part 2

• Pre-requisites for transmission of a communicable disease

  The six pre-requisites for the transmission of communicable diseases are;

  1. Presence of reservoir for infection

  2. Presence of microbiological agent

  3. Portal of exit through which the microbiological

  agent leaves the reservoir

  4. Mode of transmission

  5. Portal of entry (inlet) through which the

  microbiological enters the host

  6.Presence of susceptible host
• The Reservoir

  • It is the habitat where the infective agent survives grows and multiplies in such a manner that it can be transmitted to a susceptible host.
• Reservoir of infection can be:

  • Human => case or carrier

  Carrier=person with unapparent infection that transmit the disease to others

  • Animal=>caseorcarrier •
  • Environmental reservoir

  Water => e.g. legionnaire's disease

  Soil => e.g. Botulism, Tetanus

  • Plants
• Mechanism of disease production (pathogenesis)
• Invasiveness: ability of the organisms to invade the tissues and multiply
• Toxigenicity: ability of the organism to produce toxins
• 1. Exotoxins: (released by living organisms): Heat labile; highly immunogenic and converted to antigen or toxoid by formalin, heat and acid.
• 2- Endotoxins: (released after disintegration of the organism): Heat stable, poorly immunogenic and not converted to toxoid.
• Pathogenicity: The power of an infectious agent to produce disease
• Virulence: Ability to produce severe pathological reaction. Measured by the ratio of clinical to subclinical disease and case fatality rate
• Dose of infection (inoculum): high probability of severe disease with higher dose of infection
• Viability of the organism (resistance): Ability of the organism to live outside the body
• Spore formation: Maintain viability for a long period in unfavorable environmental conditions
• Antigenic power of the organism: Ability to stimulate the immune system to produce antibodies or antitoxin with subsequent immunity. Measured by the second attack frequency
• Ease of communicability is measured by the secondary attack rate, which is the number of secondary cases, occurring within the range of incubation period following exposure to a primary case expressed as a percentage of susceptible.
• 1. Direct Transmission

  • Direct contact skin-to-skin (e.g. STDs Droplet spread)
  • spray with droplet over a few feet (e.g. pertussis)
• Indirect Transmission

  • Airborne- droplet nuclei or dust suspended in air
  • Vehicle – food, water, biological products, fomites
  • Vector- insects

  – may support growth or change to the agent
• INCUBATION PERIOD

  It is the period between the entry of the organism and the appearance of the first symptom of the disease
• Knowledge of the incubation period is important for

  • Surveillance and quarantine in some diseases
  • Application of preventive measures to abort or modify the attack.
  • Identification of the source of infection
• Portal of entry: it is the path by which the infectious agent enters that host
• Portal of exit: is the path by which the infectious agent exits the infected host
• The Portal of Entry and Exit

  These could be:

  • Skin > Direct contact; e.g. scabies, fungal, staph
  • Mucous membrane > e.g. HBV, STDs
  • Respiratory tract > rhinovirus, EBV
  • GIT > E-coli, enteric virus, HAV
  • GUT > gonorrhoea, syphilis
  • Blood > HIV, HCV, HBV, malaria
• Susceptibility to infection is universal but susceptibility to disease depends on:

  ➢ Immunity

  ➢ Dietaryandnutritionalfactors

  ➢ Genetic-factors
• Types of Immunity

  • Natural- Natural resistance of the body offered by skin, mucous membranes, gastric acidity, respiratory cilia
  • Acquired ;
  • Passive: acquired through transferred antibodies from mother to infant (natural) or by administration of immunoglobulin or anti-sera (artificial)
  • Active: post infection immunity (natural) or following vaccination (artificial)
• Control

  Activities conducted to bring a disease or a health problem at a very low level till it becomes no longer a public health problem
• Elimination

  Termination of all modes of transmission to a reduction of the incidence of the disease to the zero in a confined or specific geographic locality as a result of deliberate efforts yet, continued intervention methods are required
• Eradication

  Termination of all modes of transmission of infection by extermination of the infectious agent
• Measures That Directed to the Agent

  • Sterilization

  • Disinfection

  • Proper treatment of infected individuals to kill the agent at its source
• Measures Directed to the Reservoir

  • Cases
  • Carriers
  • Animal reservoir
• Measures Directed towards Breaking Transmission

  • Isolation if indicated > to interrupt direct transmission
  • Decontaminating of fomites > vehicle transmission
  • Promote handwashing > prevent fecal-oral transmission
  • Modify ventilation and air pressure > prevent airborne transmission
  • Control vector population > control vector-borne transmission
  • Environment: sanitation of water, food, proper sewage handling
• Measures Directed towards Protecting Portal of Entry

  • Using bed-nets

  • Wearing masks and gowns to prevent entry of infected body secretions or droplets through skin or mucous membranes

  • Covering skin and using insect repellents
• Measures Directed to the Host

  • Health education

  • Adequate personal hygiene

  • Sound nutrition

  • Immunization

  • Chemoprophylaxis
• What is the Benefit of Complete Immunization in the Community?

  ➢ Completeimmunizationcoveragecanhelppreventthe agent from reaching a susceptible host
• Herd immunity

  • State of immunity within the community
• If a high proportion of individuals in the community are resistant to an agent, then susceptible people will also be protected by the resistant majority
• The level of susceptibility increases as new infants are born, an epidemic will develop after accumulation of susceptible
• RA No. 11332

  • An Act Providing Policies and Prescribing Procedures on Surveillance and Response to Notifiable Diseases, Epidemics, and Health Events of Public Health Concern, and Appropriating Funds Therefor, Repealing for the Purpose Act No. 3573, Otherwise Known as the "Law on Reporting of Communicable Diseases"
• Infectious disease surveillance is an important epidemiological tool to monitor the health of a population. The goals of infectious disease surveillance are threefold:

  (1) to describe the current burden and epidemiology of disease

  (2) to monitor trends

  (3) to identify outbreaks and new pathogens
• Responsible Agency: Disease Prevention and Control Bureau - Department of Health.