K2+The+Dynamics+of+Parasitism


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Parasite-host Dynamics -
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 * Many vertebrate become **immune** to many microparasite diseases. Because the vertebrates become immune to the diseases, there is a significant reduction in the disease-susceptible population. THEREFORE the number of disease incidents declines.
 * As new vulnerable vertebrates make their way into the population, however, the number of disease incidents increases again.
 * This becomes a **cycle**.********
 * As the reproductive rate of the disease slows, the # of disease incidents vs. # of susceptible vertebrates is LOW.
 * The cycle will make the transition of being **synchronous** (short period between peaks) to having reduced synchrony, to eventually completely dying out (with the help of vaccines and other forms of human intervention). See graph below...
 * The number of disease incidences also causes the host population to cycle.

Host-parasite Evolution -
The cuckoo bird is known as a "brood parasite" because instead of raising its own young, it lays eggs that mimic the appearance of other birds' eggs and abandons them in the other birds' nests. Because the cuckoo was able to adapt to make its own eggs look like the eggs of the host bird's, the host birds aren't able to tell the cuckoo eggs from their own, and in caring for the extra egg, they neglect their own. Because of co-evolution, however, the host birds were soon able to adapt as well by being able to recognize the imposture egg, and the cuckoos parasitic plan has been thwarted.
 * Adaptations created by the host to protect itself from parasites causes an adaptation in the parasite to be able to still attack this host. This is an example of ******coevolution******.
 * The increase of "defensive moves" by the host creates an increase in "offensive moves" by the parasite. This creates an '******arms race******' between the two.********
 * Coevolution doesn't always end up in an arms race. Coevolution may actually result in the reduction of harmful attacks by the parasite.******
 * For example:**

A Model of Microparasite Disease -

 * The reproductive rate of a microparasite is expressed through the # of new cases that arise from an infected host. This is shown through the equation:
 * If the reproductive rate < 1, the disease incidence is falling in host population. If the reproductive rate > 1, the disease incidence is rising.
 * If the disease wants to **persist**, the transmission rate, the number of susceptible hosts, and the duration of infectious period must be relatively large numbers.
 * This model can predict:
 * 1) The evolution of reduced virulence (harmful effects) when host deaths occur rapidly
 * 2) Altered host behavior to maximize parasite fitness
 * 3) That there is a threshold density of hosts for a given parasite, below which the parasite will not survive
 * 4) That diseases with short periods of infectivity should not persist in small populations
 * **Herd Immunity** is when a number of at-risk individuals are reduced, thus causing the spreading rate of the disease to be reduced as well. **Vaccination** creates herd immunity.

Heterogeneity in Parasite Populations -

 * Some diseases have created "escapes" from host defenses. **Anti-genetically stable diseases** (such as measles, mumps, chickenpox...) only occur once because after an individual has experienced the disease, the anti-body defense in that individual will prevent the disease from reoccuring.
 * Parasites like influenza, rhinoviruses (common cold), and salmonella have multiple **strains** which are constantly being added to. The variation among the strains means that the host is constantly battling off different parasites while appearing to have the same disease.
 * There can be different degrees of virulence within the different strains. Some strains can be more/less harsh than others.**

Heterogeneity in Host Populations -

 * **Those within the same host population do not have the same chances of being attacked by a given parasite.
 * **age, behavior, state of health, genetic predisposition, and proximity of the infected individual** will all be influences. This is an example of **genetic variation in immunity**.
 * Example**: //Sickle cell hemoglobin//////- a safeguarding gene modifies hemoglobin in the human so that it polymerizes at low oxygen levels, which interferes with red blood cells and therefore killing or “sickling” them. Those with two forms of this gene (homozygotes) usually die at an early age because it causes a delicateness in heart, kidney, spleen and brain damage. Heterozygotes, however, usually just suffer complications of anemia.

Parasites as a Reason for Sex - the 'Red Queen'

 * **The main benefit of sex is to overcome parasite attack** by producing new genetic variation in the host community.
 * New phenotypes created from reproduction cause major problems for parasites.
 * The "**Red Queen**" effect is the idea for the need of continuous evolutionary change.**