Epidemiology of Communicable Disease

3 components

1. Agents
2. Route of transmission
3. Host

• Cause diseases
• They can be viruses, bacteria, fungi, protozoa, and helminths
• Infectious agent survival depends on its ability to
• Multiply
• Emerge from host
• Reach a new host
• Infect a new host

Reservoir of infection

• Defined as the specific ecological niche upon which infectious agents depends for survival. These niche could be human, animal or non-living reservoir
• Pathogens specifically adapted to man include infective agents of measles, AIDS, typhoid, meningococcal meningitis, syphilis, gonorrhea
• Human reservoir includes both ill persons and healthy carriers.
• In some cases (salmonellosis), humans share the reservoir with other animals.
• If reservoir is shared with animals, it is termed as Zoonosis infection

Carrier

• A person who habours the infective agents without showing signs of the disease but is capable of transmitting the agents to other persons
• Types
1. Healthy carrier
2. Incubatory or precocious carrier
3. Convalescence
4. Chronic
• Importance
• Determines spread of the disease from the magnitude of the problem in the population e.g. HBV

Animal reservoir

• Some infective agents that affect humans have their reservoir in animals. Zoonosis is defined as those infectious diseases of vertebrate animals which are transmissible to man under natural conditions.
• Examples
Where humans use the animal for food e.g. taeniasis Where a vector transmit infections from animal to human e.g. viral encephalitis (by mosquito), Plague, etc. Where animal bites human being e.g. rabies Where animal contaminates human environment including food e.g. salmonellosis

Non-living reservoir

• Many of these agents are saprophytes living in the soil and they are fully adapted to living free in nature.
• They can withstand changes in environmental temperature and humidity.
• Some of them develop resistant forms such as spores which can withstand adverse environmental conditions e.g. Clostridium tetani (tetanus), Clostridium welchi (gas gangrene) and C. botulinum (botulism)

Defined as the mechanism by which an incfectious agent is transferred from one person to another or from the reservoir to a new host. For example,

• Contact e.g. Scabies, STI, etc.
• Penetratioin of skin e.g. schistosomiasis, hookworm, malaria, tetanus, etc.
• Inhalation e.g. Meningitis and many other respiratory infections
• Transplancetal e.g. syphilis, toxoplasmosis, HIV
• Ingestion e.g. Typhoid, etc.

Types

• Non-specific host factors
• Reflexes e.g. sneezing, coughing, etc. that protect against infections
• Specific
• Genetic or Acquired
• Acquired through vaccination, etc

Factors that determine host immunity

• Age,
• Gender,
• Pregnancy,
• Nutrition,
• Trauma,
• Fatigue

Herd Immunity

• Ability of population to resist infection
• It is a form of indirect Acquired protection from infectious disease that occurs when a large percentage of a population has become immune to an infection, thereby providing a measure of protection for individual who are not immune
• The greater the proportion of individuals in a community who are immune, the smaller the probability that those who are not immune will come into contact with an infectious individual
• When Herd immunity is high, the burden of the disease in that population is low
• When herd immunity is low, the burden of the dx is high

• Elimination of reservoir
• This can be done depending on the reservoir considered
• Interruption of R.O.T
• Personal hygiene
• Environmental sanitation
• Protect susceptible host
• Immunization
• Chemoprophylaxis

Question: Describe the epidemiology and control of a named communicable disease e.g. MEASLES

Acute infectious disease of children

Organism is measles virus

Case Definition: Measles is suspected in any person who develops

• Fever,
• Maculopapular rash, and
• Any of these 3:
• Cough
• Corrhyza
• Conjunctivitis

This is confirmed by a positive laboratory test result of the infection

• Infectivity/Communicability
• Infectivity period of measles occurs from the appearance of prodromal symptoms (fever, running nose, cough, loss of appetite and conjunctivitis) to 4 days after the onset of rash
• However, Infectivity of measles is greatest in the 3 days before the onset of rash (around the time koplik spots appears), and 75%–90% of susceptible household contacts develop the disease
• Virulence
• The virulence of measles is attributable to the presence of Hemaglutinin and fusion glycoprotein on the envelope of the virus
• Host Factors?
• Environmental Factors?

Control

4 STRATEGIES for measles control

1. To strengthen Routine Immunization Activity i.e. the child must be vaccinated by 9 months of age (so that seroconversion is 85-95% )
2. Supplemental Immunization Activity (SIA) i.e. giving a 2nd opportunity for the child to be protected. This is achieved through
• Catch-up campaigns
• Age-wide immunization that is done once in a life cycle of a population
• Between 9 month to 14 years
• Follow-up campaigns
• Vaccinate child between 9 months to 5 years
3. Disease Surveillances (= Measles case-based surveillance)
• First, Detection of disease based on clinical features
• Next, Report the disease
• Take blood samples to laboratory to confirm
4. Management of cases with:
• Antibiotics- for treatment of secondary infections (e.g. otitis media or bacterial pneumonia)
• Vitamin A- especially in children with clinical signs of vitamin A deficiency assoc. with measles
• Good nutrition