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