Supplemental Lecture (98/05/09 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Acquisition of Disease
    1. A list of vocabulary words is found toward the end of this document
    2. Infectious disease may be acquired from numerous sources, by numerous mechanisms. However, for individual pathogens, both the source (reservoir) and the method of transmission are fairly limited. This is because pathogens tend to have evolved one or, at most, only a few methods of acquisition of a host.
    3. Often disease can be and is prevented by specific blocks on pathogen transmission. Often these blocks involve reservoir elimination, prevention of direct contact with the reservoir, or interruption of chains of contact between the susceptible person and the reservoir.
  2. Sources of infectious disease
    1. General categories of where one may acquire an infectious disease include:
      1. inanimate reservoirs
      2. sick individuals
      3. asymptomatic carriers
      4. other species
  3. Reservoir
    1. Source of disease:
      1. A reservoir is simply the source of an infecting microorganism.
      2. An infectious disease spreads from a reservoir to individuals or sites of infection on individuals.
    2. May be no disease at source:
      1. A reservoir can be something which either is not susceptible to disease caused by the pathogen in question, not currently sick, or able to sustain the infection, even if only temporarily, while sustaining some pathogen-induced damage.
      2. It is possible for a pathogen to replicate on or within a reservoir without effecting a depletion of reservoirs from the environment.
    3. Animal, vegetable, mineral, human:
      1. Remember that a reservoir can be anything so long as it is the source of an infection.
      2. This includes inanimates, animates (such as animals), or humans.
      3. Intraspecific reservoirs may also be referred to as carriers.
  4. Carrier
    1. Intraspecific reservoir:
      1. A carrier is a person (in the case of human disease) who carries (i.e., is infected by) a communicable disease and serves as an "intraspecific" reservoir.
      2. Overall, carriers are the most important reservoirs of human disease.
    2. Asymptomatic carriers:
      1. During the incubation of and convalescence from an infection, a person can still be the carrier of some diseases.
      2. That is, an individual need not be symptomatic to serve as a reservoir.
      3. In fact, the term carrier is often used synonymously with asymptomatic carrier.
  5. Zoonosis [n, zoonotic adj]
    1. Animal reservoir:
      1. A zoonosis is a communicable disease which is transmitted from a non-human animal to a human.
      2. Here the non-human animal is the reservoir.
    2. Uni- or bi-directional:
      1. Zoonotic reservoirs can be either unidirectional (e.g., transmitting only from non-humans to humans; ) or bi-directional (i.e., back and forth between non-human animals and humans).
      2. Note that the kind of replication the microorganism does might differ between its multiple hosts (e.g., see Plasmodium spp.).
    3. The passage of pathogens between populations can range from rare to common (e.g., see emerging infetious diseases vs., for example, Lyme disease).
    4. About 150 zoonotic human disease are known.
    5. These include:
      1. anthrax, brucellosis, bubonic plague, cat-scratch fever, chagas' disease, hydatid cyst, influenza, leptospirosis, Lyme disease, malaria, pneumonic plague, psittacosis, Q fever, rabies, ringworm, Rocky Mountain spotted fever, salmonellosis, tapeworm, trichinosis, toxoplasmosis, tularemia, typhus fever, western equine encephalitis, yellow fever.
  6. Endogenous infection
    1. An endogenous infection is one which is caused by an opportunistic pathogen from an individual's own normal microbiota.
    2. Typically this is a consequence either of the individual being in a weakened state, or in the opportunist being deposited in a location other than that in which it typically benignly resides.
  7. Portals of entry
    1. Pathogen route of entry:
      1. A portal of entry is the route a pathogen takes to enter a host.
      2. Typical routes include:
        1. mucous membranes
        2. skin
        3. parenteral
    2. Typical portals of entry:
      1. Just as with portals of exit, many pathogens have preferred portals of entry.
      2. Many pathogens are not able to cause disease if their usual portal of entry is artificially bypassed.
  8. Mucous membrane portal of entry\
    1. Mucous membranes line or cover:
      1. the respiratory tract
      2. the gastrointestinal tract
      3. the genitourinary tract
      4. the eyes
    2. The gastrointestinal and respiratory tracts are particularly common mucous membrane portals of entry.
    3. Microbes gain access to these mucous membranes by the expected means:
      1. during inspiration
      2. upon ingestion
      3. touching the eyes
      4. touching the genitourinary tract
  9. Skin portal of entry
    1. Normally very impenetrable to most microorganisms, the skin can serve as an efficient portal of entry for many pathogens only when broken.
    2. Some pathogens are capable of entering through otherwise intact skin either via natural pores or by actually boring through, however.
    3. Examples of pathogens which can enter through intact skin include the hookwarm larvae of Necator americanus and some fungi.
  10. Parenteral portal of entry
    1. Bypassing barriers:
      1. A parenteral portal of entry is one of direct deposit of pathogens into body cavities or wounds.
      2. Note that a portal of entry through a break in skin is actually considered a parenteral portal of entry.
  11. Portals of exit
    1. Route out of body:
      1. A portal of exit is the route a pathogen takes out of an infected host.
      2. Portals of exit tend to be fairly well defined.
      3. What serve as a portals of exit are often not terribly surprising, at least once something is known of how and where a pathogen replicates and enters new hosts.
    2. Respiratory infections tend to utilize the mouth and nose as portals of exit.
    3. Gastrointestinal infections tend to exit in feces (or in saliva depending on the site of replication).
    4. Sexually transmitted diseases tend to have portals of exit at the urethra or genital region.
    5. Blood-borne diseases tend to exit via:
      1. arthropods
      2. needles*
      3. bleeding
      4. *i.e., hyperdermic syringe.
    6. A more general portal of exit occurs when an infected animal is butchered or an infected person undergoes surgery.
  12. Transmission
    1. Movement between environments:
      1. Transmission is the transfer of a disease-causing microorganism from one environment to another, particularly from an external environment to a susceptible individual.
      2. For example, from a portal of exit to a portal of entry.
    2. Categories of transmission:
      1. There are three general categories of transmission:
        1. contact
        2. vehicle
        3. vector
  13. Contact transmission
    1. Contact transmission is transmission by either:
      1. direct contact (person-to-person)
      2. indirect contact
      3. droplets
  14. Direct contact [person-to-person transmission]
    1. Person-to-person transmission:
      1. Direct contact (or person-to-person) transmission is a form of contact transmission.
      2. For example, direct contact transmission includes:
        1. touching
        2. kissing
        3. sexual intercourse
    2. Prevention by barrier:
      1. Note that the best way to prevent direct contact transmission is through the use of barriers such as:
        1. gloves
        2. masks
        3. condoms
        4. etc.
  15. Indirect contact transmission [fomite]
    1. Contact with fomites:
      1. Indirect contact transmission is a form of contact transmission where transmission occurs from a reservoir via inanimate objects.
      2. These objects generally are referred to as fomites.
      3. Fomites are basically almost anything an infected individual (or reservoir) can touch, upon which can be left a residue of contagious pathogen.
    2. Exceptions include the various inanimates referred to as vehicles:
      1. food
      2. air
      3. liquids
    3. Typically it is more difficult to avoid indirect contact transmission than it is to avoid direct contact transmission.
    4. A certain degree of organismal durability may be necessary to survive passage on a fomite.
    5. The best way to prevent indirect contact transmission is by:
      1. avoiding contact with fomites
      2. avoiding contact of hands with mucous membranes especially when handling or potentially handling fomites
      3. the use of barriers when handling fomites
      4. disinfecting fomites before handling
  16. Droplet transmission
    1. Achoo!:
      1. Droplet transmission is a form of contact transmission.
      2. Basically it is the consequence of being:
        1. coughed on
        2. sneezed on
        3. spit on
    2. Still moving:
      1. Note that to be considered droplet transmission the mucous droplet must still be traveling with the velocity imparted on it upon leaving the mouth.
      2. As a rule of thumb this is up to one meter post-mouth.
      3. Any further and this is considered airborne transmission.
    3. Given interaction within one meter of people it is certainly more difficult to avoid droplet transmission than it is to avoid either direct or indirect contact transmission.
    4. Not surprisingly, it is especially respiratory diseases that are transmitted by droplets.
  17. Vehicle transmission
    1. Different medium than fomites:
      1. Vehicle transmission transmission is via a medium such as:
        1. air
        2. food
        3. liquid
      2. These are all things routinely taken into the body.
      3. They thus serve as vehicles into the body.
  18. Airborne transmission
    1. Floating droplets:
      1. Airborne transmission is a form of vehicle transmission.
      2. Particularly it is airborne transmission which occurs via droplets (typically mucous droplets) where droplets are liquids that remain airborne whether as:
        1. aerosols (very small droplets)
        2. associated with dust particles
      3. An example where airborne transmission is a problem is within airliners where economizing measures reduces the turnover of cabin air and consequently increases air recycling. Organisms which can find their way into the air and remain viable thus have repeated opportunities to infect passengers.
    2. Requirement for greater organismal durability:
      1. Airborne transmission requires greater organismal durability than droplet transmission simply because of the length of time the microorganism is exposed to the air, before infecting a new host, is longer.
      2. Increased durability is to the effects of dessication, exposure to sunlight, etc.
      3. This is why breathing does not typically result in the acquisition of disease.
  19. Foodborne transmission
    1. Transmission from food:
      1. Foodborne transmission is a form of vehicle transmission.
      2. Any number of pathogens that are found in food and not killed during processing may be transmitted via food.
      3. Generally pathogens that undergo foodborne transmission are either being transmitted from the:
        1. food preparer
        2. flora originally associated with the food product
    2. Salmonella spp. tends to be part of the normal flora of chickens and consequently associated with chicken products.
  20. Waterborne transmission
    1. Fecal contaminated water:
      1. Waterborne transmission is a form of vehicle transmission.
      2. Generally, this is via sewage (i.e., fecal) contaminated water supplies.
    2. It is especially gastrointestinal pathogens that are present in feces and therefore which rely on waterborne transmission.
  21. Vector transmission
    1. No entry
  22. Portals of entry to nervous system
    1. Four common routes of entry:
      1. The brain typically is fairly resistant to bacterial infection.
      2. There are four common portals of entry to the nervous system.
      3. For an organism to take advantage of these routes they must display increasingly specialized adaptations as read from top to bottom:
        1. parenteral
        2. via the blood*
        3. via the lymphatic systems*
        4. up peripheral nerve axons
      4. *Ordering of blood and lyphatic system was arbitrary and is not intended to imply that one serves as a significantly more difficult portal to take advantage of then the other.
  23. Vocabulary
    1. Airborne transmission
    2. Carrier
    3. Contact transmission
    4. Direct contact transmission
    5. Droplet transmission
    6. Endoginous infection
    7. Fomite
    8. Foodborne transmission
    9. Indirect contact transmission
    10. Mucous membrane portal of entry
    11. Parenteral portal of entry
    12. Person-to-person transmission
    13. Portal of entry
    14. Portals of entry to nervous system
    15. Portals of exit
    16. Reservoir
    17. Skin portal of entry
    18. Transmission
    19. Vector transmission
    20. Vehicle transmission
    21. Waterborne transmission
    22. Zoonosis
    23. Zoonotic infection
  24. Practice questions
    1. In terms of either specifics or generalities (your choice), what's the difference between a vehicle and a fomite?[PEEK]
    2. In terms of a zoonotic diseases, which of the following is not true: [PEEK]
      1. they are communicable
      2. they can be passed back and forth between human and animal
      3. passage of the disease to humans can be common
      4. they have a predominantly human reservoir
      5. all of the above
      6. none of the above
    3. Compare and contrast carrier with reservoir.[PEEK]
    4. Tapeworm, cat-scratch fever, toxoplasmosis, brucellosis, Q fever, typhus fever, and Western equine encephalitis are all examples of?[PEEK]
    5. Direct contact transmission may be avoided by avoiding such contact or, should such contact be inevitable, then by employing __________. [PEEK]
    6. Which demands the greater organismal durability: (i) direct contact transmission or (ii) fomite mediated transmission? [PEEK]
    7. The best way to avoid direct contact transmission is through the use of __________ (in addition, name one specific example). [PEEK]
    8. In terms of mechanisms of disease transmission, food, water, and air (but not droplets) may all be classified as __________. [PEEK]
    9. __________ is a name for the general category of human diseases which have animal reservoirs. [PEEK]
    10. A break in the skin through which a pathogen may enter the body is called a __________. [PEEK]
    11. What category of disease transmission occurs via the uptake into the body of air, food, or water? [PEEK]
    12. What kind of transmission characterizes common source outbreaks but less so propagated epidemics. [PEEK]
  25. Practice question answers
    1. A vehicle is a substance capable of carrying (and/or likely to carry) a microorganism into your body such as air, water, or food. A fomite is an inanimate object capable of retaining a microorganism.
    2. iv, they have a predominantly human reservoir. If nothing else, a zoonotic disease is defined in terms of its having a non-human reservoir from which the infection is acquired.
    3. a reservoir is the source of an infecting microorganism while a carrier is a type of reservoir, particularly consisting of infected individuals of the same species. Note that a carrier need not be assymptomatic.
    4. Zoonoses
    5. some kind of barrier.
    6. fomite-mediated transmission.
    7. barriers, e.g., gloves, condoms, masks, etc.
    8. vehicles
    9. zoonosis
    10. parenteral portal of entry
    11. vehicle transmission
    12. Vehicle transmission.
  26. References
    1. Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 422-433.
    2. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 373-382.