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Signs & lesion interpretation

Diagnostic Signs Guide

Interpret visible, microscopic and necropsy findings by anatomy, distribution and pattern before jumping to a disease name.

50 practical guidesEvidence-first health interpretationReviewed 2026-08-14

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Describe the sign before naming the disease.

Use these pages with the disease, symptom and diagnostics libraries rather than as stand-alone diagnoses.

Signs & lesion interpretationReading gill colour during a fish-health examinationShows how pale, bright, dark, brownish or uneven gill colour can be recorded as evidence while avoiding a disease label from colour alone.

Signs & lesion interpretationReading gill mucus and surface textureExplains how mucus, roughness, debris and surface irregularity can be described before microscopy or other tests narrow the cause.

Signs & lesion interpretationReading gill-filament shape and spacingExplains what shortening, clumping, fraying or uneven filament appearance can mean as an anatomical observation rather than a diagnosis.

Signs & lesion interpretationInterpreting lamellar hyperplasia on fish gill microscopyExplains epithelial thickening on gill microscopy and why hyperplasia is a common response pattern shared by many environmental and infectious insults.

Signs & lesion interpretationInterpreting lamellar fusion on fish gill microscopyExplains how neighbouring lamellae can become bridged or fused and why the finding indicates altered architecture without identifying one unique cause.

Signs & lesion interpretationInterpreting epithelial lifting in fish gillsExplains separation of gill epithelium from supporting structures and the need to distinguish true injury from handling or fixation artefact.

Signs & lesion interpretationInterpreting gill erosion and necrosisExplains tissue loss and cell death as serious but non-specific lesions requiring context from water quality, distribution and laboratory evidence.

Signs & lesion interpretationInterpreting ionocyte or chloride-cell hyperplasiaExplains an increase in specialised ion-transport cells and why salinity, acid-base stress and other gill insults may contribute.

Signs & lesion interpretationWhen one gill looks different from the otherUses left-right asymmetry to separate focal obstruction, trauma or local lesions from processes more likely to affect both sides.

Signs & lesion interpretationSeparating true gill lesions from post-mortem changeExplains how rapid tissue deterioration after death can alter colour and microscopic appearance and why fresh sampling matters.

Signs & lesion interpretationReading skin mucus during examinationExplains how excess, patchy or reduced mucus is documented and how skin scrapes and the wider clinical picture help interpret it.

Signs & lesion interpretationDistinguishing scale loss from scale liftingExplains the anatomical difference between a scale being absent and a scale being pushed outward by changes in the tissues beneath it.

Signs & lesion interpretationReading redness around scale pocketsExplains how focal or diffuse erythema around scale bases can be mapped and why trauma, inflammation and systemic processes can look similar.

Signs & lesion interpretationDistinguishing superficial erosion from a deeper skin ulcerExplains how lesion depth, margins and exposed tissue help describe skin loss before a cause is assigned.

Signs & lesion interpretationDistinguishing fin-ray damage from fin-margin erosionExplains how a broken supporting ray differs from progressive soft-tissue loss between rays, improving injury and disease descriptions.

Signs & lesion interpretationReading redness at the base of a finExplains how focal fin-base redness is localised and compared with whole-body vascular changes, trauma and nearby skin lesions.

Signs & lesion interpretationInterpreting darkening and pigment lossExplains why stress, background response, nerve injury, inflammation and systemic illness can alter pigmentation without one colour change being diagnostic.

Signs & lesion interpretationInterpreting white material on the fish surfaceSeparates mucus, sloughed tissue, attached debris and true growths by location, texture and microscopy rather than assuming every white patch is fungus.

Signs & lesion interpretationInterpreting skin nodules by depth and tissue layerExplains how surface, intradermal and deeper masses are described differently and why depth can reshape the differential list.

Signs & lesion interpretationInterpreting pits and erosions along the lateral-line regionExplains how to map lesions around sensory canals and pores while considering nutrition, water, trauma and infectious differentials.

Signs & lesion interpretationDistinguishing corneal clouding from lens opacityExplains how to decide whether cloudiness lies on the eye surface or deeper within the lens, which leads to very different differential pathways.

Signs & lesion interpretationInterpreting one swollen eye versus both eyesUses unilateral versus bilateral distribution to separate local orbital injury or disease from broader systemic and environmental possibilities.

Signs & lesion interpretationReading eye redness by anatomical layerExplains how surface vessels, the anterior chamber and deeper ocular tissues can produce different red appearances that should not be merged into one sign.

Signs & lesion interpretationInterpreting mouth plaques, erosions and surface lesionsExplains how oral lesions are mapped to lips, jaw margins, oral mucosa or deeper tissues before infection, trauma or neoplasia are considered.

Signs & lesion interpretationInterpreting jaw swelling and asymmetryUses location, firmness, feeding ability and left-right comparison to organise traumatic, inflammatory, skeletal and mass-like differentials.

Signs & lesion interpretationReading abdominal contour and symmetryExplains how dorsal and side views help separate general body condition, symmetrical distension and one-sided enlargement.

Signs & lesion interpretationDistinguishing free abdominal fluid from organ enlargementExplains why a rounded abdomen can reflect fluid, gonad, intestine, liver or another mass and what imaging or veterinary examination can add.

Signs & lesion interpretationInterpreting liver size, edge and surface at necropsyExplains how enlargement, rounded edges, surface irregularity and colour are recorded together instead of using one gross feature as a diagnosis.

Signs & lesion interpretationInterpreting mottling and focal changes in the fish liverExplains how focal versus diffuse patterns and lesion distribution help separate local masses, vascular change, inflammation and artefact.

Signs & lesion interpretationInterpreting spleen size and colour at necropsyExplains why splenic enlargement, pallor or darkening must be interpreted with blood status, infection, congestion and species variation.

Signs & lesion interpretationInterpreting kidney enlargement along the dorsal body wallExplains how the kidney is located and assessed so renal tissue is not confused with adjacent gonad, fat, pigment or other structures.

Signs & lesion interpretationInterpreting thickened intestinal walls at necropsyExplains how focal versus diffuse wall thickening and associated contents or fluid guide inflammatory, parasitic, neoplastic and obstructive differentials.

Signs & lesion interpretationInterpreting an unexpectedly empty intestineExplains why fasting, anorexia, obstruction, recent feeding and post-mortem interval all matter before an empty gut is treated as a disease sign.

Signs & lesion interpretationInterpreting intestinal gas and distensionExplains how gas, fluid and digesta can enlarge bowel loops and why gross distension alone does not prove a swim-bladder or infectious problem.

Signs & lesion interpretationInterpreting gallbladder distension and bile appearanceExplains how feeding state and post-mortem timing can influence gallbladder fullness and why bile colour by itself is weak diagnostic evidence.

Signs & lesion interpretationInterpreting swim-bladder collapse, overexpansion and asymmetryExplains how gross shape is described and why buoyancy signs can arise from anatomy outside the swim bladder as well as from the organ itself.

Signs & lesion interpretationInterpreting fluid around the fish heartExplains how pericardial fluid is recorded and why inflammation, circulatory failure, trauma and post-mortem change require different supporting evidence.

Signs & lesion interpretationInterpreting heart size and shape at necropsyExplains why chamber proportions, species, body size and post-mortem handling matter before apparent enlargement is considered abnormal.

Signs & lesion interpretationInterpreting muscle pallor, bruising and haemorrhageExplains how focal trauma, systemic bleeding, degeneration and post-mortem change can alter body-wall muscle appearance.

Signs & lesion interpretationInterpreting spinal curvature by pattern and locationExplains how smooth lifelong curvature differs from abrupt angulation or progressive deformity and how pattern helps organise developmental, nutritional, traumatic and infectious possibilities.

Signs & lesion interpretationSeparating post-mortem stiffness from abnormal rigidityExplains how rigor and cooling after death can create stiffness that must not be confused with the posture or muscle tone observed while a fish was alive.

Signs & lesion interpretationInterpreting gross changes around the brain and meningesExplains the limits of gross brain examination and why swelling, fluid, haemorrhage or discoloration generally require histology or laboratory work to interpret.

Signs & lesion interpretationInterpreting melanomacrophage pigment in kidney and spleenExplains dark pigment aggregates in haematopoietic tissues and why age, species, prior inflammation and tissue turnover influence their prominence.

Signs & lesion interpretationInterpreting gonad size and left-right asymmetryExplains how reproductive stage, sex and species affect gonad size and why marked asymmetry should be recorded before tumour or retention is assumed.

Signs & lesion interpretationReading ovary colour and texture at necropsyExplains how oocyte stages, atresia, inflammation and post-mortem change can alter ovarian appearance and why histology is often needed.

Signs & lesion interpretationReading testis size, colour and texture at necropsyExplains how season, maturity and tissue change affect testes and why gross appearance alone cannot distinguish inflammation, degeneration and neoplasia.

Signs & lesion interpretationRecognising autolysis in fish tissuesExplains rapid self-digestion after death and the common ways autolysis can mimic epithelial sloughing, softening, colour change and loss of cellular detail.

Signs & lesion interpretationRecognising sampling and handling artefacts in fish tissuesExplains how crushing, scraping, delayed fixation, freezing and poor section orientation can create lesions that were not present in the living fish.

Signs & lesion interpretationUsing symmetry as a diagnostic clue in fishExplains why paired eyes, gills, fins and body sides provide an internal control: unilateral findings often point toward a different process than bilateral change.

Signs & lesion interpretationBuilding an anatomical lesion map before naming a diseaseShows how to record every affected organ, tissue, side and body region first, then use that distribution to choose tests and disease differentials.