Aquaculture Disease Processes

1. Aquaculture Disease Processes Dr. Craig Kasper FAS 2253/FAS 2253L

2. Lecture 1: Introduction to Disease • What is disease? • Types of diseases • Dynamics of infectious disease • Epizootiology of infectious diseases • What you have to do to be a disease agent • Disease reservoirs • Transmission • The host • Stages in an epizootic

3. What is Disease? • Definition: any alteration of the body or one of its organs so as to disturb normal physiological function • opposite of health = unhealthy or dysfunctional

4. Why are diseases of such concern in aquaculture? • 1990: WSSV, a virus, devastates shrimp culture in China, $600 million lost • 1971: Flexibacter columnaris, a bacterium, kills 14 million wild fish in Klamath Lake • the Idaho trout industry loses 10 cents on every dollar made to disease (death, weight loss) • future of finfish and shrimp culture may hinge on our ability to control vibriosis* Transmissible to humans.

5. Types of Diseases • infectious: diseases due to the action of microorganisms (animal or plant): • viruses: CCV, WSSV, TSV, YHV • bacteria: Vibrio sp. • protozoans • metazoans • fungi: Saprolegnia sp. • crustaceans: O. Isopoda

6. Types of Diseases 2) non-infectious: diseases due to non-living causes (environmental, other) • even a moderately adverse environment can lead to stress, stress leads to epizootics • a very adverse environment can cause disease and mortalities directly (e.g., nitrogen gas bubble disease, brown blood disease) • the “other” category refers to nutritional, genetic and developmental diseases

7. Types of Diseases 3) treatable vs. non-treatable • non-treatable diseases are some of the worst • include pathogens such as viruses, drug-resistant bacteria, myxozoans • white spot syndrome virus (shrimp) has no known treatment • Vibrio sp.: because of rampant over-use of antibiotics in Central America, South America, new, more virulent strains are developing

8. Dynamics of Infectious Diseases • First mode of infection demonstrated by Robert Koch (1876) and his work with Bacillus anthracis(anthrax) • reached epidemic proportions in cattle, sheep and other domesticated animals • also can occur in man (as we are well aware!) • Koch showed that a bacterium caused the disease by using the following method:

9. Koch’s Method (Postulates) • 1) find the organism common to all infected animals, demonstrate its absence in healthy ones • 2) isolate the organism in pure culture • 3) reproduce the disease in suitable experimental animals • 4) reisolate the same organism from experimentally infected animals

10. Dynamics of Disease: Germ Theory • Koch’s work lead to what is known as the germ theory: germs cause disease • if you have germs you are diseased • Renes Dubos (1955) refined the concept in the following statement: “There are many situations in which the microbe is a constant and ubiquitous component of the environment but causes disease only when some weakening of the patient by another factor allows infection to proceed unrestrained, at least for a while. Theories of disease must account for the surprising fact that, in any community, a large percentage of healthy and normal individuals continually harbor potentially pathogenic microbes without suffering any symptoms or lesions.”

11. Dynamics of Disease: stress • Definition: any stimulus (physical, chemical or environmental) which tends to disrupt homeostasis in an animal. • The animal must then expend more energy to maintain homeostasis: less energy to combat disease • Aquatic organisms are fundamentally different from terrestrials: they are immersed in their environment, can’t go somewhere else • some disease agents are almost always present in the water (ubiquitous) • examples: Aeromonas sp., Pseudomonas sp., Vibrio sp.

12. Dynamics of Infectious Disease: how it occurs • Three-set model: • susceptible host • pathogenic agent • environment unfavorable to host/favorable to agent • exceptions??: extremely large numbers of bacteria, extremely virulent agent • stress throws a wrench into it all

13. Dynamics of Infectious Diseases • infection  parasitism  disease (infection can result from parasitism, but neither necessarily results in disease • symbiosis: any association between 2 species involving an exchange of matter and energy • commensalism: symbiosis in which one partner benefits, the other is neutral • parasitism: symbiosis in which the parasite (usually smaller) is metabolically dependent on the host (larger); some harm intuitive, but not necessary

14. Epizootiology of Infectious Diseases: terminology • epidemiology: branch of medicine describing occurrence, distribution and types of diseases in populations of animals at distinct periods of time and at particular places (usually refers to humans) • epizootiology: same as above (non-human) • epidemiology is the study of the who, what, when, where, how and why of disease outbreaks

15. Epizootiology of Disease: outbreak terminology • enzootic vs. epizootic (endemic vs. epidemic) • incidence: frequency of disease in a population over time in relation to the population in which it occurs (cases/yr) • rate: number of new cases per number of population (per thousand) • prevalence: the expression of the frequency of a disease at a particular point in time in relation to the population in which it occurs (%) • proportion: number affected/population • mortality: the percentage expression of the frequency of deaths over a period of time in the total population (not a rate, a proportion)

16. How to Become a Disease Agent:6 Commandments of Parasitism • Find a proper host • Somehow get in or access inside • Find a home • Be fruitful and multiply • Get out once done or developed • Be transmitted to a new host • all this obviously involves specificity in the host:parasite relationship

17. Potential for Disease via Infection: contributors • numberof organisms (overwhelming) • infectivity (ability to get in) • virulence (ability to produce disease) • susceptibility of the host • agent’s ability to overcome host’s defenses • level of stress (REM!) • probablility of disease (Theobald Smith Model) = (# agents x virulence of agents)÷(resistance of host)

18. Possible Fates of an Agent within its Host • host dies: agent proliferates, overwhelms host, good parasites don’t do this, $$$$$ • host lives: largely dependent on stress • host gets sick, but recovers (defense worked) • host doesn’t get sick (agent not virulent, wrong host) • survivors: • agent either eliminated or • carrier state established (host infected, but no obvious disease, big problem) • latent(not easily observed) • patent (ongoing/observable)

19. Mortality Curves: bell shaped • Infectious agent or toxic substance moves into the population and then, after time, no longer affects events in population. • Transmission is horizontal with width of curve proportional to incubation time and period of communicability. Agent??: typically bacterial

20. Mortality Curves: sigmoidal • Slight deviation from bell-shaped curve due to lag period in course of disease (lag phase of growth) • Also, periods in which the disease is not communicable. lag Agent??: typically bacterial

21. Mortality Curves: point source • Population at risk was exposed to agent at a single point in time. • All susceptible members affected. • Highly virulent infectious type disease of toxic agent • Exposure to toxin. Agent??: chemical, viral

22. Mortality Curves: plateau- shaped • Indicates exposure over a long period of time • slow incubation • slow transmission Agent??: possibly nutritional

23. Mortality Curves: multiple spiked • Due to frequent but intermittent exposure to disease agent • Data usually or eventually indicate plateau effect • Must take care re frequency of sample Agent??: physical parameter (e.g., low D.O.)

24. Degree of Infection • Acute: high degree of mortality in short period of time, external signs might be completely lacking (e.g., CCV, IHNV, TSV, WSSV) • Chronic: gradual mortality, difficult to detect a peak (Aeromonas septicemia, furunculosis) • Latent: disease agent present, but host shows no outward sign, little or no mortality, sometimes associated with secondary pathogen/infection (CCV and Edwardsiella ictaluri)

25. Transmission • Definition: mode of transfer of disease to a new host • Method 1)direct transmission: from one host to another, either a) vertically or b) horizontally • vertical transmission: from parent to offspring • via male (Girodactylus, trematode in pipefish) • via female (IHN) • horizontal transmission: from one member of a population to another, one offspring to another • contact: typically water borne (e.g., fish to fish) • ingestion of agent or of infected aquatic

26. Transmission • Method 2)indirect transmission: infection via an inanimate vehicle, vector or intermediate host • vehicle: an inanimate object such as handling equipment (nets, waders, etc.) or feed (e.g., aflatoxin) • vector or intermediate host: animate object • mechanical: vector is not essential to life cycle of agent • biological: agent spends some part of life cycle in vector(e.g., water boatman and WSSV)

27. Disease Transmission: getting in the door Portals of entry, not as easy as they sound: • ingestion: e.g., Ceratomyxa shasta, BKD, Myxobolus cerebralis • gill lamellae: e.g., Schizamoeba salmonis, Ichthyobodo necatur • lesions: bacteria (Vibrio sp.), fungi (Saprolegnia sp.) • active penetration: some metazoans, dinoflagellates

28. The Host • The ability of a host to acquire a disease agent and demonstrate disease symptoms can be expressed both qualitatively and quantitatively • qualitatively: resistance(ability of a host to withstand the effects of an agent; e.g., Litopenaeus stylirostris to TSV) • quantitatively: susceptibility (a measure of the host’s ability to tolerate an agent)

29. Resistance: Primary Factors Physical barriers, inflammation, natural immunity, acquired immunity • physical barriers: refers to innate characteristic of animal body to penetration (e.g., mucous slime layer, intact skin, mucous membranes, exoskeleton) • for fish, the mucous slime layer itself displays an immune response (phagocytic properties, antibodies)

30. Resistance: Primary Factors • inflammation: basic response to any wound, designed to seal off the area and reduce further infection/damage • manifestations (humans) include swelling, reddening, loss of function, heat, pain • manifestations (fish) possibly include heat and pain • histological changes: local edema(swelling); infiltration of neutrophils (type of white blood cell produced in bone marrow) , lymphocytes (lymph proteins), macrophages; fibroplasia (formation of fibrous tissue in wounds)

31. Resistance: Primary Factors 3) Immune Response • natural immunity: inherited (discussed in detail later) • acquired immunity: either active or passive • active: obtains antibody via contact with antigen • passive: antibody obtained via donor (vaccination) • discussed in following lecture

32. Resistance: secondary factors • Secondary factors associated with disease resistance are either environmental in nature orsomatic (associated with host, itself) • environmental factors: mainly stress resulting from deviation in temperature, dissolved oxygen, ammonia; inadequate nutrition; mechanical, etc. • somatic factors: age, sex, species (e.g., IPN affects only largest fry, potential for exposure, immune experience via exposure, black spermataphore, TSV)

33. Stages in Epizootic (disease) • REM: epizootic is an outbreak of disease • incubatory: agent has penetrated host barrier, found home and multiplying • clinical or subclinical: host adversely affected (manifestations) • depression (reduced activity) • color change • interrupted feeding behavior • body contortions • respiratory change • mortality

34. Stages in Epizootic • terminal: host either dies or recovers • exception: in some very acute, highly pathogenic diseases (e.g., MBV) death may occur so fast that obvious signs don’t develop • NEXT: Immune Response in Aquaculture Organisms