Infectious Diseases Flashcards
What is an infectious disease?
an illness due to a specific infectious agent 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
What are the 2 basic mechanisms of transmission of infectious agents?
- DIRECT TRANSMISSION from an infected host to a susceptible host
- INDIRECT TRANSMISSION to a vector IH that typically does not show clinical signs and then to a susceptible host
What are the 6 features of infectious diseases?
- dseases are caused by microorganisms
- a case may also be a risk factor (presence of an infected individual starts the spread)
- individuals may be immune
- a case may be a source without being recognizedas a case (no clinical signs)
- preventative measures usually have a scientific bases
- there is sometimes a need for urgency
What is infectivity? How is it calculated?
ability to invade a host
(# infected / # exposed to agent) x 100
What is pathogenicity? How is it calculated?
ability to induce disease
(# with clinical disease / # infected) x 100
What is virulence? How is it calculated?
ability to cause severe disease
(# severely ill / # with clinical disease) x 100
What are the most common infectious agents? Vectors?
- viruses
- bacteria
- parasites
- fungi
- prions
biting arthropods - mosquitos, ticks, lice, fleas, mites, blood sucking flies/bugs (midges, sandflies)
What are the 3 types of hosts of epidemiological importance?
- domestic animals/humans
- sympatric (exist in the same area, like rodents)
- wild
What is the epidemiological triad?
the idea that diseases result from the interactions between the HOST, AGENT, and ENVIRONMENT
(sometimes including a vector or intermediate hosts/reservoirs/carriers)
What are 2 common factors that determine the host susceptibility to and infectious agent?
- genetics
- immunological condition (stressed/well-nourished/vaccinated)
What drives pathogen life cycles? What conditions fvor survival and transmission of infectious agents?
modes of transmission and maintenance of infection (hosts, vectors, agents)
interactions among host, vectors, and the environment
JEV life cycle:
Trichinella life cycle:
Body surfaces as sites of horizontal infection and shedding of infectious agents:
How does vertical transmission occur?
transplacental transmission with direct contact through blood from mother results in birth of offspring with the infectous agent
What are the 2 modes of contact (horizontal) transmission?
- DIRECT - handshaking, bites, breeding, colostrum/milk
- INDIRECT - droplets from sneezing/coughing, fomites (needles, dirty boots, palpation gloves, milking equipment, transport trucks)
What are the 3 modes of vehicle (horizontal) transmission?
- AIRBORNE - dust particles, strong winds (influenza, PRRS, histoplasmosis)
- WATERBORNE - water troughs, streams, pools (Campylobacter, cholera, Giardia)
- FOODBORNE - contaminated food, “food poisoning,” under-cooked contaminated food
What are the 2 modes of vector (horizontal) transmission?
- MECHANICAL - present ON insect bodies, like on the feet and mouthparts (E. coli, salmonellosis, pink eye)
- BIOLGICAL - present IN insect bodies, like gut and salivary glands (Lyme disease, malaria)
What is the chain of infection? What affects the agent’s ability to cause infection, transmission, and host’s ability to be infected?
how the infection moves in a population from agent to host
AGENT - pathogenicity, infection dose, reservoir, source
TRANSMISSION - contact, common vehicle, airborne, vectors
HOST - individual host factors, non-specific resistance, specific immunity
What is the difference between infection and disease?
infection must include invasion of agent into host, multiplicaion of agent, and reaction of the host tissues to the agent +/- toxins, leading to inflammation and WBC reaction
disease is a disorder (so much of a host reaction) of the structure or function of the host, associated with clinical signs
You submit a sample for PCR test and it comes back positive. What do you actually know?
agent DNA/RNA is present —> don’t know if it’s alive or causing disease
What are the transmission time periods of infection and disease in the first patient?
INFECTION: latent period —> infectious period —-> extended period leading to carrier/reservoir status OR infection of a second susceptible host
DISEASE: incubation period —> clinical disease —> dead/suscpetiblility/immunity
What is propagative spread? What 3 types of cases are found?
spread of an infectious agent from one animal to another
- INDEX = first case identified
- PRIMARY = case that brings the infection into a population (can be the index)
- SECONDARY = infection from primary
(tertiary and onward)
What does maintenance of population-level infection depend on?
successful transmission to new host
- characteristics of infectious agents
- susceptibility of host
- population dynamics (size and density)
If the number of susceptible individuals in a population is low, what is the probability of the infective host to contact susceptible hosts?
LOW
How do agents with short infective periods spread?
quickly - require rapid contact with susceptible hosts
(HIV lasts long, so it doesn’t need to jump from person to person immediately)
What is a common intervention strategy to decrease spread of an infectious agent?
resducing the susceptible population
What is herd immunity? What happens when herd immunity is high? Low?
collective immunological status of a population with regard to the amount of susceptible individuals
HIGH = low susceptible population = less chance of spread (vaccination)
LOW = high susceptible population = high chance of spread (new crop of animals entering a farm)
What does the critical threshold to determine the amout of susceptible/immune in a population to determine herd immunity status depend on?
disease-specific and related to the potential for transmission
What is basic reproductive number (Ro, or R-naught)?
mean number of secondary cases an infectious individual will cause when introduced into a SUSCEPTIBLE (naive) population
if Ro = 2-7, one individual infected 2-7 other individuals on average
What is the effective reproductive number (Rt)?
mean number of secondary cases an infectious individual will cause when introduced into the WHOLE population
disease progression = change over time
What is happening in a population when R < 1, R = 1, and R > 1?
R < 1: disease is no longer spreading to others, it has been snuffed out and disappears
R = 1: disease is passing on average to one other individual - becomes ENDEMIC
R > 1: disease spread is growing exponentially - becomes epidemic
What is the relationship between herd immunity and reproductive number?
a higher reproductive number (Ro) means that a higher number of individuals need to be vaccinated in order to control spread and reach herd immunity status —> need to get a proper amount of individuals immune so that R </= 1
What is the compartmental model?
mathematical model for propagating epidemics where individuals are put into specific states (Susceptible-Infectious-Recovered or Susceptible-Exposed-Infectious-Recovered) and the rate of change between disease states are estimated
What is the agent-based model?
mathematical model for propagating epidemics where free agents (individual entities, NOT infectious agents) are simulated with set parameters, taking into account different individual characteristics that can affect infeciton or spread
- requires huge computational power!
What is the overall goal of population models that propagate epidemics? What should be assumed about all models?
better understand and predict disease spread in a dynamic population
all models are wrongs, but some may be useful - they use assumptions and biological basis
What is the basic Reed-Frost model?
compartmental model - case —> spread —> recover —> immune
What are the 3 main goals when coming up with mitigation strategies?
- reduce the number of infected/shedding animals - treating, culling, screening/testing before moving or breeding
- reduce transmission of agent - cleaning the environment, removing vectors/reservoirs, quarantining/isolating infected, avoiding high risk practices
- reduce number of susceptible - vaccinate, isolate, cull, don’t move/mix populations
