2Infectious DiseasesDisease: conditions that impair normal tissue functionGenetic or Metabolic diseases: ex Cystic FibrosisDisease of aging: ex atherosclerosisInfectious Disease: caused by the invasion of a host by agent’s whose activities harm the host’s tissues
3Pathogens: microorganisms that are capable of causing disease Opportunistic pathogens are potentially infectious agents that rarely cause disease in individuals with a healthy immune system.
4The terms infection & disease are not synonymous An infection results when a pathogen invades and begins growing in a hostDisease results only if and when, as a consequence of the invasion and growth of a pathogen, tissue function is impaired
5Contagious 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.
6Other 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.
7How 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 tissuesPathogens usually enter through natural orifices or through breaks in the natural skin barrier such as wounds.
8Corynebacterium diphtheria Bacteria that causes diphtheriaGrows only on nasal and throat surfacesDamage to tissues is due to the production of enzymes or toxinsToxin is distributed to other systems via the circulatory system resulting in damage to the heart, liver and nervous tissues
9Streptococcus 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
10Microbes 5 major types of infectious agents Bacteria Viruses Fungi ProtozoaHelminthsIn addition, a new class of infectious agents, the prions, has recently been recognized
11BacteriaHave no organized internal membranous structure such as nuclei, mitochondria, or lysomes.Most bacteria reproduce by growing and dividing into two cells (binary fusion)
12Variety of morphologies. 3 most common Bacillus (rod shaped)Coccus (round)Spirillum (helical rods)
13Some bacteria require oxygen Some bacteria can not tolerate oxygen aerobesSome bacteria can not tolerate oxygenanaerobesSome bacteria can grow either with or without oxygenfacultative anaerobes
14Gram StainBacteria are generally divided into 2 broad classes based on their cell wall structure, which influences their Gram stain reactionGram Negative bacteria appear pink after the staining procedure
16Gram positive bacteria Appear purple after gram stain procedureStaphylococcus aureus (causes skin, respiratory & wound infections)Clostridium tetani (produces a toxin that can be lethal for humans)
17VirusesNot organisms themselves, cannot reproduce apart from the host cellCause disease by disrupting normal cell functionClassified using a variety of criteria including shape, size, and type of genome.
18Viruses Examples of viruses: Herpes viruses that cause chicken pox, cold sores and genital herpesPoxvirus that causes smallpoxRhinovirus that causes the common coldMyxoviruses and paramyxoviruses that cause influenza, measles and mumpsRotaviruses that cause gastroenteritisRetroviruses that cause AIDS and several types of cancer
19Fungi Reproduce primarily by forming spores Together with bacteria, fungi work to be the “decomposers” in our environmentExamples of diseases caused by fungiRingwormHistoplasmosis (lung infection caused by bat or bird droppings)
20FungiYeasts 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 therapyAntibiotics reduce the bacterial population normally present in the throat and vagina, allowing the yeast to grow unchecked
21ProtozoaDo have cell walls so are able to have rapid and flexible movementsCan be acquired through contaminated food or water or by a bite of an infected anthropod such as a mosquito
22Protozoa DiseasesGiardia lamblia & cryptosporidium parvum are two protozoa parasites that cause diarrheal disease in the USMalaria, a tropical illness, is caused by several species of the protozoan Plasmodium
23HelminthsAre simple, invertebrate animals, some which are infectious parasitesBecause 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
24Helminths Have complex reproductive cycles and many require a host Examples:Schistosoma, Caused by a flat wormResults in diarrhea and abdominal painTrichinella spiralis, caused by a roundwormResults in vomiting, diarrhea and fever, later symptoms include muscle pain, congestive heart failure and respiratory paralysis
25Prions Infections particles that consist only of proteins Pronounced (pree-ons)Linked to some CNS degenerative disorders in humans and animals
26Prions Examples of diseases: Creutzfeldt-Jakob disease in humans Scrapie in sheepBovine spongiform encephalopathy (“mad cow disease”) in cattleSome prion diseases are inherited, others are caused from eating infected tissue
27EpidemiologyDefined as the study of the occurrence of diseases in populationsEpidemiologist are concerned not only with infectious disease but also with non-infectious disease such as cancer and environmental diseases such as lead poisoning
28Disease reservoirsThe reservoir for a disease is the site where the infectious agent survivesExample: Humans are the reservoir for the measles virus because it does not infect other organismsWild rodents are the reservoir for Yersinia pestis, the bacteria that causes the plagueSoil is the reservoir for the bacteria Clostridium tetani which causes tetanus
29Modes of transmission Transmitted by either direct or indirect contact Direct contact is when an individual is infected by contact with the reservoirExample: touching an infected person, eating infected meat, being bitten by an infected animal, inhaling infectious droplets from a sneeze or cough, intimate sexual contact
30Direct contact diseases Some diseases spread by direct contact include:RingwormAIDSTrichinosisInfluenzaRabiesMalaria
31Modes of TransmissionIndirect contact occurs when a pathogen can withstand the environment outside its host for a long period of time before infecting another individualExamples: tissues or toys handles by sick people, ingesting food or beverages contaminated by contact with a disease reservoir
32Indirect contactFecal-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
33Role of research in prevention Understanding the infectious cycle is critical in order to identify accessible targets for control strategiesExamples: 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
34Host Defenses against Infectious Disease The humans body has several mechanisms for preventing diseasesThese 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)
35ImmunityWhen 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.
36VaccinationA vaccine is either a killed or weakened strain of a particular pathogenThe 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
37Vaccination 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
38Public 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 immunizedUnnecessary in the US and developed countries because the water used for drinking, washing and preparing food is purified before it goes into homes.
39Water purificationPurification 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
40Sewage treatment and disposal Sewage includes wash water, water from toilets and storm run-offThese 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
41Food Safety programsThe US has many standards, inspection plans and regulations about food preparation, handling and distribution.Meat packing plants, restaurants and supermarkets are all inspectedMilk is pasteurized and dated for sale and is analyzed periodically for contaminationCanning and preserving foods is maintained through quality control checks.
42Animal control programs Aniamls are carriers of many diseases that also affect humansInspecting 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
43Animal Control programs Before dogs can be licensed, owners must show proof of rabies vaccinationMost cases of rabies in the US is due to bites from wild and stray animals, these animals are usually impounded and destroyedBubonic plague is spread by rats. Rat control in urban areas is a major component of public health efforts
44Animal 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
45Vaccination ProgramsMost 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.
46Public 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
47Public Health Organizations NIH: National Institute of Health. One of the 8 health agencies of the US Public Health ServiceSupports health related research aimed at understanding, preventing, treating and controlling infectious and other diseases of humankind
48Public 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 laboratoriesFood & Drug Administration (FDA)Monitors the safety of our food, medicines, and many other products we use dailyWorld Health Organization (WHO)Provides international surveillance and control of disease
49Treatment of infectious diseases Antibiotic literally means “destoyer of life”Antibiotics refer to a chemical substance used to treat bacterial infectionsAntimicrobial refers to anything that inhibits the growth of microbes which does not include antihelminthics because worms are not microscopically small
50Antibiotics Drugs used to treat bacterial diseases Grouped into categories based on their modes of actionIn general, these drugs inhibit cell wall synthesis, nucleic acid synthesis, or other enzyme-catalyzed reactionsExamples: penicillins, cephalosporins, tetracycline and erythromycinSome antibiotics can be toxic to human tissues when used in high doses or prolonged periods of time
51Antibiotics 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.
52Treatment 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.
53Antiviral drugs Drugs that are typically target virus specific Acyclovir-used to treat outbreaks of genital herpesAmantadine-used to prevent or moderate influenza among those at high risk of severe illness from the disease
54Antiviral 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
55Treatment 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 membraneAntihelminthic drugs target the adult worm
56Protozoa 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