Fish histopathology is not easy.

I came across a very good article on fish histopathology and grabbed some of the well-worded phrases from the introduction.

The “degree of expertise cannot be taught in a matter of weeks to months, nor can it be learned simply by rote through dedicated self-study… requires initial broad-based training and comparative (multi-species) pathology, which is then followed by a steady and diverse case load, continuing education, and most notably, many years spent laboring at the microscope.

The authors go on to explain that some published fish histopathology findings may be of “dubious accuracy”, and some authors “may support their findings with evidence gleaned from prior reports that are equally questionable… this iterative circular process can create a body of unsound literature that ultimately becomes scientific dogma.” Thus it is difficult to filter-out what is factual information, if you don’t already have the knowledge or expertise. Isn’t this the case for so many of the fish-related things we find on Google?

A diagnosis in the field of veterinary fish pathology this often arrived at by “an open discussion of diagnostic and interpretive issues.” It is not something you put in a machine that goes “bing” and spits out a result. Histopathology is a qualitative test that relies in the experience of the diagnostician.

Many of the lesions detected may not necessarily be able to be supported by citations “because applicable references based on unbiased empirical research or historical control data are simply not available.” So much of the information in fish pathology still resides inside the brains of those who are practicing, and haven’t had the time/luxury to publish their findings. Moreover, we can’t simply draw conclusions based on works on other species.

The challenge of fish pathology is compounded by:

  • the profound anatomic diversity of fishes,
  • misdiagnosis (when a morphologic observation is considered incorrectly to be abnormal, or when inaccurate or imprecise terminology is used to describe it particular morphologic observation) and misinterpretation (derivation of inappropriate conclusions from morphologic findings) in the literature,
  • artifactual changes due to poor sampling technique or inadequate tissue preservation (fish histology is so dependent on this, much more than any other animal species).

Veterinary fish pathologists rely heavily on consensus of opinions; and, more important than tenure, the fact that these individuals have had the opportunity to examine thousands of disease-free fish over the course of their careers (e.g. fish sacrificed animal health screens, negative-control subjects from bio-assays).

This said, histopathology has stood the test of time and is used in almost all veterinary investigations into diseases. So if you have the patience and time (say, more than 10 years), you won’t be disappointed in the puzzles it can help unlock. Otherwise, you can submit your fish to us for diagnostics.

Check out what you can see under the microscope –

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screenshot 20180409 231423

Toxicol Pathol. 2015 Apr;43(3):297-325. doi: 10.1177/0192623314540229. Epub 2014 Aug 11.

Non-lesions, misdiagnoses, missed diagnoses, and other interpretive challenges in fish histopathology studies: a guide for investigators, authors, reviewers, and readers.

Wolf JC1, Baumgartner WA2, Blazer VS3, Camus AC4, Engelhardt JA5, Fournie JW6, Frasca S Jr7, Groman DB8, Kent ML9, Khoo LH10, Law JM11, Lombardini ED12, Ruehl-Fehlert C13, Segner HE14, Smith SA15, Spitsbergen JM16, Weber K17, Wolfe MJ18.

Author information


Differentiating salient histopathologic changes from normal anatomic features or tissue artifacts can be decidedly challenging, especially for the novice fish pathologist. As a consequence, findings of questionable accuracy may be reported inadvertently, and the potential negative impacts of publishing inaccurate histopathologic interpretations are not always fully appreciated. The objectives of this article are to illustrate a number of specific morphologic findings in commonly examined fish tissues (e.g., gills, liver, kidney, and gonads) that are frequently either misdiagnosed or underdiagnosed, and to address related issues involving the interpretation of histopathologic data. To enhance the utility of this article as a guide, photomicrographs of normal and abnormal specimens are presented. General recommendations for generating and publishing results from histopathology studies are additionally provided. It is hoped that the furnished information will be a useful resource for manuscript generation, by helping authors, reviewers, and readers to critically assess fish histopathologic data.


artifacts; diagnostic accuracy; fish histopathology; misdiagnosis.; nonlesions; tissue fixation


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