Last revised: Wednesday, April 30, 2003
Reading: Ch. 30 in text
Epidemiology
- Epidemiology = tracking the incidence and patterns of disease.
- Epidemiology depends critically on data. Accuracy much better in developed countries than in most developing countries.
- Epidemiologists try to determine how prevalent any given pathogen is, and where it is. Requires many tools from clinical microbiology (see below).
- Global: World Health Organization (WHO) in Geneva maintains records on health statistics, infection rates, epidemics in most of World.
- View WHO Statistical Information System
- Example: SARS resource page
- View The World Health Report 2002
- U. S.: Centers for Disease Control and Prevention (CDC) maintains records on health statistics, infection rates, epidemics in U. S. View CDC website.
- 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.)
- 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
Features of Epidemics
Take a look at "Steps of an Outbreak Investigation" to see how scientists go about investigating and epidemic and identifying its cause.
- Common-source epidemic: typical of food poisoning, water-borne poisoning (e.g. cholera outbreak after water source is contaminated). View graph of outbreak of Leginonella.
- Propagated epidemic: typical of diseases transmitted by direct contact, or person-to-person. View graph showing cases of Dengue Fever in Cambodia.
- 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. View graph of measles incidence in Japan over 10 years.
Propagation of Diseases
- Reservoirs: 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.
- Carriers
- 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.
- Inanimate Reservoirs
Transmission Routes
- 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
- Diseases of the GI tract: contaminated food and/or water carry many pathogens into the host. Many GI tract pathogens produce some form of diarrhea, the most common type of infection on a worldwide basis.
- Diseases of the urogenital tract: include sexually transmitted diseases (STDs) as well as urinary tract infections.
- Nosocomial and Iatrogenic Diseases : acquired in hospitals or from health care workers. About 3% of hospital patients acquire an infection they didn't have when they entered the hospital!
- Arthropod-borne Diseases and zoonoses: a number of serious diseases are transmitted by fleas, mosquitoes, and other biting insects. Ex: Typhus fever, caused by bacterium Rickettsia prowazekii, is transmitted by body lice -- this disease flourishes during times of crises such as wars, when personal hygiene breaks down. Contact with other animals or their products can lead to diseases called zoonoses.
Controlling 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)
- Quarantine
- Seven diseases are so serious that infected individuals may be quarantined by
international agreement until no longer infectious:
- smallpox
- cholera
- plague,
- yellow fever
- typhoid fever
- relapsing fever
- SARS
- Seven diseases are so serious that infected individuals may be quarantined by
international agreement until no longer infectious:
How 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.
- extensive encroachment of
civilization into tropical forests
Clinical Microbiology
- Rapid detection of pathogens is big business! At annual meetings of the American Society for Microbiology, Clinical Microbiology is one of the largest groups, and vendors of equipment and test kits number in the 100's.
- Starting next year, UConn will reestablish a lab course in "Pathogenic Microbiology" (MCB 233) taught by Dr. Graf. Will include considerable clinical micro.
- Isolating Clinical Specimens: the first task for pathogen ID is to obtain a patient sample (blood, sputum, urine, tissue biopsy, pus)
- Identifying Pathogens: this is a complex and extensive topic, outside the scope of our class. Skim text pp. 744-758 to get an idea of the range and variety of techniques.
- Antibiotic Sensitivity Testing: before prescribing an antibiotic for a bacterial pathogen, it is best to determine if the strain is sensitive or resistant. Use Kirby-Bauer test.
