Download The take home message: The burden of infectious disease in the

Agents of infectious disease in
developing countries
February 20, 2007
Infectious diseases in the
developing world
• Comprise 20% of all deaths worldwide
• The burden of disease comes from a wide range
of microbes
• The take home message: The burden of
infectious disease in the developing world is
immense, and comes from many different
sources
The study of individual diseases:
why?
• There are many, many infectious diseases that
burden the developing world
• They have unique biological characteristics
• They cause unique kinds of damage
• In order to defeat the enemy, you must first
understand what it is
• Understanding why these diseases survive and
thrive is key to controlling them
Microbes: the unseen universe
• Microbes are incredibly diverse and incredibly
well adapted to their niches in the world
• To know how to control microbes, we need to
know how they live
– Where do they live?
– What are their life cycles: how do they ensure their
own survival?
– How do they survive both inside and outside
humans?
Key concepts for understanding
microbial life
• There are three main classes of microbes
• They differ in their biological complexity
– Single celled
– Multi-celled
– No cells at all!
• They differ in their ability to survive outside their
human host
• Some need us; some don’t
• They differ in the complexity of their life cycles
– Some live entirely in a human host
– Some have complex cycles that go from us to the
environment and back to us
Microbial life: a hierarchy
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There are three broad categories of microbes
The viruses
The bacteria
The parasites
30-200 nm
1-2 microns
5 microns
The Viruses
• The smallest microbes
• Consist of nucleic acid (DNA or RNA) and protein
• Sometimes surrounded by membrane (envelope)
– Affects how well they survive in the environment
• Taxonomy based on
– Physical characteristics
– Host range (not so much any more)
– Nucleic acid similarity
• Have family, genus, and species, somewhat like bacteria
The Viruses
• Incapable of reproducing on their own
– Totally dependent on a living host (human or animal) to
produce more viruses
– Hijack the machinery of living cells in order to reproduce
themselves
• Can survive free in the environment, but do not
reproduce there
– Survive in an inert state, waiting for contact with a host
– Some remain infectious longer than others outside a host
– Can survive in food, water, soil, on surfaces and objects
Control of viral diseases
• Too small to be caught by size filtration
• Filtration by charge interaction (stick to stuff)
• Susceptible to chemical disinfectants
• Can die off naturally in environment, but this is
highly variable and influenced by many different
factors (temperature, humidity, etc)
• Vaccination can successfully control many
serious viral diseases
The Bacteria
• Single-celled organisms
• Less complex internal structures than animal
cells
• Cell walls made of sugars/lipids
• Replicate by binary fission
The Bacteria
• Huge amount of diversity; many different cell
types, habitats, survival strategies
• Some live exclusively in association with living
hosts; others can live free in the environment
Control of Bacteria
• Can be removed by size filtration
• Can be removed by coagulation and settling
processes
• Susceptible to chemical disinfectants
• Can die off in the environment, but;
– Some bacteria naturally live free in the environment
– Some bacteria have very tough environmental forms
capable of living for a long time
The Parasites
• Very diverse group of microbes
• Microscopic single celled organisms
– More complex cellular structures than bacteria
• Multicellular organisms: worms that can be seen
with the naked eye
The Parasites
• Have complex life cycles
– Life cycles consist of several distinct stages
– Some stages take place in living hosts
– Some stages take place in the environment
– Sexual and asexual stages of reproduction
• Have forms that can live in the environment,
but need living hosts for their complete life cycle
and reproduction
Example: a parasite life cycle
Control of parasites
• Can be removed by size filtration
• Can be removed by coagulation and settling processes
• Not susceptible to chemical disinfectants
• Can die off in the environment, but;
– Some have very tough environmental forms capable of living
for a long time
– Some live in insect vectors when they’re not living in the
human body
Respiratory Diseases: the viruses
• Adenovirus
– Family Adenoviridae, genus adenovirus
• Influenza
– Family Orthomyxoviridae, genus influenzavirus
• Parainfluenza
– Family Paramyxoviridae, genus respirovirus
• Respiratory syncytial virus
– Family Paramyxoviridae, genus pneumovirus
Respiratory Diseases: the bacteria
• Streptococcus pneumoniae
• Haemophilus influenzae
• Both causes of both pneumonia and meningitis
• Often set in as secondary infections after viral
respiratory infection weakens the host
• Vaccine preventable
Respiratory disease
• Bacterial and viral: both causes of pneumonia
• The leading cause of infectious disease death
among children (2 million/year)
• Bacterial infections are more likely to cause
death (often secondary infections)
• At highest risk are newborns, children with
malaria, and malnourished children
Human immunodeficiency virus
• Second biggest killer after respiratory
infections
• Death is not from HIV, but from the
opportunistic infections that devastate the
human body once the immune defenses are
eliminated by the virus
• Those with HIV are vulnerable to every one
of these infections
Diarrheal Disease: the viruses
• Rotavirus
– Family Reoviridae, genus rotavirus
• Norovirus
– Family Caliciviridae, genus norovirus and saprovirus
• Astrovirus
– Family Astroviridae, genus astrovirus
• Adenovirus
– Family Adenoviridae, genus adenovirus
• Hepatitis A virus
– Family Picornaviridae, genus hepatovirus
• Polio
– Family Picornaviridae, genus enterovirus
Diarrheal Disease: the bacteria
• Cholera (Vibrio cholerae)
• Shigella (S. dysenteriae, S. flexneri, S. sonnei, S. boydii
• Escherichia coli
• Salmonella (S. typhi, S. typhimurium)
• Campylobacter (C. jejuni)
Diarrheal Disease
• Can be both endemic and epidemic
• Fecal-oral and person-to-person transmission
• Can survive outside the human body in water,
soil, feces
• These are also vehicles for transmission
• Death by dehydration, especially in children
Tuberculosis
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Causative agent: Mycobacterium tuberculosis
Acid-fast rod-shaped bacterium
person-to-person transmission by aerosols
Does not live in the environment, but can live in
some animals
Tuberculosis
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Becomes chronic once it establishes itself
Fever, weight loss, night sweats
A disease that causes a slow decline
Half of untreated patients die within 5 years
Those who are already debilitated by other
diseases are most susceptible
• Lack of water and sanitation creates populations
vulnerable to TB
The Childhood Diseases
• Whooping cough
– Bacteria: Bordatella pertussis
• Measles
– Virus: Family Paramyxoviridae, Genus
Moribillivirus, measles virus
• Tetanus
– Bacteria: Clostridium tetani
Whooping cough
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Person-to-person by contact and droplets
Paroxysmal coughing fits
Can cause vomiting
Characteristic “whoop” from the inability to get enough
air through coughing fits
• Most dangerous to very young infants
• Vaccine preventable
• In nonimmunized populations with malnutrition
and underlying respiratory and enteric infection, it
is one of the most lethal childhood diseases
Measles
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Respiratory infection
Spread by droplet and contact
Cause of pneumonia and encephalitis
Fatality rates can read 10-30% in susceptible
populations
• Exacerbates complications of malnutrition
• Vaccine preventable
Tetanus
• Caused by the toxin produced by C. tetani
• Muscle spasms (“lockjaw”) and respiratory
failure
• Lives in soil and feces
• Introduced into the body through dirty wounds
• Children: tetanus neonatorum
• Vaccine preventable
Malaria
• A single-celled parasite of the genus Plasmodium
• Four species: falciparum, vivax, ovale, malariae
• Vector: the Anopheles mosquito
• Tropical and subtropical disease
Malaria
• Not only is malaria a killer in and of itself, but it
leaves people susceptible to many other diseases
• Disease control requires vector control
• Elimination of breeding sites of the vector
• Improved sanitation can help achieve this
The parasitic diseases
• Ascaris
– Intestinal nematode (roundworm)
– Ascaris lumbricoides
– Infectious stages is the egg, which is excreted in feces and survives in
warm, moist areas
– Can cause stunting, migrate to other organs in the body
• Hookworms
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Intestinal nematodes (roundworms)
Ancylostoma duodenale and Necator americanus
Eggs are excreted in feces and grow in warm, moist areas
Become the infectious stage, the larvae
Infect by penetrating the skin
Attach in intestine and cause blood loss
The parasitic diseases
• Schistosomiasis
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Trematodes (flatworms)
Schistosoma haematobium, S. japonicum, and S. mansoni
Eggs are shed in feces
Life cycle takes place partly in water, requiring a snail as an intermediate
host
– People become infected by coming in contact with water
– Worms migrate to organs
• Cryptosporidium
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C. parvum: coccidian single-celled parasite
Infectious form, the oocyst, is shed in feces
Infection acquired by ingesting contaminated water
Cause of chronic diarrhea
The parasitic diseases
• Cysticeriosis
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Caused by cestodes (tapeworms)
Taenia solium
Infectious stage, the egg, is shed in feces
Pigs ingest eggs from contact with human feces
People ingest eggs from fecal-oral transmission or ingestion of
undercooked pork
– Worms migrate to organs
• Dracunculiasis
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Nematode (roundworm)
Dracunculus medinensis
Transmitted through water
Humans ingest water with copepods (small crustaceans), which contain
the larvae
– Worms migrate though the body
– Worms escape through the skin, leading to contamination of water when
infected individuals come in contact with it
The parasitic diseases
• Entamoeba
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Single celled amoeba
Entamoeba histolytica
Mature cyst is the infectious form
Shed in feces
Spread through contaminated food, water, hands
Spread through the body
• Giardia
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Single celled protozoan
Giardia lamblia
Cyst is the infectious form
Shed in feces
Spread through contaminated food, water, hands
Diarrheal disease; can be chronic and cause malabsorption
Infectious diseases: the take
home message
• The burden of disease is an interplay among
different kinds of diseases
• Those who are already weak and unhealthy are
vulnerable to infection
• Those who are already debilitated by infection
are prime hosts for even more microbes
• Improvements in water and sanitation have
ripple effects far beyond just the prevention
of enteric disease