Koi Gill Biopsy: How to Sample for Gill Parasites

By KoiQuanta Editorial Team|Updated Mar 2026

Gill flukes are often missed on skin scrapes -- gill sampling is the gold standard for gill parasite diagnosis. This is a consistent finding in koi health work: hobbyists run skin scrapes, find nothing significant, and conclude the fish is parasite-free -- while the fish continues to show gill-related symptoms like rapid breathing, surface congregation, and flared operculae. The gill population of Dactylogyrus and other gill parasites requires gill biopsy to detect reliably.

If your koi shows gill signs and your skin scrape was negative or low-level, a gill biopsy is the appropriate next diagnostic step.

TL;DR

Adequate sedation -- the fish must not be moving during the procedure 2.
Minimal tissue removal -- clip only 3-5 secondary lamellae from the end of one filament, never cut deeply into the gill arch 3.
Swift procedure -- the fish should not be sedated longer than necessary 4.
It should begin recovering from sedation within 1-5 minutes.
Using clean small scissors, clip 3-5 secondary lamellae from the tip of one gill filament on the outermost arch.
Transfer the small tissue sample to a drop of pond water on a glass slide, add a coverslip, and examine immediately at 40-400x.
Removing 3-5 secondary lamellae from the tip of one filament causes minimal bleeding (which stops quickly) and heals rapidly.

Why Gill Biopsy Differs from Skin Scraping

The gill lamellae are anatomically separated from the external skin surface. Gyrodactylus (skin flukes) can infest both skin and gills, but Dactylogyrus (gill flukes) preferentially inhabit the gill lamellae and are rarely found in sufficient numbers on skin scrapes to provide an accurate assessment of gill parasite burden.

Beyond parasites, gill biopsies can reveal:

Gill damage from bacterial gill disease (bacterial filament fusion, club-shaped lamellar tips)
KHV-related gill necrosis (pale, friable, necrotic gill tissue)
Hyperplasia (thickening of gill tissue from chronic irritation)
Fungal gill disease
Gill flukes including larval stages not detectable in skin scrapes

KoiQuanta's diagnostic records include gill biopsy results linked to subsequent treatment choice, creating a record of what you found, what you treated, and what the outcome was.

Is a Gill Biopsy Safe for Koi?

Yes, when done correctly with proper sedation and minimal tissue removal. Gill tissue is highly vascular and will bleed if cut, but a proper gill clip for sampling removes only a small number of secondary lamellae from one filament, causing minimal bleeding and rapid healing.

The key safety requirements:

Adequate sedation -- the fish must not be moving during the procedure
Minimal tissue removal -- clip only 3-5 secondary lamellae from the end of one filament, never cut deeply into the gill arch
Swift procedure -- the fish should not be sedated longer than necessary
Clean instruments -- use clean scissors or tissue scissors, not contaminated equipment

Fish handled gently with proper sedation and a brief, clean gill biopsy recover quickly and show no lasting effects from the procedure.

Equipment Required

Clove oil or commercial anesthetic at appropriate sedation concentration
Small curved scissors (fine tissue scissors or small embroidery scissors work well)
Glass slides and coverslips
Pond water for the slide sample
Container for brief sedation
Bowl of clean, aerated water for recovery
Microscope (100-400x)

The Gill Biopsy Technique

Step 1: Sedate the fish. Add clove oil to a small container of pond water at 30-50 mg/L. Place the fish in the sedation solution and wait until it stops actively swimming and loses balance. This typically takes 1-3 minutes. The fish should be sedated but still breathing.

Step 2: Lift the operculum. Gently lift the gill cover (operculum) on one side to expose the gill arches. You'll see red, feathery gill tissue arranged in arches.

Step 3: Select a sampling site. Look at the outermost gill arch (the first one visible). At the distal (tip) end of one gill filament, you'll see the secondary lamellae -- tiny projections along the filament.

Step 4: Take the sample. Using clean small scissors, clip approximately 3-5 secondary lamellae from the tip of one filament. The sample will be very small -- a few millimeters of tissue. You should see a minimal amount of blood, which will stop quickly.

Step 5: Prepare the slide. Place a drop of pond water on a clean slide. Transfer the gill tissue clipping to the water drop. Use the scissors or a slide to gently tease the tissue apart slightly, then add a coverslip.

Step 6: Return the fish to recovery. Place the fish in a clean, well-aerated bowl of pond water. It should begin recovering from sedation within 1-5 minutes.

Step 7: Examine immediately. Fresh gill tissue with live organisms is much easier to examine than tissue that has dried or been sitting. Start with 40-100x magnification.

What to Look For in a Gill Sample

Dactylogyrus (gill flukes): Elongated flatworms, 0.3-1mm long, with four characteristic eyespots visible near the head end. Distinguished from Gyrodactylus by these eyespots and by the presence of eggs rather than embryos. They'll often be visible moving actively through the slide.

Gyrodactylus on gills: Same appearance as in skin scrapes -- oval flatworms with hooks and embryos, no eyespots.

Trichodina on gills: Same saucer-shaped appearance with denticle ring as in skin scrapes.

KHV damage: Look for pale, necrotic gill tissue, fusion of lamellae, and cellular debris. This is a histopathology finding that suggests viral damage and warrants PCR testing for confirmation. Healthy gill tissue should have regular, distinct lamellae with good spacing.

Bacterial gill disease: Lamellar club formation (the tips of lamellae appear club-shaped or swollen), excessive mucus, and lamellae fusing together. This differs from the mechanical damage pattern seen in parasite-heavy gill infestation.

Hyperplasia: Thickened, irregular-looking lamellae from chronic irritation. Not a specific pathogen sign but indicates ongoing gill stress.

Quantifying the Gill Fluke Burden

As with skin scrapes, you're looking for density:

1-5 flukes per field (100x): Low burden, may not be clinically significant
5-20 flukes per field: Moderate burden, treatment warranted if fish shows symptoms
20+ flukes per field: Heavy infestation, treatment required

For the skin scraping procedure that typically complements gill biopsies, see the koi skin scrape guide. For treatment of the gill flukes and other parasites you find, the koi flukes treatment guide covers the complete treatment protocol.

Frequently Asked Questions

How do I take a koi gill sample?

Sedate the fish adequately with clove oil (30-50 mg/L). Lift the operculum to expose the gill arches. Using clean small scissors, clip 3-5 secondary lamellae from the tip of one gill filament on the outermost arch. Transfer the small tissue sample to a drop of pond water on a glass slide, add a coverslip, and examine immediately at 40-400x. Return the fish to a recovery container immediately after sampling. The biopsy is safe when done correctly with minimal tissue removal and adequate sedation.

Is a gill biopsy safe for koi?

Yes, with proper sedation and minimal tissue removal. Removing 3-5 secondary lamellae from the tip of one filament causes minimal bleeding (which stops quickly) and heals rapidly. The fish experiences no lasting effects from a correctly performed gill biopsy. The risks come from inadequate sedation (a moving fish during the procedure causes more trauma) and removing too much tissue (cutting into the gill arch itself rather than just the filament tips). Done correctly with clove oil sedation and small scissors, gill biopsies are a routine diagnostic procedure.

What parasites does a gill biopsy reveal?

Gill biopsies are the reliable diagnostic method for Dactylogyrus (gill flukes), which are often missed or underestimated in skin scrapes. They also reveal Gyrodactylus (which can infest gills as well as skin), trichodina on gill tissue, and signs of KHV gill necrosis, bacterial gill disease, and lamellar hyperplasia from chronic irritation. If your fish shows gill-related signs (rapid breathing, flared operculae, surface congregation) and your skin scrape was negative, a gill biopsy is the appropriate next step before concluding the fish is parasite-free.

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association