Download Human Ecology

Disease Ecology
Today we will discuss:
– History and Overview of Emerging Infectious
Diseases
– How are human exposures to disease affected by
ecosystem attributes?
• Lyme disease
– How does climate change influence the
prevalence and severity of certain diseases?
• Cholera
– Evolution of pathogens to antibiotic resistance
Major Factors Contributing to Emerging
Infections: 1992
1.
2.
3.
4.
5.
6.
Human demographics and behavior
Technology and Industry
Economic development and land use
International travel and commerce
Microbial adaptation and change
Breakdown of public health measures
Institute of Medicine Report, 1992
More Factors Contributing to Emerging
Infections: 2003
7. Human vulnerability
8. Climate and weather
9. Changing ecosystems
10. Poverty and social inequality
11. War and famine
12. Lack of political will
13. Intent to harm
Institute of Medicine Report, 2003
Emerging Infections:
Human Demographics, Behavior, Vulnerability
•
•
•
•
More people, more crowding
Changing sexual mores (HIV, STDs)
Injection drug use (HIV, Hepatitis C)
Changing eating habits: out more, more
produce (foodborne infections)
• More populations with weakened immune
system: elderly, HIV/AIDS, cancer patients
and survivors, persons taking antibiotics
and other drugs
Emerging Infections:
Technology and Industry
• Mass food production (Campylobacter,
E.coli O157:H7, etc…)
• Use of antibiotics in food animals
(antibiotic-resistant bacteria)
• More organ transplants and blood
transfusions (Hepatitis C, WNV,…)
• New drugs for humans (prolonging
immunosuppression)
Organ Transplantation
Year-end Waiting Lists vs. Transplanted
(kidney, liver, pancreas, heart, lung)
70,000
60,000
50,000
40,000
30,000
20,000
10,000
1988
0
1989
1990
1991
1992
1993
Patients Waiting
1994
1995
1996
1997
1998
Transplantations
Source: UNOS
CDC
Emerging Infections:
Economic Development, Land Use, Changing Ecosystems
• Changing ecology influencing waterborne,
vectorborne disease transmission (e.g.
dams, deforestation)
• Contamination of watershed areas by cattle
(Cryptosporidium)
• More exposure to wild animals and vectors
(Lyme disease, erhlichiosis, babesiosis,
HPS,…)
Emerging Infections:
International Travel and Commerce
• Persons infected with an exotic disease
anywhere in the world can be into major US
city within hours (SARS, VHF,…)
• Foods from other countries imported
routinely into US (Cyclospora,….)
• Vectors hitchhiking on imported products
(Asian tiger mosquitoes on lucky
bamboos,….)
Emerging Infections:
Microbial Adaptation and Change
• Increased antibiotic resistance with
increased use of antibiotics in humans and
food animals (VRE, VRSA, penicillin- and
macrolide-resistant Strep pneumonia,
multidrug-resistant Salmonella,….)
• Increase virulence (Group A Strep?)
• Jumping species from animals to humans
(avian influenza, HIV?, SARS?)
Emerging Infections:
Poverty, Social Inequality, Breakdown of Public Health
Measures
• Lack of basic hygienic infrastructure (safe
water, safe foods, etc..)
• Inadequate vaccinations (measles,
diphtheria)
• Discontinued mosquito control efforts
(dengue, malaria)
• Lack of monitoring and reporting (SARS)
Emerging Infections:
Intent to Harm
• Bioterrorism: Anthrax in US 2001
• Bio-Crimes: Salmonella in OR, Shigella in TX.
• Potential agents: Smallpox, Botulism toxin,
Plague, Tularemia, ….
Infectious Disease Ecology
Epidemiology: the study of the causes of diseases and
injuries to humans
−What causes disease?
−How do you identify the causes?
−Mechanistic
Disease ecology: the study of causes of emerging
diseases to natural populations and communities;
- Spatio-temporal patterns of diseases
−Why do the patterns of disease occur as they do?
−Conceptual: what variables are important?
Types of Infectious Human Diseases
• Infectious Disease – illness caused by a pathogenic
microbe (bacteria, virus, fungi, parasite, prions
(aberrant protein))
• Emerging Infectious Disease (EID) – “New,
reemerging or drug-resistant infections whose
incidence in humans has increased within the past
two decades or whose incidence threatens to
increase in the near future.” IOM 1992
• Zoonotic Disease
• Vector-borne Disease
• Non vector-borne
Zoonotic Disease
• Definition –disease caused by a pathogen that
primarily resides in a second species and is
transmitted to humans without an
intermediary species
• Examples: rabies, H1N1, many others . . . . . .
Vector-Borne Disease
• Definition -Infectious agents transmitted to
humans through action of another species
(often arthropods)
• Examples: Lyme disease, bubonic plague,
WNV, malaria, Rabies, many others . . . . .
• Also non vector-borne diseases (e.g. Cholera,
influenza, HIV-AIDS
Diseases can have 3 stages:
1.Endemic -number of cases maintained in the
human population (i.e. each infected person
transmits disease to only one other person)
2.Epidemic – excessive rise in number of new cases
of infectious disease (i.e. each infected person
transmits disease to more than one other person)
3.Pandemic – spread of infectious disease over a
large area (global)
Emerging infectious diseases
• Usually zoonotic
• Appear in areas undergoing ecological
transformation
• Result from adaptation to new hosts OR
• Reemerge as a result of antimicrobial resistance
• Increased in the past 2 decades
# EIDs by a) pathogen type, b) transmission type, c) drug resistance, d) transmission mode
EID hotspots -the human factor
# EID events
1
2–3
4–5
6–7
8–11
EID events from 1940 to 2004
Nature, 2008
Risks of Different Types of EIDs
Zoonotic (wildlife)
Zoonotic (non-wildlife)
Drug-resistant pathogens
Vector-borne pathogens
Trends in EIDs
•
•
•
•
2008 study published in Nature
EID events have risen significantly over time
peak incidence in the 1980s (HIV pandemic).
60.3% EID events are zoonotic (which include vector-borne
diseases in this study)
• 71.8% zoonotic diseases originate in wildlife (for example,
severe acute respiratory virus, Ebola virus), and are increasing
significantly over time.
• 54.3% EID events caused by bacteria or rickettsia (bacteria
that only grow inside living cells), reflecting a large number of
drug-resistant microbes
• EID origins are significantly correlated with socio-economic,
environmental and ecological factors (SEE MAP)
Lyme Disease
– Vector-borne disease
– Lyme disease is caused by a spiral-shaped bacterium, Borrelia
burgdorferi
– Bacterium is carried by the black-legged tick
– These ticks move about on mammal hosts such as deer and mice
– environmental factors that affect these non-human hosts have
implications for human exposure to Lyme disease
– Lyme disease is very serious in the northeast U.S.; if untreated it
leads to severe joint and nervous system problems
Lyme Disease Transmission
• The Lyme disease bacterium,
Borrelia burgdorferi, normally
lives on mice, squirrels and other
small animals.
• Transmitted among animals – and
to humans – through bites of
certain species of ticks.
• In the northeastern and northcentral US, the black-legged tick
(or deer tick, Ixodes scapularis)
transmits Lyme disease.
• In the Pacific US, spread by the
western black-legged tick (Ixodes
pacificus).
• Other major tick species in US
have not been shown to transmit
Borrelia burgdorferi.
Life cycle of blacklegged ticks
• Live for 2 years
• 3 feeding stages: larvae, nymph,
adult.
• Tick eggs are laid in the spring and
hatch as larvae in the summer.
• Larvae feed on mice, birds, and
other small animals in the
summer/fall.
• When a young tick feeds on an
infected animal, the tick takes
bacteria into its body along with
the blood meal, and it remains
infected for the rest of its life.
• After 1st feeding, the larvae
become inactive as they grow into
nymphs.
• In spring, nymphs seek
blood meals to grow into
adults. When nymph feeds, it
can transmit bacterium to a
new host (animal or a
human).
• Most human illness occurs
in late spring and summer
when nymphs are most
active and human outdoor
activity is greatest.
• Adult ticks feed on large animals or humans.
• In spring, adult female ticks lay eggs on the ground.
• Deer do not become infected by adult ticks, but deer are
important in transporting ticks and maintaining tick
populations.
Acorns and Lyme Disease: An Ecological Chain
Reaction
Acorns and Lyme Disease: An Ecological Chain
Reaction
• Oaks periodically produce large acorn crops, followed
by a few years of poor acorn production. This
phenomenon is called masting.
• Acorns are a high quality food for many vertebrate
consumers
• Masting results in a flush of resources available to
wildlife every 2-5 years
• Acorn production sets off a chain reaction that
affects the populations of gypsy moths (an
introduced forest pest that defoliates forests), and
black-legged ticks (vector of Lyme disease)
• In mast years, white-tailed deer specialize on acorns and are
attracted to oak-dominated forests.
• Autumn is also the peak activity period for the adult stage of
the black-legged tick.
• Deer are the preferred host for adult ticks;
• In oak forests, adult ticks take their final blood meal, drop off
the deer, and lay eggs (hatch the following summer).
• The density of larval ticks hatching from eggs in summer is
highly predictable based on acorn availability the prior
autumn. However, these ticks hatch from eggs free of Lymedisease;
• Larval ticks that feed on white-footed mice are much more
likely to acquire the Lyme-disease spirochete than are ticks
that feed on a variety of other vertebrate hosts.
• Therefore, the white-footed mouse is considered the principal
natural reservoir for Lyme disease.
• High acorn densities (in mast years) increases the number of
white-footed mice and white-tailed deer in oak forests
• Mice harbor the Lyme disease bacterium. More mice= more
Lyme disease
• Lyme disease infection is higher in summers following oak
mast years (high acorn productions)
• Acorn biomass could be used to predict Lyme disease risk.
• Mice eat gypsy moth larvae. More mice = fewer/less severe
gypsy moth outbreaks.
• Jones et al. (1998), Science
Cholera
• Vibrio cholerae
• Gram negative bacteria
abundant in freshwater and
estuaries around the world
• waterborne and attach to
crustacean zooplankton
• Toxin alters sodium pump in
intestinal cells  fluid loss
• gastrointestinal disease
• climatic and environmental
factors that affect water sources
and ecology of aquatic food
webs influence the dynamics of
cholera
Copepod Carrying Vibrio cholerae
First Cholera Pandemic
• Endemic to areas north of Bay of Bengal
(Bangladesh and eastern India)
Cholera
The Disease
• Enters from water or
food
• Colonizes small intestine
• Symptoms: nausea,
diarrhea, muscle
cramps, shock, severe
dehydration
Treatment
• Rehydration therapy,
antibiotics
Prevention
• Water treatment
– Filtration
– chlorination
World Cholera 2000-01
Why Has Cholera Re-emerged?
• Deteriorating sanitary facilities as larger
population moves into shanty towns
• Trujullo, Peru – fear of cancer from
chlorination so water untreated
• Use of wastewater on crops
• Africa – civil wars and drought caused
migrations into camps
How Has Cholera
Re-emerged?
• Simultaneous appearance along whole
coast of Peru
• Traveled in ship ballast?
• Traveled in plankton from Asia?
• Can remain dormant in local zooplankton
(copepods) until triggered by ???
Cholera and El Niño
• Periodic warming of water near coast of
Central and South America
• Large plankton blooms, especially in coastal
waters with nutrients from sewage runoff
Cholera and El Niño
• Cholera in Bangladesh also seen to fluctuate with
El Niño, but with 11 month lag
• Rita Colwell and multinational group studying link
between climate and cholera
• Satellite and surface data used to show cholera
incidence is related to sea surface temperature
Cholera and Sea Surface
Temperature
Cholera Antibiotic Resistance
• Cholera is
becoming
resistant to several
antibiotics
Will migratory birds continue to follow traditional flyways as the climate
changes, or will they adapt and perhaps in the process transport
avian influenza to new locations?
Graphic: UN Food and Agriculture Organization.
Malaria and Climate
• Vector-borne
• life-threatening, caused by 4 species
of Plasmodium parasites
• transmitted to people through the bites of
infected
mosquitoes.
• A child dies of malaria every 30 seconds.
• 247 million cases of malaria in 2006, 1 million deaths, mostly
among African children.
• Malaria is preventable and curable.
• Approximately half of the world's population is at risk of
malaria, particularly those living in lower-income countries.
• Travelers from malaria-free areas to disease "hot spots" are
especially vulnerable to the disease.
• Malaria takes an economic toll - cutting economic growth
rates by as much as 1.3% in countries with high disease rates.
Factors that Increase Malaria
Transmission
•
•
•
•
•
rainfall
floods
proximity of mosquito breeding sites to people
types of mosquito species in the area
Introduction of mosquito-borne parasite into areas
where people have had little prior contact/immunity
(causes large and devastating epidemics)
• mass population movements driven by conflict
Symptoms and Treatment
• fever, headache, chills, vomiting (10-15 days after infected).
Can cause severe illness and is often fatal without prompt
treatment.
• DRUG TREATMENT/RESISTANCE:
– combination of drugs known as ACTs
– However, the growing resistance of parasite to these meds
is undermining malaria control
• VECTOR CONTROL:
– increasing mosquito resistance to key insecticides DDT and
pyrethroids, particularly in Africa;
– a lack of alternative, effective insecticides;
– changing behaviours of local malaria-bearing mosquitoes,
which can result from vector control efforts (as insects
move to more hospitable areas).
Net Reproductive Ratio (R0)
Net Reproductive Ratio (R0)
• R0 < 1, each ‘infection generation’ is SMALLER than the last
• R0 = 1, each ‘infection generation’ is the same size as the last
• R0 > 1, each ‘infection generation’ is LARGER than the last;
what happens eventually?
Some Estimated Values of R0
R0
• What happens to R0 when
. . . . . disease results in immediate death to infected
individuals?
. . . . . vaccination programs are adopted?
. . . . . Population density increases?
. . . . . People don’t use good hygiene practices (washing
hands, covering coughs)
Vaccinations