Infectious Diseases Flashcards
What is the definition of infectious disease?
An illness due to a specific infectious agent and/or its toxic products that arises through transmission …from an infected
person, animal or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector or inanimate environment.
- Last JM, Dictionary of Epidemiology (1988)
Describe the basic mechanisms of transmission of infectious agents?
Agent that is living and reproducing and is looking for another host to live in.
When goes directly from infected to susceptible = direct
When goes from infected to vector to susceptible host = indirect.
What are the general features of infectious diseases?
Features
* Diseases are caused by micro-organisms
* A case may also be a risk factor.
* Individuals may be immune. If they have not had the infection does not mean they won’t get it.
* A case may be a source without being recognized as a case. Agent can live happily in host and show no clinical signs at all.
* Preventive measures usually have a scientific basis. Usually based off evidence.
* There is sometimes a need for urgency
Define infectivity. How is this calculated?
Infectivity
* Ability to invade a host
(# infected / # exposed to agent) x 100
Tells you how well it is able to infect in a population.
Define pathogenicity. How is it calculated?
Pathogenicity
* Ability to induce disease
(# with clinical disease / # infected) x 100
You have to first be infected before you can cause disease. How many have clinical disease vs. infected?
Has to be in the host first then it has to be able to cause disease.
Define virulence. How is it calculated?
Virulence
* Ability to cause severe disease
(# severely ill / # with clinical disease) x 100
Severe illness out of all of those that have clinical signs. Bit of subjectivity to this.
Can also be defined as the ability to infect and cause pathogenicity.
What do you need for disease to spread?
Multiple populations (at least 2) You need multiple populations for an infection to spread.
1. Infectious agents
* Viruses, bacteria, parasites, fungi, prions
2. Vectors
* Typically biting arthropods (insects including: mosquitos, ticks, lice, fleas, mites, blood sucking flies/bugs (e.g. midges, and sandflies)
3. Hosts
* Domestic animals / humans
* Sympatric (exist in the same area, e.g. rodents)
* Wild
What is the epidemiological triad?
What do diseases result from?
- Diseases result from interactions between the host, agent, and the
environment - A vector may be involved in transmission
- May include Intermediate hosts / Reservoirs (Carriers)
How is host susceptibility determined?
Host susceptibility to the agent is determined by a variety of factors (e.g.):
* Genetic background
* Immunological condition
* Stressed / well nourished / vaccinated, etc.
Pathogen life cycle is driven by?
- Driven by modes of transmission and maintenance of infection
* Hosts, vectors, and agents - Ecological conditions that favor survival and transmission of
infectious agents
* Interactions among host, vectors, and environment
japanese encephalitis visur
amplified by pigs and wild birds
lives happily in mosq as vector and doew not necessaarily cause a lot of disease and death but does cause infection once in host adn reproducing. If mosq bites a humn, horse, or cattle –> causes disease b/c in host it is not normally in and is interacting wiht host that causes severe disease.
triconella (parasite)
lives among domestic cyel (almost exc. pigs b/c we farm pigs intensely) and the pigs will die from this and mice/rats will eat decomposig tissues from dead pig -> infect rat/mouse –> incorporated into feed and cycles this way in domestic cyel. in sylvatic cyel –> in the north the nativa strain that lives (can overwinter/freeze so it can live up there) –> humans eat some infected animals }(diaphragm or tongue is affected) and then we get trichonella by accident.
What are the different modes of transmission of infectious agents?
Define vertical transmission.
Vertical
* Trans-placental transmission ONLY
* Direct contact through blood from mother to offspring through placenta. Some agents do not make it through bloodflow but sometimes it is through childbirth itself, milk/colostrum. Is this pure vertical? Debate with this. Bottom line: mother –> offspring.
* Born with the infectious agent
Define horizontal transmission.
- Contact
- Direct contact
- Handshaking, bites, breeding, colostrum/milk
- Indirect contact
- Droplets from sneezing/coughing (within 1 meter)
- Fomites (inanimate objects): sharing needles, dirty boots, palpation gloves, milking equipment, transport trucks, etc.
E.g. common cold –> droplets from sneezing or coughing (1 m/3 feet)
- Direct contact
needles –> drug users
dirty boots –> feces on boots that may have FMD and mycoplasma in pigs
Blood during rectal exam, use same gloves in another cow –> pass virus
milking equip: mastitis is passed this way.
Define vehicle in relation to horizontal transmission.
Vehicle = medium that helps transmit it.
What are the different types of “vehicles” in horizontal transmission?
- Airborne
* Dust particles, strong winds
* e.g. Influenza, PRRS, histoplasmosis
if it can pass through the air, not just through the droplets then it is considered airborne.
Ebola has curly shape it is able to take flight with wind. - Waterborne
* Water troughs, streams, swimming
pools
* e.g. Campylobacter, Cholera = drinking water, Giardia - Foodborne
* Contaminated food (“Food
poisoning” from un-/under-cooked
contaminated food) Typhoid Mary = diarrhea, did not wash hands, contaminated food –> fed to fever.
What are the different types of vectors in horizontal transmission?
- Mechanical
* Present on OUTSIDE insect bodies
* e.g. E.coli, salmonellosis, ‘Pink eye’
(Moraxella bovis is the bacterium that causes infection of eyes in cattle; the bacterium does not live inside housefly but on the outside on their feet/mouth), etc. - Biological
* Lives inside a vector (insect) usually along guts or salivary glands. when insect bites they are excreted from salivary glands into blood supply.
* e.g. Lyme dz, malaria, etc.
What is the chain of infection?
How does the agent travel to the host?
What is the difference between infection vs. disease?
- Infection (all of these must occur)
* Invasion of agent into the host –>
* Multiplication of agent –>
* Reaction of host tissues to the agent +/- toxins produced; neutorphils, monocytes, eosinophils, etc. Reaction of host to agent = infection. Just seeing an agent present in tissue without reaction is NOT an infection. - Disease
* Disorder of the structure or function of the host
* Associated with clinical signs
When there is so much of a host reaction that the structure or function of host is compromised AKA a clinical sign present in host for it to be called a disease. Agent is what is passed from person to person and CAUSES disease.
You submit a sample for PCR test and it comes back positive for agent X…
What do you know?
Takes sample and amplifies a bit of the DNA and or the RNA of the agent. Takes small piece and if + states that is present. We know that the agent is present in the individual. Do not know if animal is alive or dead? No If it is causing infection? No. We only know that a bit of DNA and/or RNA is present. Do not have a lot of agent to know if it is present or not.
Define propagative spread.
An infection will disappear if …
will become endemic if…
will be come an epidemic if…
See below.
Calculate the reproductive number. Wont need to do math on exam but understand concept. Is this infection effective?
Calculate the reproductive number. Wont need to do math on exam but understand concept. Is this infection effective?
No because 0.25
What type of model is pictured below? Explain its principles.
SIR model
Time period (x) is the incubation period.
Within every incubation period there is a turnover. During the time a person was infectious, they infect someone else and it takes x amount of time for the contagious person to recover and the infected person to develop clinical signs.
