Chapter 16- Disease and epidemiology Flashcards
Epidemiology
The study of factors and mechanisms that are involved in the frequency and spread of diseases which affect humans, plants, and animals. When studying a disease, the first task of an epidemiologist is to determine the cause of disease- the etiologic agent. The goal is to obtain information useful in preventing the spread of disease. Methods used are descriptive, analytical, and experimental
Epidemiologic analysis is always carried out in regards to
Reference to the risk population. Susceptible individuals can be defined by behaviors (IV drug use, etc)
Morbidity rate
indicates the number of cases as a percentage of the population
Mortality rate
the measure of deaths in the population- the number per one hundred thousand people
Prevalence rate
Number of people infected at any one time- the number of people who have a given disease. Expressed in numbers per 100,000 people per year or as percent of the population
Incidence rate
New cases per unit population per unit time period. Expressed in numbers per 100,000 people per year or as percent of the population
How are incidence and prevalence rates related?
Every new case in the incidence rate adds to the prevalence, so prevalence will keep increasing even as incidence is decreasing. Prevalence will only decrease once incidence is basically nonexistent
Sporadic diseases
only seen occasionally and a random and unpredictable manner
Endemic diseases
constantly present is the population
Epidemic diseases
Larger than expected number of cases in a short period of time. You can have an endemic disease that becomes an epidemic due to additional cases. Occurs when you suddenly have a high peak in a graph
Pandemic diseases
Worldwide epidemic. COVID, HIV, smallpox
Propagated epidemics
person to person spread. Spread of disease can occur due to environmental conditions
John Snow
First epidemiological study- John Snow and the water pump on Broad Street. Attempted to determine where everyone who had been infected with cholera was getting their water from, and tracked the cholera outbreak to one contaminated water pump. His work supported the germ theory of disease
Common source
single source for all of the individuals infected
Point source
common source operates for a short period of time
Continuous common source spread
the infection occurs for an extended period of time
Intermittent common source spread
infections occur for a period, stop, and then begin again
Common source related spread of disease (4)
- Common source
- Point source
- Continuous common source
- Intermittent common source
With common source outbreaks, the cases quickly peak. Once the source is determined and removed, the cases quickly decline. The plateau period is very short
Propagated spread
Direct or indirect person to person contact. There is no single source of infection- each infected individual becomes a source for one or more subsequent infections. Difficult to stop and cause longer outbreaks. With propagated outbreaks, the number of cases rises and falls very slowly. There is a long plateau period. The only way to stop the outbreak would be to isolate everyone with the pathogen, but this is typically impossible due to incubation periods
Propagated spread
Direct or indirect person to person contact. There is no single source of infection- each infected individual becomes a source for one or more subsequent infections. Difficult to stop and cause longer outbreaks. With propagated outbreaks, the number of cases rises and falls very slowly. There is a long plateau period. The only way to stop the outbreak would be to isolate everyone with the pathogen, but this is typically impossible due to incubation periods
Observational studies
Data are gathered from study participants through measurements. Methods and types of studies include descriptive epidemiology, analytical epidemiology, retrospective studies, prospective studies, case control studies, and cross sectional studies
Descriptive epidemiology
Gathers information about the disease outbreak and the infected individuals, how the disease spreads over time. Involves examination of samples and medical records and interviewing patients and their families
Analytical epidemiology
Involves careful selection (choice) of groups of individuals in an attempt to develop a more convincing hypothesis for the disease source
Retrospective studies
Gather data from the past for present day cases. Includes medical history, age, gender. Tries to determine the relationship between past events and current disease
Prospective studies
Follow individuals and monitor the disease state during the course of the study. The individuals are followed for a long period of time
Case control studies
typically retrospective and compare a group of individuals with a disease to a group of individuals without a disease
Cross sectional studies
Analyzes randomly selected individuals in a population that have a disease to individuals that don’t have a disease in a specific point in time
Experimental studies
Use laboratory or clinical studies in which the investigator manipulates the study subjects to study connections between diseases and potential causative agents
Carriers of infection
individuals carrying the infection agent, does not have symptoms
Passive carrier
Contaminated with a pathogen and can mechanically transmit it to another host. The carrier is not infected itself. A person who does not wash their hands and then touches another person to transmit the pathogen would be a passive carrier
Active carrier
An infected individual that can transmit the disease to others but may or may not have symptoms- they can transmit the pathogen during the incubation period or during convalescence (after symptoms are gone). They do not have signs or symptoms at this point
Asymptomatic
Have no symptoms, a carrier can be active and asymptomatic. They can still be in the early phase of disease or the convalescent phase
Zoonotic diseases
Diseases and infections that are naturally transmitted between vertebrate animals and humans. Mosquitoes are a zoonotic vector
Nonliving reservoirs
water supplies, soil that are environmental factors and can harbor pathogens
Direct contact
body contact between individuals, can be horizontal or vertical
Horizontal contact
occurs due to shaking hands, sexual contact, animal bites, contact with fecal matter, etc
Vertical contact
From parent to offspring through the placenta, like HIV or herpes
Indirect contact by fomites
Nonliving objects that can harbor and transmit an infectious agent. Droplets can become indirect when they land on surfaces and the pathogen can survive. Tetanus, common cold, enterovirus are examples
Droplet transmission
Sneezing or coughing where the droplets are directly transmitted to another person. Whooping cough, measles, common cold, influenza are examples
Vehicle transmission
Transmission via a medium like air, food, or liquid. These are all things routinely taken into the body and serve as vehicles into the body. Cholera is an example, it is transmitted through contaminated water. Dust particles are another example
Mechanical vector
The pathogen is passively transmitted through feet or body parts, does not require that the pathogen multiplies in the vector. E. coli and salmonellosis are examples
Biological vector
Active transmission, the pathogen must complete a form of its life cycle in the vector
R0
Indicates how contagious a disease is when a susceptible individual is exposed to it. If R0 is below 1, the infection is contained and is not being transmitted to anyone
3 factors that influence R0
- Duration of infectiousness- how long the agent is infectious
- Probability of infection being transmitted during contact- what are the chances that an infected person will transmit the disease
- Average rate of contact- between- how many particles are necessary for the person to be infected, and how long do the two people need to be in contact for
3 factors that influence R0
- Duration of infectiousness- how long the agent is infectious
- Probability of infection being transmitted during contact- between a susceptible and infected individual- what are the chances that an infected person will transmit the disease
- Average rate of contact- between susceptible and infected individuals- how many particles are necessary for the person to be infected, and how long do the two people need to be in contact for
Why are severe or fatal diseases easier to contain?
If the disease is severe or fatal, it is easier to contain- this is because the patient will seek healthcare and will be diagnosed and treated. Their contacts can be traced
Quarantine
Applied to healthy people exposed to a disease during the incubation period. It limits the movements of such people and not necessarily to precautions during treatment
Isolation
when a person is confirmed to have the disease
Notifiable diseases
infectious diseases that are potentially harmful to the public’s health and must be reported by the physicians
Herd immunity
Constitutes the amount of people that are immune to a particular disease. As you increase the percentage of the population that is immune, the less of a likelihood there is that an infected person will transmit the disease to a susceptible person. Herd immunity breaks the cycle of disease. Different diseases have different requirements for herd immunity. For measles, it is 97-98%
Hospital acquired infections (HAI)
Acquired during medical treatment, in the hospital. Can just be acquired from hospital personnel
Common agents: E. coli, staph aureus, Streptococcus, candida, enterobacter, and many others
Modes of transmission of HAI
There are many modes of transmission. Elevator shafts (particles stay in elevator and pass around the hospital), bacteria can enter from outside, bacteria can be transmitted from infected patients, equipment can be contaminated, bacteria can be transmitted from visitors or hospital personnel, bacteria can be carried by insects, and bacteria can be passed to rooms through ventilation ducts
Universal precautions against HAI (7)
- Wear gloves and gowns
- Wear masks, eyewear, and face shields
- Wash hands before and after patient contact, and after removal of gloves
- Change gloves in between each patient
- Use a disposable mouthpiece for CPR
- Discard contaminated needles and other sharp objects immediately into a special puncture-proof container
- Clean spills of blood or contaminated fluids by putting on gloves and any other necessary barriers, wiping up with disposable towels, washing with soap and water, and disinfecting with a solution of bleach and water
Why is it difficult to eradicate disease? (4)
- Available medical expertise is not always applied- patients might not correctly prevent the disease, or may be treated incorrectly
- Infectious agents adapt- can resist medical treatment
- Unknown or rare diseases become more significant
- Immigration, international travel, and commerce
How to investigate an outbreak in 7 steps
- Case investigation- case by case of each patient, determine histories
- Cause investigation- what is causing the disease
- Control measures- limit exposures
- Conduct analytic study if necessary
- Conclusion (causal inference)
- Continue surveillance
- Communicate findings
