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Factors Involved in the Outbreak of Disease
Fish Diseases Lesson 2 Factors Involved in the Outbreak of Disease
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Factors Involved in the Outbreak of Disease
A susceptible host A virulent pathogen Proper environmental conditions
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1. A susceptible host Good diet and clean water supply
Avoid introducing fish from another hatchery - Quarantine Host’s defense system
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Fishes’ Lines of Defense
Mechanical - Skin, Scales and Mucus Physiological - White blood cells (Phagocytosis) Liver detoxifies chemicals from water Local Tissue reactions Immune system Active against bacteria, parasites & viruses
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2. Virulent Pathogen Not all bacteria cause disease
Some strains of known fish pathogens may not always cause disease Bacteria can become more virulent by being “passed through” a fish
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3. Adverse Environment Changes in environment
Certain temps favor or hinder pathogen Facility design - tanks vs ponds Biological factors Hatcheries with well water vs surface water
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Critical Levels of Water Quality
Oxygen Nitrites Ammonia Total Alkalinity Total Hardness pH Temperature
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Critical Temperatures
Optimum varies with species Tilapia - above 21 C (70 F) Trout C (50-61 F) Catfish C Optimum C (77-86)
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Critical Temps for Pathogens
Ichthyopthirius Multifilis (Ich) Most severe C ( F) Can occur down to 5 C Seldom above 23 C ( 73) Channel Catfish Virus (CCV) occurs above 23 C Enteric Septicemia (ESC) - 23-27 C (73-81) Aeromonas C (77-86)
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Factors in Disease Development
1. Source of infection 2. Mode (method) of infection 3. Portal of entry 4. Virulence of the organism 5. Resistance of the host Break the Chain
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Source of Infection Infected fish, dead fish , sick fish, carriers
Infected fish eggs from carrier broodstock Contaminated water supply Contaminated Feed Man and his activities
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Mode of Transmission Through the water
water with all the waste products from the fish make ideal conditions for pathogens to survive
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Portal of Entry Break the Chain
Gills - parasites can pierce gills and bacteria can enter Skin - breaks in skin and mucus by parasites or handling Ingestion-Don’t feed wild, uncooked fish Break the Chain
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Virulence of Pathogen Varies by species of virus or bacteria
Different strains Pass-through effect
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Resistance of Host Different than immunity Natural Resistance
Immunity is acquired by previous exposure to the infection Natural Resistance substances in the fish Non-specific
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Factors Effecting Natural Resistance
White Blood cells Tissue integrity Nutrition Age of Animal Strain of species Temperature
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Resistance Does not mean that the animal cannot be infected, but that the host has the ability to subdue the pathogen to the point where it will not cause disease.
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