WHY WE GET SICK
THE EVOLUTIONARY
ECOLOGY OF DISEASE
A FACT
•  Medical science rarely employs an
evolutionary perspective
DEFINE DISEASE
•  Abnormal or low performance
SOMETHING TO REMEMBER
•  Symptoms and causes of diseases need
not be synonymous
CAUSES OF DISEASE
•  Non-infectious - selfgenerated or non-biological
agent
•  Infectious - biological agent
that can be transmitted
4 QUESTIONS TO ASK
•  Function of the symptoms (why?)
•  Cause of the symptoms (how? Mechanism)
•  Ontogeny of symptoms (time course)
•  Lineage (of victim and if appropriate agent)
Ex. 1 - MORNING SICKNESS
•  Most common during early pregnancy
•  Nausea, vomiting, aversion to many foods
especially “rich” foods
•  Mechanism - hormonal shifts
•  Lineage ?
CLASSIC PERSPECTIVE
•  Morning sickness is a side effect of
hormonal change
•  Is it?
FUNCTIONAL QUESTION
•  Could morning sickness be adaptive?
•  This sickness leads to elimination of
various foodstuffs from the mother’s diet
and by association from the fetus’ nutrition
EXAMINE REJECTED
FOODSTUFFS
•  Margaret Profet classified food groups
•  Commonality is that many are mutagens
•  Mutagens cause greatest impact during
early development - later stages of
pregnancy are primarily growth related
MORE RECENT STUDIES
•  In a Korean study, women with morning
sickness ate less and ate less diverse diets
•  Those women gained less weight and
produced lighter, smaller babies
•  A US study showed that women with
m.sickness had same rates of malformation
as those without
PREDICTION
•  Women who suffer from morning sickness
are less likely to bear children with
abnormalities
Ex. 2 - ALLERGIES
•  Symptoms - sneezing, coughing , weeping,
inflammatory response
•  How - Class E immunoglobulins (antibody)
•  Response occurs after antibodies have
bound to ingested or inhaled compounds
•  Ontogeny - allergies can be gained or lost at
any age
WHAT HAPPENS?
•  Symptoms - sneezing, coughing , weeping
all cause elimination of the foreign bodies
•  Inflammation can isolate foreign bodies
A FACT
•  Many allergins are carcinogenic
A PREDICTION
•  Allergy sufferers should be less likely to be
stricken with cancer A PREDICTION
•  Allergy sufferers should be less likely to be
stricken with cancer •  A CONFOUND
A PREDICTION
•
Allergy sufferers should be less likely to be
stricken with cancer •  A CONFOUND
•  People with high allergy rates may
be found in areas with very high
levels of carcinogens
THE HYGIENE HYPOTHESIS
Medical - Early lack of exposure to
infectious agents and parasitic worms
(helminths) suppresses natural
development of the immune system
•  Darwinian – immune system has evolved to
expect mild suppression of the immune
system – good hygiene removes that
suppression
•
TEST THE HYGIENE
HYPOTHESIS
Compare allergies (e.g. asthma) in
developed vs. developing countries or in
developed countries now vs. 100 years ago
•  Confounds?
•
INFECTIOUS DISEASES
•  A perspective: disease problems are first and
foremost problems of population and
evolutionary biology and and second a problem
of symptoms
INFECTIOUS DISEASES
•  EXPLANATION FOR SYMPTOMS:
– Host defense
– Manipulation by infectious organism
– Interaction between host and infectious
organism
FEVER: GOOD OR BAD?
•  Until last century, fever was considered as a
positive event
•  New, anti-fever drugs changed that view due to
concerns of heat damage from extreme fever
•  However, many viruses succumb to heat at
moderate fever temperatures FEVER: GOOD OR BAD?
•  Raising body temperature in mammals leads to:
Increasing resistance to: herpes simplex virus,
poliovirus S. pneumoniae ,gastroenteritis virus ,
and of ferrets to influenza virus.
•  Preventing fever can lead to longer lasting
symptoms of chicken pox and more viral export
from common cold.
FEVER: GOOD OR BAD?
•  Aside from damage of extreme fever, bacterial
infections do not seem to suffer from exposure to
high temperatures and might even prosper
•  However, raising body temperature does not
simulate all aspects of fever in mammals so
reports of positive and negative effects of heat
must be viewed with caution
FEVER: GOOD OR BAD?
•  Fever reducing drugs may also:
– Reduce pain - host becomes more mobile
– Reduce inflammation - infectious agent
moves through host
ANOTHER SYSTEM
•  A FACT: (cold blooded) lizards and
grasshoppers create a behavioral fever upon
exposure to pathogens
•  Lizards manipulate their temperature until it
reaches that of fevered mammals •  Lizards and grasshoppers experience much
higher recovery when fevered
BEHAVIOUR FEVER
Feed
Rest
Bask
BEHAVIOUR FEVER IMPACT
PARASITE MANIPULATION
•  Cholera Facts:
– Bacterial disease
– Acquired orally from untreated water or
untreated foods
– Bacteria can live for up to 5 days on food
– Symptoms include severe diarrhea
– Vaccines are short lassting
INPACT OF CHOLERA
•  Last major outbreak of cholera in Latin
America caused illness in 400,000
people with 4000 deaths
•  Outbreak in Peru caused economic
losses of approximately 1 billion
dollars in trade embargo
PARASITE MANIPULATION?
•  Cholera bacterium releases toxin at a
very high rate that causes host
intestinal distress
•  Humans respond by releasing large
amounts of the bacteria via diarrhea
•  Toxin doesn’t harm
human but dehydration
does
A SOLUTION?
•  Rehydration therapy reduces harm
from dehydration but doesn’t stop
bacteria from spreading
•  So, combine sugar water with rice
starch