reservoirs
John Snow and cholera
"Miasma" theory of disease was predominant In 1849, Snow published evidence that cholera is transmitted by the fecal‐oral route and by the water supply Predated the germ theory of disease!
Anthrax - earliest reports in
1491BC
Large outbreak in London in 1854
616 fatalities, up to 12.8% of the people living in the most heavily affected areas
Vector
A living organism that serves to communicate disease. For example, mosquitoes and other arthropods.
Vehicle
An inanimate object which serves to communicate disease. For example, a glass of water containing microbes, or a dirty rag, etc.
Biological vectors examples
Dirofilaria development (L1-->L3) in a mosquito
"Chain" of Infection
Disease transmission can be represented as a chain of events Intervention at any link in the chain can break the cycle Veterinary medicine usually focuses on reservoirs, transmission, and host susceptibility
exampkles of mechanical vectors
Fleas transmitting myxomatosis between rabbits
Can this source transmit the disease to a new, susceptible host? NO
If not, it is a DEAD‐END host
Is it naturally infected with the pathogen? NO
If not, it is an ACCIDENTAL host
typhoid mary asymptomatic carrier
Irish immigrant who worked as a cook Caused several outbreaks of typhoid fever (Salmonella Typhi = anthroponotic) between 1900‐1915
Recognizing a Reservoir An animal (or soil, or water, etc.) is a reservoir if you answer YES to all three of these questions:
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?
fomites
Object that can be contaminated and transmit disease on a limited scale - surgical instruments, door handles, boots, etc. Most nosocomial and iatrogenic infections are transmitted by fomites
Anthrax
Outbreaks were, over time, blamed on: the bite of pregnant shrews; miasma; divine punishment
Direct contact
Skin‐skin contact, mucous membrane contact (including sexual transmission), direct contact with a soil reservoir, bite, scratch, etc.
John Snow's Cholera Map
Went door‐to‐door and mapped cases of cholera in London Outbreak was centered around one water pump Removing the handle from the pump (so people had to go elsewhere for water) stopped the outbreak One of the "fathers" of epidemiology!
Direct projection (droplet spread)
Wet, large, and short range aerosols (sneezing, coughing or talking)
Pathogens can evade immunity
allowing reinfection to occur after a short time period Ex: canine Bordetella bronchiseptica
If there is a very large population, there will
always be new, susceptible animals: either by birth or by geographic spread
1881: Pasteur developed and tested
an early vaccine in sheep, goats, and cattle
Pathogens can mutate to escape immunity, so that
animals become "susceptible" again, over time Ex: influenza virus
If the pathogen persists long enough in the environment
another susceptible animal will eventually be available
example of persistant pathgoen
antrhax in soil
Latent carriers
are persistently infected, for years/life Equine herpesvirus can remain latent for years. Latent infections can be re‐activated by stress--> shedding!
most vectors are
arthropods(mosquitoes flies fleas ants and ticks)
WNV can also be transmitted by
blood transfusion Since 2003, over 1,000 WNV‐infected donations have been intercepted
direct transmisison requires
both spatial and temporal association bewtene the infected host and the naïve host
vertical transmisison examples
congential perinatal
reservor for environmental pathogens the habitat that sevres as
coninuous source of disease
direct
directly from the reservoir to a susceptible host
1897 ‐ Ronald Ross
discovered that malaria is transmitted by mosquitoes; some credit the discovery to the Italian scientist, Grassi
perinatal
during parturition, via colostrum
Common vehicle
food water contamonated IV drugs several hosts are exposed by one shared(common) vehicle
vertical transmisison
from a reservoir host to its offspring
horizontal transmission
from the reservoir to a new host
Reservoir
habitat in which an infectious agent normally lives, grows, and multiplies (humans, animals, or the environment)
environemnt reservoirs
legionella coccidioides
vectors are
living organisms
reservoir living animal/species that
maintains a poathogen under normal conditons
example of large population of pathogens
measels in epople parvovirus in dogs
Pathogens can cause chronic infections with
minimal symptoms ("balanced pathogenicity") Ex: rabies or Ebola virus in bats
reservoirs maintain pathogens
over time from year to year or generation to generation
pathogen chronic infection min symptoms often seen in natural host
pathogen relationships that have remained stable over many, many generations
Clinically ill animals that are reservoir competent are
probably infectious
some asymptomatic animals = carriers
probably infectious
not all sick animals=
reservoirs
Disease transmission is a
result of the interaction between the host, agent, and environment
animals(non-humans) reservoirs
salmonella spp toxoplasma gondii feline leukemia virus foot and mouth disease virus
Convalescent carriers
shed pathogens during recovery Chickens shed END virus for up to 30 days after clinical recovery
Incubatory carriers
shed the infectious agent during the incubation period (prior to symptoms appearing) Many respiratory viruses are infectious 24‐48 hours before symptoms are evident
humans reservoir diseases
smallpox measles mumps taenia solium
congenital
some pathogens can cross the placenta, infect eggs, etc
Pathogens use different strategies to
survive, over time, in the reservoir as a population
mechanical=
the agent does not multiply or undergo part of it's life cycle while in/on the arthropod
French physcian Laveran discovered
the agent plasmodium 1880
Biological =
the agent undergoes changes or multiples while in the vector; these activities are required for transmission
WNV is primarily spread through
the bite of infected mosquitos The virus replicates in the mosquitos...
In disease prevention, knowing
the mode of transmission is generally more important than identifying the specific agent Especially when facing an outbreak of a newly emerging disease!
William Gorgas used
this information to rid Havana of Yellow Fever, and later for the Panama Canal
Robert Koch first isolated the Anthrax bacterium - and
used experimental infection of naïve animals to prove that B. anthracis causes anthrax Published in 1876 The beginning of modern "germ theory"
Animal and human diseases do not arise in a
vacuum
indirect
via any sort of intermediary, animate or inanimate
1900 ‐ Walter Reed discovered that
yellow fever is transmitted by mosquitoes