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 - self-generated 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