Emerging & Re-emerging
Infectious Disease
Infectious Diseases
• Disease: conditions that impair normal
tissue function
• Genetic or Metabolic diseases: ex Cystic
Fibrosis
• Disease of aging: ex atherosclerosis
• Infectious Disease: caused by the invasion
of a host by agent’s whose activities harm
the host’s tissues
• Pathogens: microorganisms that are
capable of causing disease
• Opportunistic pathogens are potentially
infectious agents that rarely cause disease
in individuals with a healthy immune
system.
• The terms infection & disease are not
synonymous
• An infection results when a pathogen
invades and begins growing in a host
• Disease results only if and when, as a
consequence of the invasion and growth
of a pathogen, tissue function is impaired
Contagious vs. Virulent
• Some infectious agents are easily
transmitted (very contagious) but are not
likely to cause disease (not very virulent)
• Example: Polio-probably infects most
people it contacts, however only 5-10%
actually develop the disease.
• Other infectious agents are very virulent,
but not terribly contagious.
• Ex: Ebola hemorrhagic fever, virulence of
the virus is 50-90% fatality among those
infected, however the virus is not
transmitted easily by casual contact.
How do infections happen?
• In order to cause disease, pathogens must
be able to enter the host body, adhere to
specific host cells, invade and colonize
host cells, invade and colonize host
tissues, and inflict damage on those
tissues
• Pathogens usually enter through natural
orifices or through breaks in the natural
skin barrier such as wounds.
Corynebacterium diphtheria
• Bacteria that causes diphtheria
• Grows only on nasal and throat surfaces
• Damage to tissues is due to the production
of enzymes or toxins
• Toxin is distributed to other systems via
the circulatory system resulting in damage
to the heart, liver and nervous tissues
Streptococcus pyogenes
• Infectious agents associated with “strep
throat” and “flesh eating disease”
• Produces enzymes that break down
barriers between epithelial cells and
remove fibrin clots, helping the bacteria
invade tissues
Microbes
• 5 major types of infectious agents
– Bacteria
– Viruses
– Fungi
– Protozoa
– Helminths
In addition, a new class of infectious agents, the
prions, has recently been recognized
Bacteria
• Have no organized internal membranous
structure such as nuclei, mitochondria, or
lysomes.
• Most bacteria reproduce by growing and
dividing into two cells (binary fusion)
• Variety of
morphologies. 3 most
common
– Bacillus (rod shaped)
– Coccus (round)
– Spirillum (helical rods)
• Some bacteria require oxygen
– aerobes
• Some bacteria can not tolerate oxygen
– anaerobes
• Some bacteria can grow either with or
without oxygen
– facultative anaerobes
Gram Stain
• Bacteria are generally divided into 2 broad
classes based on their cell wall structure,
which influences their Gram stain reaction
• Gram Negative bacteria appear pink after
the staining procedure
Gram Negative Bacteria
• Salmonella typhi
(causes Typhoid
Fever)
• Yersinia pestis
(causes the plague)
Gram positive bacteria
• Appear purple after
gram stain procedure
• Staphylococcus
aureus (causes skin,
respiratory & wound
infections)
• Clostridium tetani
(produces a toxin that
can be lethal for
humans)
Viruses
• Not organisms themselves, cannot
reproduce apart from the host cell
• Cause disease by disrupting normal cell
function
• Classified using a variety of criteria
including shape, size, and type of genome.
Viruses
• Examples of viruses:
• Herpes viruses that cause chicken pox, cold
sores and genital herpes
• Poxvirus that causes smallpox
• Rhinovirus that causes the common cold
• Myxoviruses and paramyxoviruses that cause
influenza, measles and mumps
• Rotaviruses that cause gastroenteritis
• Retroviruses that cause AIDS and several types
of cancer
Fungi
• Reproduce primarily by forming spores
• Together with bacteria, fungi work to be
the “decomposers” in our environment
• Examples of diseases caused by fungi
– Ringworm
– Histoplasmosis (lung infection caused by bat
or bird droppings)
Fungi
• Yeasts of the Candida genus are
opportunistic yeast infections and cause
diseases such as vaginal yeast infections
and thrush (a throat infection) among
people who are immunocompromised or
undergoing antibiotic therapy
Protozoa
• Do have cell walls so are able to have
rapid and flexible movements
• Can be acquired through contaminated
food or water or by a bite of an infected
anthropod such as a mosquito
Protozoa Diseases
• Giardia lamblia & cryptosporidium parvum
are two protozoa parasites that cause
diarrheal disease in the US
• Malaria, a tropical illness, is caused by
several species of the protozoan
Plasmodium
Helminths
• Are simple, invertebrate animals, some
which are infectious parasites
• Because they are animals, their
physiology is similar to humans, making
this parasite difficult to treat because
drugs that kill helminths are frequently
very toxic to humans
Helminths
• Have complex reproductive cycles and
many require a host
• Examples:
– Schistosoma, Caused by a flat worm
• Results in diarrhea and abdominal pain
– Trichinella spiralis, caused by a roundworm
• Results in vomiting, diarrhea and fever, later
symptoms include muscle pain, congestive heart
failure and respiratory paralysis
Prions
• Infections particles that consist only of
proteins
• Pronounced (pree-ons)
• Linked to some CNS degenerative
disorders in humans and animals
Prions
•
•
•
•
Examples of diseases:
Creutzfeldt-Jakob disease in humans
Scrapie in sheep
Bovine spongiform encephalopathy (“mad
cow disease”) in cattle
• Some prion diseases are inherited, others
are caused from eating infected tissue
Epidemiology
• Defined as the study of the occurrence of
diseases in populations
• Epidemiologist are concerned not only
with infectious disease but also with noninfectious disease such as cancer and
environmental diseases such as lead
poisoning
Disease reservoirs
• The reservoir for a disease is the site
where the infectious agent survives
• Example: Humans are the reservoir for the
measles virus because it does not infect
other organisms
• Wild rodents are the reservoir for Yersinia
pestis, the bacteria that causes the plague
• Soil is the reservoir for the bacteria
Clostridium tetani which causes tetanus
Modes of transmission
• Transmitted by either direct or indirect
contact
• Direct contact is when an individual is
infected by contact with the reservoir
• Example: touching an infected person,
eating infected meat, being bitten by an
infected animal, inhaling infectious
droplets from a sneeze or cough, intimate
sexual contact
Direct contact diseases
• Some diseases spread by direct contact
include:
• Ringworm
• AIDS
• Trichinosis
• Influenza
• Rabies
• Malaria
Modes of Transmission
• Indirect contact occurs when a pathogen
can withstand the environment outside its
host for a long period of time before
infecting another individual
• Examples: tissues or toys handles by sick
people, ingesting food or beverages
contaminated by contact with a disease
reservoir
Indirect contact
• Fecal-oral route is a significant form of
indirect transmission for gastrointestinal
diseases such cholera, rotavirus infection,
cryptosporidiosis, and giardiasis.
• Also included in indirect transmission are
diseases transmitted from parent to child
such as AIDS and herpes encephalitis
Role of research in prevention
• Understanding the infectious cycle is critical in
order to identify accessible targets for control
strategies
• Examples: Vector-borne diseases may be
prevented by control methods that either kill the
vector or prevent it’s contact with humans.
Vaccines may prevent the development of a
pathogen within a host, drugs may be used to
prevent infections or suppress the disease
process
Host Defenses against Infectious
Disease
• The humans body has several
mechanisms for preventing diseases
• These include anatomical barriers to
invading pathogens (skin, bone, nasal
passage), physiological deterrents to
pathogens (tears, sweat, blood, vaginal
secretions), and the presence of normal
flora (microorganisms present on and in
the body)
Immunity
• When a host encounters an antigen that
triggers a specific immune response for
the second or later time, the memory
lymphocytes recognize it and quickly begin
growing and dividing. The immune
response occurs so quickly the second
time that the pathogen does not have time
to reproduce before the body has
destroyed it.
Vaccination
• A vaccine is either a killed or weakened
strain of a particular pathogen
• The body’s immune system will respond to
these vaccines as if they contain the
actual pathogen.
• As a result, memory lymphocytes will
rapidly respond when the actual pathogen
is encountered, destroying the pathogen
before it can reproduce
Vaccination
• Many diseases are not easily prevented by
vaccination
• Some pathogens have a way to evade the
immune response either by not allowing
antibodies to pass through their cells, by
disguising themselves as a host cell or by
mutating.
• Cold and influenza viruses are examples
of rapidly mutating pathogens
Public Health measures to prevent
Infectious Disease
• Safe water: Many pathogens that cause
gastrointestinal diseases (cholera &
typhoid fever) are transmitted in water.
• Travelers to developing countries are
asked to be immunized
• Unnecessary in the US and developed
countries because the water used for
drinking, washing and preparing food is
purified before it goes into homes.
Water purification
• Purification methods include settling, filtration
and chlorination.
• Well water is usually safe if guidelines about
distance from sewage disposal facilities is
followed.
• When purification systems breakdown, such as
with unusual flooding, drinking water may not be
safe and should be boiled or treated with
chlorine prior to ingesting.
• Public water must be protected from sewage
Sewage treatment and disposal
• Sewage includes wash water, water from
toilets and storm run-off
• These fluids may carry pathogens for
many waterborne diseases, including
giardiasis and hepatitis A.
• To ensure safety, the US government
requires that sewage be treated to
eliminate pathogens
Food Safety programs
• The US has many standards, inspection
plans and regulations about food
preparation, handling and distribution.
• Meat packing plants, restaurants and
supermarkets are all inspected
• Milk is pasteurized and dated for sale and
is analyzed periodically for contamination
• Canning and preserving foods is
maintained through quality control checks.
Animal control programs
• Aniamls are carriers of many diseases that also
affect humans
• Inspecting herds animals for tuberculosis (due to
the bacterium Mycobacterium bovis) and
brucellosis (a disease that causes spontaneous
abortion in herd animals and abscesses of the
liver, spleen, bone marrow and lymph nodes in
humans) has helped prevent passage of those
organisms in milk and infected meat
Animal Control programs
• Before dogs can be licensed, owners must
show proof of rabies vaccination
• Most cases of rabies in the US is due to
bites from wild and stray animals, these
animals are usually impounded and
destroyed
• Bubonic plague is spread by rats. Rat
control in urban areas is a major
component of public health efforts
Animal control measures
• Insect-borne diseases such as malaria can
be controlled by eliminating breeding
areas for insects and using pesticides.
• Examples: draining areas where stgnant
water collects
Vaccination Programs
• Most states now require parents show proof of
vaccination before enrolling their children in
daycare or public schools.
• If a certain proportion of the population is
immune to the disease, the pathogen that
causes the disease will be unable to reproduce
itself at a high enough level to maintain itself in
the population. Eventually the pathogen cannot
spread any further and may be eliminated from
the population.
Public Health Organizations
• City and state agencies are responsible to
enforce regulations, provide public health
services such as vaccinations, and
monitor and report the incidence of a
particular disease to state and federal
agencies.
• All these agencies report data to the US
Public Health Service
Public Health Organizations
• NIH: National Institute of Health. One of
the 8 health agencies of the US Public
Health Service
– Supports health related research aimed at
understanding, preventing, treating and
controlling infectious and other diseases of
humankind
Public Health Organizations
• Centers of Disease Control (CDC)
– In Atlanta, Ga. Investigates disease outbreaks,
publish epidemiological reports, sponsor a variety of
educational programs, research projects and
reference laboratories
• Food & Drug Administration (FDA)
– Monitors the safety of our food, medicines, and many
other products we use daily
• World Health Organization (WHO)
– Provides international surveillance and control of
disease
Treatment of infectious diseases
• Antibiotic literally means “destoyer of life”
• Antibiotics refer to a chemical substance
used to treat bacterial infections
• Antimicrobial refers to anything that
inhibits the growth of microbes which does
not include antihelminthics because
worms are not microscopically small
Antibiotics
• Drugs used to treat bacterial diseases
• Grouped into categories based on their modes
of action
• In general, these drugs inhibit cell wall synthesis,
nucleic acid synthesis, or other enzymecatalyzed reactions
• Examples: penicillins, cephalosporins,
tetracycline and erythromycin
• Some antibiotics can be toxic to human tissues
when used in high doses or prolonged periods of
time
Antibiotics
• Rifampicin-used to treat TB (tuberculosis)
• Sulfonamides interfere with the synthesis
of folic acid, a vitamin necessary for
nucleic acid synthesis. Most bacteria must
synthesize their own folic acid because
their membranes are impermeable to
external folic acid.
Treatment of Viral Diseases
• In general, drugs that effectively inhibit
viral infections are highly toxic to host cells
because viruses use the host’s metabolic
enzymes in their reproduction.
• For this reason, most viruses are treated
symptomatically until the host’s immune
system controls and eliminates the virus or
the host dies.
Antiviral drugs
• Drugs that are typically target virus
specific
• Acyclovir-used to treat outbreaks of genital
herpes
• Amantadine-used to prevent or moderate
influenza among those at high risk of
severe illness from the disease
Antiviral Drugs
• AZT-inhibits that replication of the HIV
genome
• AIDS patients today are prescribed
proteases that interfere with the packaging
of the HIV genome into virus particles
Treatment of fungal and parasitic
disease
• The development of drugs to treat fungal,
protozoan, and helminthic diseases is
challenging because the agents that kill or
inhibit growth in the organisms are also
highly toxic to human cells.
• Amphotericin B is an antifungal that
disrupts the cell membrane
• Antihelminthic drugs target the adult worm
Protozoa Treatment
• Malaria was successfully treated with
chloroquine
• In recent decades, Plasmodium species
that are resistant to this drug have
appeared and spread to areas where
malaria is a common threat. In those
areas, a combination of the drugs
sulfonamide and pyrimethamine is
frequently used to treat the disease
Resistance to antimicrobial agents