Health Does Not Mean Sterility

“By the toll of a billion deaths, man had earned his immunity —
his right to survive among this planet’s infinite organisms.”

War of the Worlds (2005), final monologue.

Presentation on this topic

Показания и ограничения к применению лекарственных препаратов в аквариумных системах

An organism is often called healthy when nothing “bad” is found in it: parasites, bacteria, fungi, injuries, or deviations from an assumed norm. Yet a living organism does not exist separately from its environment. Its body surface, gills, intestine, and even internal tissues constantly interact with other organisms. Absolute sterility is therefore not a natural state for an animal (Gomez et al., 2013).

Health is not the absence of all foreign organisms. It is the ability to preserve function, maintain internal balance, and compensate for environmental effects without a pronounced loss of condition.

An organism may be infected without being diseased.

The presence of a pathogen is not the same as disease

Detecting a parasite or bacterium answers only one question: the organism is present in the examined material. It does not yet show:

whether it is causing the observed changes;
how numerous it is;
whether the host can control its effects;
whether it is the main cause of the disorder;
whether immediate treatment is required.

A single ciliate on the surface of a fish and tens of thousands of ciliates on its skin and gills are biologically very different situations, even though the name of the finding is the same.

In natural environments, fish regularly carry small numbers of external parasites. Detecting them does not mean that every such fish is diseased. Otherwise, a substantial proportion of wild animals would have to show severe clinical signs continuously.

The boundary between carriage and disease is determined not by the mere presence of an agent, but by the consequences of its presence.

Disease begins not when a parasite appears, but when its effects exceed the host’s capacity to cope.

Immunity controls, but does not necessarily eliminate

The immune system is often imagined as an army whose task is to find and completely destroy everything foreign. Yet continuous full mobilisation against every microorganism would be too costly and too dangerous for the host itself.

A defensive response requires energy, alters circulation, increases mucus production, recruits inflammatory cells, and may damage the animal’s own tissues. The organism therefore does not have to respond with equal intensity to a single parasite and to a massive infection.

In many cases, the outcome of an immune response is not sterility but controlled coexistence (Luo et al., 2007; Bai et al., 2008; Yin et al., 2015):

parasite numbers remain low;
tissue damage is limited;
organ function is preserved;
clinical signs are absent or minimal.

This is not a failure of immunity. On the contrary, maintaining balance may be more advantageous than a destructive attempt to eliminate every last cell.

A defensive response also has a cost

Mucus protects the skin and gills of fish. However, excessive mucus production may also impair gas exchange and retain parasites, bacteria, and cellular debris. Inflammation limits infection, but marked oedema and cellular infiltration damage tissues. Sloughing of epithelium helps remove an affected layer but disrupts the integrity of the body surface (Dash et al., 2018).

The intensity of a defensive response therefore does not always directly reflect the number of pathogens. In a susceptible species, even a relatively low parasite burden may cause marked skin cloudiness, increased respiratory effort, and behavioural changes. Another fish carrying the same number of parasites may remain almost normal in appearance (Yin et al., 2018).

The severity of the condition is therefore determined not only by the aggressiveness of the parasite, but also by the way in which the host responds.

Sometimes an animal is harmed not only by the pathogen, but also by an excessive attempt to defend itself against it.

An aquarium can turn presence into disease

In nature, a parasite that leaves a fish enters an enormous volume of water. Its next stage may fail to encounter a suitable host, be carried away by currents, die, or settle far from the fish.

In an aquarium, the situation is different:

the volume of water is limited;
animal density is high;
the same surfaces are used repeatedly;
detached stages accumulate near the host;
water movement returns them to the zone occupied by the skin and gills of fish;
the same fish encounters the offspring of parasites that were recently living on it.

A parasite that existed in nature as a low-intensity infection may, in a closed system, repeatedly complete its cycle on the same animals.

Disease in an aquarium is therefore often associated not with the arrival of an entirely new pathogen, but with changes in space, density, and the probability of repeated contact.

An aquarium does not merely contain a fish and a parasite. It changes the probability that they will meet (How et al., 2015).

The absence of signs does not prove the absence of a parasite

After treatment, a fish may look clean, breathe normally, and resume feeding. This is an important clinical result. But it does not prove that no viable stage remains on the fish or in the system. Acquired protection can sharply reduce the intensity of subsequent infection and its clinical consequences (Luo et al., 2007; Yin et al., 2015).

We see disease only after a certain threshold has been exceeded. While parasite numbers remain low, they may:

cause no visible clouding of the skin;
cause no marked hypersecretion of mucus;
be detected only inconsistently in a limited skin scrape;
persist in folds, beneath scales, in the gills, or in other difficult-to-sample locations;
pass between fish at an intensity below the clinical threshold.

The disappearance of symptoms therefore means that the animal’s condition has improved, but not necessarily that the agent has been completely eliminated.

Below the detection threshold does not mean equal to zero.

Not every finding must be destroyed immediately

When a ciliate, bacterium, or fungus is found in a skin scrape, the natural reaction is to add a medication immediately. But the decision should not depend only on the name of the organism that was found.

The following must be considered:

the condition of the fish;
infection intensity;
lesion location;
progression over time;
species susceptibility;
water quality;
the possibility of direct treatment;
the risk posed by the treatment itself;
consequences for the whole system.

A single finding in a fish behaving normally and a massive infection in an animal that is breathing heavily and has stopped feeding should not automatically lead to the same treatment protocol (Yin et al., 2018).

A medication is required not because other life is present in the system, but because further damage to the animal is expected without intervention.

A wound in water remains a wound — but water does not make it a catastrophe

Damage to a fish’s skin remains in constant contact with water. To a human observer this seems unusual: we try to protect our own wounds from contamination and prolonged wetting. As a result, the mere appearance of a wound under water may be perceived as an emergency.

Fish, however, are evolutionarily adapted to this environment. A small, clean wound may close on its own under appropriate conditions. Not every lesion requires an antibiotic, an antiseptic applied to the entire system, or a prolonged course of medication.

First, it is necessary to determine:

whether tissue destruction is continuing;
whether necrosis is present;
whether inflammation is increasing;
whether fungal or bacterial growth has appeared;
whether the animal’s general condition is deteriorating;
whether the lesion can be cleaned directly.

If pathological material is accessible, it can be removed. If a wound is clean and healing, normal water quality and observation may sometimes be the best course of action.

Treat the loss of balance

The existence of parasites, bacteria, and fungi is not a mistake of nature. Hosts and the organisms around them evolved through constant interaction. Immunity, body surfaces, mucus, behaviour, and tissue repair all developed in a world that was never sterile.

The aim of treatment should therefore not be formulated as the destruction of all other life around the animal.

The aim is to return the system to a state in which:

the fish remains viable;
damage is not progressing;
the parasitic or bacterial burden is controlled;
repeated infection is limited;
treatment does not cause more harm than the original problem.

Treatment should target not the presence of other life, but the loss of balance that makes this presence dangerous.

References

Bai J. S., Xie M. Q., Zhu X. Q., Dan X. M., Li A. X. 2008. Comparative studies on the immunogenicity of theronts, tomonts and trophonts of Cryptocaryon irritans in grouper. Parasitology Research, 102(2): 307–313. https://doi.org/10.1007/s00436-007-0766-6
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): 72–81. PMID: 30046316; PMCID: PMC6056142.
Gomez D., Sunyer J. O., Salinas I. 2013. The mucosal immune system of fish: the evolution of tolerating commensals while fighting pathogens. Fish & Shellfish Immunology, 35(6): 1729–1739. https://doi.org/10.1016/j.fsi.2013.09.032
How K. H., Zenke K., Yoshinaga T. 2015. Dynamics and distribution properties of theronts of the parasitic ciliate Cryptocaryon irritans. Aquaculture, 438: 170–175. https://doi.org/10.1016/j.aquaculture.2014.12.013
Luo X. C., Xie M. Q., Zhu X. Q., Li A. X. 2007. Protective immunity in grouper (Epinephelus coioides) following exposure to or injection with Cryptocaryon irritans. Fish & Shellfish Immunology, 22(4): 427–432. https://doi.org/10.1016/j.fsi.2006.04.011
Yin F., Sun P., Tang B., Dan X., Li A. 2015. Immunological, ionic and biochemical responses in blood serum of the marine fish Trachinotus ovatus to poly-infection by Cryptocaryon irritans. Experimental Parasitology, 154: 113–117. https://doi.org/10.1016/j.exppara.2015.04.010
Yin F., Liu W., Bao P., Jin S., Qian D., Wang J., Tang B. 2018. Comparison of the susceptibility and resistance of four marine perciform fishes to Cryptocaryon irritans infection. Fish & Shellfish Immunology, 77: 298–303. https://doi.org/10.1016/j.fsi.2018.03.052

Health Does Not Mean Sterility

The presence of a pathogen is not the same as disease

Immunity controls, but does not necessarily eliminate

A defensive response also has a cost

An aquarium can turn presence into disease

The absence of signs does not prove the absence of a parasite

Not every finding must be destroyed immediately

A wound in water remains a wound — but water does not make it a catastrophe

Treat the loss of balance

References

See also