Origins of major human

infectious diseases

N.D. Wolfe, C.P. Dunavan, J. Diamond

Nature 2007

Questions of interest

• Where did these major infectious diseases come from?

• Why do most of them originate from the Old World?

• How does a disease progress from existing exclusively in animal pops into human pops?

• Focus: unicellular microbial pathogens

Diseases considered in the study:

▫ 8 temperate: hepatitis B, influenza A, measles, pertussis, rotavirus A, syphiis, tetanus and TB

▫ 9 tropical: AIDS, Chagus, cholera, DHF, E. and W. African sleeping sickness, malaria, leishmaniasis

▫ 8 others: diphtheria, mumps, plague, rubella, smallpox, typhoid and typhus, tropical yellow fever

Animal pathogen human pathogen

in 5 evolutionary stages

• 1. Microbe is present in animals but not humans

• 2. Animal pathogen is transmitted to humans but not between humans (anthrax, rabies, etc)

• 3. animal pathogens have a few cycles of secondary transmission in humans but soon die out (ebola, Marburg, etc)

Prof. Durham’s ebola

• Stage 4:

▫ Disease exists in animals & humans can get infected through contact w/ the sylvatic cycle

▫ but also, the disease can be transmitted between humans w/o animal hosts

▫ Three substages:

� a) sylvatic cycle more important than direct human to human spread (Chagas disease, yellow fever)

� b) both sylvatic and direct transmission important (dengue)

� c) spread mostly through humans (typhus, influenza, cholera)

• Stage 5: pathogen is exclusive to humans

▫ Malaria, measles, mumps, rubella, smallpox and syphilis

▫ 2 ways pathogen became confined to humans:

� A)ancestral pathogen already present in the common ancestor of chimps and humans split

� B) animal pathogen recently colonized humans and evolved a separate human form

measles

Temperate/Tropical zone differences

• Greater proportion of diseases are transmitted by insect vectors in tropics than temperate zones

• Higher proportion of temperate diseases confer lasting immunity (upon survival) than in tropical regions

• Animal reservoirs are more common in tropics & an environmental reservoir is more common in temperate

• Temperate infectious diseases tend to be more acute (a few days duration) rather than chronic

• Tropical diseases are more chronic, some last weeks, months and years

• There are more stage 5 diseases in temperate zones

• There are more ‘crowd epidemic diseases’ in the temperate zones

Pathogen origins• >half of temperate diseases are from domestic animals

• Rise of agriculture helped build up crowds of people and livestock living close to each other

• Fewer tropical diseases originate from domestic animals

• Half of the tropical diseases have wild non human primate origins, compare to only one temperate

• Most diseases with animal origins are from warm-blooded animals

One exception to the last point

Geographic origins• Majority of 25 major pathogens considered originate from the Old World

▫ Historical significance in the colonization of the New World

▫ Why? : “More temperate diseases arose in the Old world than new World because far more animals that could furnish ancestral pathogens were domesticated in the Old World.”

▫ Human density: Agriculture led to higher human population densities infectious diseases thrive

Diamond is one of the authors of the paper

Future research and

steps to be taken

• 2 avenues

▫ Continue clarifying origins of existing diseases

▫ Increase surveillance for early detection of new diseases: global early warning system

Photo credits

• http://www.afip.org/Departments/infectious/mp/images/72main.jpg

• http://webs.wichita.edu/mschneegurt/biol103/lecture17/Ebola_EM.jpg

• http://katelewis.files.wordpress.com/2007/12/ebolaplush.jpg

• http://www.nlm.nih.gov/medlineplus/ency/images/ency/fullsize/2943.jpg

• http://weblogs.variety.com/photos/uncategorized/2007/11/18/cholera2.jpg

• http://www.skeptic.com/Merchant2/graphics/audio_video/av559_lg.jpg

• http://www.crimethinc.com/tools/downloads/preview_big/surveillance_big.gif

• The End• Thank you for staying awake!