Epidemiology of Bacterial Infections Flashcards
EPIDEMIOLOGY
The study of the spread of infectious disease within a community or population.
ROUTES OF INFECTION
DIRECT CONTACT- inhalation, aerosol, contaminated food/water, ingestion, inoculation (vector borne or injected), sexual transmission. These are all examples of HORIZONTAL TRANSMISSION.
VERTICAL TRANSMISSION- From mother to young eg. TRANSPLACENTAL TRANSMISSION.
Many bacteria use multiple routes eg. M. bovis is shed in faeces, milk, and from the respiratory tract.
RESERVOIR OF INFECTION
Where the infectious agent lives, multiplies and/or survives. Can be within a host animal or within the environment.
SOURCE OF INFECTION
Can be ENDOGENOUS, ZOONOTIC, or EXOGENOUS.
eg. Animals, food, water, humans, environment.
Pathogens can travel between all of these sources via various routes.
ENDOGENOUS source of infection
Comes from within the animal eg. An opportunistic infection caused by the animal’s own microflora.
ZOONOTIC source of infection
Human -> animal transmission of infection. Can also see animal -> human.
EXOGENOUS source of infection.
Infection is obtained from an environmental source.
VEHICLE OF INFECTION
When the infection comes from an inanimate source eg. food, water, or fomites- these are a vehicle for infection.
Which method of transmission can infect more animals- Horizontal or vertical?
HORIZONTAL transmission can infect more animals as it uses more routes of transmission- those which fall under the ‘direct contact’ heading. Animals become infected via infected water, food, inhalation, inoculation, close contact, vectors etc.
VERTICAL transmission refers to transplacental transmission; the mother can only infect her young and so on and so on, so fewer animals are infected each time than in horizontal.
DIRECT TRANSMISSION FROM THE ENVIRONMENT
Host is infected by bacteria that MULTIPLY within the environment. Environmental exposure may lead to transmission then disease.
Occasionally this disease may spread to other animals either directly or indirectly.
INDIRECT TRANSMISSION FROM THE ENVIRONMENT
Host is infected by bacteria that can SURVIVE in the environment.
Survival of these bacteria may depend on conditions and/or spore formation.
FACTORS WHICH COMPLICATE DISEASE TRANSMISSION
- Vaccination (colostrum from vaccinated mothers- food producing animals)
- Herd immunity- level of immunity within a population. Affects epidemiology of all infectious diseases.
- Normal bacterial flora- eg. Normal skin flora can infect a wound.
- Animals with active disease- Shed organisms in to the environment or directly to other animals, causing disease. Non immune animals will become ill, immune animals may become carriers.
- Symptomless carriers- Subclinical animals which are still carrying the bacteria- this may be as part of their normal flora, or because the pathogenic bacteria is still in it’s incubation stage.
ADVANTAGES OF CARRIAGE
TO BACTERIA- Facilitates transmission, bacteria are less likely to be eliminated, and are spread to susceptible hosts.
TO HOST- Do not become diseased- Only infected. Immunity is boosted by ‘natural vaccination’
Can disease transmit from one species of animal to another?
YES. Disease can cycle in the host species and form a reservoir of infection for other species.
Host of origin may be actively showing disease, or be a symptomless carrier.
eg. Bordetella in pigs and dogs.
FOMITE
A fomite is any inanimate object which can be contaminated with bacteria and can serve in their transmission.
