Epidemiology & Public Health Microbiology
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Last revised: Monday, May 8, 2000
Ch. 35 in Prescott et al, Microbiology, 4th Ed.Note: These notes are provided as a guide to topics the instructor hopes to cover during lecture. Actual coverage will always differ somewhat from what is printed here. These notes are not a substitute for the actual lecture!Copyright 2000. Thomas M. Terry
Introduction to Epidemiology
- "Epi" + "demos" + "logos" = upon + population + study
- Concerned with transmission, spread, control and prevention of infectious disease in populations
- Epidemiologists = "disease detectives" -- often physicians, not necessarily.
- Epidemiology is typically a graduate field of study; Yale has epidemiology program.
Terminology and Methodology
- Sporadic: disease occurs occasionally, irregularly
- Endemic: disease stays in population at low frequency
- Epidemic: sudden outbreak in disease above typical level
- Pandemic: epidemic over wide area (may be entire world). 1918-19 influenza pandemic killed 20 million people worldwide
- Morbidity: all reported cases of disease, illness + deaths, in some specific time period (e.g. 1 month, 1 year)
To calculate: # of disease cases/total # people, usually over some time
- Mortality: reported deaths due to a disease
To calculte: # of deaths/# of people infected
- Prevalence rate: total # of infected individuals in population at one time
Measuring the incidence of infectious diseases
- Epidemiology depends critically on data. Accuracy much better in developed countries than in most developing countries.
- Global: World Health Organization (WHO) in Geneva maintains records on health statistics, infection rates, epidemics in most of World.
- U. S.: Centers for Disease Control and Prevention (CDC) maintains records on health statistics, infection rates, epidemics in U. S.
- Every licensed clinic, physician, hospital must submit weekly reports of every instance of reportable diseases (about 50 on current list) to state public health office, which forwards this information to CDC.
- CDC publishes weekly reports (now available on Web and via e-mail): Morbidity & Mortality Weekly Reports.
- View Morbidity & Mortality Weekly Report summary from CDC (Centers for Disease Control)
- U.S. and other countries in developed world have cut many diseases by sanitation, public health (vaccines, etc.)
Features of Epidemics
- Common-source epidemic: typical of food poisoning, water-borne poisoning (e.g. cholera outbreak after water source is contaminated)
- Propagated epidemic: typical of diseases transmitted by direct contact, or person-to-person.
- Herd immunity: typically, when substantial % (e.g. for polio, ~70%) of population has become immune to disease (through exposure or immunization), disease ceases to be epidemic, even though many sensitive people still in population. Why? When infected person finds too few uninfected people to pass disease on to, disease loses its "foothold". Useful concept in predicting when epidemic will pass.
- Attenuation of virulence. Diseases and their host populations evolve together (co-evolution). Well-studied example was myxoma virus in Australian rabbit population. Rabbits introduced to Australia in 1859, quickly became major pest, no natural predators. In 1950, virus from S. America was introduced ---> rapid and enormous die-off of rabbits. Initially, over 99% of infected rabbits died from virus; but within a few years the virulence changed; only 84% of infected rabbits die today, and virus has demonstrably lower virulence. Rabbit population has stabilized at level ~20% of what it was before virus introduction.
- Epidemic cycles. Many epidemics occur in more or less predictable cycles. Ex: measles (before vaccine introduction) would occur in high % of children in first year of school at age 5 or 6. As new kids entered large school population, virus spread during school year, until every kid was immune and epidemic dropped off. Next fall, new population of susceptible individuals entered, new epidemic outbreak.
Propagation of Diseases
- Where disease is typically found. Important to know in order to control. May take some detective work.
- Ex: Yale scientists had to sample various animal populations to find out that Borrelia burgdorferi had reservoir in field mice, picked up by certain ticks and transmitted to deer and other large animal hosts, including humans).
- Most pathogens are well adapted to life in/on their animal hosts, don't compete successfully in nature with other microbes found in freshwater, soil.
- Ex: E. coli is transmitted via water; can be found in streams, ponds after fecal contamination. But within a week the numbers of viable organisms decline noticeably; other microbes are better adapted to the ecology of freshwater than E. coli. So the real reservoir for E. coli is animal GI tract, where it can persist indefinitely, not a polluted lake where it is present transiently.
- Inanimate Reservoirs
- Some pathogens are found primarily in inanimate habitats; e.g. Clostridium tetanus, common soil organism, does not require animal bodies. If no further animal infections, organism would still survive.
- Animate Reservoirs
- For many diseases, only humans are effective reservoirs (e.g., gonorrhea, syphillis)
- For other diseases, other animals can be important reservoirs (e.g., plague in rats and wild rodents, lyme disease in mice.
- Diseases that mainly afflict animals other than man are called zoonoses. (sing. zoonosis). Ex: anthrax is primarily a disease that affects animals such as cattle. Humans are occasionally infected.
- Carrier = infected individual who is not obviously diseased. Potential sources of infection.
- May be individual still in incubation period, who will come down with it = acute carrier.
- May be person who remains infected for long time = chronic carrier (typhoid Mary was prime example).
- Can identify carriers through routine surveys, e.g. X-rays, or by immunological screening. Especially important to know who are carriers for typhoid fever and tuberculosis, very dangerous diseases.
Routes of Transmission
- Usually related to habitats of organism in the body. E.g., respiratory tract pathogens usually spread through air, GI tract pathogens spread by contaminated water.
- Infectious dose = minimal # of pathogens needed to establish a disease. Can range from a single cell to hundreds of thousands. E.g. for typhoid fever, need very low doses. Cells quickly enter lymphatic system, are not killed by phagocytes but multiply inside them. But for cholera, need >108 cells (100 million) to establish successful infection.
- Airborne diseases
- Most common route of infection. More deaths due to various respiratory diseases than any other category
- Exs: common cold, influenza, tuberculosis
- Arthropod-borne diseases
- Carried by arthropod = vector.
- Ex: Typhus fever, caused by bacterium Rickettsia prowazekii, is transmitted by body lice. Reservoir is other humans; louse necessary vector to infect new host. Big problem in crowded, unsanitary conditions: trenches in WWI, concentration camps in WWII.
- Direct Contact diseases
- Typical of sexually transmitted diseases (STD).
- Exs: AIDS epidemic.
- View current statistics on AIDS/HIV worlwide (December 1999 WHO report)
- From December 1999 WHO report, 33.6 million infected people in world today, and additional 16.3 million AIDS deaths since beginning of epidemic.
- Although highest risk in U.S. is from homosexual contacts and drug users, world statistics show equal numbers of men and women infected, disease transmitted principally through normal sex.
- Other STD examples: Gonorrhea, Syphillis, Nongonococcal urethritis (mostly due to Chlamydia trachomatis)
- Some direct contact diseases involve skin contact. Exs: warts (due to virus), leprosy (over 3 million cases in world in 1980), ringworm (a fungal disease -- typically infects feet = athlete's foot, or groin = jock itch, or scalp, or nails, or whole skin. Remains localized.)
- Food and Waterborne diseases
- Two distinct processes:
- food poisoning = intoxication. Organism doesn't need to grow in body; it has already produced some toxin(s) in food, which can be adsorbed into bloodstream and quickly cause damage.
- Ex 1: Botulism (discussed earlier)
- Ex. 2: Staphylococcal food poisoning (due to heat stable enterotoxin). Often transmitted by food handlers. Common in summers, when picnic foods are left out in warmth for hours. Symptoms: nausea, vomiting, abdominal pain within a few hours of eating food. Especially found in foods with high salt or sugar (custards, processed meats, potato salad, etc.)
- ingestion of pathogenic microorganisms that grow in GI tract.
- A number of enteric bacteria cause this: e.g. Salmonella species, E. coli, Campylobacter, Shigella, others.
- Salmonella became big news around 1991, 15 people developed diarrhea, fever, abdominal cramps, nausea, chills, lasted for several days. All had eaten at same restaurant within 9 days, and interviews with staff found another 23 employees with symptoms. Disease was traced to Caesar salad, made with raw eggs. Eggs were traced to one single flock of chickens, led to major change in many restaurant policies (no more sauces with uncooked eggs). symptoms typically begin 8-24 hours after eating contaminated food, lasts no more than one week.
Control of Epidemics
- Airborne diseases
- block spread of aerosols
- isolate patients with contagious diseases
- wear masks (common in Japan)
- Arthropod-transmitted diseases
- control vector populations by insect control measures
- Direct Contact diseases
- wash hands frequently
- minimize contact
- use condoms to prevent STD
- Food and Waterborne diseases
- sanitation: sewage treatment and water disinfection (e.g. chlorine)
- careful food preservation
- Wounds and Cuts
- antisepsis: cleaning with soap and water; topical antiseptics
- careful surgical procedures; use of antibiotics
- Reducing or eliminating reservoirs
- If reservoir is domestic animals, this is attainable. E.g. bovine tuberculosis has been essentially eliminated.
- If reservoir is wild animal, not easy. Rabies = classic problem. Major concern for British in building Chunnel -- have a long history of rabies-free island, quarantine. Rabies has been spreading through East coast.
- If reservoir is humans, normally can't do anything.
- Breaking the transmission route
- Public health measures are effective in controlling food- and waterborne diseases.
- Respiratory transmission difficult to control. In Japan, face masks are worn by many people with respiratory disease. Good, but need major changes in social behavior for this to be accepted and useful.
- Reducing the number of susceptible individuals
- Vaccinations have been enormously successful for many diseases.
- Ex: measles, polio, diphtheria no longer significant diseases because of immunization programs. Only major source of continued disease is immigrants; still screen immigrants for contagious diseases (e.g. TB)
- Six diseases are so serious that infected individuals may be quarantined by international agreement until no longer infectious:
- yellow fever
- typhoid fever
- relapsing fever
- 5% of patients in hospitals acquire infection they did not have on entering
- 20,000 deaths directly, another 60,000 indirectly
- cost over $2 billion/year
- many patients already ill, impaired defenses
- most virulent forms of disease concentrated in hospital environment
- crowding increases risk of infection
- many hospital procedures (surgery, catheters, injections, etc.) involve risk both to patient and to personnel
- newborn infants especially susceptible, lack functional immune system many drugs in use, drug-resistant pathogens common
- Typical infections
- E. coli and Pseudomonas aeruginosa --most common urinary tract infections
- Staph. aureus, Streptococcus sp common in surgical infections (also E. coli and Pseudomonas)
Where do new diseases originate?
- Several hypotheses, probably several different types of sources. Only recently has technology (DNA sequence analysis) made it possible to investigate this question with precision tools -- likely to learn much in coming decades.
- Disease existed for long time, but went undetected. Ex: Lyme disease
- Disease results from mutation/recombination of existing organisms, producing new levels of virulence. Ex: influenza
- Disease originates by crossing species boundaries from some other animal. Ex: AIDS, probably moved from primates into humans. Also Hantavirus, summer of 1993.
- Disease results from ecological changes. Example:
- extensive encroachment of civilization into tropical forests --> allows diseases previously confined to spread quickly.
- This was found in Brazil in 1950's, previously unknown viruses infected highway workers, spread to populations.
- 1961, Oropouche virus caused 11,000 infections. But where did it come from?
- In 1980, virus was isolated from biting midges. Midges had population explosion during highway construction because of habitat changes --> explosion of vectors.
- Similar changes may explain AIDS, Ebola fever, yellow fever, and others.
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