Fish 117

• Factors that cause stress to fish

  • Higher stocking densities
  • Increased feeding
  • Environmental fluctuations
  • Poor water quality
• Disease


  • Any abnormality of structure or function displayed by living organisms through a specific or non-specific sign
  • Any condition that can decrease the survivability of an organism in a natural setting
• Requirements for a disease situation to exist

  • Potential pathogen
  • Susceptible host
  • Environmental conditions that bring about either increased virulence of the pathogen or decreased resistance of the host
• Factors that influence disease development in the host

  • Species
  • Age
  • Size
  • Sex
  • Immunocompetence of host
  • Host response
  • Target tissue
• Host defense mechanisms


  • Structural (skin, scale, exoskeleton, mucus)
  • Physiological (phagocytes, avoidance mechanisms, detoxification of chemicals from water or diet by the liver, local tissue reactions)
  • Immunological (immune system and its specific activity against biological agents)
• Factors that influence disease development in the pathogen

  • Portal of entry
  • Mode of transmission
  • How it multiplies
  • Infective stages
  • Primary or secondary
• Factors that influence disease development in the environment

  • Temporal aspect (season)
  • Spatial aspect (location)
  • Factors which stress the host
  • Factors favoring the pathogen
• Classification of agents of diseases

  • Physical agent
  • Chemical agent
  • Biological agent
• Physical agents


  • Extreme temperature changes
  • Radiation (ultraviolet rays from the sun, mechanical, thermal)
• Chemical agents


  • Environmental contaminants
  • Toxins
  • Nutritional imbalances
  • Drugs and chemical overdose
• Biological agents


  • Viruses (obligate intracellular parasites)
  • Bacteria (unicellular, micrococcus, staphylococcus, streptococcus)
  • Fungi (mushroom, yeasts, heterotrophic, saprophytes)
  • Parasites (protozoans, nematodes, leech)
• Major characteristics of biological agents

  • Capability for direct transmission
  • Ability to multiply in the host tissue
• Modes of transmission for biological agents

  • Vertical (parent to offspring)
  • Horizontal
• Portals of entry for biological agents

  • Waterborne (contaminated water)
  • Airborne (migratory birds, insects, wind)
  • Overland transport route (infected seeds and broodstocks, feeds, humans, animals, vehicles, farm equipment)
• Humans are a factor in disease development because aquaculture technicians must be knowledgeable and have a great understanding of the species being cultured to prevent costly mistakes and poor yield
• Disease development process


  1. Host-pathogen-environment interaction

  2. Entry of pathogen

  3. Pathological changes in target tissue

  4. Functional failure of affected tissue

  5. Clinical manifestation (disease expression)

  6. Morbidity and Mortality

  7. Outbreak
• Signs


  Something that can be observed externally
• Symptoms


  Something that can be felt internally
• Fish cannot show symptoms, they can only show signs
• Typical signs of disease in fish

  • Lack of appetite
  • Respiratory difficulties
  • Behavioral changes
  • Weakness or lethargy
  • Flashing or scraping at surfaces
  • Physical abnormalities (lesions, warts, discoloration, slow growth rate, fin rot, opaque eye, exophthalmia, opaque muscle/tissue, swollen belly)
  • Parasites
  • Mortality
• Stress


  The sum of the physiological responses the fish makes to maintain or regain its normal balance
• Stages of response and adaptation to stress

  • Alarm stage
  • Adaptive stage
  • Exhaustion stage
• Alarm stage


  The fish attempts to escape from the problem
• Adaptive stage


  If escape is not possible, the fish's body attempts to react to the environmental change (adjust to change and reach a new equilibrium, deals with the stress as a priority). May not be able to maintain growth, reproduction, and disease immunity.
• Exhaustion stage


  The environmental change is so great that the fish cannot adapt to it and the fish dies
• Laboratory procedures


  • Presumptive identification of pathogens
  • Positive identification and confirmation
  • Test for drug sensitivity and effectiveness
  • Evaluation of recommendations and reports of additional analysis (histopathology, toxicology)
• On-site investigation


  • Examine fresh materials from healthy, moribund, and dead fish
  • Collect fish tissue samples
  • Measure environmental conditions
  • Investigate physical factors and rearing conditions
  • Gather information on time-course of mortalities
  • Deliver suitable samples with accompanying information to the diagnostic laboratory as soon as possible
• Correct diagnosis is essential in selecting the best management approach to correct the problem and the best possible treatment for the disease
• Mortality patterns


  • Acute Environmental Failure
  • Acute Infectious Disease
  • Chronic Infectious Disease
• Basic procedures in disease diagnosis

  • Recognize early or consistent signs of disease
  • Gather information on pattern of mortalities
  • Measure/record water quality conditions
  • Evaluate rearing conditions, sanitation and management practices
  • Examine fresh materials from healthy/normal, moribund and dead fish
• Levels of disease diagnosis

  • Level I (farm-site monitoring, macroscopic examination or ocular observation of diseases)
  • Level II (caused by a variety of infectious and non-infectious agents, not readily recognized by gross examination using naked eye, requires specialized training and access to necessary equipment like bacteriology, mycology, parasitology, histopathology)
  • Level III (problematic pathogens and those that are difficult or impossible to identify at Levels I and II, requires virology, electron microscopy techniques, immunology and molecular diagnostics)