Macroecology: global patterns and effect of climate change on disease
January 29, 2014 Discussion
• “The ecology of climate change and infectious diseases” (2009). Review article.
Infectious diseases transmit better when the climate is warmer.
Climate change (making places warmer) might make it easier for
things like malaria to be transmitted in traditionally colder climates
Climate change ▯expansion of tropical diseases’ ranges. Mosquito
and tick born tropical diseases in particular.
Aims of the review article
• Introduce how temperature drives important biological
• Consider how climate might affect spatial (latitude and
altitude) and temporal (seasonal, interannual, historical)
patterns in disease.
• Figure out whether climate change has affected infectious
• Review models that project how climate change might
affect infectious diseases in the future.
o Thermal physiology
Some parasites prefer cooler temperatures, because their
environmentally resistant stages are cannot deal with extended
exposures to warmer temperatures
Range limits for parasites are affected by the increase in
temperatures through out the world. Eg: since Sweden is warming
up, the range of tickborne encephalitis is increasing.
The exact effects depend on a number of variables within the
complex system that governs transmission and stuff
o Evidence for effects of climate in infectious diseases
• Tropical regions ▯higher species diversity
• Diversity of infectious diseases is higher in countries that
are closer to the equator
• High diversity of infectious diseases in the tropics could be
related to the diversity of the vectors in the region.
o Lots of infectious diseases have a specific vector.
So, if you live in a temperate zone, the vector will
not be around ▯you will not become infected
• altitude is also related to infectious disease prevalence.
Malaria is more common in lower altitudes than in higher
altitudes, because the cold that comes with the higher altitudes makes it difficult for the mosquitos to develop
• GDP and wealth of nations may also affect the rate of
transmission (think: control measures being put in place
and stuff of that sort)
• Restriction of malaria to the tropics suggests that climate is
really important when it comes to the range of an infectious
• Malarious countries have GDPs one fifth that of non
malarious contries. Suggesting that economic forces,
particularly environmental destruction, have pushed
malaria out of temperate regions
o This makes sense. More industrialized regions have
less of the natural environments that would promote
the survival of vectors and other things of that sort.
• There are seasonal patterns in malaria.
• Since temperature and precipitation change as seasons
change, climate change will affect infectious diseases that
• Eg: Black spot (fish trematode metacercariae) is more
common in salmon when it is warm.
• Endemic cholera is more common when the water
temperatures are higher.
• Some infectious diseases have declines when the
temperature is warmer and the weather is wetter. Specific
examples are referred to in the paper there.
• Seasonality is not necessarily related to the effects of
climate on disease. Has more to do with the lengths of
days, which is important for biological processes.
Climate change does not affect day length
• Time lags between climate and species abundances create
statistical challenges for investigating seasonal effects on
o Conditions that make it easier for mosquito larvae
to develop do not necessarily enhance disease
• Epidemics can vary from year to year.
• Epidemics have been linked to the strong interannual
variation in weather associated with the Southern
Oscillation (El Nino) o Malaria cases increase after El Nino.
• Interannual associations with disease relate to precipitation
o For aquatic vectors, more rain makes transmission
o More rain ▯mosquito larvae develop better ▯more
• This is not a simple relationship though. There are way
more things that have an effect.
• Historical records of disease can provide long time series
for investigating interannual associations between climate
• The yellow fever epidemic that hit the US in 1878
happened after a large El Nino event.
o More precipitation ▯more reproduction of the
yellow fever vector Aedes aegypti ▯more chance of
transmission of the virus
• To be able to fully link El Nino to the yellow fever
epidemic, you have to look at the years around that year to
see what was happening.
o Found the most deadly yellow fever epidemics were
more likely to follow an El Nino event.
o El Nino did not explain everything though
o Other factors = accumulation of susceptible hosts
during the nonepidemic years, accumulation of
enough susceptibles for an epidemic after 3 years.
How would you be able to know that for
sure, though? How did the research that
was being reviewed determine that there
were other things that were causing this
• Malaria in England: history
o Common during the medieval warm period (1200)
o Outbreaks corresponded with unusually warm
o Declines in malaria deaths correspond to landuse
changes more than to climate
Again, how were they able to determine
o Ironically, when malaria was declining, the climate
was getting warmer and wetter.
o Decrease in malaria was related to an increase in
Experiments • Use of experiments = to investigate the effects of climate
variables on vital rates of some infectious diseases and their
• Eg: Schistosomiasis. Research has been done to look at the
rate at which cercariae shedding from snails changes with
the changes in temperatures.
• “Changes in temperature” experiments have been done to
determine whether or not temperature change will affect a
parasite’s ability to cause disease in the host.
• Experiment work suggests that climate may directly affect
influenza epidemic rates.
o Dry and cold environments favour the transmission
of the flu virus.