Human Disease: Factors, Agents & Transmission

Factors Causing Human Disease

• Human disease factors are:
  • Biologic
  • Physical
  • Chemical
  • Stress, and other hard to classify factors

Factors That May Be Associated with Increased Risk of Human Disease

• Host Characteristics:
  • Age
  • Sex
  • Race
  • Religion
  • Customs
  • Occupation
  • Genetic profile
  • Marital status
  • Family background
  • Previous diseases
  • Immune status
• Types of Agents and Examples:
  • Biologic agents
  • Bacteria, viruses
  • Chemical agents
  • Poisons, alcohol, smoke
  • Physical agents
  • Trauma, radiation, fire
  • Nutritional agents
  • Lack, excess
• Environmental Factors:
  • Temperature
  • Humidity
  • Altitude
  • Crowding
  • Housing
  • Neighborhood
  • Water
  • Milk
  • Food
  • Radiation
  • Air pollution
  • Noise

Modes of Disease Transmission

• Diseases can be spread directly or indirectly.
• Direct transmission occurs through:
  • Person-to-person contact
• Indirect transmission occurs through:
  • Common vehicle
    • Single exposure
    • Multiple exposure
  • Continuous exposure
  • Vector

Portals of Entry

• Conjunctiva
• Mouth
• Respiratory tract
• Alimentary tract
• Urogenital tract
• Anus
• Skin
• Scratch or injury
• Capillary
• Arthropod

Clinical and Subclinical Disease

• Clinical disease exhibits signs and symptoms.
• Nonclinical (inapparent) disease includes:
  • Preclinical disease that is not yet clinically apparent but is destined to progress to clinical disease
  • Subclinical disease that is not clinically apparent and is not destined to become clinically apparent, often diagnosed by serologic (antibody) response or culture of the organism
  • Persistent (chronic) disease where a person fails to "shake off" the infection, persisting for years or life.
  • Latent disease where there is no active multiplication of the agent and only the genetic message is present in the host

Carrier

• An individual who harbors the organism but is not considered infected as measured by serologic studies or clinical illness
• Carriers can still infect others.
• Carrier duration has a limited or chronic existence, lasting months or years.
• Typhoid Mary is the best-known long-term carrier, carrying Salmonella typhi and dying in 1938.

Endemic, Epidemic, and Pandemic

• Endemic is the habitual presence of a disease within a given geographic area.
• Epidemic refers to the occurrence in a community/region of a group of illnesses of similar nature and are derived from a common or propagated source.
• Pandemic refers to a worldwide epidemic.

Disease Outbreaks

• Characteristics include:
  • Explosiveness with sudden and rapid case increase
  • Limitation to people sharing a common exposure in such cases
• Foodborne outbreak having rare cases where the disease transmits between persons from a primary case

Food Contamination

• Single-exposure, common-vehicle outbreak happens with multiple exposures
• Common-vehicle exposure occurs if an outbreak is among an exposed food group.
• Single exposure means food is served to people once.
• Multiple exposures mean food is served to people it more than once.

Immunity and Susceptibility

• The amount of illnesses in a population depend on:
  • The balance of susceptible people at risk for a disease
  • The number of people that are immune and therefore at no risk
  • Whether people are immune because they've had it and those who are now immunized to it
  • Some are not susceptible on a genetic basis.

Herd Immunity

• The resistance of a group of people to an attack by a disease to which a large proportion of the members of the group are immune
• The whole population (including non-immune) is generally protected.

Herd Immunity Importance

• Highly effective protection can be attained by immunizing a large part of the populace
• The remaining part will be protected because of herd immunity.

Incubation Period

• Period from infection receipt to clinical illness.
• It is affected by time to replicate, site of organism replication, and infectious agent dosage.

Quarantine

• Isolation duration is dependent on the person being no longer infectious to others.

• Incubation periods vary for diseases

• Incubation period length indicates the infective organism.

• Analogue has the incubation period for infectious and noninfectious diseases.

Epidemic Curve

• Epidemic curve defines disease onset distribution.

• Single-exposure, common-vehicle epidemic has an epidemic curve which represents distribution of incubation period.

• Critical variables in an outbreak or epidemic investigation are:

  • Exposure time
  • Disease onset time
  • Disease incubation period

Attack Rate

• Attack Rate is useful for assessing disease risk in groups with different exposures.
• Attack rate is calculated as (number of people at risk in whom an illness develops) / (total number of people at risk).
• Food-specific attack rate is calculated as (number of people who ate food and became ill) / (total number of people who ate that food)

• Primary case : A person that acquire the illness from exposure.
• Secondary case : A person that acquire the the illness from exposure to a primary case.
• Secondary attack rate : the rate in susceptible people whom have been exposed to a primary case.

Exploring Occurrence of Disease

• Questions include:
  • Who was attacked by the disease?
  • When did the disease occur?
  • Where did the cases arise?
• Factors including sex, age, and race indicate a major disease risk effect

When Diseases Occur

• Periodic diseases occur over time, or seasonally.
• Disease patterns and seasonal variations are addressed by inspecting disease incidence trends over time.

Where Diseases Occur

• Disease does not have a random timing or location distribution

Outbreak Definition

• An incidence in a community or area of illness cases exceeding the number of cases within a normal range.
• Case number dictates if there is an outbreak regarding:
  • Infectious agent
  • Size/type exposed population
  • Disease's previous exposure
  • When and where it occurred
• Outbreak status is proportional to the commonness of a disease within the same place, among the same population, within the same year season.

Outbreak Investigation Team

• To investigate a suspected outbreak an Outbreak Control Team (OCT) should include:
  • Consultant in Communicable Disease Control (CCDC) / Consultant in Health Protection (CHP)
  • Environmental Health Officer (EHO)
  • Consultant Microbiologist or Virologist
  • Secretarial/Administrative support
• Also take consideration of these people:
  • Regional epidemiologist
  • Media/press officer
  • Infection control nurse
  • State veterinary service
  • Food chemist/microbiologist/Food Standards Authority
  • Toxicologist
  • Director of Public Health
  • Water company

Steps in Outbreak Investigation

• Establish the existence of an outbreak:
  • current information compared with previous incidents within the community that exceeded the expected number of cases
  • compare existing data about new cases relative to a predetermined definition of an outbreak
  • look for increased cases in a population or time/place occurrence
  • check and interview cases and review laboratory findings
  • determine further investigation including urgency is required
• Confirm the diagnosis / Verify Diagnosis:
  • analyze case history and standard test
  • confirm suspect diagnosis to determine agent causing illness such a bacterial or viral or other type of infection if needed obtain medical records and lab reports such as a repeat and further clinical tests
• *Establish Case Definition and Count Cases (Case Definition)**:
  • establish case definition and methods to identify cases.
  • document the outbreak timeline in terms of person(s) and location.
• Determine who is at risk of falling ill (case finding)
  • Count instances and correlate with the suitable population to locate the relevant group
  • Contact individuals who supply further information on their illness, and about the environmental conditions that gave rise to the onset
  • Interview existing cases
  • Find vulnerable party member who can provide the likely risk agent
  • Check surveillance/ laboratory results
  • Contact related parties from CCDC to medical workers
• : Performing Descriptive Epidemiology*
  • To be familiar with the data
  • Note any trends in this data concerning time, population, or area.
  • Use information gathered from descriptive epidemiology to create a hypothesis and identify potential exposure time periods.

Epidemic Curve (for investigation steps)

• Look up the average and minimum incubation periods of the disease based on the disease being investigated
• Identify the peak of the outbreak (or the median case) and count one average incubation period back on the graph
• Start by starting with the initial instance of the epidemic and then count to check the minimum time range (incubation time) back to note down this date
• Time
  • The actual outbreak period
  • The likely exposure
  • Any source
• There are a variety of elements when studying a case of person vs place
• Place is most significant.
• Person factors include specifics on attack rate, age/gender grouping and symptoms within a population

Interpreting an Epidemic Curve:

• This curve gives potential sources, patterns and suggestions for its timing
• The process covers how it spread, existing situation and how it formed including incubation timeframe

Epidemic Patterns: Common and Propagated

• Common: A point or always around.

  • Source is where all victims share the same or closely related origin.
  • People are given equal amounts of the same origin over limited time and within standard incubation.
  • Shows rise then decline.

• Continual: An established source showing for a long time.

• Prolonged. Incubation is not identical among cases. Dropoff is rapid if it was a rapid event such as cholera outbreak.

• Propogated: Transmissions have one cause setting off secondary instances,

  • Curve increases. Control steps can be a benefit and pool dropoffs start to take effect.

• Questions are formed regarding various elements which include:

  • Does it derive from single, various or unapparent resources?
  • Is it from a host from animal source?

• How much does it take for incidents to affect an spread?
  • Includes affected people as it spreads.
  • What are main and minor periods?

Formulate Tentative Hypothesis

• To explain the cause, source and distribution of cases with knowledge of: - Source - agent’s reservoir? - Mode of transmission - How is it transmitted? - Exposure - What are commonly implicated vehicles? - Risk Factors -Time, Place, Person

Testing the hypothesis.

There are several methods to testing the hypothesis.

• The first is by simple comparison to the facts.
• When all clinical, lab, and epidemiologic evidence clearly supports the hypothesis this may be sufficient.
• The second is by quantification of the relationship using analytic epidemiology.
• This method is required if simple comparison is insufficient.

Analytic Epidemiology

Analytic Epidemiology is the study to test the hypothesis.

There are two methods of carrying such a study:

• Retrospective Cohort method: This is the study of choice for the outbreak of a small, well-defined population. (eg: A wedding reception)
• The cohort is tested for outcomes based on common exposure within the cohort, such as foods served at the wedding
• The attack rate is calculated to deduce relative risk within the cohort
• Case Control Study
• When studying a widespread outbreak without a small, well-defined population (eg: A city wide Salmonella outbreak) the population is not suitable for this type of work
• The investigator asks cases and controls about their exposure
• An Odds Ration is calculated
  • The key feature of analytic epidemiology is a comparison group. The comparison group allows epidemiologists to compare the observed pattern among case-patients or an exposed group with the expected pattern among non-cases or unexposed persons.

Refining Hypotheses

• Refining Hypotheses can entail studies which will include:
• Epidemiologic
• Laboratory
• Environmental

Implementing Control and Prevention Measures

• Effective control measures must be developed using gathered evidence
• Collect available info over the course for better mitigation that will avoid further community onset
• Set standards to the spread of an agent and be active
• Set control to tentative based onset

Communicating Findings.

A written report should summarize investigation and be well prepared. It needs to communicate and address: - Methods - Findings

Methods of control

- Effectiveness
- Prevention over next steps for standards

Outbreak Elements

• In the case of a food/waterborne gastro-enteritis issue get involved with the local EHO to start the initial action.
• Find how the infection began
• Gather information from all available outbreak parties
• Find data on the patients

Prevention

• Hygiene: handwashing and towel control.
• Dispose of used materials and plastic.
• Clean and maintain sanitary practices for both food prep and areas such as toilets and showers for personal hygiene
• Make an active effort to educate parties with food and water.

Exclusions

• Remove instances from works
• Get assistance from all forms of health care personnel

Control Measures

• Exposure to polluted resources
• Inspect at the beginning of: ➤ Work ➤ Recreation ➤ Where one Lives ➤ In food at place and areas of infection

Communicable Disease

Chain of disease transmission

1. The agent
2. Its Reservoir
3. It's mode of entry
4. There can be a human case set off from an entrance of disease
5. It involves cases with individuals or within certain members.
6. Host resistance can be at the community (population) which is known as herd immunity

Communicable Chain Parts

These various components may be ➤ Part 1 : An agent that carries the disease from sources ➤ Part 2 : Place of growth agent as a water setting or people animal or things ➤ Part 3: The method of agent exits which secretions blood or urine. A good example is feces carrying eggs. ➤ Part 4 : Delivery systems include by contact or direction

• 1 - Person to others such as HIV. It has saliva and causes colds

• 2 - Inanimates carrying issues from touching to handling of food

• 3 - Agent being transport by mechanical carriers like flies.

• 4 - Agent being transport by mechanical carriers like an environment issues containing dust. This creates the entry site for the disease into any members, while sites including:

  • Nose
  • Wounds A: The amount a carrier effects to the members

Direct and Projection Transmissions

• Mucosa or Conjunctiva are touched due to kissing, sexual contact, rabies and so on
• Direct: - Saliva by coughing like colds
• Mother and babies such the placenta to deliver syphilis.
• In most situations take safe steps by looking hygiene in hands by disposal
• Cleaning from toilets is recommended