Video 2.1: Ecology and evolution of infectious diseases

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En provenance du cours de The University of Hong Kong

Epidemics

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The University of Hong Kong

Epidemics

29 notes

“If history is our guide, we can assume that the battle between the intellect and will of the human species and the extraordinary adaptability of microbes will be never-ending.” (1) Despite all the remarkable technological breakthroughs that we have made over the past few decades, the threat from infectious diseases has significantly accelerated. In this course, we will learn why this is the case by looking at the fundamental scientific principles underlying epidemics and the public health actions behind their prevention and control in the 21st century. This course covers the following four topics: 1. Origins of novel pathogens; 2. Analysis of the spread of infectious diseases; 3. Medical and public health countermeasures to prevent and control epidemics; 4. Panel discussions involving leading public health experts with deep frontline experiences to share their views on risk communication, crisis management, ethics and public trust in the context of infectious disease control. In addition to the original introductory sessions on epidemics, we revamped the course by adding: - new panel discussions with world-leading experts; and - supplementary modules on next generation informatics for combating epidemics. -------------------------------------------------------- (1) Fauci AS, Touchette NA, Folkers GK. Emerging Infectious Diseases: a 10-Year Perspective from the National Institute of Allergy and Infectious Diseases. Emerg Infect Dis 2005 Apr; 11(4):519-25. What you'll learn - Demonstrate knowledge of the origins, spread and control of infectious disease epidemics - Demonstrate understanding of the importance of effective communication about epidemics - Demonstrate understanding of key contemporary issues relating to epidemics from a global perspective

À partir de la leçon

Theme One: Origins (Emergence and ecology of infectious diseases)

Week 2: Introduction1:28

Video 2.1: Ecology and evolution of infectious diseases8:54

Video 2.2: Emerging infectious diseases at the human-animal interface8:21

Video 2.3: Phylogenetics9:35

Video 2.4: Emergence of highly pathogenic H5N1 avian influenza virus in Asia10:04

Video 2.5: Emergence of the H7N9 influenza A virus in China9:14

Video 2.6: Swine Influenza and the 2009 Pandemic H1N16:51

Rencontrer les enseignants

Gabriel M. Leung (HKU)
Dean
Li Ka Shing Faculty of Medicine
Joseph T. Wu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Kwok-Yung Yuen (HKU)
Professor
Li Ka Shing Faculty of Medicine
Tommy Lam (HKU)
Assistant Professor
Li Ka Shing Faculty of Medicine
Maria Huachen Zhu (HKU)
Associate Professor
Li Ka Shing Faculty of Medicine
Guan Yi (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Benjamin Cowling (HKU)
Professor
Li Ka Shing Faculty of Medicine
Thomas Abraham (HKU)
Associate Professor
Journalism and Media Studies Centre
Mark Jit (LSHTM)
Professor
Department of Infectious Disease Epidemiology
Malik Peiris (HKU)
Chair Professor
Li Ka Shing Faculty of Medicine
Marc Lipsitch (Harvard)
Professor
Department of Epidemiology

This session introduces the concept

of ecology and evolution of infectious diseases.

I will first talk about the view

of host-pathogen interactions as part of the ecosystem.

Second, the role of evolution in such

a disease ecosystem, and its impact

on disease control and prevention measures.

Ecology is the study of interactions

among organisms and their environment.

A food web here shows an example

of such interactions in an ecosystem.

If the population of a prey, in this case, the rabbit,

increases, the population of the predator,

the fox, will start to increase because of more food.

The predator population will drop

after it depletes the prey population.

Sometimes, animals have infectious diseases,

which are often caused by parasitic pathogens.

In microscopic view, the infectious diseases

involve complex interactions of at least two species,

the pathogen and the host.

The pathogen lives in and obtains resources from a host,

which can result in host death or development of immunity.

It also relies on an individual host to transmit

to other individuals,

which is often a host density dependent process.

This means more individuals get infected

when the host population is denser,

and less when the density is lower.

When the pathogen spreads through and depletes

the host population,

the transmission will eventually slow down.

Then the natural birth of host may restore the population,

and subsequently the pathogen transmission

will increase again when the host density becomes higher.

This two-way regulation between the pathogen and host

abundance forms the basic disease ecosystem

and represents an important integral part

of the larger ecosystem.

In a disease ecosystem, some pathogens

can infect more than one host.

Other pathogens are transmitted from one host to the other

through at least one species of vector.

For example, malaria is a mosquito-borne disease

caused by Plasmodium parasites and requires mosquitos

as a vector for human and animal transmission.

As a part of the larger ecosystem,

environments also affect the emergence of infectious disease

with the effect on the hosts, pathogens and vectors.

For example, in some countries,

a lower temperature and humidity

facilitates human influenza transmissions.

Level of rainfall also affects the occurrence

of some water and vector-borne diseases.

For example, malaria epidemics tend to occur

during rainy seasons in the tropical countries

because of the higher mosquito abundance.

Cholera epidemics occurred in some poor areas

during severe drought because of the water scarcity

that results in poor hygiene;

hence more people are exposed to the contaminated water.

Such climate factors often drive

an infectious disease into seasonal epidemics.

Disease ecologists are often concerned about the burden

to a host population by an infectious disease.

Host extinction is perhaps the worst consequence

caused by an infectious disease.

However, in theory, a single-host pathogen

with density dependent transmission

seldom leads to host extinction

because the transmission will naturally slow down

when the host population drops.

The disease will then fade out

if all transmission chains are broken.

In contrast, a multi-host pathogen

could potentially cause the extinction

of a host population.

An example is the introduction of North American

grey squirrels to England,

which is resistant to the squirrel poxvirus.

But that poxvirus is lethal to

the native English red squirrels.

Therefore, the poxvirus was maintained

in the grey squirrels, and continuously transmitted

to and infected the red squirrels even when

the red squirrel population became very low.

The poxvirus eventually wiped out the English red squirrels.

All organisms, including pathogens, evolve.

A new phenotype is sometime expressed from random mutations

that occur in the reproductive process.

Evolution takes place when nature tends to retain

a new phenotype because of

the advantage conferred to the organism.

An example here shows how evolution works

in a host-pathogen system.

After a host recovers from the infection by a pathogen,

adaptive immunity against this specific pathogen

is often developed.

Such immunity mainly recognizes

the surface antigen of the pathogen –

which is usually a protein encoded

by the pathogen genome.

When this recovered host is exposed

to the same pathogen again, the adaptive immunity

will protect the host from infection.

However, pathogens are smart - they mutate.

So, on the one hand, there is continuous natural selection

of pathogen mutants that encode a varied antigen

to escape from the pre-existing immunity.

On the other hand, the host immune system

continues to develop new immunity

when the mutated pathogen invades the host.

Such an arms race between pathogens and host immune system

drives their continuous co-evolution,

and shapes the patterns of disease emergence and re-emergence.

An example of this is the human seasonal influenza.

In addition to the annual influenza epidemics,

which are probably driven by the climate,

larger epidemics usually occur every several years.

They were caused by the emergence of new antigenic

variants of the influenza virus that were able to

effectively escape from the pre-existing immunity

in the human population.

Vaccines developed for previous influenza variants

often become useless to protect

from these antigenic variants.

While evolution of influenza is inevitable,

one of the global efforts is to detect

the new antigenic variants

and produce the corresponding vaccine

as soon as they emerge, so as to reduce

the burden of these larger epidemics.

So, viruses escape host immunity by their evolution.

In a similar way, bacteria pathogens also develop

drug resistance through their evolution.

This has been highlighted by the recent expansion

of multi-antibiotic-resistant bacteria.

This becomes a global concern whether our evolution

in drug development will win over

the evolution of bacteria and viruses.

In summary, pathogen-host interaction

is an important integral part of the ecosystem.

Better understanding of disease ecology and evolution

help us to develop a more effective control

and prevention measure against the disease.

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