Basic Field Epidemiology

AUSTRALIA INDONESIA PARTNERSHIP FOR EMERGING INFECTIOUS DISEASES

Basic Field Epidemiology

Session 6 – How disease progressesRecorded PowerPoint file


In Session 6 we will explore:

• How disease occurs and the progression of disease states in an individual animal

• The progression of disease in a population of animals


Soleh’s place

• All Soleh’s chickens have been sick with influenza in the past

• He brought a sick chicken home and 3 days later most of his other chickens were sick


Definitions• Infectious agents include:

– Bacteria– Viruses– Parasites– Protozoa– Fungi– …

• Contagious diseases include: – Foot and Mouth disease – Highly Pathogenic Avian Influenza– …

• Non-contagious infectious diseases include:– Tetanus– Anthrax– Liver fluke– …

Living agents that are capable of causing disease

Infectious agents that can be spread directly from one animal to another

Diseases causes by infectious agents that are not spread directly from one animal to another


Definitions• Susceptibility

– An animal must be susceptible to the infectious agent for infection to occur

• Exposure– Contact or interaction between infectious agent and

animal

Some agents only infect one species, others infect multiple species.

Some agents only infect the young or pregnant females or very old or sick.

Not every exposure will result in infection.


Disease progression in an individual animal

Time

Animal with no infection

Incubation period (no signs of infection)

Clinical disease (showing signs of infection)

Chronic disease(ongoing)

Carrier state

Death

Recovered - signs go away & infectious agent eliminated

Susceptible Infected Outcomes of infection

Start of infection

Exposure


Soleh’s place

• Some cows had diarrhoea• All Soleh’s cow were infected with worms

– Not every infected cow was showing clinical signs of disease


Soleh’s place

• 2 years ago Soleh had some cows die from Anthrax– All the cows that were infected died

Anthrax spores in soil

Animal eats spores

Spores activate, bacteria grow and infectAnimal. Animal dies.

http://www.cdc.gov/anthrax/basics/index.html


• Carrier animals– Bacteria lives in:

• Eyes• Nose• Vagina

– Can spread the bacteria to other susceptible animals and cause disease in other animals

Pinkeye

Animal with pinkeye recovers from infection but can still carry the bacteria (one of the causes of pink eye)


Recovered animals

• Immune

• Susceptible


Progression of disease in a population

Introduce one animal infected with a contagious diseaseInto a susceptible population

Over time – the whole populationmay get sick.


Progression of disease in a population

Immune animal

Susceptible animal

Infected animal that is infectious to others


1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

100

200

300

400

500

600

Susceptible Recovered Clinical

Time (days)

Num

ber o

f ani

mal

sProgression of disease in a populationHerd immunity


Session 6 - Summary• Within an animal there are a number of steps that determine if the

animal develops disease after being exposed to an infectious agent

• Infected animals may develop chronic disease, die from the disease or may recover

• Animals that recover often develop immunity to the infectious agent. Immunity may last a lifetime or it may be shorter. If the immunity declines the animal may become susceptible to infection again

• Herd immunity describes a form of immunity that occurs when a significant portion of a population of animals is immune and this provides protection for the susceptible animals


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Basic Field Epidemiology

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