Emerging

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Emerging Re-emerging Infectious Disease
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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 agents whose activities harm the hosts
  tissues

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• 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.

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• 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

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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.

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• 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.

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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.

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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

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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

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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

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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)

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• Variety of morphologies. 3 most common
• Bacillus (rod shaped)
• Coccus (round)
• Spirillum (helical rods)

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• Some bacteria require oxygen
• aerobes
• Some bacteria can not tolerate oxygen
• anaerobes
• Some bacteria can grow either with or without
  oxygen
• facultative anaerobes

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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

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Gram Negative Bacteria

• Salmonella typhi (causes Typhoid Fever)
• Yersinia pestis (causes the plague)

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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)

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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.

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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

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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)

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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

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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

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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

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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

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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

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Prions

• Infections particles that consist only of
  proteins
• Pronounced (pree-ons)
• Linked to some CNS degenerative disorders in
  humans and animals

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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

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Epidemiology

• Defined as the study of the occurrence of
  diseases in populations
• Epidemiologist are concerned not only with
  infectious disease but also with non-infectious
  disease such as cancer and environmental diseases
  such as lead poisoning

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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

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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

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Direct contact diseases

• Some diseases spread by direct contact include
• Ringworm
• AIDS
• Trichinosis
• Influenza
• Rabies
• Malaria

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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

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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

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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 its 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

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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)

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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.

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Vaccination

• A vaccine is either a killed or weakened strain
  of a particular pathogen
• The bodys 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

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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

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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.

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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

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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

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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.

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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

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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

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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

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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.

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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

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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

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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

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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

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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
  enzyme-catalyzed 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

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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.

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Treatment of Viral Diseases

• In general, drugs that effectively inhibit viral
  infections are highly toxic to host cells because
  viruses use the hosts metabolic enzymes in their
  reproduction.
• For this reason, most viruses are treated
  symptomatically until the hosts immune system
  controls and eliminates the virus or the host
  dies.

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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

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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

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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

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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

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Resistance to antimicrobial agents