Disease Reservoirs and Transmission Flashcards
Def. Disease Transmission
Disease transmission is a result of the interaction between the host, agent, and environment
John Snow
John Snow and cholera – In 1849, Snow published evidence that cholera is transmitted by the fecal-oral route and by the water supply
Large outbreak in London in 1854… – 616 fatalities, up to 12.8% of the people living in the most heavily affected areas
Created the first spot map- IDed the waterpump as the main transmission route
Anthrax and Germ Theory
Anthrax – earliest reports in 1491BC Robert Koch first isolated the bacterium – and used experimental infection of naïve animals to prove that B. anthracis causes anthrax – Published in 1876 – The beginning of modern “germ theory” 1881: Pasteur developed and tested an early vaccine in sheep, goats, and cattle
Typhoid Mary
Irish immigrant who worked as a cook
Caused several outbreaks of typhoid fever (Salmonella Typhi = anthroponotic) between 1900-1915 – Outbreaks followed wherever she worked
ASYMPTOMATIC CARRIER!
Ronald Ross
1897 - Ronald Ross discovered that malaria is transmitted by mosquitoes; some credit the discovery to the Italian scientist, Grassi – The French physician Laveran had discovered the agent, Plasmodium, in 1880
Walter Reed
1900 - Walter Reed discovered that yellow fever is transmitted by mosquitoes – William Gorgas used this information to rid Havana of Yellow Fever, and later for the Panama Canal
What is more important to learn- the specific agent or how it is transmitted?
In disease prevention, knowing the mode of transmission is generally more important than identifying the specific agent
Reservoir
Reservoir = habitat in which an infectious agent normally lives, grows, and multiplies (humans, animals, or the environment)
Reservoirs maintain pathogens
Reservoirs maintain pathogens over time, from year to year or generation to generation!
How do you recongize a reservoir?
An animal (or soil, or water, etc.) is a reservoir if you answer YES to all three of these questions: 1. Is it naturally infected with the pathogen? 2. Can that species of animal (etc.) maintain the pathogen over time? 3. Can this source transmit the disease to a new, susceptible host?
What are 3 ways that pathogens have evolved to be more virulent?
Pathogens can mutate to escape immunity, so that animals become “susceptible” again, over time
– Pathogens can evade immunity, allowing re- infection to occur after a short time period
– Pathogens can cause chronic infections with minimal symptoms (“balanced pathogenicity”- i.e. BATS or rodents)
When there is an infection, who is a reservoir?
Clinically ill animals that are reservoir competent are probably infectious
But so are some asymptomatic animals = carriers
And not all sick animals are reservoirs
REMEMBER, INFECTION DOES NOT ALWAYS EQUAL DISEASE
Vertical Transmission
Vertical: from a reservoir host to its offspring – Congenital – some pathogens can cross the placenta, infect eggs, etc. – Perinatal – during parturition, via colostrum
Horizontal Transmission and the 2 main types
Horizontal: from the reservoir to a new host – Direct – directly from the reservoir to a susceptible host – Indirect – via any sort of intermediary, animate or inanimate
Horizontal / Direct Transmission
Direct contact – Skin-skin contact, mucous membrane contact (including sexual transmission), direct contact with a soil reservoir, bite, scratch, etc. Direct projection (droplet spread) – Wet, large, and short range aerosols (sneezing, coughing or talking) Airborne – Considered to be a form of direct transmission because disease agents do not generally survive for extended periods within aerosolized particles
