Selected Readings on Human Health, Disease, and the Environment

The following are responses and summaries of various readings related to Human Health, Disease, and the Environment. 

Linkages Between Climate, Ecosystems, and Infectious Disease

Weather and climate are different ideas in similar fields of study.  Weather is the measurement of temperature, humidity, precipitation, and wind daily whereas climate measures these factors over a long period of time.  Climate is an average and varies with geography and time.   The average temperature of an area decreases by 6.5 degrees C with every 1000 meter change vertically and decreases 5 degrees C with every 1000 kilometer change from the equator.   The range with which temperature fluctuates due to the seasons is greater towards the poles, with much less change experienced near the equator.  There are many climate cycles occurring from yearly to millennially and some occurring as little as every 100,000 years; these oscillations can cause temperature shifts as high as 10 degree C.  Since the Industrial Revolution, and more notably in the last twenty years, the global average temperature has begun to increase yearly and is predicted to continue increasing several degrees in the next century.  The basis for good predictive weather and climate models does not rest solely on the atmosphere but also in surface variations, especially the oceans.  Currently weather prediction is only effective on a timescale of less than two weeks, while climate models are limited to seasonal or greater timescales.  Including the effects of ocean, atmosphere, and land may make climate models sufficient for shorter timescales and weather prediction for greater timescales.  With infectious disease, the number of cases that indicates and epidemic will vary with each particular disease, region, and time.  Emerging disease can be due to climatic or ecological change, which causes a greater number of people to come into contact with a natural reservoir of infection.  Diseases can be transmitted either directly, due to contact with an agent, or indirectly, through a natural reservoir or host.  Many studies of infectious disease use the SEIR framework:  S. The proportion of people who are able to become infected E. Within that same group, those who have had contact with the agent but who have not become infected I.  The number of people within S group who have contracted the disease R. The number of people who have been removed from the original group S due to either death or immunity.   SEIR attempts to account for the many different factors in disease studies such as population size and density, demographics, connectivity patterns, and immunity.  Diseases and natural reservoirs have environmental conditions that are both favorable and not for the spread of particular diseases which include precipitation, temperature, humidity, and ultraviolet radiation.  Most diseases are sensitive to changes in temperature with regards to their dissemination, replication, and movement.  Climate change over the long-term may result in increased intensity and occurrence in extreme weather events, where the direct impact is obviously more deaths due to disasters, though potentially and indirectly an increase in infectious disease outbreaks.  The diseases associated with flooding are well known while droughts are not nearly as studied, though a disease would not affect a crisis area should it not have been present in the system before the extreme event and had not be introduced.  There are many other factors that influence disease dynamics including land use, migration of disease and hosts, the societal makeup of populations, and public health services.

Climate Influences on Specific Diseases

Dengue fever is currently the biggest disease worldwide that is transmitted via insects.  It causes fever, headache, muscle and bone pain, and hemorrhage in humans with about five percent of its cases being fatal.  The tropical regions are the most affected by dengue with about 50 million occurrences of infection every year.  The mosquito is effective as a vector because it is able to both infect humans and acquire the disease from already infected humans, thereafter replicating in the stomach of the mosquito.  The mosquito finds stagnant water, especially in manmade containers the most appealing for breeding, thus areas with much excess garbage and rainfall are most susceptible, though rainfall is not requisite should the containers be filled by people.  Temperature is the strongest factor in dengue fever, because the mosquito biting frequencies are much lower in cooler times, breeding and hatching is much less frequent, if at all possible, and the virus itself takes longer to form in the gut of the mosquito.  An increase in temperature by as little as one degree could see the at risk population increase by as much as fifty percent in areas already containing the dengue fever. 

Malaria is a disease transmitted via mosquito in the tropics which causes chills and fever and potentially death.  Upward of 300 million cases are reported yearly with around one million deaths.  Ninety percent of cases are found in sub-Saharan Africa, with most of the remainder taking place in the tropics.  Malaria is on the rise due to poor control of mosquito populations, insecticide resistance, and drug resistant strains appearing in recent years.  Malaria acts very similarly to dengue fever in that higher temperature generally results in more cases due to the behavior of the carrying mosquitoes and its virus.  The spread of malaria seems most affiliated with economics, considering that most of the cases occur in areas that have poor sanitation and life style. 

St. Louis encephalitis is an inflammation of the brain due to a virus carried by mosquitoes.  St. Louis encephalitis can cause headache, fever, stiffness, stupor, disorientation, coma, and paralysis.  In the United States there are about 128 cases on average every year.  The disease is dominant in the southern U.S. where the temperature is generally warmer and more prevalent in years of greater rainfall or in areas of recent flooding. 
Rift Valley Fever can cause abortions and death in the young of animals, while causing fever, headache, myalgia, and incapacitation in people, with one percent of cases being fatal.  Rift Valley Fever is transmitted by mosquitoes, mostly on the African continent, after periods of great rainfall where they generally only infect animals. 

Hantavirus causes fever, muscle aches, and respiratory problems, which all develop over a period of less than a week after initial infection.  The largest outbreak of the virus in the last fifteen years saw 80 cases in 1993.  The virus is transported in the urine, feces, and saliva of deer mice in the southwestern United States.  The virus is greatly influenced by rainfall with large population jumps in years directly following big rain seasons. 

Lyme disease is a bacteria transmitted by ticks which can cause fever, fatigue, headache, and muscle and joint aches, though it is rarely the primary cause of death.  It is most prevalent in the suburbs, or in areas near new forest.  The ticks generally feed on field mice and deer, which have found abundance in the areas of new forests which only fifty years ago were home to farms and industry thereby leaving a vacant lot for new life to emerge.  There is currently insufficient data to conclude what effect the climate has on Lyme disease and its vectors. 

Influenza is a virus of the respiratory tract in humans.  The virus is transmitted through the air, enabling it to move quickly through populations.  Millions of cases are identified every year, with three major outbreaks occurring in just the last century.  Currently there is no human immunity to influenza due to the main strain regularly incorporating pieces of the infected DNA into its genome.  Infection seems to be more prevalent during colder months and in areas of the world closer to the poles than the equator, though it has not been shown to be directly influenced by temperature; many theories include that people crowding inside in the wintertime enables for easier transmission and also that there is a greater humidity in the winter months which influences the virus. 

Cholera is a bacteria causing diarrheal disease in people.  Cholera is most often transmitted through the water supply, though modern filtration and chlorination techniques have virtually eliminated the disease in developed countries.  The developing and third-world nations contribute to almost all the cases of cholera today.  Increased temperature, or temporary climate shifts such as El Nino, have shown an increase in the number of cholera cases. 

Hanta Virus

http://en.wikipedia.org/wiki/Hanta_virus

The Hantavirus is transmitted through the air, via the excretions of rodent urine, feces, and saliva and through rodent bites.  The virus was previously referred to as the Korean hemorrhagic fever, though that term is in use no longer.  The disease was initially recognized by modern Western medicine during the Korean war in the 1950s and recognized in its current species form in 1993 during an outbreak of the “Sin Nombre virus” in the Four Corners region of the United States.  The virus is found most often in from China up to Russia, Northern Europe, South America, and North America.  In the United States the virus is dominant in the Southwestern states, including Texas and California, as well as the Northwestern states Montana, Idaho, and Utah.  The Hantavirus has an incubation time of two to four weeks in humans.  Symptoms include fever, chills, headaches, nausea, stomach and back pain, respiratory problems, and gastro-intestinal problems.  The disease can cause renal failure, hypoxemia, and tachycardia.  The virus is most occurrent in years after excess rainfall when the mouse and rat populations thrive due to the excess food put off by the area’s plant life.  Housing areas near forests and in rural areas are most susceptible to the disease due to the increased likelihood of rodent infestation and life within a home.  Increased flooding and equally increased sunlight during moist times has shown an increase in Hantavirus occurrences because of increased food for rodent populations in the areas.

WHO: Climate Change and Human Health

http://www.who.int/globalchange/publications/cchhsummary/en/

With climate change extreme weather events are expected to increase in both intensity and number and have the greatest impact on poorer communities and nations.  The ENSO cycle, which occurs twice per ten years, influences much of the world’s weather and climate change will likely increase how often this occurs and its amplitude.  Warmer temperatures will increase the breeding of mosquitoes and allow for more rapid maturation of pathogens within the organism.  An increase in temperature could cause for mosquito carried diseases to have a greater geographic range of infection.  While an increase in extreme events such as flooding could also increase the occurrence of mosquito vector disease as well as rodent spread diseases and an increase in diarrheal diseases.  Climate change will come with more often and stronger heat waves, warmer summers, and less cold winters.  Heat wave related deaths is predicted to increase many times its current rate over the next fifty years, though the decrease in the number of cold related deaths may outweigh the heat related deaths.  The number of natural disasters and extreme weather events is on the rise, with nearly triple the number of events occurring annually when compared with the number in the 1960s while the number of people affected by these events is proportionally increasing.  It is estimated that the risk of diarrheal disease will increase by up to ten percent over the next twenty years due to climate change.  Proper modeling techniques need to be further perfected in order to predict more clearly. 

Readings above may have been drawn from the following sources:

Six Modern Plagues and How We are Causing Them, Mark Jerome Walters; Shearwater Books, 2003, ISBN 155963992X

Life Support, The Environment and Human Health, Michael McCally, editor, MIT Press, 2002, ISBN 0-262-63257-8

Rx for Survival,  Philip Hilts, Pengquin Books.  ISBN 0-7394-6974-6

Emerging Infectious Diseases,  Stuart A. Hill, Pearson Eductation Inc., publishing as Benjamin Cummings.  ISBN 0-8053-3955-8

Under the Weather:  Climate, Ecosystems and Infectious Disease, National Research Council, National Academy Press.  ISBN 0-309-07278-6

This article originally written November 11th, 2008 for OU IPE 3913 - Human Health, Disease, and the Environment.