Microbiology, Lec 26.
Epidemiology is the study of the distribution and prevalence of communicable disease in populations. There have been some amazing developments in our knowledge regarding infectious disease in the last 100 years. In the USA, public health measures and the development of antibiotics has greatly reduced the toll of infectious diseases, but worldwide infectious diseases are still a major cause of death.The ultimate goal of epidemiology is to keep disease at bay.
When a disease breaks out, epidemiologists gather data on occurrences of the disease and try to relate the disease to "risk factors". In other words, they study the relationship between pathogens and their environments, like medical detectives. Epidemiology is also sometimes refered to as "medical ecology".
Here's an epidemiological detective story. I. Some basic terminology of epidemiology:
Endemic
epidemic
pandemic
.common source vs. Propagated (person-to-person transmission) (Figure 35.3).
Reservoir (pg. 727): a place where infectious agents can live outside of hosts. In many diseases, the only reservoir is other humans.
Zoonosis (pl. zoonoses): Sometimes the major reservoir for a disease is animals (as with plague, Rocky Mountain Spotted Fever, Lyme disease, etc.) (p. 722, Table 35.1).
Vector: living agents which transmit disease but do not necessarily have the disease (usually biting insects).
Fomites: inanimate objects which transmit disease after being handled by carriers.
An example using most of these terms:
Plague (p. 777):
.Yersinia pestis(Gamma Proteobacteria) inhabits prairie dogs and other wild rodents (the reservoirs). Fleas (the vector) on the wild rodents can transfer plague to the occasional human that comes near. In communities where this happens from time to time, plague is endemic. This results in sporadic occurrence of "wild" plague or "sylvatic" plague. When fleas pass Y. pestison to domestic rats, the fleas on these rats can then pass Y. pestis on to large numbers of humans. This can result in an epidemic (India, 1994) or even a pandemic (middle ages).
Y. pestisinfects lymph nodes to make bubos (thus"bubonic" plague). Subcutaneous hemorrhaging makes dark areas. Because bubonic plague results from the bite of the flea (vector) from a rodent, bubonic plague is considered to be a zoonosis.
When Y. pestisinfects the lungs it results in pneumonic plague (pneumon = Gk. for lung). Pneumonic plague can spread from person to person independently of animal reservoirs or vectors. Pneumonic plague is very deadly with an almost 100% mortality if not diagnosed within 24 hrs. Y. pestisproduces several toxins including murine toxin (exotoxin) that prevents respiration..
Here's a link to a discussion of the recent increase in plague cases in the South Western U.S.A.
another example of a zoonotic disease:
Rocky Mountain Spotted Fever: The wood tick or dog tick is both reservoir and vector for Rickettsia rickettsii (Gm-, proteobacterium, See Lec. 25). Almost all Rickettsiaspp. are obligate intracellular parasites.
II. Modes of disease transmission...
1. Animal vectors and reservoirs (see examples above)2. Transmission by personal contact
Examples: "Childhood diseases"Diphtheria (Corynebacterium diphtheriae- see Lec. 7)3. Transmission by sexual contactPertussis or whooping cough (Bordatella pertussis, Gm-, strict aerobe, Beta Proteobacteria)
These have been effectively controlled with vaccines. Vaccines can be very effective. Even if not everyone is immune, "herd immunity" helps to prevent disease.
Streptococcus pyogenesinfections (streptococcal pharyngitis (strept throat), scarlet fever, rheumatic fever) are controlled with antibiotics. Before antibiotics, isolation was all one could do.
The pathogens which cause these diseases are generally obligate pathogens. Humans are the only known reservoirs so transmission is by intimate contact only. These diseases are controlled by individual treatment and public education about safe sex or abstinence. We have already talked about some of these diseases (AIDS, Chlamydia, Genital Herpes, Genital warts ---- Lec. 23, Lec. 24 and Lec. 25).4. Transmission by ingestion: the "fecal-oral" pathway.Gonorrhea Neisseria gonorrhoeae (Gm-, coccus, beta Proteobacteria, Lec. 6) is easily killed by drying or UV. Primarily genitourinary tract infection but can spread to eyes, throat etc. There is also a concern about transmission to newborns. It is difficult to develop immunity or a vaccine because many possible pili expressed on the surface (phase variation, see Lec. 22).
A. Water - We owe our high quality drinking water to political/environmental activism of 100 years ago, before the age of antibiotics:
Alyce Hamilton and Chicago typhoid epidemic: Typhoid fever caused by Salmonella typhi. - the only reservoir is humans so this disease is transmitted primarily by "fecal-oral" route. In early 1900's, Chicago water supply was from Lake Michigan which was also sewage disposal system. In rainy year of 1902, Chicago rain washed sewage further out into the lake where water-supply intake occurred. Dr. Hamilton also provided evidence that the epidemic was worse in areas of town with outdated "outdoor privies" which allowed flies to act as vectors transmitting S. typhi. This resulted in a crackdown on certain employees of the Chicago Health Department.
Typhoid fever epidemic in Denver ~1873 prompted early city counselors of Boulder to secure Arapahoe glacier as a safe (sewage-free) source of drinking water.
John Snow and London cholera epidemic (see box 35.1). As with S. typhi,the only major reservoir for Vibrio choleraeis humans, so major transmission is the "fecal-oral route". The cholera toxin increases cyclic AMP in epithelial cells which causes pumping of Cl- ions and therefore lots of water. So the major symptom is diarrhea causing death by dehydration if not treated. Cholera is still endemic in countries where drinking water is not chlorinated.
B. Food
Food infection vs. food poisoning (intoxication) - see example of Salmonella spp. vs. Staphylococcusfrom Lec. 22
Other e.g. botulism (Clostridium botulinum - Lec. 7) and E. coli0157:H7, etc.
III. Special problems for epidemiology:
1) Carriers: people who can pass disease on to others either because they have the disease or because they have a chronic subclinical infections (no symptoms). Example: "Typhoid Mary" (box 35.2), HIV, HPV, etc....Here is a link to the Morbidity and Mortality Weekly Report a site where you can get the latest news on disease epidemics and outbreaks in the U.S.A.
2) Nosocomial infections / antibiotic resistance (read p. 808-809) - Susceptible patients can contract diseases in hospitals. We can't make vaccines for everything. We can't use antibiotics indiscriminately.
3) Global change- Are "tropical" diseases moving north?