AEC 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 population size or growth rate
        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?
        a. >25% of annual human deaths worldwide are the direct result of 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. Community ecology of infectious disease

        g.  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.       Population ecology of infectious disease

        B.   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

             4.  R0 – the basic reproductive ratio (Pages 356-357)
                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 (see Figure 15.17)




VI.  The ecology and evolution of AIDS

A.   How does HIV work?

         --  Attacks CD4 cells, or helper T cells, that initiate the immune response

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

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

B.   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