Disease Transmission and Pathogen Reservoirs (block 2)

disease

deviation from normal physiological conditions where one or more organs is affected (disrupted homeostasis)

four stages of infectious disease

incubation, clinical disease, convalescence, recovery

classification by duration (in order)

per-acute, acute, sub-acute, chronic

source / reservoir

where the agent lives and muliples

susceptible population

hosts capable of infection

modes of spread

pathways connecting source to host

infectious disease

caused by pathogens (bacteria, viruses, fungi, parasites)

non-infectious diseases

result from metabolic disorders, hormonal imbalances, immunological reactions, neoplasia, deficiencies, or toxicities

disease develops only when...

a susceptible host meets an infectious agent

incubation period

pathogen establishes and multiplies in host tissue, no clinical signs are visible but changes at cellular level

three phases of clinical disease

prodromal, fastigium, defervescence

prodromal

early, non-specific signs like fever, lethargy

fastigium

peak illness with characteristics clinical signs

defervescence

declining symptoms as immunity develops

convalescent period

recovery phase where clinical symptoms resolve, animal may still shed pathogens (critical for disease transmission to naive animals)

recovery

complete return to health with pathogen clearance, some animals become carriers

periods of infectivity

latent, patent, shedding, carrier state

latent

agent hard to detect (incubation)

patent

agent detectable (in secretions, excretions, blood, or tissues) and trasmissible

shedding

late incubation, end of covalescence

carrier state

some hosts shed long after recovery

infectiousness

ability of pathogen to invade and establish infection in a new host

highly infectious diseases

short shedding period, rapid spread

slowly spreading diseases

long incubation, prolonged shedding

healthy carrier type

recovered or never symptomatic, still harbor agent

incubatory carrier type

transmit during incubation period

convalescent carrier type

transmit during recovery after clinical signs resolve

intermittent shedders

unpredictable shedding, evade detection

temporal classifications

incubatory carriers, convalescent carriers, chronic carriers

incubatory carriers

infectious during incubation before showing signs

convalescent carriers

continue shedding after clinical recovery

chronic carriers

long-term shedders

shedding patterns

continuous shedders, intermittent shedders, non-shedders

continuous shedders

constantly releasing pathogens

non-shedders

harbor agent but don't transmit

reservoir

natural habitat where pathogen lives, multiplies, and persists long-term

types of reservoirs

animal, human, environmental, arthropod (ticks, mosquitoes, etc)

source

immediate source of infection in a specific case

direct transmission

intimate physical contact (ex: sexually, bite, skin-to-skin)

indirect transmission

through contaminated fomites, equipment, or handlers

airborne transmission

pathogens suspended in droplet nuclei that reach lung alveoli directly (favored by overcrowding and poor ventiliation)

vehicle borne transmission

non-living objects carry pathogens, created explosive outbreaks

epidemic patterns from vehicle-borne spread

explosive onset, common source, limited geography, sharp decline

vector borne transmission

mechanical, biological

mechanical transmission

vector carries pathogen on body parts without biological interaction

three types of biological transmission

propagative, cyclo-propagative, cyclo-developmental

propagative

pathogen multiplies only

cyclo-propagative

pathogen undergoes development and multiplies

cyclo-developmental

pathogen develops without multiplying

extrinsic incubation period

time required for pathogen development in the vector before transmission capability

transovarial transmission

pathogens pass through the ovaries to the next generation

transstadial transmission

pathogens survive molting between life stages

overwintering

infected vectors hibernate (agent through winter), seasonal reappearance (returns after long quiet period)

overwintering survival mechanisms

adult vector hibernation, transovarial persistence, environmental reservoirs

adult vectors hibernation

infected adult vectors enter dormancy, harboring pathogens through winter

transovarial persistence

female vectors pass infection to eggs before dying

environmental reservoirs

pathogens form resistant stages that survive in soil, water or organic matter

vertical transmission

parent to offspring through multiple routes

routes of vertical transmission

transplacental (cross placenta), transmammary (through milk), transovarial (through eggs), during parturition (contact with birth fluids)

horizontal transmission

spread between animals sharing the same environment or exposure

zig zag transmission

alternates between vertebrate and invertebrate hosts, neither of which can maintain the pathogen alone