of Principles Disease Part 2: Disease Transmission Chain of Disease Transmission Pathogen in Reservoir Portal of exit Mode of transmission Portal of Entry to new (susceptible) host Reservoirs of Infection Natural host or habitat of a pathogen (place(s) where a pathogen normally lives/multiplies) Human Reservoirs 1. 2. People with clinically recognized disease (have signs & symptoms) Carriers: No clinical disease, but pathogens inconspicuously harbored and shed to others: A. B. C. D. Subclinical infections During incubation or convalescence stage of disease Chronic carriers (long-term shedding) Passive carriers (contaminated hands)
Animal Reservoirs Zoonoses: Diseases that occur primarily in wild or domestic animals that can be transmitted to humans Nonliving Reservoirs Food, Water, Soil
Disease Transmission Movement of a disease agent from reservoir (source) to host Direct Contact Transmission Microbes spread by physical contact between source and host (no intermediate objects) Direct Transmission by Droplets Microbes spread in mucous droplets (sneezing, coughing, talking) that travel less than 1 meter
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Vertical Transmission Microbes cross placenta from mother to fetus Transmission by Vectors (Vector-borne) Vector: Nonhuman, living organisms capable of transmitting infectious disease Ex: Arthropods (Ticks, Mosquitoes, etc); rodents carried (Pathogens on the vector's body/feet or pathogens multiply within the vector)
Indirect Transmission by Vehicles Vehicle: Nonliving material capable of transmitting infectious disease 1. 2. 3. 4. Water (Water-borne) Food (Food-borne) Air (Air-borne): Microbes in droplet nuclei from fine spray from sneezing/coughing (travel more than 1 meter from reservoir to host), in water aerosols, or by dust Fomites: Contaminated objects (Bedding, dishes, tissues, etc.)
Portals of Entry/Exit Pathogens have definite routes of entry and exit from infected hosts (often are the same) 1. 2. Mucous membranes (mucosa) of Respiratory, Gastrointestinal, Genitourinary tracts Skin (usually has to be damaged in some way: nicks, abrasions, punctures, insect bites, etc.)
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Breaking the Chain of Transmission Key to disease control Examples of Breaking the Chain at Reservoirs 1. 2. Reduce # animal reservoirs with rabies vaccine Treatment of human reservoirs with antimicrobial drugs
Examples of Breaking the Chain at Mode of Transmission 1. 2. 3. Reduce vector populations by drying out wet areas or use of insecticides Reduce vehicle contamination by chlorination of water or cooking of food Interrupt direct contact transmission by hand washing or condom use
Example of Breaking the Chain at New (Susceptible) Hosts 1. Reduce # susceptible hosts by vaccination
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