epidemiology of infectious diseases Flashcards
what is an infectious disease
is a disease caused by an infectious agent/organism such as bacteria, fungi or parasites
what is epidemiology?
- is the study of disease in populations & includes investigating factors that determine the occurrence of disease
- involves studying patterns & causes of diseases in populations
what is infectious disease epidemiology?
- is the application of epidemiology principles & methods to the study of infectious diseases in groups of animals
- important for determining control measures to reduce & prevent disease in animals
when does infection occur?
- occurs when an infectious agent gains entry & multiplies within a living host
- an animal can be infected but show no signs of disease
define clinical disease
- when a susceptible animal has detectable symptoms
define subclinical disease
- no detectable symptoms in patient
- need further tests to detect
what does the natural history of a disease refer to?
- refers to the progression of a disease in an individual over time in the absence of treatment
- epidemiologists want to identify the important factors affecting natural history in order to identify possible methods of prevention & control
define agent aș a determinant of disease
- refers to the infectious pathogen
- biological properties of infectious agents (infectivity, pathogenicity, virulence) are important contributors to an agent’s ability to cause disease
define virulence, pathogenicity & virulence
- virulence = % of susceptible individuals exposed to a particular agent who become infected
- pathogenicity = % of infected individuals who develop clinical disease due to a particular agent
- virulence = % of individuals with clinical disease who become seriously ill or die
define gradient of infection
- is the variety of responses an animal can have after exposure to an infectious agent
- exposure to infectious disease can lead to: no infection, clinical disease, subclinical disease or carrier
what are the potential outcomes of infection?
- death
- carrier
- recovery
define stability/survival
- is the length of time that an infectious agent can remain infective outside its natural host
define latent period
is the time between infection to the onset of shedding of infectious agent
define incubation period
- is the time between infection to the development of clinical signs
how could an infected animal be spreading agent whilst still appearing healthy?
- when the infectious agent has a latent period that is shorter than the incubation period
define host as a determinant of disease?
- host is the animal capable of being infected with agent & supporting replication/development of agent
what are key host determinants for disease?
- genotype, age, sex, breed, immunological state, nutritional status, behaviour
- some infectious agents have a narrow host range (high host specificity)
- some have a broad host range
what are key environmental determinants of disease?
- weather, geographic location, topography, water system, flood, vector distribution & husbandry factors
how could changes in weather conditions produce patterns of disease occurrence?
- may change disease risk by affecting survival & spread of the infectious agent or by changing health & susceptibility to infection of hosts
what is the ecology of disease?
- is the study & understanding of the complex interactions between infectious agents, hosts & their environment &
- how these interactions effect occurrence & impact
communicable disease vs non-communicable disease
communicable disease
- a disease that can be spread, directly or indirectly from one individual to another
- agent needs to make contact with host for infection to be transmitted
non-communicable disease
- a disease that is not transmissible from the infected host to another
define primary (natural/maintenance) host?
host that maintains infection in endemic area & is responsible for ensuring long-term survival of infectious agent
define reservoir host
- is the host the infectious agent normally lives & multiplies
define definitive host
host in which parasite reaches maturity & sexually reproduces
intermediate host
- development & sometimes asexual reproduction occurs
define paratenic host
- no development of parasite, only mechanical transfer can occur
define amplifer host
- amplifies the amount of agent, often associated with host population dynamics
define accidental/incidental host
- does not transmit infection
define superspreader
- disproportionally infects more secondary contacts than other hosts
define link host
- host that forms a link between 2 other host species eg horses between bats & humans with Hendra virus
describe the alimentary tract (mouth to anus) as a route of infection
- infection via the mouth is a common route of entry & many infectious agents are excreted in waste which can contaminate feed & water
describe the urogenital tract as a route of infection
- urogenital tract contains common portals of entry/exit for infectious agents & includes mammary glands & during birthing
describe the respiratory tract as a route of infection
pathogens in respiratory tract may be shed during breathing, coughing or sneezing
describe the skin & cornea/conjunctiva as routes of infection
- some infectious agents selectively target skin or cornea and can penetrate through
what is vertical transmission?
- occurs between generations
- when parent transmits infectious agent to offspring in utero or after birth via colostrum or milk
what is direct horizontal transmission?
- is transmission between animals by physical contact, close proximity or in contact with discharges
what is indirect horizontal transmission?
- transmission between animals mediated though a vector or a fomite
what is airborne transmission?
- transmission of an infectious agent in the air
- spores of some infectious agents can be inhaled in air
define maintenance of infection?
- is how an infectious agent can continue to maintain existence over a long period of time
eg
- infected host can pass an agent to external environment or a vector which then infects host
- or infected host can directly infect host
name 5 maintenance strategies infectious agents can use to maintain its existence
- persistence in the host
- extension of host range
- avoid a stage in external environment
- development of resistant forms
- rapidly in & rapidly out of host
list the 4 strategies that allow persistence of an infectious agent in the host
- immunosuppression or immunotolerance of host immune response to increase likelihood of agent surviving within the host
- antigenic variation
- localise to immunologically privileged sites where host immune system is unable to operate
- persist in arthropod vectors for long periods
how does the host use rapidly in & rapidly out as a maintenance strategy?
- based on speed from initiation to shedding of infectious agent
- aim is to complete infection, replication & shedding before host can either mount an effective immune response or die
what does the chain of infection refer to?
- refers to the way in which infectious agents exist & spread from one host to another
- a spread of infection can be described as a chain of interconnected steps
- a managed response to an outbreak aims to break chain of infection to stop further spread
what are the 4 basic strategies for disease control?
- detecting the agent (surveillance, tracing)
- reducing the number of infected hosts (treating, quarantining or culling sick animals)
- increasing resistance of susceptible hosts (vaccinate, breeding resistance)
- reducing contact between infectious & susceptible hosts (biosecurity, disinfection)
