Internal parasites
Microsporidia, myxozoans and internal cysts
These are distinct groups of parasites. Tissue nodules or cysts may need laboratory examination to identify them.
Why ordinary checks may miss them
A negative external mucus sample cannot rule out organisms in internal tissues. An unexplained lump is not enough to choose a treatment.
Get a diagnosis
Ask a fish vet whether imaging, tissue examination or laboratory tests are appropriate. Do not open or squeeze cysts.
Keep records
Record growth, appetite, location of swellings and previous fish losses. Provide the examiner with the fish’s purchase and housing history.
Sources & further reading
Original WakeyKoi wording, informed by the references below. Linked clinical resources explain the evidence and their scope; they have not reviewed or endorsed this website.
Related guides
Educational guidance cannot diagnose an individual fish. Breathing distress, severe injury or unexplained losses need prompt professional assessment.