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?
clinical perception of disease vs herd health -perspective
Concept of an iceberg
the presumptive number of infected versus the true number
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.
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.
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.
