Dynamics of disease Transmission

1. Dynamics of disease Transmission DrPracheth

2. Communicable diseases are transmitted from the reservoir/ source of infection to susceptible host . • Basically there is link in the chain of transmission between reservoir, mode of transmission and the susceptible host.

3. Source and Reservoir Source of infection is the person, animal , object or substance from which an infectious agent passes or disseminated to the host. Reservoir is defined as any person , animal , arthropod , plant , soil or substance in which an infectious agent lives and multiplies ,on which it depends primarily for survival , and where it reproduces in such a manner that it can be transmitted to susceptible host . The terms reservoir and source are not always synonymous .

4. Types of reservoir • Human reservoir. a. cases b. carriers • Animal reservoir. • Reservoir in non-living things.

5. Human reservoir • Human is most important source of reservoir of infection for himself. May be as case or carrier. • Case - A person identified as having the particular disease, health disorder as per clinical , bio chemical, laboratory criteria. Clinical case. Sub clinical case. Latent infection.

6. carriers An infected person or animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others. • Carriers are less infectious but more dangerous. • Presence in the body • Absence of recognizable signs /symptoms. • Shedding of disease agent in discharges or excretions- source of infection to others.

7. Carriers A ) TYPE (a) Incubatory (b) Convalescent (c) Healthy B ) DURATION (a) Temporary (b) Chronic c) PORTAL OF EXIT (a) Urinary (b) Intestinal (c) Respiratory (d) others

8. Animal reservoir Infections which are transmissible to man from vertebrates are called zoonosis. • There are over 100 zoonotic diseases which may be conveyed to man from animals and birds. • Source of infection sometimes may be animals and birds Reservoir in non living things soil and in animate matter can also act as reservoir of infection. eg. tetanus , mycetoma etc.

9. Modes of transmission • DIRECT TRANSMISSION 1. Direct contact 2. Droplet infection 3. Contact with soil 4. Inoculation into skin or mucosa 5. Trans placental (vertical) • INDIRECT TRANSMISSION 1. Vehicle – borne 2. Vector – borne 3. Air – borne 4. Fomite – borne 5. Unclean hands and fingers

10. Direct contact • Infection may be transmitted by direct contact from skin to skin, mucosa to mucosa ,or mucosa to skin of same or another person. I. e, Direct transfer of infectious agents from reservoir or source to a susceptible individual without intermediate agency. • Skin to skin contact - Touching. kissing or even sexual intercourse.

11. Droplet infection • Direct projection of a spray of droplets of saliva and naso-pharyngeal secretions during coughing, sneezing or speaking and spitting. • The expelled droplets may impinge directly upon conjunctiva, oro- respiratory mucosa or skin of a close contact. • Disease transmitted by droplet spread include – many respiratory infection, eruptive fevers, common cold, whooping cough, tuberculosis etc.

12. Contact with soil • Disease agent may be acquired by direct exposure of susceptible tissue to disease agent in soil, compost or decaying vegetable matter in which it normally leads to saprophytic existence. Eg Hook worm larvae, tetanus , mycosis.

13. Inoculation into skin or mucosa • The disease agent may be inoculated directly into the skin or mucosa. Eg- rabies virus by dog bite, Hepatitis B virus through contaminated needles and syringes.

14. Trans-placental transmission • Disease agents can be transmitted trans- placentally. Another form of direct transmission. Eg , TORCH agents , Varicella virus , Hepatitis B , Coxsackie B, and AIDS.

15. Indirect Transmission • Flies • Fingers • Fomites • Food • Fluid. Infectious agent must be capable of surviving outside the human host in the external environment and retain its basic properties of pathogenesis and virulence till it finds new host.

16. Vehicle-borne • Transmission of the infectious agent through the agency of water, food, ice, blood, serum, plasma or other biological products such as tissues and organs. • Infectious agent may have multiplied or developed in the vehicle before being transmitted or only passively transmitted in the vehicle. Eg Cholera, Polio, Hepatitis A, Malaria, Syphilis, Cytomegalovirus infection

17. Vector - borne • An arthropod or any living carrier that transports an infectious agent to a susceptible individual. • Transmission by vector may be mechanical or biological.

18. Air borne • Droplet nuclei are tiny particles of dried residue of droplets formed by evaporation of droplets coughed or sneezed into air. or generated by aerosols. • Droplet nuclei may remain air borne for long periods of time • They not only keep floating in air but may be disseminated by air currents • Eg tuberculosis influenza chickenpox measles.

19. Dust • Some larger droplets which are expelled during talking coughing or sneezing settle down on floor carpet ,furnitures and become part of dust. • Variety of infectious agents bacteria viruses and fungal spores found in dust. • During the act of sweeping, dusting etc they become airborne – air borne dust may be inhaled or may settle on un covered food and water Eg tuberculosis, pneumonia, Q fever coccidioidomycosis.

20. Fomite - borne • Fomites are inanimate articles or substances other than water or food contaminated by infectious discharges from a patient and capable of harbouring and transferring the infectious agent to healthy person. • Fomites includes soiled clothes towels, linen, handkerchiefs, cups, spoons, pencils, books toys, door handles, taps, lavatory chains. etc. • Disease transmitted – Bacillary dysentery, hepatitis A , Eye and Skin infections.

21. Unclean hand and fingers • Pathogenic agents are transferred to food from skin, nose bowel, etc. • Transmission takes place both directly and indirectly • Unclean hands and fingers imply lack of personal hygiene • Staphylococcal and streptococcal infections, typhoid fever, dysentery etc.

22. Susceptible host • Successful parasitism • Incubation period • Serial interval • Generation time • Period of Communicability • Secondary attack rate

23. Thank you