Module 2 - One Health Diseases Flashcards
epidemiology
The branch of medicine which deals with the incidence, distribution, and possible control of diseases and other factors relating to health.
- provides a profile of the type of
individual who is likely to contract a
disease after being exposed to an
agent.
* Time, place, demographics - quantifies the amount of disease that
is associated with an agent (measures
of disease frequency) - identifies risk factors that are associated with an increased risk of disease in groups of individuals
(measure of disease association)
Epidemiology of infectious diseases
- Host factors
- Pathogen factors
- Environmental factors
Sufficient and necessary causes
of infectious diseases
Sufficient: is not a single factor, but a minimum set of factors and circumstances that, if present in a given individual, will produce the disease.
Necessary: A component that appears in every pie or pathway is called a necessary cause, because without it, disease does not occur.
Disease Ecological Equilibriums
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Heterogeneity in disease outcomes
Heterogeneity between strains:
– Virulence (defined as host mortality or severity of disease)
– Vulnerability to host immune response
– Competition via cross-immunity
Within host heterogeneity:
– Immunogenic variability
– Different location within host leads to different effects
(invasive infection versus carrier)
Agent classifications
– Virulence: describes severity of disease produced by different strains of an infectious agent in the same host type. Virulence is a characteristic of the organism.
- Pathogenicity: severity of disease of the same agent strain in populations of other host types (Heterogeneities of HOST RANGE)
Bluetongue
in sheep and cattle in the UK 2008 (single strain outbreak)
Avian influenza
- Reservoir host:. Reservoir hosts are those that serve as a habitat for the infectious agent and are a common source of infection to other species.
– Definitive host: a species in which a parasite undergoes the sexual phase of reproduction.
– Intermediate host is one in which the infectious agent undergoes development, and possibly replication, without sexual reproduction.
– Paratenic host is one in which the agent is transferred mechanically, ie
without further development.
A paratenic host is not essential for the
long term survival of the infectious agent species.
SEIR models
S – Susceptible
E – Exposed (infected)
I – Infectious
R –Recovered (Immune)
exposure to agent occurs between S and E
S-E interaction:
Routes and Mechanisms of Transmission
- Vertical
- Horizontal
- direct
- indirect- fomites
- vectors
- biological
- mechanical
- waterborne
- airborne
- Nosocomial
- Iatrogenic
Contagious
Infectious agent transferred from one
animal to another by direct contact or
indirectly via fomites
e.g. Foot and Mouth Disease - transferred via touch and indirect e.g. contaminated feeds
Non-contagious
Infectious agent not
transferred from one
animal to another by
direct contact or via
fomites
e.g Malaria - only transferred between particular insect species
“True” Zoonosis
Anthropozoonosis
Animal Reservoir
e.g. rabies
humans are typically a dead end host and cant transfer to one another
“Reverse” Zoonosis
Zooanthroponosis
Human Reservoirs
e.g. cysticercosis
animals are typically a dead end host and cant transfer to one another
Indirect Transmission
- Natural
- Air (Q-fever)
- Water (giardia)
- Soil (toxoplasma)
- Vehicle
- Food (Salmonella)
- Fomite (ringworm) = object/utensil
- Vector
- Arbovirus’
- Rickettsias
The force of infection
the probability for a susceptible
host to acquire the infection.
* In a simple model with homogeneous “mixing”, it has 3 “factors”:
Y = m x (I / N) x t
- m : “mixing” rate (contact pattern)
- I / N : proportion of contacts with infectious hosts
– t : probability of transmission of the infection once a contact is made between an infectious host and a susceptible host
Number of new infections (E) = Y x S
(“catalytic model”)
Infectivity
Infectivity of an infectious agent can be assessed using the ID50:
number of organisms required to produce infection in 50% of individuals of a particular species under specified conditions.
Modes of entry (and exit) of
infectious agents
Circulatory System
Skim
Urinary System
Anus
Genital System
Mammary Gland
Respiratory System
Mouth
CNS
Conjunctivita
Latent and Incubation period
(E-I Interaction)
Latent period (A): from exposure to onset of agent shedding
Incubation period (B): from exposure to
onset of clinical signs
B-A = Prodormal period: disease process has begun but it is not clinically apparent
Factors influencing infectiousness
- Probability of contact
- Contact rate
- Infectivity
- Duration of shedding
Speed of clinical disease onset
- Peracute:
Very rapid onset of clinical signs,
often with severe clinical expression
and often of short duration - Acute:
Severe signs and a short course
(several hours to several days) - Sub Acute:
Slower onset, with less intense
clinical signs - Chronic:
Disease of long duration (weeks to
years) - Subclinical:
Production and productivity may or
may not be affected
What is Disease?
Traditional:
Any deviation from or interruption of the normal structure or function of any body part, organ or system that is manifested by a characteristic set of symptoms
PROBLEMATIC:
Sometimes we don’t show clinical symptoms
Modern:
– Failure to produce at expected levels
– Residues of particular chemicals
Carrier state
Upon recovery individuals may be
permanently infected with agent:
– Carrier state animals remain infected but are not always infectious
Arboviruses (arthropod-borne)
Enzootic: JE, RRV, MVE, Kunjin, West Nile
Anthroponotic: Dengue, Chikungunya, Yellow Fever
