Dynamics of Diseases Transmission : 6/10/2009 Dr Muhammedirfan 1 Dynamics of Diseases Transmission Dr. Muhammedirfan H. Momin
Community Medicine Department
Government Medical College
Surat. Definition of communicable diseases : 6/10/2009 Dr Muhammedirfan 2 Definition of communicable diseases A communicable disease is an illness due to a specific infectious (biological) agent or its toxic products capable of being directly or indirectly transmitted from man to man, from animal to man, from animal to animal, or from the environment (through air, water, food, etc..) to man. Importance of Studying Communicable Diseases Epidemiology : 6/10/2009 Dr Muhammedirfan 3 Importance of Studying Communicable Diseases Epidemiology Changes of the pattern of infectious diseases
Discovery of new infections
The possibility that some chronic diseases have an infective origin. Infectious Disease Model : 6/10/2009 Dr Muhammedirfan 4 Infectious Disease Model Host : 6/10/2009 Dr Muhammedirfan 5 Host A person or an animal that affords subsistence or lodgement to an infectious agent under natural conditions. Types include: an obligate host, definitive (primary) host, intermediate host and a transport host. Infection : 6/10/2009 Dr Muhammedirfan 6 Infection Infection is the entry and development or multiplication of an infectious agent in the body of man or animals. An infection does not always cause illness.
There are several levels of infection (Gradients of infection):
Colonization (S. aureus in skin and normal nasopharynx)
Subclinical or inapparent infection (polio)
Latent infection (virus of herpes simplex)
Manifest or clinical infection (I): Source or Reservoir : 6/10/2009 Dr Muhammedirfan 7 (I): Source or Reservoir The starting point for the occurrence of a communicable disease is the existence of a reservoir or source of infection.
The source of infection is defined as “the person, animal, object or substance from which an infectious agent passes or is disseminated to the host (immediate source). The reservoir is “any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host. It is the natural habitat of the infectious agent.” Types of reservoirs : 6/10/2009 Dr Muhammedirfan 8 Types of reservoirs Human reservoir : 6/10/2009 Dr Muhammedirfan 9 Human reservoir Cases : 6/10/2009 Dr Muhammedirfan 10 Cases A case is defined as “a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation” Carriers : 6/10/2009 Dr Muhammedirfan 11 Carriers It occurs either due to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state.
It is “an infected person or animal that harbors a specific infectious agent in the absence of discernible (visible) clinical disease and serves as a potential source of infection to others.
Three elements have to occur to form a carrier state:
The presence in the body of the disease agent.
The absence of recognizable symptoms and signs of disease.
The shedding of disease agent in the discharge or excretions. Attack rates and primary/secondary cases : 6/10/2009 Dr Muhammedirfan 12 Attack rates and primary/secondary cases Attack rate: proportion of non-immune exposed individuals who become clinically ill.
Primary (index)/secondary cases: The person who comes into and infects a population is the primary case. Those who subsequently contract the infection are secondary cases. Further spread is described as "waves" or "generations". Animal reservoirs : 6/10/2009 Dr Muhammedirfan 13 Animal reservoirs Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis…..
There are over a 100 zoonotic diseases that can be conveyed from animal to man. Reservoir in non-living things : 6/10/2009 Dr Muhammedirfan 14 Reservoir in non-living things Soil and inanimate matter can also act as reservoir of infection.
For example, soil may harbor agents that causes tetanus, anthrax and coccidiodomycosis. (II): Modes of transmission : 6/10/2009 Dr Muhammedirfan 15 (II): Modes of transmission (III): Susceptible host : 6/10/2009 Dr Muhammedirfan 16 (III): Susceptible host An infectious agent seeks a susceptible host aiming “successful parasitism”.
Four stages are required for successful parasitism:
Portal of entry
Site of election inside the body
Portal of exit
Survival in external environment Slide 17: 6/10/2009 Dr Muhammedirfan 17 THANK YOU THANK YOU