Lecture 2: Who Gives And Who Gets?

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Disease in humans

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any semi-permanent condition that has a negative effect on an individual’s well-being (i. e. causes discomfort, incapacity, illness)

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Disease in In biological organisms

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any semi-permanent condition that has a negative effect on an individual’s fitness (genetic contribution of an individual to subsequent generations via survival and reproduction).

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Non-infectious causes of disease

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caused by internal and external conditions
e.g. developmental abnormalities (many cancers, but some exceptions)
e.g. genetic abnormalities (hemophilia, sickle cell anemia)
e.g. nutrient or vitamin deficiency (scurvy, ricketts)

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Infectious causes of disease

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caused by infectious agents

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Infectious Agents

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Replicating entities (usually organisms) that multiply in or on hosts, and are transferred from host to host.

When these agents cause harm to the host, we call them pathogens or parasites.

When they enter a host we say the host is “infected”.

The manifestation of the harm that they cause is termed a “disease”.

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An infectious disease is

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the product of the interaction between the host and the pathogen or parasite.

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A disease is not the causal agent but...

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a product. AIDS – Acquired Immune Deficiency Syndrome is the disease. HIV – Human Immunodeficiency Virus is the pathogen

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Etiology

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technical term for the cause of disease

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Same agent can...

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cause different diseases. Agent = pathogen (if they're in different individuals, we call them different things)

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Prions (cause different diseases)

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abnormal proteins that cause neural degeneration but are transmissible
In cows, “Mad Cow Disease”
In humans, “Creutzfeldt-Jacob Disease”

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Varicella-Zoster Virus (cause different diseases)

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is the Human Herpes Virus 3.

Chicken pox in young children
Shingles when it resurges in older individuals

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Meningitis (Same disease can be caused by different agents)

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infection of the fluid and membranes enclosing the brain
Bacterial: Neisseria meningitidis
Viral: enteroviruses, herpes viruses
Fungal: Cryptococcus neoformans

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Malaria (Same disease can be caused by different agents)

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caused by 4 different species of protozoans in the genus Plasmodium
P. falciparum, P. vivax, P. ovale, P. malariae

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A successful infection requires: (what 3 things)

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1. a susceptible host
2. an infectious pathogen
3. an appropriate environment

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The disease triangle

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a useful tautology

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Problem of causation in disease triangle

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Disease triangle for ulcers

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Host genetics example: People with A or O blood type are more susceptible to ulcers than other blood types.

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Difficult to say that a disease is pure genetic, environmental or infectious. It’s usually a combination of the three.

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Atherosclerosis (where genetics, environment, and infection are confused)

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hardening of arteries may be a reaction to sub-clinical Chlamydia pneumoniae infection

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Cystic fibrosis (where genetics, environment, and infection are confused)

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genetic defect in membrane protein causes accumulation of mucus,; lungs invaded by Pseudomonas aeruginosa

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Schizophrenia (where genetics, environment, and infection are confused)

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early infection with Toxoplasma gondii (toxoplasmosis)

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Alzheimer’s disease (where genetics, environment, and infection are confused)

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- reports suggest association with Herpes Simplex Virus 1 (HSV1)

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What is a pathogen/parasite?

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Organism that forms a close semi-permanent association with another organism (the host) and decreases the health or fitness of the host.

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Pathogens and parasites Broad generalities (but no hard and fast definitions):

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Pathogens usually thought of as smaller than parasites

Micro-parasites (= pathogens) vs. macro-parasites (= parasites)

(Endo-)parasites (intimate association) vs. ecto-parasites (less intimate association)

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“Defining” pathogensvs. parasites vs. predators

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Parasitoids - intermediate between predators and parasites. or few on one parasitismusually insects

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Parasitoids: when one organism goes in another organism, eats the other organism and comes out of the organism
There are also non-tophic interactions: e.g. epiphytic, phoretic

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A wasp parasitoid of aphids

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e.g. another parasitoid: The horrid phorid (Phoridae)

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Infectious disease and ecological interactions

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Parasites and ecosystem functioning
Trophic position Food webs Diversity
Study effort Predators vs. parasites

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Ecological interactions A normal “trophic pyramid”

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Trophic pyramid of football

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Pathogens and parasites are always at the top of the food chain

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An example of a parasite food chain

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Parasites and pathogens themselves form food chains

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Food webs and ecosystem connectivity

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Study of a California salt marsh

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Predators/prey = 87 species
Links = 505 out of 7,569 possible
6.7%

Include parasites (47 additional species)
Links = 2313 out of 17,956 possible
12.9%
Biomass of parasites may exceed that of the birds


Only counted macro-organisms (top predator/prey count).

Pathogens and parasites were showing more connection in the ecosystem.

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Parasitic molluscs:Kidney shell mussels and darters

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Beneficial effects on the environment.

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Numbers of predatory and parasitic species (insects)

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Parasitoids are one of the most common organisms in the world.

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Host specificity of insects

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Parasites are highly specialized = specific form of life.
Preadators are the recover, they have to eat a lot of organisms to survive so they’re highly non-specialized

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Detection problems with pathogens “Plant diseases are like the stars”

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X = how many times it’s been studied. For about 1000 citations, you get about 100 diseases. Lower solid line = potato famine.
For any particular number of citations, there are more fungal diseases in the muscle family than the potato family.
The longer you look the more you see
The more you research and look into an organism, the more pathogens you will find.
Number of diseases (only fungal diseases).
The more the organism has been studied, the more diseases there are.

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Monkey diseases are like the stars

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Parasite species richness vs. sampling effort for primate hosts.

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Number of recorded STDs

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STDs primarily occur in mammals. Humans and mammals are studied more than the other groups.
STDs in snails (in France) –studied because people eat them.
Dung beetles = in Australia. Reduce dung.
Lockhart et al. 1996 Biological Reviews

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How many infectious diseases in a well-studied species, Homo sapiens?

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Recent count in Emerging Infectious Diseases 2005
Lists 1407
58% shared with animals

GIDEON database — Global Infectious Disease and Epidemiology Network
Lists 342

Best studied organism = humans.

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3. What organisms cause disease?4. What organisms get diseased?

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3. Who gives disease to who? It’s not random at all.
Mosses, Rotifers, Helminthis don’t get many diseases. Mosses (the government will not fund research on mosses).
Archabacteria = do not get many diseases. Small things tend to parasitize small things and other large things parasitize large things.
We have yet to answer all these questions.

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Previous diagram shows that:

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Smaller organisms parasitize larger organisms.
Species are often parasitic on similar organisms.
Some groups of organisms appear to be relatively free of disease/parasitism – we don’t know why! Rotifers/Helminths/Mosses
Some groups seem to cause more disease than others. These will be considered in lectures to come.

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Conclusions

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1. Environmental, genetic and pathogen causes of disease are sometimes difficult to distinguish
2. Pathogens are small, specialized and nutritionally dependent on their hosts
3. Parasites and pathogens play an important role in ecosystems
4. Parasites are specialized, much more so than predators
5. Most species in the world are parasitic and cause disease
6. Most species also become diseased