TV4001 - Epidemiology 1

Epidemiology is the study of?

disease in populations and of factors that determine its occurrence, the key word being populations

Endemic means?

May also refer to?

Usual presence of a disease within a geographic area

The usual prevalence (number of cases) of a disease within a geographic area

Epidemic means?

Disease within a geographic area in excess of the normal expectancy (the endemic level), and transmitted either from a common (single) source or from sources propagated from a single source

Pandemic means?

epidemic that affects several countries or continents or much of the world.

Five objectives of epidemiology are?

The acceptance of Koch's postulates by the scientific community resulted in a stereo-type line of thinking that lasted into the 1960s, that being?

More recent thinking has once more recognised the concept that disease is? What are these factors?

Each disease had a single cause and thus a suitable control strategy is directed at that causal agent

Usually caused by multiple factors.

These factors are determinants of disease. A determinant is any characteristic that affects the health of a population

Determinants can be classified in 3 ways, as?

1. primary or secondary;

2. intrinsic or extrinsic;

3. associated with host, agent or environment.

Primary vs Secondary Determinents

Difference between the two?

Primary: Factors that exert major influence on causing disease, often a necessary cause for disease

Secondary: correspond to predisposing, enabling or reinforcing factors

Primary Determinants Example Table

Secondary Determinants Example Table

Intrinsic vs Extrinsic Determinants

(Primary and Secondary Determinants can be intrinsic or extrinsic)

Intrinsic: Internal to the host (aka endogenous)

Extrinsic: External to the host (aka Exogenous)

The triad consists of what?

How does it work?

The Host, Agent and Environment

Early emphasis on microbes as primary disease causes made them be separated from other factors such as husbandry, trauma and toxic agents (environmental factors)

The Host, Agent and Environment triad:

Recently there has been a fourth category considered for what type of industry?

Why?

Intensive animal farming

Management and husbandry are considered to be important enough to be separated from the environemtn into its own category

The Host, Agent and Environment triad - What is a simple disease?

Examples?

An agent is the main determinant, and host and environmental factors are of relatively minor importance

Acute toxicities like arsenic or strychnine poisoning, where a multifactorial nature is not obviou

The Host, Agent and Environment triad - What is a complex disease?

Examples?

Disease where multifactorial nature dominates - interaction btw host, agent and environment necessary to cause disease

Bovine Respiratory Disease Complex

The Host, Agent and Environment triad

Agent Factors

Causative (etiologic) agents are not limited to biological agents; they may also be?

Chemical or Physical

The Host, Agent and Environment triad

Agent Factors Table

Examples for Biological Chemical and Physical Agents

The Host, Agent and Environment triad

Host Factors

Why are they important?

Examples of Host Factors?

They affect the risk of exposure to infection source AND host’s resistance to disease

The Host, Agent and Environment triad

Environmental Factors

The infectious disease process

Infectious disease results from the interaction of the agent, host, and environment in a process

which involves six components, what are they?

1. Causative agent

2. Reservoir of the agent

3. Portal of exit of the agent from the host

4. Mode of transmission of the agent to a new host

5. Portal of entry into the new host

6. Host susceptibility

The infectious disease process

1. Causative Agent

There are five basic categories of biological or etiologic agents. They are?

Those agents that are capable of causing disease are termed?

Factors that enhance ability to cause disease include?

The capacity of these agents to infect and to produce subsequent disease is called? What does it manifest as?

Protozoa

Bacteria & Rickettsia

Viruses & Prions

Fungi

Metazoa

Pathogens

Host specificity, ability to survive or multiply outside the host, and virulence

Pathogenicity

Manifest as signs and symptoms

The infectious disease process

1. Causative Agent

The severity of apparent infections is measured in terms of?

morbidity and mortality

The infectious disease process

2. Reservoirs of the Agent

What is this?

The two major categories of sources of infection are?

What does each mean?

Reservoir = normal habitat where agent lives, grows and multiplies (human, animal, environment)

Acute clinical cases and carriers.

Acute Clinical Cases

• Transmission less likely as less contact and early diagnoses and specific TX

Carriers

• Animal/human harbours infectious agent with no clear signs/symptoms

The infectious disease process

2. Reservoirs of the Agent

Carriers may be characterized as?

Significance of carriers?

inapparent infections, incubatory, convalescent, or chronic

More risk of disease transmission as contacts unaware of infection presence

The infectious disease process

2. Reservoirs of the Agent

Carrier classes and descriptions?

Inapparent infections (subclinical cases) - Infection but no symptoms

Incubatory Carriers - Can spread infection prior to disease symptoms

Convalescent Carriers - Remain infectious after acute illness and are now “healthy”

Chronic carriers - Harbour infectious agent 1+ yrs

The infectious disease process

2. Reservoirs of the Agent

Environment aspect?

Plants, soil, and water in the environment can be reservoirs of infection

The infectious disease process

3. Portals of exit of the agent from the host

What is it?

Different paths with examples?

Path where agent leaves the host = portal of exit

Resp tract - enzootic pneumonia

Genitourinary tract - Transmissable venereal tumours

Alimentary tract - Rabies, Salmonella

Skin - Smallpox

Transplacental - Rubella, Hepa B

The infectious disease process

4. Modes of transmission of the agent to a new host

Classes anf Examples?

Vertical Transmission - Via placenta or milk to next gen e.g. Toxocara canis and Akabane virus

Horizontal Transmission - from one individual to another, two forms (direct/indirect)

The infectious disease process

4. Modes of transmission of the agent to a new host

Horizontal - Direct vs Indirect Transmission?

Direct

• Immediate via contact or droplet spread

• Agents with this method - ideal to separate the diseased animal/environment

Indirect

• Via animate/inanimate mechanisms

• Animate - vectors like fleas

• Inanimate - Spread via air or vehicles (fomites)

The infectious disease process

5. Portals of entry into the new host

Describe it

Same as portals of exit, disease may use different ones on entry/exit or the same one

The infectious disease process

6. Host susceptibility

Host resistance to disease is increased to the greatest extent by?

Specific acquired immunity either Natural or Artificial

Natural - Obtain protective ABs from getting infection (active) or transplacental transfer (passive)

Artificial - Admin of vaccines/toxoids (active) or Antitoxins/Immune serum globulin (passive) to get ABs

Prevention vs control vs eradication

Prevention is the first line of defence against a disease. It occurs at three levels:

Give an example for each

1. National - quarantine stations for livestock

2. Provincial/State - certification for transport that states free from certain disease

3. Local - separation of sick animals, maintain disease free herd

Prevention vs control vs eradication

Prevention of disease

Governmental agencies are usually involved in?

Foreign Animal Diseases are controlled at a variety of levels, those being?

Excluding pathogens from an area it is absent

1) Import requirements and health certificates.

2) Quarantine stations.

3) Veterinarians must recognise and report the disease.

Prevention vs control vs eradication

Protecting a specific population from a disease that already occurs can be achieved by?

• Immunisation

• Chemoprophylaxis

• Environmental sanitation

• Vector control

• Genetic engineering and selective breeding

• Early detection and DX

• Public awareness and education

Prevention vs control vs eradication

Control of disease

Disease control is a term used to describe?

There are four approaches to control, what are they?

Measures done to dec freq of illness already in a population to what is considered an acceptable level

1. Preventing the spread of disease - intervene at the weakest pt of organism lifecycle

2. Dec communicability

3. Inc resistance

4. Early detection and DX

Prevention vs control vs eradication

Control of disease

decreasing communicability methods include?

• Isolate the sick

• Segregation practises - separate the at-risk

• Quarantine - control movement if they have been exposed to disease but are healthy

• Environmental control - disinfection, pasture rotation, manure disposal, etc

Prevention vs control vs eradication

Control of disease

Inc resistance how?

Active immunisation, passive immunity

Chemoprophylaxis - Note: Preventative TX can cause organism resistance and drug residues in food products

Prevention vs control vs eradication

3. Eradication of disease

Eradication of a veterinary disease involves?

Selective slaughter - kill infected, protect majority

Depopulation

Quarantine

Mass TX

Mass immunisation

Prevention vs control vs eradication

3. Eradication of disease

When is depopulation used?

a) A diagnostic test cannot be applied to an affected population in order

to carry out selective slaughter.

b) The population is inaccessible to perform other measures.

c) The disease is spreading too rapidly to control by other methods.

d) There are no other control methods available and depopulation of a

specific group is the only way to protect the species as a whole

Prevention vs control vs eradication

3. Eradication of disease

Use of mass TX depends on?

safe and cheap chemotherapeutic agents and early detection and diagnosis

Prevention vs control vs eradication

3. Eradication of disease

Which procedure is most effective of directed action against disease?

Mass immunisation

Prevention vs control vs eradication

3. Eradication of disease

Issues with selective slaughter?

How is it oft implmented?

Monetary costs outweigh benefits due to high involvement of animals

Disrupted economy

Unavailability of replacing animals

Mass immunisation then selective slaughter

To vaccinate or not to vaccinate

When should eradication be considered over using a biological product i.e. vaccine?

incidence of disease is low, the technology for eradication is present, and the industry support is available

To vaccinate or not to vaccinate

It is important to know whether or not the vaccine will further the chances of spreading disease due to?

reversion to virulence (in the case of a modified live vaccine) by creating carriers

To vaccinate or not to vaccinate

If safe and effective biological products are available their use should be considered if?

1) Incidence is widespread or increasing in spite of present control measures.

2) The technology to eradicate is not available.

3) It does not cause subclinical or carrier states and does not interfere with disease detection.

4) The disease is endemic