Basic Field Epidemiology

1. Basic Field Epidemiology Session 6 – How disease progresses Recorded PowerPoint file

2. In Session 6we will explore: • How disease occurs and the progression of disease states in an individual animal • The progression of disease in a population of animals

3. 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

4. Definitions Living agents that are capable of causing disease • 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 • … 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

5. 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.

6. Disease progression in an individual animal Susceptible Infected Outcomes of infection Time Death Incubation period (no signs of infection) Clinical disease (showing signs of infection) Chronic disease (ongoing) Animal with no infection Carrier state Exposure Recovered - signs go away & infectious agent eliminated Start of infection

7. 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

8. Soleh’s place Anthrax spores in soil • 2 years ago Soleh had some cows die from Anthrax • All the cows that were infected died Animal eats spores Spores activate, bacteria grow and infect Animal. Animal dies. http://www.cdc.gov/anthrax/basics/index.html

9. Pinkeye Animal withpinkeyerecoversfrom infection but canstill carry the bacteria (one of the causes of pinkeye) • Carrier animals • Bacterialives in: • Eyes • Nose • Vagina • Can spread the bacteria to other susceptible animals and cause disease in otheranimals

10. Recovered animals • Immune • Susceptible

11. Progression of disease in a population Over time – the whole population may get sick. Introduce one animal infected with a contagious disease Into a susceptible population

12. Progression of disease in a population Immune animal Infected animal that is infectious to others Susceptible animal

13. Progression of disease in a populationHerd immunity

14. 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

15. Close of video