PU520: Principles of Epidemiology Unit 7 Infectious Disease and Outbreak Investigation

Question 1

What is defined as a disease due to an infectious agent? Such agents include bacteria and viruses.

Answer 1

Infectious disease

Question 2

What is a disease transmitted by direct or indirect contact with a host that is the source of the pathogenic agent?

Answer 2

Contagious disease

Question 2

What is an illness due to a specific infectious agent or its toxic products that arises through transmission of such agent or products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment?

Answer 2

Communicable disease

Some writers use the terms infectious disease and communicable dis-ease as synonyms. Technically speaking, these terms can have different meanings.

Question 3

What is an infection caused by a parasite, which “… is an animal or vegetable organism that lives on or in another and derives its nourishment therefrom.”?

Answer 3

Parasitic disease

Question 3

What is a defined as the entry and development or multiplication or an infectious agent in the body or persons or animals?

Answer 3

Infection

Question 3

What is the epidemiologic triangle?

Answer 3

It includes three major factors - agent, host, and environment - and is one of the longstanding models used to describe the etiology of infectious diseases.

Although this model has been applied to the field of infectious disease epidemiology, it also provides a frame-work for organizing the causality of some other types of health outcomes, such as those associated with the environment.

Question 4

Considering the epidemiologic triangle, what is an agent?

Answer 4

An agent refers to “[a] factor (e.g., a microorganism, chemical substance, form of radiation, mechanical, behavioral, social agent or process) whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease.

A disease may have a single agent, a number of independent alternative agents (at least one of which must be present), or a complex of two or more factors whose combined presence is essential for or contributes to the development of the disease or other outcome.

Question 4

Considering the epidemiologic triangle, what is the term environment?

Answer 4

The term environment is defined as the domain in which disease-causing agents may exist, survive, or originate; it consists of all that which is external to the individual human host.

Question 5

Considering the epidemiologic triangle, what is a host?

Answer 5

The host is [a] person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions.

A human host is a person who is afflicted with a disease; or, from the epidemiologic perspective, the term host denotes an affected group or population.

Question 6

What does infectivity refer to?

Answer 6

It refers to the capacity of an agent to enter and multiply in a susceptible host and thus produce infection or disease.

Question 7

What does the term virulence refer to?

Answer 7

It refers to the severity of the disease product (i.e., whether the disease has severe clinical manifestations or is fatal in a large number of cases)

Question 8

What is a toxin?

Answer 8

It usually refers to a toxic substance (a material that is harmful to biologic systems) made by living organisms.

Foodborne intoxications are examples of illness caused by the actions of microbial toxins.

Question 9

What is a host characteristic that can limit the ability of an infectious disease agent to produce infection?

Answer 9

Immunity

This refers to the host’s ability to resist infection by an agent. This status is usually associated with the presence of antibodies or cells having a specific action on the microorganism concerned with a particular infectious disease or on its toxin.

Question 10

What is the difference between active and passive immunity?

Answer 10

Active immunity refers to the immunity that the host has developed as a result of natural infection with a microbial agent. This can also be used to describe immunity gained by an injection of a vaccine (immunization) that contains an antigen (a substance that stimulates antibody formation).

Examples of antigens are live or attenuated microbial agents. (Jenner’s development of an immunization against smallpox was an early example of using a vaccination to protect against a disease.)

Active immunity is usually of long duration and is measured in years.

Passive immunity refers to the immunity that is acquired from antibodies produced by another person or animal.

For instance, the newborn infant’s natural immunity conferred trans-placentally from its mother. Another example is artificial immunity that is conferred by injections of antibodies contained in immune serums from animals or humans. Passive immunity is of short duration, lasting from a few days to several months.

Question 10

What does herd immunity mean?

Answer 10

Denotes the resistance (opposite of susceptibility) of an entire community to an infectious agent as a result of the immunity of a large proportion of individuals in that community to the agent.

Herd immunity can limit epidemics in the population even when not every member of the population has been vaccinated.

Question 11

A clinically apparent disease is one that produces observable clinical signs and symptoms. What does the term incubation period denote?

What terms may be used when the infection does not show obvious clinical signs or symptoms?

Answer 11

This term refers to the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease.

Subclinical, also called inapparent.

For example, hepatitis A infections among children and the early phases of infection with HIV are largely asymptomatic. Nevertheless, individuals who have inapparent infections can transmit them to others; thus inapparent infections are epidemiologically significant and part of the spectrum of infection.

Question 12

What is the term generation time mean?

Answer 12

It is defined as the time interval between lodgment of an infectious agent in a host and the maximal communicability of the host.

The generation time for an infectious disease and the incubation time may or may not be equivalent.

For some diseases, the period of maximal communicability precedes the development of active symptoms. Incubation period applies only to clinically apparent cases of disease, whereas generation time applies to both inapparent and apparent cases of disease.

Question 13

What is a person or animal that harbors a specific infectious agent without discernible clinical disease and that serves as a potential source of infection?

What would you call if this host’s status is lonstanding?

Answer 13

A carrier or referred to as carrier status.

A chronic carrier.

Example:

“Typhoid Mary” Mallon, who worked as a cook in New York City during the early 1900s and was alleged to be a typhoid car-rier. Several cases of typhoid fever were traced to households where she was employed. Typhoid fever, caused by Salmonella bacteria (S. typhi), is a systemic infection associated with a 10 to 20% case fatality rate when untreated. After the first cases of typhoid were associated with her, Mallon was quarantined for 3 years on Brother Island in New York City and then released with the proviso that she no longer work as a cook.

Question 14

What is the difference between quarantine and isolation?

Answer 14

Quarantine—Well persons who have been exposed to an infectious disease are prevented from interacting with those not exposed, for example, preventing medical personnel who have been exposed to Ebola virus from leaving their place of residence. This is different from isolation.

Isolation—Persons who have a communicable disease are kept away from other persons for a period of time that corresponds generally to the interval when the disease is communicable, for example, maintaining isolation of patients with Ebola in special isolation units.

Question 15

What term is used in an epidemiologic investigation of a disease outbreak to denote the first case of a disease to come to the attention of authorities?

Answer 15

Index case

Question 16

What are some examples of how environmental determinates may act as potential influences associated with the occurrence of diseases and other health outcomes? (4 main categories/examples)

Answer 16

Physical environment - The availability of clean and abundant water supplies is instrumental in maintaining optimal sanitary conditions; waterborne diseases such as cholera are associated with pathogens that can contaminate water. Other pathogens such as fungi may be present naturally in the soil in some geo-graphic areas. An example is the fungus Coccidioides immitis, found in California’s San Joaquin Valley. This fungus is the agent for San Joaquin Valley fever.

Climatologic environment - In warm, moist, tropical climates, disease agents and arthropod vectors such as the Anopheles mosquito, the vector for malaria, are able to survive and cause human and animal diseases. These same vectors and the diseases associated with them are not as common in drier, colder, temperate climates. However, with global warming observed in recent years, it may be possible for disease vectors to migrate to regions that formerly were much colder.

Biologic environment - The biologic environment includes the presence of available plant and animal species that can act as reservoirs for disease agents. These species may be part of the cycle of reproduction of the disease agent. An example is the disease schistosomiasis, which depends on the presence of intermediate hosts (certain species of snails) in order to reproduce. Schistosomiasis, a major cause of illnesses including liver cirrhosis, is found in Africa, the Middle East, parts of South America and Asia, as well as some other geographic areas.

Social and economic environment - While the world becomes increasingly urbanized as inhabitants search for improved opportunities, cities will become ever more crowded. The overcrowded urban environment can contribute to the spread of infections through person-to-person contact and creation of unsanitary conditions such as improper disposal of human wastes.

Question 17

What do you call an infectious disease that is habitually present in an environment (either a geographic or population group)?

Answer 17

It is said to be endemic.

Plague is an endemic among certain species of rodents in the western United States.

Question 18

What is another term to describe the presence of an infectious agent in the environment which is a place where infectious agents normally live and multiply?

This place can be human beings, insects, soils, or plants.

Answer 18

Reservoir

Question 19

What is a term that refers to an infection transmissible under natural conditions from vertebrate animals to humans?

Answer 19

Zoonosis

An example of a zoonotic disease is rabies, a highly fatal viral disease that affects the brain (causing acute viral encephalomyelitis) and can be transmitted by the bite of an infected dog or other rabid animal.

Question 20

What is a portal of exit?

Answer 20

For an infected person, it is the site from which the agent leaves that person’s body; portals of exit include respiratory passages, the alimentary canal, the genitourinary system, and skin lesions.

Question 21

What refers to direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place?

Answer 21

Direct Transmission.

This may be by direct contact such as touching, kissing, biting, or sexual intercourse or by the direct projection (droplet spread) of droplet spray…

Question 22

What do you call the site in which an agent enters the body or organism?

Answer 22

Portal of entry

Examples of portals of entry are the respiratory system (through inhalation), a skin wound (such as a break in skin), and the mucous membranes, which line some of the body’s organs and cavities—e.g., nose, mouth, and lungs.

Depending on several factors—including the type of microbial agent, access to a portal of entry, the amount of the agent to which the potential host is exposed, and the immune status of the host—an active infection may result.

Question 23

What type of transmission of infectious disease agents involves intermediary sources of infection, such as vehicles, droplet nuclei (particles), and vectors?

Answer 23

Indirect transmission

Question 24

What are the three types of indirect transmissions?

Answer 24

Vehicle-borne infections
Airborne infections
Vector-borne infections

Question 25

In vehicle-borne infections, how are vehicles defined?

What might these vehicles be?

Answer 25

Vehicles are contaminated, nonmoving objects.

Vehicles can include fomites, unsanitary food, impure water, or infectious body fluids.

For example, used injection needles may contain bloodborne pathogens. This was the case during a 2008 suspected hepatitis C virus (HCV) transmission by unsafe injection practices.

In January 2008, the Nevada State Health Department reported three cases of acute hepatitis C to the Centers for Disease Control and Prevention (CDC). Investigations by state and local health departments in collaboration with CDC revealed that all three individuals had procedures performed at the same endoscopy clinic.

Question 26

What exactly is a fomite??

Answer 26

It is an inanimate object that carries infectious disease agents; fomites include the classroom doorknob, used towels found in a locker room, or carelessly discarded tissues.

Question 27

What term refers to unsanitary linens in hospitals that are contaminated with medical wastes in which causes outbreaks?

Answer 27

Hospital-acquired (nosocomial) infections

Question 28

What are enteric protozoal parasites?

Answer 28

Parasites that live in the intestinal tract.

Both giardiasis and cryptosporidiosis are diseases caused by these organisms.

Question 29

What is an inanimate, living insect or animal that is involved in the transmission of disease agents?

Answer 29

Vector

Transmission of an infectious disease agent may happen when the vector feeds on a susceptible host. Examples of vectors are arthropods (insects such as lice, flies, mosquitoes, and ticks) that bite their victims and feed on the latter’s blood. Rats are an example of a rodent species that can be a reservoir for fleas that transmit plague.

Question 30

T/F Diseases can be categorized in multiple categories such as foodborne, STD, emerging infections, vector-borne, etc.

Answer 30

True

Question 31

What are a few categories of infectious diseases?

Answer 31

STDs
Food-borne diseases
Water-borne diseases
- Bacterial conditions
- Parasitic diseases
Vector-borne diseases
Vaccine-preventable diseases
Zoonotic diseases
Emerging infections
Bioterrorism-related diseases

Question 32

What is an example of an emerging infectious disease in our recent history?

Infectious disease that has newly appeared in a population or that has been known for some time but is rapidly increasing in incidence or geographic range.

Answer 32

Examples of emerging infections are HIV infection, Ebola virus disease, hepatitis C, avian influenza, and E. coli O157:H7. In addition to being emerging infections, these diseases also fit into other categories (for example, foodborne, vector-borne, or sexually transmitted).

Question 33

Steps in the Investigation of an Infectious Disease (See attached)

Epidemic Curve - is defined as a graphic plotting of the distribution of cases by time of onset.

An epidemic curve may reflect a common-source epidemic, which is defined as an outbreak due to exposure of a group of persons to a noxious influence that is common to the individuals in a group.

A point source epidemic is a type of common-source epidemic that occurs when the exposure is brief and essentially simultaneous, and the resultant cases all develop within one incubation period of the disease.

Incubation period: As noted previously, the incubation period is the time interval between invasion of an infectious agent and the appearance of the first signs or symptoms of disease. As part of the investigation of a disease outbreak, the incubation period for each affected person is estimated.

From this information, the average and range of incubation periods for the affected group can be computed. In conjunction with information about symptoms, the incubation period provides clues regarding possible infectious disease agents that caused the outbreak. For example, in a foodborne illness outbreak caused by Salmonella bacteria, the incubation period would range from 6 to 72 hours, with most cases having an incubation period of 12 to 36 hours.

Answer 33

Case mapping - is the process that involves plotting cases of disease on a map. Although mapping is a simple concept, it can yield powerful data. Early in the history of epidemiology (mid-1800s), John Snow used this method to show the location of cholera cases in London. Mapping procedures can be used to locate cases in relation to environmental exposures to pollution, identify contacts of cases of infectious diseases, and conduct many other innovative health research investigations. The process of case mapping is facilitated by computer hardware and software known as geographic information systems (GIS).

Hypothesis formulation and confirmation: With the foregoing types of information at hand, the epidemiologist is now in a position to suggest (hypothesize) the causative agent for the outbreak and attempt to confirm the hypothesis by trying to locate additional cases and conducting additional laboratory analyses.

Draw conclusions: Once the cause of an outbreak has been determined, the final stage in the investigation is
to develop plans for the prevention of future outbreaks. For example, if the outbreak was a foodborne illness that occurred in a restaurant, the epidemiologist could recommend procedures to the management for improved methods of storing and preparing foods. Public health authorities in many localities are required to shut down restaurants that maintain unsanitary conditions until the deficiencies have been corrected.

Detecting outbreaks

• Passive and active surveillance
• Sentinel surveillance
• Syndromic surveillance
• Other social means

Confirm —> Describe —-> Determine Cause —–> Control

Confirm
1. Baseline level of disease and compare actual level
1a. Rule out other causes such as increased levels of testing, lab error, increase in pop, etc.

Step 2. Prepare for fieldwork - scientific, investigative, management and operational preparations.

Step 3. Verify diagnosis by considering clinical findings, lab results, and patient interviews.

Step 4. See below. Step 5. Use descriptive epidemiology.

Describe
1. Who is a case? All cases found? Describe cases.
2. Case definition - Time, place, person.

For case definition
- Criteria for person, place, time, and clinical features
- Criteria really for who and who should not be considered a case
- Before clinical diagnosis, you can use “probable”. After diagnosis, you can use “confirmed”.

Step 5. Develop hypothesis - Consider what you know. Consider unnatural patterns as well as intentional, biologic disaster, etc.

Step 6. Evaluate the Hypothesis - Use environmental evidence, laboratory science, and epidemiology to conduct research and perform statistical testing.

Step 7. Refine the hypothesis - Did the evidence support the hypothesis or was it rejected?

Step 8. Implement Control and Prevention Measures - As early in the process as possible, maintain confidentiality, direct measures toward breaking the chain of infection, interventions should be geared toward protecting portals of entry or increasing host defenses.
For Control (See attached)

Step 9. Communicate findings - Oral briefings to the public, written reports, media (when appropriate), know what should be communicated to whom