnosis its weight. Vetofish can support public displays, specialist ponds and professional koi collections with clinical examination, anaesthesia, biopsy, laboratory submission and interpretation.