What are the two meanings of sickness?
- 1. Can cause a part of the body of the organism to fail. (heart attack or diabetes)
- 2. Can result from the invasion of the host by another organism, leading to an infection, the other organism generally finds resources for reproduction at the host’s expense. In some vertebrates and humans, an acquired immune response may take place.
What is epidemiology?
It is the study of how the population ecology of infectious diseases and their hosts shape the spread of disease.
What is an epidemic?
It occurs when a disease infects an increasing and unusually high number of individuals in a population over time.
What is a pandemic?
Epidemics can turn into pandemics. It is an epidemic that has spread on multiple continents or is occurring worldwide.
What factors are important for assessing the impact of a disease?
- 1. The mode of infection transmission.
- 2. How easily the disease infects new hosts.
- 3. How well the disease can be fought off through natural immune responses or modern medicine.
- 4. The environment in which the hosts live.
What are microbes?
- 1. These are organisms that are too small to see with the naked eye.
- 2. The invading microbes move through a population of organisms, taking advantage of the nutrient-rich environments that are the bodies of the hosts to multiply.
- 3. Microbes include all bacteria, single celled eukaryotes and small multicellular eukaryotes. Viruses are also considered microbes.
- 4. Most microbes don’t cause disease and perform important functions such as decomposition, digestion etc. Almost 90% of our bodies isn’t human cells but microorganisms.
What are pathogens?
- 1. It is a biological agent that causes a disease.
- 2. Most pathogens are microorganisms, however the word can also refer to non-infectious disease causing agents such as chemicals.
- 3. Pathogenic microorganisms are by definition, parasites.
- 4. Pathogens have evolved numerous ways of causing infection in their hosts. The method a particular pathogen uses determines how the disease progresses and what the organism / medicine can do to combat it.
What are some examples for pathogenic microorganisms?
- 1. Bacteria: 1/10 human cell size, single celled organisms. They reproduce through binary fission. They colonize areas on or around the cells of the host. Bacteria use the nutrients around the host cell to reproduce. Salmonella, tuberculosis, pneumonia, cholera are examples.
- 2. Viruses: obligate intracellular parasites. dna or rna covered by a protein coat and sometimes a lipid envelope. HIV, polio, smallpox. Bacteria can reproduce in a dead organism but viruses cannot.
- 3. Protozoans (single celled eukaryotes) and helminths (multicellular parasitic worms). dIseases caused include malaria, hookworm, schistosomiasis (snail fever, menstruating egyptian boys)
- 4. Fungi : generally good for nutrient cycling and drug and food production. However, may cause problems for those with compromised immune systems.
- 5. Prions: infectious agents made of misfolded proteins. Contain neither dna or rna and are simply 3 dimensional proteins that can cause other proteins to be misfolded as well. Misfolded proteins may cause severe pathology as they accumulate. Mad cow disease.
What are some examples of pathogenic macroparasites?
- 1/ Worms (flatworms, segmented worms, roundworms): hookworm, roundworm, tapeworm.
- 2. Arthropods: ticks, fleas, crustaceans
- 3. Vertebrate parasites: eurasian cuckoo (laying eggs in other nests), sea lamprey, vampire bat
- 4. Plants: dwarf mistletoe, rafflesia arnoldii
What are holoparasites?
Plant parasites that lack chlorophylls and are fully dependent on other plants for energy, water and nutrients. (rafflesia arnoldii)
What are hemiparasites?
Have chlorophyll but lack root systems and depend on other plants for water and nutrients (dwarf mistletoe)
What are the population wide effects of parasites?
- 1. Parasites can decimate populations. (rinderpest virus decimated cattle populations in the 1800s. Even though only cattle were vaccinated, wildlife aka giraffes, buffalo etc. showed major recovery, as cattle were the reservoir for the virus.)
- 2. Parasites can alter morphology of host. (puccinia monoica fungus alters the color of boechera holboellii and forms attractive pseudo-flowers to spread the fungal spores)
- 3. Parasites can modify behavior: snails eat fluke eggs from cow feces, ants that consume the slime balls climb to the tip of the blade of grass where it can be eaten by a cow, completing the cycle.
- 4. Parasites can affect species interactions: Reduce or increase competition.
- 5. Parasites can affect host geographic range.
- 6. Parasites can alter ecological communities.
- 7. Parasites can alter the reproductive success of the host.
- 8. Parasites can alter the physical environment.
What are the methods for combating bacterial infections?
As bacteria diverged from eukaryotes a long time ago, many processes and structures are different than that of eukaryotes. These uniquely prokaryotic features can be exploited. For example, antibiotics poison prokaryotes but not eukaryotes. Eukaryotic pathogens are harder to combat due to their similarities to host cells.
What is an antibody?
Specific type of protein produced by the body in response to a pathogen. Each antibody has two attachment sites that attach to a specific antigen. This marks the pathogen for destruction by other cells. The presence of two attachment sites also helps create clumps of pathogens, making it easier for macrophages to combat.
What are antigens?
Foreign proteins embedded in the cell membranes of pathogens.
What are plasma cells?
Plasma cells are a specific type of leukocyte that produces a specific antibody. As there generally isn’t enough plasma cells present in the body to respond to an infection and produce the necessary amount of antibodies, the body has a specific reaction to primary encounters.
Explain a typical primary immune response (polyclonal response)
- Macrophage engulfs the pathogen.
- The antigen presents the antigen on its cell membrane to be determined by helper T cells.
- T cells activate the antibody producing B lymphocytes in accordance with the antigen is identified. Many different antibodies are produced, thus this is called a polyclonal response.
- This specific type of plasma cell rapidly undergoes mitosis as an army of plasma cells is created.
- This army rapidly produces antibodies which later on circulate in the blood system until they reach the antigen carrying pathogen and eliminate it.
- Some plasma cells (B cells) remain in the blood system and continue to provide protection against the specific pathogen. These long lived cells are called memory cells.
- Memory cells respond quickly if the same antigen is encountered again.
What are the two main modes of transmission?
- Direct transmission: the pathogen moves directly from an infected individual to an uninfected individual. Such contact might include an infected and susceptible host engaging in sexual activities, touching, sneezing on each other etc.
- Indirect transmission: the pathogen moves from one host to another through some intermediate agent. This agent can be something inanimate (water, food, commonly contacted surface) or it could be a vector organism such as a mosquito. These pathogens will be transmitted more quickly throughout a population.
What is a disease vector?
- A living intermediate that transmits a pathogen from an infected individual to a susceptible individual.
- Biological vector: also becomes infected (malaria carrying mosquito)
- Mechanical vector: doesn’t become infected (fly transmitting bacteria on its body)
What is a patch?
In ecology, a patch is a distinct area of habitat for a type of organism. Each patch is occupied by a distinct population.
What is a metapopulation?
This is a collection of spatially distinct subpopulations of the same species that are connected via dispersal. The metapopulation persists over time, even if subpopulations go extinct.
How does disease spread in a population look like on the graph.
It shows logistic growth if no one is cured and everyone is infected. If people are cured OR they die before they spread infection (equivalent for viruses), not everyone is infected and the disease spreads more slowly.
What is an SIR model?
Susceptible, infected, recovered (resistant or removed) model is a type of disease outbreak simulation used by epidemiologists. It has 3 types of individuals, namely S I and R. v is the recovery rate. L = 1’v is the average infectious period.
What assumptions do simple SIR models make?
- Once infected and recovered, each individual is permanently immune to the disease.
- Individuals in the population are perfectly mixed - everyone comes into contact with everyone.
- As soon as individuals become infected, they become infectious themselves.
What factors influence the proportion of the population that get infected?
- Population density : This determines how often people come into contact with each other.
- Transmission rate (beta): the probability that contact will result in disease transmission.
- Infectious period: the average period of time an infected person remains infectious.
- High density, high transmission rate, long infectious period leads to the highest proportion of the population getting sick.
What is basic reproductive rate?
R0 for a disease in a population is the number of individuals that get infected from each person in the population that is already infected. This depends on transmission rate, infectious period and population density. R0 = susceptible x transmission rate x infectious period. R0 must be greater than 1 for an epidemic to occur.
What are the limitations of the SIR model?
- 1. It can’t account for mutation, multiple epidemics in a year, indirect transmission, diseases affecting multiple populations.
- 2. More broadly, diseases affect more than the host population and have ecosystem consequences. This can negatively or positively affect the predator, prey or competitors of the host species.
What is the geometric model of disease spread?
N1 = N0 * R0
What factors can functionally lower R0 in communities?
- Low density host population.
- Host immune systems
- Environmental conditions
- Issues with intermediate vectors
- A population with few susceptible individuals.
What is symbiosis?
Intimate relationship between different species in which one lives in or on the other. Doesn’t have to be mutualistic. Any species that lives inside another is a symbiont.
How many parasite species exist?
More parasite species than host species exist.
How are parasites categorized?
- Based on where they live (endoparasites/ectoparasites)
- Based on size (macroparasites/microparasites)
What is the evolutionary relationship between hosts and parasites?
There is an evolutionary arms race of defense and counter defense between hosts and parasites.
What is the main trade-off for the sake of parasites?
The trade-off between virulence and transmission. If parasites are too virulent, they may kill of the host before further transmission. Parasites do not have an evolutionary interest in harming the host species.
Where do microparasites and macroparasites generally reproduce?
- Microparasites generally reproduce within the host.
- Macroparasites generally grow but do not reproduce within the host and persist by continuous reinfection.
Are microparasites and macroparasites intracellular or extracellular?
- Microparasites are intracellular
- Whereas macroparasites generally live in body cavities or on body.
How do microparasites and macroparasites differ in terms of immune response?
- Microparasites often induce immunity to reinfection.
- Macroparasites induce a short term response.
How do microparasites and macroparasites differ in terms of host impact?
- Microparasites have high or moderate ability to regulate host population.
- Macroparasites have low ability to regulate host population.
What are the advantages of ectoparasites?
- Ease of dispersal.
- Protected from host’s immune response
What are the disadvantages of ectoparasites?
- Vulnerability to predation.
- Unstable environment
- Feeding is more difficult.
What are the advantages of endoparasitism?
- Ease of feeding
- Protected from natural environment
- Safer from predation.
- Stable environment and constant source of food.
What are the disadvantages of endoparasitism?
- Vulnerability to host’s immune system.
- Dispersal is difficult.
What are the ortak nokta of parasites and predators.
- Both can reduce the fitness of the host / prey
- Both can reduce the competitive impacts of otherwise dominant species