lecture 9: dhaka & dengue fever, the black death

in the digestive tract (stomach) what has co-evolved with humans?

helicobacter pylori

helicobacter pylori: (4)

• colonized humans at least 100k ya (1/2-1/3 infected, declining in western populations)

• considered a ‘dominant member of the human gastric microbiota’

• an amphibiont bacteria - promotes/protects from pathology (theodor rosebury)

• stomach cancer/ulcer/esophageal cancer

dhaka, bangladesh: (4)

• ‘order’ & ‘nature’ in the urban environment

• dhaka - megacity (14 million ppl); 1 of 28 in the world

• every year ~300,000-400,000 of bangladesh’s urban poor relocate to dhaka for employment and wages

• as a result = massive rate of (unplanned) urbanization

in this megacity (dhaka, in a developing country) the concept of ‘order’ is challenged along 4 lines:

• reliable access to water, food, housing

• regular collection of waste

• natural disasters (earthquakes/flooding)

• urban hazards (fires, car accidents, crime and violence)

2 examples of hazardous/order issues in dhaka:

• raza plaza; 2013, garment factory collapse - 1129 dead and 2515 injured (most workers - women/children)

• nimtali, old dhaka; 2010, fire in apartment building - fire trucks could not even access the site, streets too narrow, no fire escapes (building infrastructure issues) left 124 dead

dhaka is said to have a proliferation of…

shanty towns - some established and others ephemeral (coming and going)

dhaka: the problem and the response

• the problem: lack of a reliable water supply (even piped water access relies on electricity)

• the response: to store water

stored water is an issue because?

its an opportunity for aedes aegypti - clean still water as an ideal breeding grounds

recommendation for dhaka:

to make stored water ‘safe’ (changing water, scrubbing vessels, using lids)

a. sultana (2014) (masters thesis investigating dhaka’s dengus problem): the main problem and theme

• problem: underestimating the risk of dengue

• theme: contextualizing dengue: daily stressors - forget about dengue listen to our sufferings (dengue - not a current stressor, consider daily life; we’re worried about building collapses, gridlock, fires, etc. dengue is way lower on the list)

2nd theme in dhaka (masters thesis):

dengue - a disease of 8-10 years ago; there is no news of dengue deaths anymore, 8-10 years back dengue is as the headline of every newspaper now we hardly see mention in the news about people dying of dengue, and if we do, they are not even in our country - i think 8-10 years ago dengue was a killer disease in our country too, but it is at least not a killer disease anymore

3rd theme in dhaka (masters thesis):

dengue and the language of virus jor:

• dengue naming practices where virus jor means the common fever “with or without medicine, virus jor will be cured in seven days

the meaning of surviving dengue; my family was scared but i was not; i felt i was important/special

• not recognizing classic dengue fever versus dengue hemorrhagic fever

dengue hemorrhagic fever in short:

involves liver dysfunction, lack of blood clotting, much more serious disease

dengue risks in dhaka:

are connected to massively growing, unplanned urbanization

dengue in dhaka: the crux of the problem & remediation

• crux: the need to store water

• remediate: establish

• a water supply (in a megacity) (also in rural areas - irrigation of crops and aedes albopictus breeding - find this ecology particularly useful)

plague’s delicate balance review:

• the microbe

• the reservoirs & host(s)

• the vector(s)

bubonic plague bacterium: (4)

• yersinia pestis: isolated in 1894 by alexandre yersin (named after him)

• DNA sequencing suggests: bacterium have evolved over time (various gains/losses of DNA), but only seemed to become lethal ~1,500 ya (previously it was a gastrointestinal bug)

• evidence suggests yersinia pestis arose in asia

• spread and likely present, e.g., in parts of africa for last 2000 years

plague’s 2 epidemiological settings:

• sylvatic (wild) plague

• urban (domestic) plague

sylvatic plague:

aka wild plague; typically circulates among rodents resistant to infection (tolerant of plague bacterium)

urban plague:

aka domestic plague; involves humans and commensal rodents

black death transmission:

vector-borne transmission (via flea) (often but not always)

2 important resistant rodents in the black death:

• voles (field mice)

• deer mice

both important reservoirs of plague bacillus in animal populations

describe examples of each in the image above:

vector (flea most typically) —> reservoir (rodents) —> incidental (ground hogs/prairie dogs/squirrels) —> commensal & domestic (cats and dogs) —> humans

prairie dogs and plague:

are incidental hosts which are particularly vulnerable to plague bacterium (yersinia pestis) - die very easily when infected

rat-borne epidemics:

account for many human plague epidemics

for humans (rat borne epidemics):

rattus rattus (the black rat, ship rat, roof rat)

rattus rattus: (4)

• found throughout the world as a human commensal but… it is a species which can MOVE BETWEEN sylvatic and urban systems

• this makes control efforts difficult (both environments are hospitable)

• probably native to asia but established a very wide distribution (ship rat - comfortable on ships, how it got its name)

• high producer: breeds all year long, 3-5 litters/year, average of 6-10 offspring per litter

yersinia pestis originally evolved where?

in asia… and moved around with human activity

how did yersinia pestis make it to north america?

plague entered north america in ~1900: west coast port cities (through rat infested ships)

how did plague venture inland in north america (how did it cross the rockies?): (3)

• coastal prairie dogs died in large numbers (entire populations wiped out)

• farmers on the other side of the rockies/inland travelled to the rockies to get some sick prairie dogs to ‘eliminate’ more prairie dogs back home (better for farming…)

• this propagated the movement of plague bacterium into the rest of north america

last known rat borne epidemic in north america:

occurred in LA in 1924-25

does plague still exist in north america?

currently, north america = sporadic outbreaks, through contact with rodent reservoirs and incidental hosts - plague is out there in nature!

transmission in bubonic plague:

there are 3 important vectors to consider: all fleas

3 flea vectors of bubonic plague and who they affect primarily:

• xenopsylla cheopis: important in infecting black rats - historically valuable

• pulex irritans: infected humans and farm animals - less common today

• oropsylla montana: infects squirrels/prairie dogs - most important vector in america