BIO 360  Ecology

Lecture 21:  The Ecology, Evolution, and Behavior of Infectious Disease

I.  Types of interactions between species
    A.  Interactions can be classified based on whether they have positive, negative, or no effect on the other species
        1.  Interactions that have a positive effect on both species are called mutualisms
        2.  Interactions that have a positive effect on one species and a negative effect on the other are called a variety of names, including predation, disease, herbivory, parasitism
        3.  Interactions that have negative effects on both species are called competition

II.            Why do we study infectious disease?

III.            What is infectious disease?

        a.  What is a parasite or pathogen?

        b.  What is infection?

        c.  What is a host?

        d.  Examples of infectious disease and their pathogens.

        e.  Disease triangle

        f.  What infectious disease is not…

                1.  Non Infectious disease

                 2.  Predation
                         i. Doesn't necessarily kill its victim
                         ii.  After infection, victim often becomes immune
                         iii. Disease organism is much smaller than its victim
                         iv.      Disease organism is often not independently mobile

IV. How do we study infectious disease?

        A.       SIR Models
            1.      Rate of transmission

                    --  The process by which a pathogen passes from a source of infection to a new host and infects that host

                    -- Dependent on rate of contact and probability of infection given contact.

            2.      Why is it important?

                     -- Host individuals are spatially discrete

                    --  Hosts defend themselves (resistance)

                    --  Hosts die (especially if infected!)

            3.  Modes of transmission
                 Direct contact (e.g. handshake)
                 Indirect contact (e.g. sneezing)
                Vector (species that transmits pathogen without experiencing disease; usually arthropods)
                 Trophic (from prey to predator)
                Environmental reservoir (free-living stage)
                Vertical (from parent to offspring)

             4.  R0 – the basic reproductive ratio
                a.    # secondary infections produced by a single infected host in a susceptible population
                b.  Depends on:

                    -- Rate of transmission (b)
                    --  Rate of removal (g)
                    --  Number of Susceptible (S)

                                 R0 = (g/b)S

                                When R0>1 then Pathogen invades

                                When R0<1 then Pathogen no invasion

V.  Observation: In many organisms, higher population growth rate leads to higher fitness

             Question: Why would diseases not evolve high virulence?

Observation: diseases are dependent on the host for their transmission

 Question: How do diseases modify the behaviors or physical characteristics of hosts to promote transmission?

 Behaviors or modifications induced in host that cause transmission




VI.  The ecology and evolution of AIDS

A.   AIDS impact on human pop. Growth

B.   Rates of HIV infection

C.  How does HIV work?

         --  Attacks helper T cells, which are critical in the immune response

        --  Does not kill host directly, but by weakening immune system

         --  Does not kill host quickly – usually within 15 years of infection

D.  The Origin of HIV

        1.  Multiple jumps from other primates to humans

        2.  Rapid diversification makes vaccine design difficult

E.   Evolution of HIV

        1.  Retroviruses prone to error in transcription  -- 1 every 1000 base pairs (107 times typical mutation rates in humans)

        2.  Mutations provide variability within hosts

        3.  Drugs provide a selective force

        4.  Some virions are better able to survive and reproduce

         5.  Large numbers, fast population growth rate, and high mutation rate lead to rapid evolution

F.  Why is HIV fatal?

         1.  Assumption 1: More virulent strains have higher population sizes and kill host faster

        2.  Assumption 2: More virulent strains are more likely to be transmitted

                --  transmission rate related to virulence

                --  More virulent strains transmit faster, but die sooner

                --  Less virulent strains transmit slower, but die later