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Investigation of Epidemic

  1. AIM
    1. To define magnitude of outbreak
    2. To define factors responsible
    3. To identify cause & Sources​
1st step:- Confirm diagnosis and then confirm epidemic existence.

Epidemic free criteria:- No new case reported for twice the incubation period of disease since the last case.
  1. Steps Involved in Epidemic Investigation:
    1. Verification of diagnosis
    2. Confirmation of existence of epidemic
    3. Defining the population at risk
    4. Rapid search for all cases and their characteristics
    5. Data analysis
    6. Formulation of Hypothesis and initiation of preliminary control and preventive measures
    7. Testing of hypothesis
    8. Evaluation of ecological factors
    9. Further investigation of population at risk
    10. Writing the report
  1. Important Points and Definitions to be mugged up:
    1. Epidemic
      • ​​The occurrence of a disease clearly in excess of normal expectancy is called an epidemic.
      • In an area where a disease has not been seen for many years, even the occurrence of a single case may be sufficient to call it an epidemic.
      • To call a disease as an epidemic it must be more than 2SD of previous year.
​​Types of Epidemic:-
  1. Common Source Epidemic:
    1. Single exposure:- ​​​
      • ​​All cases within one incubation period
      • Epidemic curve rises and falls rapidly Q
      • No secondary waves Q
      • Clustering of cases within narrow interval of time eg. Bhopal Gas tragedy
    2. ​Continuous or Multiple Exposure:- Exposure is multiple eg. CSW and Legionnaires d/s in Philadelphia.
  2. Propagated Epidemic:
    1. Results from person to person transmission eg. Polio epidemic.
    2. Gradual rise and fall in epidemic curve over a period of time.
    3. Spread of d/s depends upon the herd immunity, opportunities for contact and SAR.
  3. Slow (Modern) Epidemic:-
    1. Secular trend
      If the pattern or trend of disease frequency changes only over many years then it is called a secular trend. A secular trend implies a consistent tendency to change in a particular direction or a definite movement in one direction. Eg: Coronary heart disease, lung cancer & diabetes which have shown a consistent upward trend in the developed countries over the past 50 years.
    2. Cyclic trend
      If the occurrence of disease changes over a short duration of time like a year, it is called a cyclic trend. Some diseases change in frequency over seasons and such changes are referred to as seasonal changes – Measles and chickenpox are examples of such diseases.
  1. Endemic diseases
    The constant, continuous or usual presence of a disease in a defined geographic area or delimited territory is called an endemic disease.
    1. Hyper endemic refers to a persistent intense transmission in an area while Holoendemic means a disease staring early in life and affecting most of the Population. An endemic disease may become an epidemic if the number of cases usually seen suddenly increase in proportion. Malaria, tuberculosis, leprosy, filariasis, etc
  1. ​​Cases
    1. Primary case
      The first case of a disease which occurs in a community/area is called the primary case. In disease like acute conjunctivitis a number of primary cases may occur almost at the same point in time in such case the primary cases are referred to as “Co–primaries”
    2. Index case
      The first case, which comes to the attention of the health authorities in an area, is referred to as the index case. Such a case may or may not be the primary case.
  2. Secondary Attack Rate
    The secondary attack rate refers to the number of cases occurring among contacts of a primary case within the known incubation period of the disease. The denominator refers to the number of susceptible contacts who are in close touch with the primary case. However, if a person among the contacts has previously suffered from the specific disease and developed immunity is not known, then all the contact should be considered in the denominator.
  1. Herd immunity
    1. The immune status of a group of people/community is called herd immunity as it is the immune status of the ‘herd’ of people.
    2. For many communicable diseases, an outbreak of disease is only possible if the level of immunity is sufficiently low and there are a large number of susceptible in the population.
    3. In diseases like poliomyelitis, diphtheria, measles etc., herd immunity plays an important role.
    4. However, in a disease like tetanus or rabies where every individual is at risk unless specifically protected, herd immunity plays no role.
  2. Nosocomial infection
    1. An infection occurring in a patient in a hospital or other health-care facility and in whom it was not present or incubating at the time of admission or arrival at a healthcare facility is called a nosocomial infection. It refers to diseases transmitted from a hospital.
    2. Usually such infections are more difficult to manage, as they are generally resistant to most of the common antibiotics.
    3. Nosocomial infections also include those infections, which were contacted in the hospital but manifested after discharge, and also infections suffered by staff members if they contacted the infection from the hospitalized patients.
  3. Period of communicability
    1. Period of communicability or communicable period refers to the time during which an infectious agent may be transferred directly or indirectly from an infected person to a susceptible person.
    2. This period is usually equal to the maximum known incubation period for that disease.
  4. Contact transmission
    When disease is spread by direct contact with an infected person, it is called contact transmission. This may be by kissing, touching, biting or sexual intercourse. Ringworm, scabies, yaws, etc.
  5. Zoonoses
    An infectious disease transmissible under natural conditions from vertebrate animals to man is called a zoonoses. There are over 150 disease common to man and animals. These include anthrax, psittacosis, liver fluke, T. solium, T. saginata, bovine TB, salmonellosis, brucellosis, scabies, plague, typhus, yellow fever and KFD.
    1. Anthropozoonoses: Disease transmitted from Animals to Man. Eg. Rabies, Plague, Anthrax.
    2. Zooanthroponoses: Disease transmitted from Man to Animals. Eg. Human TB in cattle.
    3. Amphigenesis: Disease transmitted from Man to Animals and also Animals to Man. Eg. Schistosomiasis, Trypanosoma cruzi.

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