Aquaculture Disease Aquaculture Disease ProcessesProcesses

Aquaculture Disease Aquaculture Disease ProcessesProcesses

Dr. Craig KasperDr. Craig Kasper

FAS 2253/2253LFAS 2253/2253L

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

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

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*

*more on “vibrio” in a later lecture!

Types of Diseases1) 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

Types of Diseases2) non-infectious: diseases due to non-living

causes (environmental, other)

– even a moderately adverse environment can lead to stress, stress leads to epizootics (a disease that appears as new cases in a given animal population, during a given period, at a rate that substantially exceeds what is "expected" based on recent experience)

– 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

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

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:

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

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.”

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 as easily.

• Some disease agents are almost always present in the water (ubiquitous)

• examples: Aeromonas sp., Pseudomonas sp., Vibrio sp.

REM: 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

How Disease Occurs

Three-step model: 1. susceptible host2. pathogenic agent3. environment unfavorable to host/favorable to

agent

• exceptions??: extreme numbers or extreme virulence of agent

• stress is the monkeywrench

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 where one gains, the other is neutral

• parasitism: symbiosis where one (parasite) is metabolically dependent the other (host); some harm intuitive, but not necessary

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

• epizootiology: same as above (non-human)

• epidemiology is the study of the who, what, when, where, how and why of disease outbreaks

Epizootiology of Disease: outbreak terminology

• enzootic (affects animals) vs. epizootic (disease epidemic for animals)

• incidence: a measure of the risk of developing some new condition within a specified period of time.

• incidence rate: the number of new cases per population in a given time period.

• prevalence: the total number of cases in the population, divided by the number of individuals in the population,

• proportion: number affected/population• mortality: number of deaths over a time interval in the

total population (%, frequency)

How to be a parasite:

1. Find a proper host

2. Get inside

3. Find a home

4. Grow and multiply

5. Get out once done or developed

6. Be transmitted to a new host

Host:Parasite Specificity

• Specificity is required for steps 1 and 3, above (find a proper host, find a home inside)

• host specificity example: Shasta rainbow trout are highly susceptible to Ceratomyxa shasta (myxosporean parasite) while Crystal Lake individuals are completely resistant

• reason: physiological specificity (the host must meet all of the metabolic requirements of the agent without destroying it immunologically)

Host:Parasite Specificity

• Another example: Why are bluegill and bass infected with black spot metacercariae while walleyes aren’t?

• Answer: ecological specificity -- the host and agent must overlap in time and space

• Another type of specificity: tissue specificity

What Contributes to Infection?

1. number of organisms (overwhelming)2. infectivity (ability to get in)3. virulence (ability to produce disease)4. susceptibility of the host5. agent’s ability to overcome host’s defenses6. level of stress (REM!)

• Probablility of disease (Theobald Smith Model) = (# agents x virulence of agents)÷(resistance of host)

Possible Fates of an Agent within its Host

1. host dies: agent proliferates, overwhelms host, good parasites don’t do this, $$$$$

2. 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)

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.

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Agent??: typically bacterial

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. 0

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Agent??: typically bacterial

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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.0

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Agent??: chemical, viral

Mortality Curves: plateau- shaped

• Indicates exposure over a long period of time

• slow incubation• slow transmission

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Mortality Curves: multiple spiked

• Due to frequent but intermittent exposure to disease agent

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Agent??: physical parameter (e.g., low D.O.)

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)

The Reservoir Concept

• reservoir: the sum of all sources of the agent, the natural habitat of the agent, where the agent comes from– The size of the reservoir is proportional to the chance of

spread of a pathogen

• transient reservoir: situation in which the epizootic displays a seasonal pattern of either cases or carriers

• permanent reservoir: usually associated with disease in which chronic carriers are shown– good example: water supply, itself

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

a) vertical transmission: from parent to offspring via male (Girodactylus, trematode in pipefish) via female (IHN)

b) 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

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)

Disease Transmission: getting in the door

Portals of entry, not as easy as they sound:

1. ingestion: e.g., Ceratomyxa shasta, BKD, Myxobolus cerebralis

2. gill lamellae: e.g., Schizamoeba salmonis, Ichthyobodo necatur

3. lesions: bacteria (Vibrio sp.), fungi (Saprolegnia sp.)

4. active penetration: some metazoans, dinoflagellates

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)

Resistance: Primary Factors

Physical barriers, inflammation, natural immunity, acquired immunity

1. 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)

Resistance: Primary Factors

2. 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)

Resistance: Primary Factors

3) Immune Response1. natural immunity: inherited (discussed in detail

later)

2. acquired immunity: either active or passivea) active: obtains antibody via contact with antigen

b) passive: antibody obtained via donor (vaccination)

– discussed in following lecture

Resistance: secondary factors

• Secondary factors associated with disease resistance are either environmental in nature or somatic (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)

Stages in Epizootic• REM: epizootic is an outbreak of disease1. incubatory: agent has penetrated host barrier,

found home and multiplying2. clinical or subclinical: host adversely affected

(manifestations)– depression (reduced activity)– color change– interrupted feeding behavior– body contortions– respiratory change– mortality

Stages in Epizootic

3. 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