Transcript Infectious Diseases

Political Ecology of Infectious Disease
• Many changes that are relevant to understanding reemergence of old and emergence of new infectious
diseases are due to political and economic power at a
variety of scales, ranging from:
– transnational
– down to the household and
– individual levels.
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Political Ecology of Disease …
• Some or even much of this power is influenced by
which groups control decisions over land use.
• This, in turn, influences the relationships of people
and the environment.
• This is a basic principle of political ecology which
has been used in understanding the consequences of
human environment interactions.
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Re-emerging Infectious Diseases
Antibiotic Resistant Bacteria
• The discovery of penicillin in 1928 and the
introduction of other antibiotics e.g., such as
streptomycin, in the 1940s raised hopes that cures
could be found for all infectious diseases.
• Only one family of antibiotics (quinolones) have
been developed since the 1960s.
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Antibiotic Resistant Bacteria …
• Strains of Staphylococcus aureus developed
immunity against penicillin in the 1960s.
However, methicillin was still effective.
• Methicillin resistant Staphylococcus aureus were
found by the 1980s. Vancomycin was used as a
last resort.
• Now vancomycin resistant Staphylococcus aureus
has been observed in hospitals around the world.
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Antibiotic Resistant Bacteria …
• Streptococcus A which caused scarlet fever more or less
vanished by the 1960s, but it re-emerged in a much more
deadly form in the late 1980s.
• Streptoccous pyrogenes (dubbed the ‘flesh eating bug’)
causes life threatening necrotizing fascitis unless stopped
by amputation.
• Various strains of Pneumonoccus have resistance to whole
classes of antibiotics.
• Hospitals are a major breeding ground for antibiotic
resistant bacteria.
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Re-emerging Infectious Diseases …
Some Examples
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Malaria
Yellow fever
Tuberculosis
Cholera
Bubonic plague
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Malaria
• Malaria is a 3-factor disease: host, agent
(plasmoduium) and vector (anopheles mosquito).
• The agent has a very complex life cycle.
• Part of its development takes place within the
stomach of the mosquito, and part within the human
host’s organs.
• Victims experience repeated attacks corresponding to
the destruction of red blood cells.
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Malaria …
• Resistance to chloroquine was noticed in southern
Asia in the 1950s.
• By the 1960s some strains of Plasmodium falciparum
had developed resistance to the 4 main anti-malarial
drugs.
• A new drug, mefloquine, was adopted in the 1970s,
but resistant strains of falciparum emerged by the
1980s.
• Strains of falciparum in Thailand have evolved an
enzyme which expels all hostile chemicals, making it
resistant to drugs that have not even been invented.
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Tuberculosis
• Tuberculosis has been on the increase since the
1980s. It is one of the main cause of death worldwide.
• Many people carry the bacillus but do not develop
symptoms unless their immune system is
compromised.
• The resurgence in tuberculosis is associated with HIV
infection.
• The problem is compounded by the emergence of
drug resistant strains, partly due to HIV-infected drug
users not completing antibiotic courses.
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Cholera
• There were no new pandemics for most of the 20th
century, but the 7th pandemic began in the 1960s (the
El Tor strain). It spread to Latin America in the
1980s. Developed countries have so far escaped.
• An even more virulent strain (0139) has emerged in
the Sea of Bengal. This may be the beginnings of 8th
pandemic. There is no guarantee that developed
countries will escape.
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Re-emerging old infectious diseases …
• The re-emerging old infectious diseases have made
relatively little impact upon the public imagination
• However, given the rapid development of drugresistant strains, coupled with rapid air transportation,
it may be only a matter of time before our ghosts
from the past come back to haunt us.
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Emergence of New Infectious Diseases
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Desselberger, 2000
Examples: Machupo
• Machupo (Bolivian Haemorrhagic Fever) originated in
1961 in the headwaters of the Amazon where the
traditional cattle ranching/export economy was
replaced by self-reliant peasant agriculture after a
social revolution.
• Jungle was cleared to grow corn and vegetables,
disrupting the habitat of Colomys callosis (a field
mouse).
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Machupo …
• The mouse population swelled given the
availability of corn. They invaded the villages.
• A virus carried by the mice was passed by the
mice in their urine.
• The virus in humans caused 50 people mortality.
• The disease was eventually contained by catching
the mice.
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Ebola
• Occurred almost simultaneously in two locations a
few hundred miles apart in Zaire (Congo) and Sudan.
• Ebola is a horrific disease with 90 per cent mortality
• Transmission is by direct contact, but it is very
contagious.
• The reservoir of the virus has never been discovered.
• Infected monkeys from the Phillipines almost caused
an outbreak in Reston VA, near Washington in 1989.
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Rift Valley Fever
• Originally confined to sheep and cattle in Africa, it
jumped species and affected 200,000 people in Egypt
in 1977.
• The virus is transmitted by a mosquito.
• The 1977 outbreak is believed to have been triggered
by the construction of the Aswan dam which created
favourable conditions for the mosquitoes to breed.
• There was a similar outbreak in Mauritania after the
Senegal river was dammed.
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Lyme Disease
• Unknown before 1962, but now the most common
vector disease in the USA.
• Caused by a bacteria transmitted by bites from a tick
that lives on deer.
• Lyme disease has increased due to former farm land
reverting back to scrub which favours the growth of
the deer population.
• Especially common in the suburbs where people
come in contact with the deer population.
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Examples – Other
• Dengue Haemorrhagic Fever. Originally confined to
South East Asia, but now a growing threat in other
areas including the southern USA due to the diffusion
of the vectors (Aedes aegypti or Aedes albopictus).
• Latin American Haemorrhagic Fevers (Sabia,
Guaranito, Junin). Similar causes to Junin – i.e.
expansion of agriculture into new areas.
• The main risk to developed countries is the possibility
of a new air-borne disease. Hence the concerns about
SARS (2003) and Asian bird flu (2004), both of
which originated in Asia.
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Factors Leading to the Emergence of New
Infectious Diseases
1.
Cross-species transfer of pathogens
2.
Spatial diffusion of microbes
3.
Microbial mutation through recombination
4.
The new description of a pathogen that had been
present in humans for years, but which is “newly
recognized''.
5.
Changes in the human-environment relationship.
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Cross-species transfer of pathogens
• Cross-species transfer of pathogens is the first reason
for emergence, reflecting the de novo introduction of
a pathogen into the human population.
• This is usually due to alterations in the stability of
human–environment relations, usually by human
encroachment of previously isolated environments
that are habitats for zoonotic infectious diseases.
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Cross-species transfer of pathogens …
• Frequently, this is the result of deforestation, public
works projects in previously stable ecosystems, or
other intended or unintended changes in human–
environment relations
• Thus, human movement into these areas allows
species jumps of zoonotic microbes into the human
population.
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The spatial diffusion of microbes
• The diffusion of microbes from isolated endemic
areas to new foci of endemicity, with the concomitant
identification of what is thought to be a new agent.
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Microbial mutation through recombination
• The third explanation for the emergence of new
diseases is a result of microbial mutation through
recombination, either increasing the pathogenicity of
microbes, or the transformation of microbes into
pathogens.
• This frequently allows the development of resistance
to the immune response or to effective antimicrobial
treatment.
• The latter happens periodically with the genetic drift
and shift of influenza viruses, resulting in epidemics
and pandemics
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Recognition of a previously unidentified
pathogen
• Another cause for emergence of new
disease is the new recognition or
specification of a previously unidentified
pathogen.
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Changes in Human-Environment Relatioship
• The increased demand for food has resulted in new
areas being brought under cultivation, disrupting
existing ecosystems
• Monocultivation has reduced biodiversity, forcing
viruses to seek new hosts
• Rapid urbanisation results in high population
densities and insanitary conditions
• Global warming is extending the habitats of
mosquitoes and other vectors
• Competition for resources contributes to wars which
disrupt public health systems and trigger mass
population movements.
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Climate change and emerging infectious
diseases
• The ranges of infectious diseases and vectors are
changing in altitude, along with shifts in plant
communities and the retreat of alpine glaciers.
• Additionally, extreme weather events create
conditions conducive to ’clusters’ of insect-, rodentand water-borne diseases.
• Accelerating climate change carries profound threats
for public health and society.
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Infectious Disease Emergence and Human
Action
• Most of the factors are social and geographical
• Two key factors:
– human ecology
– behaviour
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Emergence and Human Action
• There is great urgency in improving the surveillance
for newly or recently emerging diseases, and
resurgent diseases.
• The containment and control of emerging infectious
diseases are of paramount importance.
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Emergence and Human Action
• Many of these diseases are potentially preventable,
through vaccination and adequate medical treatment,
but because much of the mortality from infectious
diseases occurs in developing countries, the cost of
vaccination and treatment is high—prohibitively so—
in many cases.
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Main priorities in emerging and reemerging
diseases research
• Supporting the application of recent discoveries and
new biomedical technologies to the identification,
management and control of emerging diseases
• Providing information for developing prevention and
treatment strategies by expanding research on host
susceptibility to emerging or reemerging pathogens
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HIV/AIDS
• The major lesson to be learned from the HIV/AIDS
pandemic is that the assumption that infectious
diseases are a phenomenon of the past, largely
restricted as major health threats to developing
countries, and that “international health” consists of
the study of problem of developing countries, are all
erroneous.
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