PART 3: Microorganisms, Humans, and Disease Epidemiology and transmission of disease
Epidemiology Epidemiology is the study of disease in populations, and includes the transmission, incidence, frequency, and distribution of disease. The Centers for Disease Control in Atlanta is the main source of information in the United States: their publication Morbidity and Mortality Weekly tells us how many people are sick and how many have died from various diseases (light reading with coffee in the morning). They are also responsible for tracking and identifying emerging diseases, and report their findings to the World Health Organization.
Disease occurrence Incidence of a disease refers to the percentage of the population that gets the disease during a given time period; Prevalence refers to the percentage of a population which has the disease during a given time period.
Virulence of a pathogen Virulence refers to the DEGREE of pathogenicity for a given pathogen. Virulence can be measured as the LD50 (lethal dose for 50% of inoculated hosts) or the ID50 (infectious dose for 50% of inoculated hosts) The ID50 can range from literally 1 cell to 1 billion cells! (some pathogens are more able to cause disease, that is they are more virulent, than others)
More terms A disease is said to be communicable if it can be transmitted from one host to another. A contagious disease is very easily communicable. Contrast AIDS w/ the flu. A disease is said to be non-communicable if it cannot be transmitted from one host to another, usually because it is acquired from the environment (e.g. botulism)
Frequency of occurrence Sporadic - occurs only occasionally in a population (e.g. typhoid fever) Endemic - constantly present in a population (e.g. the common cold) Epidemic - acquired by many people in a given area over a short time (often the flu); AIDS? Pandemic - epidemic worldwide - AIDS?
The Spread of Infection A reservoir is a continual source of infective pathogens. Reservoirs may be Human - people infected with the pathogen; Animal - zoonoses are diseases that can be transmitted from animals to humans (rabies); also, vectors like mosquitoes are considered animal reservoirs; Non-living - soil or water; the pathogen can exist stably outside a host (B. anthracis because it forms endospores; Vibrio cholerae [causes cholera] often persists in water contaminated from feces)
Disease Transmission (careful, parts of this are a little tricky!) Contact transmission: Direct contact: transmission of a pathogen by physical contact between living reservoir and host; no intermediate object is involved Ex. Touching (the common cold), kissing (infectious mononucleosis), sexual intercourse (HIV) Indirect contact: transmission of a pathogen from a living reservoir to a host via an intermediate object, called a FOMITE. Example: HIV transmission via sharing hypodermic needles. Droplet transmission: transmission of a pathogen from a living reservoir to a host via droplets of mucus which travel less than 1 meter via sneezing, coughing, etc. (the flu)
Disease Transmission Vehicle transmission: transmission of a pathogen by physical contact between NON-LIVING reservoir and host. Non-living reservoirs are often food, water, or air. Foodborne transmission - foods that are incompletely cooked or unsanitary (Salmonella poisoning, trichinosis) Waterborne transmission - water is often contaminated by poorly treated sewage (cholera) Airborne transmission: transmission is considered airborne if the pathogen can travel more than 1 meter from the host (tuberculosis)
PLEASE: Recognize the difference between vehicle transmission and indirect contact!
Arthropod vectors: Biological transmission A special form of direct contact in which an arthropod harbors a pathogen and transmits it from host to host. We discussed malaria as an example, also Lyme disease.
Etiology - the cause of disease Koch's postulates: Robert Koch was the first microbiologist to come up with a set of rules to determine which microorganism caused which disease. Koch showed that Bacillus anthracis causes anthrax and that Mycobacterium tuberculosis causes tuberculosis, among others. What did he say?
Koch's Postulates The same pathogen must be present in every case of the disease; The pathogen must be isolated from diseased host in pure culture; The cultured pathogen must be capable of causing the disease in healthy lab animals; The pathogen must be re-isolated from the inoculated lab animal.
Potential Exceptions to Koch's Postulates Viruses and certain bacteria cannot be cultured normally; Some diseases (pneumonia) may be caused by a variety of different pathogens; Some pathogens may be able to cause several different diseases (S. pyogenes); Certain pathogens cause diseases in humans only, so would not infect lab animals (no, you can't experiment with humans yet!). HIV is an example.