the ecology of emerging infectious disease
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The ecology of emerging infectious disease. From the New York Times, July 16, 2012. Eastern equine encephalitis kills 4 people in Florida: Aug 18, 2010. Uganda Ebola outbreak confirmed - PowerPoint PPT PresentationTRANSCRIPT
The ecology of emerging infectious disease
From the New York Times, July 16, 2012
Eastern equine encephalitis kills 4 people in Florida: Aug 18, 2010
CDC: West Nile outbreak largest ever seen in U.S.Posted on: 5:39 pm, August 26, 2012, by Alix Bryan
Update to CDC's Sexually Transmitted Diseases Treatment Guidelines, 2010: Oral Cephalosporins No Longer a Recommended Treatment for Gonococcal InfectionsWeeklyAugust 10, 2012 / 61(31);590-594
Uganda Ebola outbreak confirmedHealth officials say mysterious illness that has killed 14 people in western Ugandan district of Kibaale is Ebola virus (July 29, 2012)
Infectious disease is a current important threat to human health and well being and to the integrity of natural and managed ecosystems
World Health Organization (1990)
37% human mortality attributable to infectious disease
1.7 billion tuberculosis infections
267 million cases of malaria
50 million reported cases of dengue fever
Institute of Medicine (1992)
54 new infectious diseases in US since1940
Nature 2008 (Jones et al.)
300 new human pathogens world-wide from 1940 – 2003
What is an emerging infectious disease (EID)?
One that has recently increased in occurrence
One that has recently expanded its geographic range
One that is caused by novel pathogen
Includes the emergence of novel pathogens and reemergence of previously controlled infectious diseases
Examples of emerging infectious diseases
Increased incidence - Lyme disease
Increased impact - Tuberculosis
Increased geographic range - West Nile virus
Evolution of new strain - Influenza viruses (H1N1)
Pathogen entering humans - Nipah virus
Newly discovered pathogen - Hendra virus
Previously controlled but now re-emerging:
Dengue fever
The rate of emergence is increasing
Emerging infectious diseases (EID)
recent increase in occurrence recent increase in geographic range, or effectcaused by a novel pathogen
What factors do you think account for emergence of
new diseases and re-emergence of old ones?
Drivers of emerging human pathogens*
Changes in land use and agricultural practices
Changes in human demography
Poor population health
Hospital and medical procedures
Pathogen evolution
Contamination of food or water supplies
International travel
Failure of public health programs
International trade
Climate change
*In order from most to least number of pathogens affected (2005)
the study of the
distribution and
abundance of organisms
Ecology
What does ecology have to do with infectious disease?
Predation and parasitismInfectious disease constitutes a classic form of species interaction (predator-prey) studied by ecologists.
Competition
FacilitationMutualism
Predation
CONSUMER - RESOURCE INTERACTIONS
Lethality Intimacy
HI LOW
HI
Host - Parasitoid Predator-prey
LOW
Host - Parasite Grazers
Two examples of how tools and concepts from ecology can be used to help predict, prevent and control outbreaks and emergence of infectious disease
1.Use of a mathematical model of species interactions to evaluate alternate means of responding to outbreak of the novel disease SARS.
2.The role of ecology in predicting the occurrence of Lyme Disease
What is a model and what can you do with it?
y = mx + b
A model is a simplified representation of something
Models can be used to describe, explain, or understand more complex reality.
Physical models
Conceptual modelMathematical model
Some questions that might be answered with a mathematical model of an infectious disease
1.What are the conditions under which an epidemic will occur?
2.What fraction of the population will become infected?
3.How would vaccination change the speed or duration of an epidemic?
4.How will treatment or other intervention affect the course of an epidemic?
S I R#susceptible #infected #removed
Ecologists use SIR models to study the interactions between parasites and their hosts
N is the total number of individuals in the population of hostsS is the number that are susceptible to a diseaseI is the number of individuals that are infected with the diseaseR is the number that are not susceptible or infected (removed)
S + I + R = N
S I R
is the rate at which the disease is transmitted
#susceptible #infected #removed
is the rate of recovery from the
disease
S + I + R = N
Ecologists use SIR models to study the interactions between parasites and their hosts
S I R
#susceptible #infected #removed
Equations describe how the numbers in each box change over time.
The change in the number of susceptible individuals through time
The change in the number of infected individuals through time
The change in the number of removed individuals through time
S I R
is the rate at which the disease is transmitted
is the rate of recovery from the disease
What information is in the sign of dI/dt?
#susceptible #infected #removed
Some questions that might be answered with a mathematical model of an infectious disease
1.What are the conditions under which an epidemic will occur?
2.What fraction of the population will become infected?
3.How would vaccination change the speed or duration of an epidemic?
4.How will treatment or other intervention affect the course of an epidemic?
SARS; Severe acute respiratory syndrome.
Reservoir host
Will there be a pandemic?
Possible responses to an emerging novel viral epidemic?
Vaccination
Isolation of infected individuals
Quarantine of contacts
Culling (of the reservoir, not the victims)
How do we decide which responses will be most effective?
Mathematical models are used to explain, explore and predict how biological systems work. We can conduct “experiments” with models that would be impossible or too slow to be used in an on-going epidemic.
Lipsitch et al. 2003 used a mathematical model to predict the effects of different control measures on the initial outbreak of SARS
Schematic diagram of a model of SARS
Indicates effect of intervention
Latent Infection designates individuals who are infected but do not have active disease and so are not (yet) infectious
The Model The ODE
Schematic diagram of a model of SARS
Use computer simulation to ask how isolation and/or quarantine would affect the course of an epidemic
Indicates effects of intervention
S
I
R
Latent Infection designates individuals who are infected but do not have active disease and so are not (yet) infectious
SARS Epilogue
Over 8000 cases world wide, over 750 deaths
Average mortality rate 9.6%, but variable among age groups
Cases reported from >2 dozen countries on 4 continents.
Last reported case in 2004 (lab acquired infection)
Development of vaccine is on-going
Early intervention is critical, greater surveillance is needed for prediction and early detection
Agent of Lyme disease
Yersinea pestis agent of plague
About 40% of all human diseases are caused by bacteria. Most cause disease via the production of toxins. Exotoxins are secreted or induced by live bacteria. Endotoxins are released when bacteria die.
Lyme disease
From CDC report 2006
How many humans view Lyme disease
Borrelia burgdorferiIxodes scapularis
Host infection
How an ecologist sees the system in which Lyme disease is embedded
vector
reservoir
reservoir
reservoir
Tick vector life cycle and host associations
uninfected
infected?
How an ecologist sees the system in which Lyme disease is embedded
vector
reservoir
reservoir
reservoir
Predicting risk of exposure from ecological data
Possible predictors
deer weather reservoirs weather mammalsacorns
Affect populations of rodents and attract deer
Affect survival of larval and nymphal ticks off hosts
Reservoir and “taxi”
Reservoir
Reservoir
The variables most closely associated with Lyme disease incidence
Chipmunk density a year earlier Acorn abundance 2 years earlier
What determines spatial variation in risk of infection?
Abundance of reservoir hosts, vectors, and humans must coincide, but the mechanisms underlying the distribution of each may vary.
“Hot spots” on North Atlantic coast, upper midwest, and northern California
How spatial variation in biodiversity of potential reservoir species influence the risk of Lyme disease
Logic: As the most competent reservoir host becomes a smaller fraction of the community, fewer tick nymphs will be infectious and the number of cases of Lyme disease will decline. This is called a dilution effect.
Ostfeld and Keesing 2000
Measured biodiversity of different components of the reservoir community in 10 “states” along the US eastern seaboard and looked for a relationship to cases of Lyme disease.
Effects of biodiversity on the incidence of Lyme disease
Suggests many birds may be competent reservoirs
Consistent with a dilution effect by less competent reservoirs
Consistent with known ability of fence lizards to clear the bacteria
Keesing and Ostfeld 2000 Conservation Biology
r2 = 0.54
r2 = 0.46
r2 = 0.47
Some contributions from ecological analyses of Lyme disease
Nymphs cause more infections than adults
The abundance of chipmunks and acorns are better predictors of the risk of Lyme disease than weather or deer abundance
Greater biodiversity of the small mammal community reduces the risk of Lyme disease
It’s not just Humans
Domestic animals: FMD in GB, Brucellosis in YNP
Wildlife: Chytrid fungus, white band disease
It’s not just animals
Dutch elm disease
Sudden oak death
Targets of agricultural biowarfare
Who should take responsibility for
prevention of disease emergence
and spread?
Roles in prediction, prevention, and control of infectious disease
Medicine: diagnosis and treatment of individuals
Biomedical research: identification of pathogens, development of treatment and defense of individuals
Epidemiology: Analysis of patterns of disease occurrence and their relationship to potential causes
Ecology: incorporate biological mechanism into prediction, prevention, and control of disease at the population, species, community, and ecosystem levels
Public health: Devising and implementing policy and practice to reduce the occurrence and spread of disease
Education:?
The study of infectious disease is inherently multidisciplinary but has fallen through the cracks, and is not taught systematically
Microbiology
Immunology
Public Health
Ecology
Mathematics
Summary
1.The emergence and reemergence of infectious disease is a serious threat.
2.Humans are not separate from the environment. Our actions and behaviors have consequences for the structure and integrity of natural ecosystems.
3.Understanding ecology provides one set of tools to address the problem of infectious disease
4.Current popular visibility provides an opportunity to educate students