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