Chapter 20 Epidemiology

Glimpse of History 1841 – Ignaz Semmelweis became the first assistant in a hospital under Professor Johann Klein. There were two parts of the hospital: one run by Professor Klein and the medical students, while the other was run by midwives and midwifery students.

Semmelweis realized that women who gave birth in Professor Klein’s section had a higher instance of fever after birth (which usually resulted in death). While conditions in both sections were the same, management was not.

Semmelweis recognized that the “poisons” that caused the fever were being transmitted from the medical students who did the autopsies to the women in childbirth. Midwives did not do autopsies, so they did not transfer any “poisons.”

Semmelweis instituted the practice of hand washing in a chlorine of lime solution before attending to patients. Using these techniques, there was a decrease in illness and death at the hospital. However, his colleagues did not agree with his findings, and he ended up leaving the hospital.

Epidemiology Combines ecology, microbiology, sociology, statistics, and psychology to study the cause and distribution of health states in populations. Epidemiologists who collect and compile data to describe disease outbreaks.

20.1 Principles of Epidemiology Communicable diseases are those that can be transmitted from one host to another. For a disease to be transmitted, specific events must occur: ▫ The pathogen must have a suitable environment, or reservoir of infection. ▫ It must leave its reservoir and enter a suitable host (portal of entry). ▫ It must leave the host to be passed on (portal of exit)

Disease that do not spread from one host to another are called non- communicable. Microorganisms that cause disease often arise from an individual’s normal flora or from an environmental reservoir.

Rate of Disease in a Population Epidemiologists are most concerned with the rate of a disease (the proportion of a given population infected). They are also concerned with attack rate, or the number of cases developing in a group that were exposed to the infectious agent.

Morbidity is calculated as the number of cases of an illness in a given time period divided by the population at risk. Mortality rate reflects the percent of a population that dies from the disease.

Occurrence of a Disease Incidence: Fraction of a population that contracts a disease during a specific time Prevalence: Fraction of a population having a specific disease at a given time Sporadic disease: Disease that occurs occasionally in a population

Endemic disease: Disease constantly present in a population Epidemic disease: Disease acquired by many hosts in a given area in a short time Pandemic disease: Worldwide epidemic

Reservoirs of Infection Human reservoirs – infected humans are the most significant reservoir of communicable diseases If humans are the only reservoir, then it is seemingly easier to keep under control. Vaccination programs and isolation of those who are infected can eliminate some diseases from nature (like small pox).

Vehicle Transmission Transmission by an inanimate reservoir (food, water, air) Figure 14.7b

Vectors Arthropods, especially fleas, ticks, and mosquitoes Transmit disease by 2 general methods: ◦ Mechanical transmission: Arthropod carries pathogen on feet ◦ Biological transmission: Pathogen reproduces in vector

Factors that influence the epidemiology of disease Infectious agents transmitted to a new host can possibly cause disease, but the outcome of the transmission is affected by different factors: ◦ Dose ◦ Incubation period ◦ Characteristics of the host

The Dose Probability of an infection is lower when an individual is exposed to lower amounts of infection Because host defenses mobilize and race to the infection, many times small doses get eliminated quickly

The incubation period Diseases with a long incubation period can spread extensively before the first cases appear

Population characteristics Immunity to a pathogen: previous exposure to an agent influences who will become ill General health: malnutrition, overcrowding, and fatigue increase susceptibility to disease Age: the very young and very old are more likely to become ill Gender

Religious and cultural practices: infants fed breast milk are less likely to get ill ◦ Groups who eat raw fish are more likely to catch a certain type of tapeworm Genetic background: natural immunity can vary with genetic background

20.2 Epidemiological Studies Epidemiologists investigate a disease outbreak to determine the causative agent, reservoir, and route of transmission After a disease outbreak, a descriptive study defines the characteristics such as person, place and the time.

The person Knowing the profile of those who become ill is critical to understanding the population at risk Factors such as age, race, sex, occupation, personal habits, previous illness, socioeconomic class, and marital status can give clues about risk factors.

The place Geographical location of the disease acquisition identifies the general site of contact between the person and the infectious agent

The time Common-source epidemic: a rapid rise in the number of people who became ill suggests that they were all exposed at one time Propagated epidemic: the number of people ill rises gradually suggests the disease is contagious with one person transmitting it to others, and then they pass it on

20.3 Infectious disease surveillance National Disease Surveillance Network relies on agencies that monitor disease development ◦ CDC is part of the US Department of Health and Human Services ◦ Public Health Departments

Worldwide Disease Surveillance ◦ WHO (World Health Organization) is devoted to achieving the highest level of health for all people

20.4 Trends in disease Humans have been successful in developing the means to eliminate or reduce the occurrence of certain diseases by improving sanitation, vaccination and antibiotics.

Emerging Infectious Diseases Diseases that are new, increasing in incidence, or showing a potential to increase in the near future

Emerging Infectious Diseases Contributing factors ◦ Genetic recombination  E. coli O157, avian influenza (H5N1) ◦ Evolution of new strains  V. cholerae O139 ◦ Inappropriate use of antibiotics and pesticides  Antibiotic-resistant strains ◦ Changes in weather patterns  Hantavirus

Emerging Infectious Diseases Modern transportation ◦ West Nile virus Ecological disaster, war, and expanding human settlement ◦ Coccidioidomycosis Animal control measures ◦ Lyme disease Public health failure ◦ Diphtheria

20.5 Nosocomial Infections Hospital acquired infections Hospital infections have been a problem since the first hospitals opened (nosocomial is Greek for hospital), but modern problems

Nosocomial Infections Are acquired as a result of a hospital stay Affect 5–15% of all hospital patients