Virology 2: Epidemiology and transmission of Viral Diseases Flashcards
Define viral epidemiology. List the three terms used to describe.
Disease in population that develops before recognition of causative agents.
1. Endemic
2. Epidemic
3. Pandemic
Define endemic
Multiple/continuous transmission in a population/region/time
Define epidemic
widespread occurrence of an infectious disease in a community at a particular time. Exceeding the endemic baseline.
Define Pandemic
Worldwide epidemic
Define rate and how to calculate
Disease in a population.
Number of cases/population
Define Incidence or “attack rate” and how to calculate
- Case to population ratio.
- Number of cases/Population over a period of time (i.e person-years)
- Acute/Short-duration diseases will have incidence
Parameters that define incidence in acute diseases
Three in acute disease (short-duration)
1. Proportion of the population which is susceptible (S/P)
2. Proportion of the infected susceptible individuals (I/S)
3. Proportion of the diseased infected individuals (C/I)
Attack Rate VS Incidence parameters
See image. The Parameters allow you to more clearly identify the incidence and rate among a specified population.
Parameters that define incidence in viral diseases
- Case/infection ratio: proportion of infections resulting in clinical disease
- Case/fatality ratio: proportion of infection resulting in lethal disease
Define Prevalence and how to calculate
“Picture” of disease.
- Onset of unknown date so no time parameters.
- Number of current cases/population.
- Chronic, long duration diseases
Characteristics of data regarding epidemiology
Difficult to acquire, Often incomplete and inaccurate, Easier to compute.
Surveillance of disease
Continuous reporting and monitoring for growth of data
Serological survey definition
Antibody data; subjects were clinical disease silent (No clinical signs).
Not informative for current infection rate
what are prospective studies?
tracking events that are supposed to happen in the future. (i.e: placebo and treatment groups)
*Expensive
What are characteristics of retrospective studies?
Tracking past events, only need a limited number of subjects
*cost effective
Steps through viral infection of an individual
Incubation period, Generation time, period of infectivity, infection
When is the incubation period?
Moment of infection to onset of clinical signs
When is the generation time?
From moment of infection to first day of shedding virus. Usually shorter than incubation period.
When is the period of infectivity?
First day of shedding to last day of shedding. Influences spread of disease.
What are characteristics of a chronic viral disease and steps through the viral lifespan?
Distinction between each period is difficult to asses.
-Little correlation among disease, generation time, infectivity.
Modes of transmission by viruses
Horizontal
Vertical
Zoonotic
Vector-borne
Iatrogenic
Nosocomial
What is Horizontal Transmission and types of contact?
With or without a vector and between same or different species.
- Direct contact
- Indirect contact
- Airborne
Airborne examples
Droplets, aresols.
Indirect contact example and what type of virus is usually spread?
Indirect contact: Fomites, easting, bedding, vehicles, needles. Usually non-enveloped viruses.
— Common vehicles: Water, feed
Direct contact example and what type of virus is usually spread?
licking, rubbing, biting, sexual. Usually spreads enveloped viruses.
Vertical Transmission and examples
Movement of virus from parents to offspring.
- Gestation via placenta, milking
- Germ-line transmission
What is Germ-line transmission?
Type of vertical transmission. Virus is integrates into the genome of an ovum, transcription and replication of virus occurs in offspring. Creates generational infection.
What is Arthropod Vector-Borne (Arbovirus) Transmission? What are the types? Example?
Uses Biological vector or Mechanical vector.
Example: Ticks, mosquitoes, sandflies
What happens in a Biological vector?
Vector-borne transmission:
Virus replicates and magnifies in the vectors = Efficient transmission
What happens in a Mechanical Vector?
Vector-borne transmission:
Virus DOES NOT replicate in vectors = NOT efficient at transmission
What is Zoonotic transmission? What is the rate?
Viral disease transmissible from animals to humans under natural condition.
pigs closely related and many zoonotic disease
58% of recognized human diseases are zoonotic (and rising)
What is Iatrogenic transmission? Example?
Patient to patient transmission under veterinary care during interaction of vets to animals -Preventable with biosecurity
Ex: Reusing needles on beef farm
What is Nosocomial transmission? Example?
Transmission occurs while the animals are in the hospital - Preventable
Ex: Sterilizing surgery tools properly
Types of Viral Infections
Acute infection, Persistent infection, Chronic infection, Latent infection, Recrudescence, Productive, abortive, restrictive, transforming
What is Acute infection? How long does it last?
Rapid production of infectious viruses, Rapid resolution and elimination.
- A few days
- May not produce disease
What is persistent infection? How long does it last?
Infection is not cleared efficiently, Virus particles are produced for a long time.
-Months or Years
-Two types: Chronic, Latent
What is chronic Infection?
Persistent infections that will eventually cleared
What is latent infection? What are the steps?
- Viral genome is integrated into the cellar genome of host but is not expressed.
- No infectious expression - Under stress animal will undergo recrudescence
-New infectious virus expression
What is recrudescence? What type of infection does it correlate with?
Activation of latency due to many reasons
- I.E: immunosuppression, stress.
-New infection expression causing disease fair-up and new immune response
What are patterns of viral shedding? Systemic vs local infectious shedding?
What does shedding determine?
-Shed through body openings and surfaces.
-Local infections shed locally.
-Systemic infections will have various routes of shedding.
-Amount of shedding defines infection outcome.
What is shedding? How does this impact virus survival?
Replication of Virus
Last phase of viral pathogenesis, mandatory for the virus to survive in the host.
** Not all shed (i.e: retrovirus = germ-line)
Routes of shedding?
Skin
Respiratory
Saliva
Feces
Genital secretions
Urine
Milk
Shedding: Skin
Route of shedding
Not a major route.
Contact, abrasions, wounds
Shedding: Respiratory
Route of shedding
VERY important
- Local and systemic
- Shedding will occur before, during, and after clinical signs
Shedding: Saliva
Route of shedding
Salivary gland/oral cavity
i.e Rabies, FIV
Shedding: Feces
Route of shedding
GI tract viruses
** Can occur without GI clinical signs
Shedding: Genital secretions
Route of shedding
Sexual activity, semen, mucus
Shedding: Urine
Route of shedding
Viruses from kidneys
Hantavirus: mice to humans
Shedding: Milk
Route of shedding
NOT important
Mammary gland replication
What is and what determines host range?
Host cells determining ability of viral transmission.
- Receptors on animal tissues/cells. - Susceptibility and permissively.
What is susceptibility?
Ability of host to become infected.
What is permissivity?
Ability of virus to replicate and produce progeny(offspring) viruses
Host factors determining the outcome of infection
Age, Sex, Reproductive status of population, immune status of population, nutritional status of population, genetics of population
