As with every organism fishes do experience diseases and can play hosts for pathogens (some fish could be considered pathogens e.g. Vandellia cirrhosa (candiru catfish)). For the fishes and the fishkeeper it is an arms race of immune systems and treatments against the adaptability and damage to the host. This can be discussed as part of the red queen hypothesis (Van Valen, 1973), as a never ending feedback cycle, without the constant adaptation a species will go extinct.

I, the editor will clearly state that I am not a fish pathologist and this article, I am an evolutionary biologist/ichthyologist although this has given me an understanding into aspects of evolutionary pathology. Having worked in the aquarium trade I have additionally experienced a wide range of different pathogens. I will also state decisions regarding discussion the diseases as a result because I feel many fishkeepers have misunderstood fish pathology, there aren’t many hobbyists with that in their background I assume is the reason why.

Right now lets look a bit into pathogens.

Host specificity

While it is quite common knowledge that pathogens within humans or mammals can be very specific this is the same for fishes. There will be several reasons for this:

• The evolutionary mechanisms to actually become a host require a variety of adaptations and some might be very specific. For example for viruses and bacteria they will have to have the correct proteins to enter cells (Cohen, 2016; A great paper introducing how viruses function). Pathogens might require the presence of specific tissues, this could be why there have been no proven cases of Lymphocytis in catfishes (siluriformes), salmonids and carp as these clades are very close and possibly lack the effected tissues.
• Simply the environment might not be suitable for transmission between individuals, maybe the salinity is too high or the temperature is out of range.

The easiest way to assume is to look half at evolutionary distance, the more distant species are from each other the less likely transmission will occur. Additionally the pathogens of marine fishes are less likely to be able to survive and cause disease in freshwater and vice versa.

The fishes weaponry against pathogens.

This is very difficult to generalise given how many fishes there are and there is a lot of convergence with other organisms.

• The first barrier, the skin, the scales and slime coat in general prevent or reduce access of pathogens to the internal environment where they can cause damage.
• The slime coat, other then a physical barrier the skin contains many goblet cells which secretes many antimicrobial compounds such as proteases and lysozymes (Dash et al., 2018). These destroy and damage microbial pathogens and even parasitic pathogens. There is also the potential that the slime coat contains many beneficial microbes who help aid in that defence against any that become pathogenic. There is the possibility that fishes shed their slime coat to dislodge pathogens on their skin/scales or in the slime coat but I cannot find any evidence in the literature.
• The immune system, this part of biology is probably the most complex and it is very species specific. So rather then me explain it very poorly I will link a very good, recent review paper on the topic https://www.mdpi.com/2410-3888/8/2/93

The latest science

There is a lot of research as of recent looking into how a microbial ecosystem in the gut and over the surfaces of fishes aid in protection against disease (Chu et al., 2014; D’Alvise et al., 2013). This can be influenced by the environment where poor water quality can change the balance, where a bacteria or fungus previously managed by the immune system and other pathogens can cause disease (Bentzon-Tilla et al., 2016). A major cause of tipping this balance is antibiotics and therefore, antibiotic resistance, this also limits ability to use antibiotics for more concerning bacteria (de Bruijn et al., 2018).

The balance of treatment

Regardless of what treatment I cannot think of one which doesn’t have a biological effect. In my first pharmacology lecture during my undergraduate degree the lecturer stated all treatments should be understood to not just effect the target tissue, they will travel throughout the body where they can. We are very limited within the aquarium trade of what we can use and it should be understood that there might be undesired long term consequences of any treatment e.g. formalin and formaldehyde are carcinogens yet very effective on protozoa. Antibiotic resistance is a real risk with their use hence strict UK regulations.

So this brings me onto my final point:

To diagnose or not to diagnose

Fishes can host many pathogens, some more common then others. They are extremely diverse and there are many more species then vets that do 5 years training normally handle, like 10-100x more species. A fish pathologist might do 3-4 years undergraduate training, 1-2 years masters training and maybe 3-6 years PhD training, just to understand maybe a set of bacteria, fungi, parasites etc. Or what most commonly hosts a particular taxa of fishes. It’s not a simple topic.

Even with identifying fishes I see the most common species misidentified, let alone pathogens of which many need to be seen under the microscope. That access to the microscope is half of the issues, most fishkeepers outside the koi world don’t own one. And I dare say if many did, can they identify what they are seeing? And it’s great if you can but then most bacteria, viruses can’t be seen using that method. For fishes it’s not so simple to do a biopsy. What makes me really think about this topic is the misdiagnosis of Epistylis, the literature diagnoses it very differently to the hobby (Valladao et al., 2015; Wang et al., 2017; Wu et al., 2021; Ksepka & Bullard, 2021) and I am not entirely sure where the myth that Epistylis can be confused with white spot started but it’s rather frustrating. From all the literature I cited there it can’t be really diagnosed using the naked eye. I am not a pathologist but I think maybe this is more a story of fact checking because a quick read of the literature tells another story.

There are a lot more diseases and pathogens then we even know as a hobby and sometimes they can appear in a wide diversity of forms e.g. Dermocystidium. There isn’t going to be a set 5-10 most common pathogens otherwise you could say other vertebrates you only need to know those few. I prefer to back off from fish pathology because honestly how easy is it to misdiagnose? Swim bladder disorders vs neurological disorders vs malnutrition. And with so little fact checking how much can be trust? There are great fish pathologists out there and a few fish specialist vets.

The other problem with diagnosis is, how much does the science even know about the fishes we keep? Individual species in our hobby often don’t hold the largest economic weight to warrant scientific study so I believe there is a lot we don’t know. Other then pathogens malnutrition is something that hasn’t particularly been looked at in tropical ornamental fishes.

Why does correct diagnosis matter?

If you want to use the correct treatment being the main reason you need to know what you are dealing with, without using one compound after another crossing them out as you go without thinking about the physiological impact on the fish. Treatment might need to be quick or need long multiple rounds of treatment. Thankfully there might be some overlap in some. For some diseases such as neurological or related to nutrition the cause or point might never be reached.

What do you recommend?

I don’t know, contacting a vet specialised in fishes is a great idea, if there is a laboratory or pathology service this can be another option.

But maybe the most important thing is prevention, great water quality so water changes! A diverse diet regarding ingredients that caters for what the fish feeds on in the wild.

I think discussing exposure to pathogens is another topic but….. there are definitely many pathogens that risks should be taken against.

References:

Bentzon‐Tilia, M., Sonnenschein, E. C., & Gram, L. (2016). Monitoring and managing microbes in aquaculture–Towards a sustainable industry. Microbial biotechnology, 9(5), 576-584.

de Bruijn, I., Liu, Y., Wiegertjes, G. F., & Raaijmakers, J. M. (2018). Exploring fish microbial communities to mitigate emerging diseases in aquaculture. FEMS Microbiology Ecology, 94(1), fix161.

Chu, W., Zhou, S., Zhu, W., & Zhuang, X. (2014). Quorum quenching bacteria Bacillus sp. QSI-1 protect zebrafish (Danio rerio) from Aeromonas hydrophila infection. Scientific reports, 4(1), 5446.

Cohen, F. S. (2016). How viruses invade cells. Biophysical journal, 110(5), 1028-1032.

D’Alvise, P. W., Lillebø, S., Wergeland, H. I., Gram, L., & Bergh, Ø. (2013). Protection of cod larvae from vibriosis by Phaeobacter spp.: a comparison of strains and introduction times. Aquaculture, 384, 82-86.

Dash, S., Das, S. K., Samal, J., & Thatoi, H. N. (2018). Epidermal mucus, a major determinant in fish health: a review. Iranian journal of veterinary research, 19(2), 72.

Diwan, A. D., Harke, S. N., & Panche, A. N. (2023). Host-microbiome interaction in fish and shellfish: An overview. Fish and Shellfish Immunology Reports, 100091.

Ksepka, S. P., & Bullard, S. A. (2021). Morphology, phylogenetics and pathology of “red sore disease”(coinfection by Epistylis cf. wuhanensis and Aeromonas hydrophila) on sportfishes from reservoirs in the South‐Eastern United States. Journal of Fish Diseases, 44(5), 541-551

Mokhtar, D. M., Zaccone, G., Alesci, A., Kuciel, M., Hussein, M. T., & Sayed, R. K. (2023). Main components of fish immunity: An overview of the fish immune system. Fishes, 8(2), 93.

Valladao, G. M. R., Levy-Pereira, N., Viadanna, P. H. D. O., Gallani, S. U., Farias, T. H. V., & Pilarski, F. (2015). Haematology and histopathology of Nile tilapia parasitised by Epistylis sp., an emerging pathogen in South America. Bulletin of the European Association of Fish Pathologists, 35(1), 14-20.

Van Valen, L. (1973). A New Evolutionary Law. Evolutionary Theory, 1(1):1-30.

Wang, Z., Zhou, T., Guo, Q., & Gu, Z. (2017). Description of a new freshwater ciliate Epistylis wuhanensis n. sp.(Ciliophora, Peritrichia) from China, with a focus on phylogenetic relationships within family Epistylididae. Journal of Eukaryotic Microbiology, 64(3), 394-406.

Wu, T., Li, Y., Zhang, T., Hou, J., Mu, C., Warren, A., & Lu, B. (2021). Morphology and molecular phylogeny of three Epistylis species found in freshwater habitats in China, including the description of E. foissneri n. sp.(Ciliophora, Peritrichia). European Journal of Protistology, 78, 125767.