HUADE INSIGHTS | Is Fish Disease Becoming Harder to Prevent? The Real First Line of Defense May be Lost Not in the Gills, but in Natural Immunity

Is Fish Disease Becoming Harder to Prevent? The Real First Line of Defense May Be Lost Not in the Gills, but in Natural Immunity

Reference

Diamond G., Beckloff N., Weinberg A., et al. β-Defensins: Farming the Microbiome for Homeostasis and Health. Frontiers in Immunology, 2019; 10:326.

Statement

This article is a translated and condensed summary based on the above review article. The content has been reorganized for scientific communication and educational purposes and does not represent the complete views of the original publication. Readers are encouraged to consult the original article for comprehensive information.

For many aquaculture farmers, the greatest concern is not slow growth, but the sudden outbreak of disease in their fish.

This is especially true during high-temperature seasons, after pond seining and restocking, or following abrupt weather changes—when many fish exhibit reduced feed intake, edge-swimming, and surface gasping, eventually followed by successive bacterial infections.

The immediate reaction for many is:

- Is there something wrong with the water quality?

- Is the pathogen load too high?

Consequently, they proceed with sediment conditioning, disinfection, sterilization, water exchange…

These measures are certainly important, yet many farmers have observed a recurring phenomenon: 

Under identical pond conditions and with the same management practices, some fish consistently show significantly lower disease incidence, while others suffer from recurrent outbreaks. If pathogens and water quality were the only contributing factors, how could this discrepancy be explained? In recent years, a growing body of research on teleost innate immunity has begun to focus on a critical question:  What determines whether fish are prone to disease is not merely the external environment, but also the fish’s own natural defense capacity—i.e., their innate immune competence.

Fish Live in a Microbe-Rich Environment

Unlike terrestrial animals, fish are constantly immersed in water environments teeming with microorganisms. Their skin, gills, and intestinal tract are in continuous contact with a wide variety of bacteria, fungi, and other microbes.

Yet, under most circumstances, fish do not fall ill as a result.

This suggests that healthy fish are not those that avoid pathogen exposure, but rather those that possess a fully functional, continuously operating defense system.

While reviewing the review article *“β-Defensins: Farming the Microbiome for Homeostasis and Health,”* we noted that the authors highlight that β-defensins are not only found in mammals and poultry, but have also been identified in multiple species of teleost fish, where they participate in mucosal immunity.

This indicates that fish do not rely solely on adaptive immunity to fend off diseases—they are also equipped with their own innate defense mechanisms.

The Fish Gill: More Than Just a Respiratory Organ

Many aquaculturists know that the gills are responsible for respiration. 

In reality, however, they are also one of the most important immune barriers in the fish body.

The gills are in constant contact with large volumes of water every day. While carrying out gas exchange, they simultaneously face continuous challenges from a wide array of microorganisms. 

Consequently, the mucus layer and mucosal tissues covering the gill surface inherently perform crucial defense functions.

The literature mentioned that β-defensins are extensively involved in teleost mucosal immunity, exerting their effects across sites such as the skin, gills, and intestine, where they help the host resist pathogen invasion and maintain local microbial homeostasis.

In other words, the fish gill is not only responsible for "breathing"—it is also constantly "defending."



Why Do Disease Outbreaks Occur More Easily After Stress?

Many fish diseases share a common feature: 

Under normal conditions, everything appears fine—but once seining, pond restocking, prolonged high temperatures, or heavy rainstorms occur, disease outbreaks begin to emerge.

Why is this?

Because these factors not only alter the external environment but also affect the physiological state of the fish themselves. 

When mucosal barriers are compromised and natural defense capacity declines, the originally stable microbial equilibrium may shift, allowing opportunistic pathogens to proliferate more readily.

Thus, what we observe as a disease outbreak may, in reality, reflect an underlying change in the stability of the fish's own defense system. This pattern closely parallels the stress responses seen in pigs, cattle, and poultry—revealing a fundamentally similar biological principle across species.

What Does This Mean for Aquaculture?

In recent years, aquaculture has placed increasing emphasis on the principle that "prevention is better than cure."

Beyond improving water quality, optimizing feeding practices, and strengthening biosecurity measures, a growing body of research has begun to focus on how to maintain the fish's own intrinsic health status.

From a management perspective, this means that farmers must not only manage the pond environment but also pay close attention to the fish themselves.

A healthy fish is not one that lives in a pathogen-free environment, but rather one that maintains its internal homeostasis despite continuous exposure to pathogens.

Huade Technical Insight

While reading this review, one observation struck us profoundly: 

Although pigs, cattle, poultry, and fish live in vastly different environments, they all share one common trait—each possesses its own innate defense system.

Different animals have evolved different strategies to adapt to their surroundings, yet the underlying logic of health remains remarkably consistent:  It is not about completely avoiding pathogens, but about continuously sustaining one's own defense capacity and microbial homeostasis.

This is precisely why mucosal immunity, host defense peptides, and microbiome research have continued to attract growing attention in recent years.