Lecture 1 - General concepts Flashcards
What is disease?
any impairment that interferes with or modifies the performance of normal functions,
including responses to environmental factors such as nutrition, toxicants, and climate; infectious agents; inherent or congenital defects; or combination of these factors (Wobeser 1981).
What is an infectious disease?
infectious disease is caused by living organisms that cause harm while residing in or on an animal’s body, and these living agents replicate and are involved in a trophic relationship with the animal
infectious diseases can be characterized as (3)
contagious
non-contagious (e.g. insect vectors required)
or
noninfectious but transmissble (TSEs)
(Contagious diseases are spread by contact, while infectious diseases are spread by infectious agents.)
Infectious means
capable of causing infection.
not all infected individuals are infectious.
(Contagious diseases are spread by contact, while infectious diseases are spread by infectious agents.)
infection means
the invasion and replication of an agent within a host animal.
but does not always equal disease because an infection may be present in the body without inflammatory reaction.
Etiologic classification of infectious diseases refers to?
nomenclature and classification based on etiology such as,
Salmonellosis
Coronavirosis
Brucellosis
Mycoplasmosis etc
Syndromic classification of infectious diseases refers to?
classification Based on clinical syndrome and animal species (and agent),
e.g.
Foot- and mouth disease
Rabies
Bovine tuberculosis
Enzootic pneumonia of swine
Avian respiratory mycoplasmosis
Contagious means
capable of being transmitted from one individual to another
not all infectious diseases are contagious.
Epidemiologic classifications (2)
By reservoir (e.g. human, animals, soil, water so biotic or abiotic reservoirs)
By transmission (direct, indirect, aerogenic etc.)
Koch’s postulates
- The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
- The microorganism must be isolated from a diseased organism and grown in pure culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.
“Classical” classification of infectious diseases refers to?
Well defined (classical) inf. disease.
One agent and one disease (pathognomonic)
Some basis on Koch’s postulates.
E.g. FMD, ASF, rabies
Evan’s postulates 1st half
Disease should follow exposure to the putative agent.
Exposure increases disease incidence prospectively.
Exposure increases disease prevalence.
Exposure to the cause more common in those with the disease than those without.
Dose-response relationship.
Evan’s postulates 2nd half
Experimental reproduction of the disease possible.
Measurable host response following exposure to the cause.
Elimination of putative cause reduces incidence.
Prevention of the host‘s response eliminates the disease.
The whole thing should make biologic and epidemiologic sense.
Zoonosis & Anthroponosis defintions
Zoonosis = any disease or infection that is naturally transmissible from vertebrate animals to humans.
Anthroponosis = an infection or disease that is transmissible from humans to animals under natural conditions.
Contemporary use of ZOONOSIS = contagious disease of animals transmissible to humans.
Anthropozoonosis & Zooanthroponosis definitions.
Anthropozoonosis = pathogen that is transmitted from animals to humans.
Zooanthroponosis = pathogen reservoired in humans that is capable of being transmitted to non-human.
infection does not equal
disease
infection refers to
the invasion and replication of an agent within a host animal
Consequences of infections are various such as (3)
Elimination of the agent by the host’s immune system
Subclinical disease
Clinical disease or death
Persistent infection refers to
chronic infection; clinical signs may or may not be present; replication and shedding of the agent is continuous.
E.g. ASF – animals surviving the dz
Latent infection refers to
chronic infection; no clinical signs; not contagious
E.g. Herpes
Infection process / stages of infection (6)
- invasion by pathogen
- incubation period (pathogen begins to replicate)
- subclinicial infection / prodromal period
- period of illness
- convalescent period
(6. host may become chronic carrier)
IP
INCUBATION PERIOD (IP) – time from incursion of the agent to appearance of clinical signs.
Every disease has its own typical IP that can vary widely from other diseases.
the period of communicability is always longer than the period of
clinical disease
Course of infection (disease) can be described as (4)
Peracute, acute, subacute, chronic
define peracute
(of a disease) very severe and of very short duration, generally proving quickly fatal.
Dissemination of the infection in the host can be described as (4)
Local(ised), generalized;
lymphogenic, neurogenic etc.
Presence of the dz agent in blood can be described as
Viraemia, bacteremia; septicemia, toxemia, Candidemia etc.
Form of the disease can be described by
various syndromes / complexes of clinical signs
e.g. meningitis (a syndrome) but the same agent can cause arthitis (a syndrome) in another
course of infection refers to
the nature of it
e.g. Peracute, acute, subacute, chronic
Epidemiologic triangle
Host-Agent-Environment
Who?-What?-Where?
Factors influencing the course of infection (3)
Properties of the agent
Properties of the host
Environmental conditions
examples of Properties of the agent influencing the course of infection (2)
Pathogenicity
Virulence
examples of Properties of the host influencing the course of infection (2)
Susceptibility
Immunity
examples of Environmental conditions influencing the course of infection
Effect on general condition of the host
Influence on presence and abundance of the agent
Effect on portal of entry of the pathogen
pathogenicity refers to
the ability of the agent to produce a disease in the host
virulence refers to
the degree of pathogenicity of a microorganism as indicated by the severity of disease produced and the ability to invade the tissues of the host
difference between pathogenicity and virulence
Pathogenicity is the ability of an organism to cause disease, virulence, on the other hand, refers to the magnitude of the disease caused.
agent pathogenicity can be e.g.
pathogenic, potentially pathogenic, apathogenic
agent virulence can be e.g.
highly virulent, moderately virulent, with low virulence
SUSCEPTIBILITY refers to the
ability to become infected (or diseased)
e.g.
Genetic susceptibility/resistance
Age dependent susceptibility
Gender specific susceptibility
IMMUNITY refers to
the ability of an organism to resist a particular infection or toxin by the immune responses of the host generated by immunization or previous infection
The development of a disease in a population is comparable to
the disease pathogenesis in an individual
clinical perception of disease vs herd health -perspective
Concept of an iceberg
the presumptive number of infected versus the true number
VIRULENCE
measure of severity of a disease
Note: High virulence -> more excretion -> higher level of contagiousness!
morbidity refers to
Proportion of diseased animals in a population
pathogenicity can initially be assessed by what factor
by morbidity (Proportion of diseased animals in a population)
case fatality rate can give you an idea about what factor
the VIRULENCE – measure of severity of a disease
High virulence =
more severe disease, higher fatality rate.
also can mean more excretion and thus a higher level of contagiousness!
CONTAGIOUSNESS (or communicability) refers to
the capability of being transmitted from one individual to another.
Describes how easily the agent spreads in susceptible population.
Level of contagiousness depends on: (5)
- Period of infectiousness of the host
- How agent is excreted
- Infectious dose
- Stability in the environment
- Herd immunity
Infectious dose (ID) refers to
minimal amount of pathogen required to cause an infection in the host.
pathogenic agent STABILITY refers to
the capability of it to resist environmental conditions
Main survival strategies of pathogens:
(5)
Avoid external environment
The development of resistant forms
E.g. Anthrax – spores
“Rapidly in rapidly out” strategy
Persistence within the host
Extension of host range
4 Properties of an agent influencing the spread of infection in population
Pathogenicity
Virulence
Contagiousness
Stability
3 Environmental conditions influencing the spread of infection in population
Effect on general condition of the host
Influence on presence and abundance of the agent
Effect on portal of entry of the pathogen
3 Properties of the host influencing the spread of infection in population
Susceptibility
Infectiousness
Immunity
2 host factors to contribute to suceptibility
genetics
immunity
INFECTIOUSNESS (or infectious period) refers to
time period during which an infected animal is able to transmit the agent to another host
enviromental factors influencing the spread of infection in population can be classed as: (3)
Abiotic, biotic and socio-economical
EFFECTIVE CONTACT refers to
the contact between the source of infection and the susceptible individual leading to successful transmission.
effective contact is measured by
probability
describe the chain of infection
a susceptible host and infectious agent are required. the agent is sourced from a reservoir.
a portal of exit from the host is required with a mode of transmission and a portal of entry to a new susceptible host.
voila.
RESERVOIR of the pathogen refers to
one or more epidemiologically connected populations or environments in which the pathogen can be permanently maintained and from which infection is transmitted to the defined target population
Reservoir vs. source
the source is not inevitably a reservoir!
E.g. Rabies
Reservoir: red fox and raccoons; bats.
Source of (human) infection: typically dogs.
“The reservoir of an organism is the site where it resides, metabolizes, and multiplies.
The source of the organism is the site from which it is transmitted to a susceptible host, either directly or indirectly through an intermediary object.”
Source of infection is typically one of 3
infected host
contaminated environment
or fomite/s
Asymptomatic carriers could be one of the following three
Incubation period carrier
Convalescent carriers – excreting after healing
Chronic carriers – individuals who shelter the agent for a long time
route of infection refers to
how agent gains entry to new host
Most agents enter through digestive, respiratory and urogenital tracts, which communicate with the external environment.
route of excretion refers to the
Anatomical route through which the pathogen leaves the body of the host
(Often related to clinical symptoms e.g. diarrhea)
mode of transmission refers to?
The method by which the pathogen is transferred from one individual to another. e.g. vector borne, aerosolized etc.
NB! Is not the same as infection route!
what is horizontal mode of transmission?
Horizontal transmission is the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship.
difference between aerosol/droplet transmission and air-borne transmission?
aerosol/droplet transmission counts as a direct mode of transmission
air-borne transmission counts as indirect contact
give 3 examples of vertical transmission
Transovarial
In utero
(Colostral)
Route of infection refers to
the portal of entry of the agent, to a susceptible host
e.g. Most agents enter through digestive, respiratory and urogenital tracts.
Generation time =
the time from the introduction of the agent until the maximum infectiousness.
incidence versus prevalence
Prevalence differs from incidence proportion as prevalence includes all cases (new and pre-existing cases) in the population at the specified time whereas incidence is limited to new cases only.
prevalence = all cases
incidence = all NEW cases
