Lecture 3 - Diagnosing & outbreak investigation Flashcards
First 2 Steps for diagnosis of infectious disease
- Anamnesis and collection of epidemiological data
- Clinical investigation of individuals and the herd/flock (necropsy)
What is the basis for a diagnostic hypothesis?
Anamnesis and epidemiological data
and/or
Clinical investigations and autopsy
+ epidemiological situation
What sort of questions might you ask of an animal keeper in order to collect anamnestic information?
What has happened?
When did it start?
Where did it start?
Is there and what are the exceptional event(s) around the time of probable introduction of the disease?
Animal movements
Introduction/change of feed
Introduction/change of bedding material
Visitors, treatments
strategy for reducing mycotoxin content in animal feed
Mycotoxin adsorbents are an effective strategy to prevent mycotoxin absorption through the gastrointestinal tract.
The most commonly used and most researched mycotoxin-binding agents are the aluminosilicates – clays and zeolites which are added to the animal feed.
An Epidemiological situation describes what?
Presence/spread of infection(s)
in a herd
in a region
in the state
in neighbouring countries
in the world
Diagnostic Laboratory investigations look for what exactly
pathological agent detection
antibody detection
or
to detect characteristic tissue changes (histological)
For agent detection, what sample materials are preferable?
organ material or full blood
From which animals?
In acute phase
Viremia
Before antibiotic treatment
For what reasons should an acute phase animal be sampled instead of a chronically ill one?
chronically ill ones will already have secondary infections so secondary agents will make diagnosis of initial disease agentmore complicated.
acute phase animal should have highest concentrations of initial disease causing agent.
So,
In acute phase
Viremia
Before antibiotic treatment
For antibody detection, what sample materials are preferable?
Blood serum
Milk
Fluids of body cavities
Tissue exudates (e.g. meat juice)
From whom?
In later stage of the disease - survivors
What does a rising and a decreasing antibody titer tell you?
rising indicates active disease/infection
decreasing titer indicates convalescence from disease. exposure has occurred previously but infection is no longer active.
When should histological samples be taken for investigation?
Max 6 hours post mortem, the fresher the better.
typing of pathological agents uses two kinds of methods:
conventional methods are based on phenotype (bacteria, viruses)
molecular and DNA based methods are based on genotyping
describe conventional Bio-typing of bacteria
specific biochemical reactions;
morphology of colonies;
tolerance of cultivation environment
describe conventional sero-typing of bacteria
antibodies against surface antigen of bacteria
(e.g., LPS, envelope, membrane; pili)
Further development –> monoclonal antibodies.
describe conventional biotyping of viruses
influence on cell culture cells or to experimental animals
e.g. virulence
describe conventional serotyping of viruses
antibodies against surface antigens of the virus
Mostly monoclonal antibodies.
name some DNA/RNA sequence based
molecular methods of agent typification
RFLP (restriction fragment length polymorphism)
PFGE (pulsed-field electrophoresis)
PCR amplification
DNA /RNA NT-sequences
Distribution of disease in time are described by
disease curves
Diseases are not randomly distributed in time and space in populations.
Epidemic curve illustrates what on the:
y-axis
x-axis
y-axis - Magnitude of the dz problem (No. of cases)
x-axis - Speed of the progression of the epidemic (time)
Name the Three main patterns of disease in time:
Endemic
Epidemic
Sporadic
Definition of an epidemic:
Exponential development in the number of diseased per time unit. E.g. No. of individuals submitted to hospital per day due to influenza: 12,24,48 …etc
OR:
A disease occurrence which exceeds the expected
What might an endemic disease curve look like?
More or less stable number of cases of disease per unit of time.
Describe what a sporadic disease curve might look like?
What is “index case” in the context of epidemiology?
“patient zero”
index case (patient) is defined as the first reported case of the disease that is diagnosed by clinician or epidemiologist.
Propagated spread
Disseminated outbreak originating from an index case with propagated spread.
This begins like an infection from an index case but then develops into a full-blown epidemic with secondary cases infecting new people who, in turn, serve as sources for yet other cases.
This produces successively taller peaks, initially separated by one incubation period, but the peaks tend to merge into waves with increasing numbers of cases in each generation. The epidemic continues until the remaining numbers of susceptible individuals declines or until intervention measures take effect.
Cyclic variation
trend in disease curves: regular changes in dz occurrence exceeding the seasons of the year
secular trends
regular trends in disease occurrence over long periods (years).
Cyclical fluctuations – regular, periodic fluctuations in the occurrence of the disease, related to, for example, changes in the size of the susceptible population, lack of effective contacts, etc.
Factors affecting the shape of the curve (4)
The incubation period (IP) of the disease
The infectivity of the agent
The proportion of susceptible animals in the population
The distance between animals (i.e., animal density)
Kendall’s threshold theorem
Defines the minimum density of susceptible individuals required to allow an epidemic to occur ie. the threshold level.
At threshold level the infectious individual is able to infect more than one susceptible individual.
The greater the density of susceptible individuals the steeper the slope of the endemic curve.
Basic reproduction number:
R0 is
the average number of secondary cases one infectious individual generates over the course of its infectious period in a fully susceptible population
The threshold level is more generally expressed in basic reproduction number R0 [R-naught]
AKA basic reproduction rate, basic reproductive ratio
Is specific for a disease agent – still needs to be evaluated in context of particular population and time point.
If R0= >1 ?
If R0= <1 ?
If R0= 1 ?
If R0= >1 – epidemic can occur
If R0= <1 – diseas fades out
If R0 = 1, then 1 organism is capable of spreading to 1 other organism on average.
R0 formula
Many different mathematical formulae have been proposed.
𝛽 is influenced by: (3)
Population density
Social behavior of the members of population
Factors influencing the probability of effective contact (agent, host and environmental factors)
d - is influenced by? (3)
depends on the host factors,
virulence of the agent, and
human interference (in anthropogenic populations)
Herd level R0
Expresses the number of herds one infected herd can infect
Is called dissemination rate or herd level R0
Herd level R0 depends on what factors? (6)
Environmental factors – geography and climate
Herd density in an area
Production technologies and habits
Contact between herds (animal movement)
Behavior of the farmer – preventative measures
Herd immunity
What is EDR?
In case of epidemics, estimated dissemination rate (EDR) is used for practical reasons.
Herd level R0 is found indirectly by using historical data from epidemics.
Calculated as number of outbreaks in two consecutive time periods:
If EDR = >1 – the disease is spreading
If EDR = <1 – the disease is fading out
estimated dissemination rate
If EDR = >1 ?
If EDR = <1 ?
If EDR = >1 – the disease is spreading
If EDR = <1 – the disease is fading out
An outbreak investigation is
An investigation conducted in case of disease epidemic to find out the cause and to restrict the spread of the disease.
In EU legislation this is called ‘epidemiological investigation’.
Who deals with outbreak investigations?
Field-epidemiology/-ologists
Goals of outbreak investigation (4)
Identification of the agent (cause)
Identification of the source
Identification of modes of transmission (introduction)
Defining the measures to stop the spread of disease
Depending on the character of the epidemic the focus of the investigation is somewhat different.
investigation of Food(feed)-borne point-epidemic focuses on?
Identify the source of infection
Identifying the agent
investigation of a well defined infectious disease with a known causative agent, focuses on?
Identifying source
Identifying mode of transmission (introduction)
-> TRACING
Implication of control measures
investigation of an endemic disease that is not well defined and probably multifactorial, focuses on?
Identifying the agent
Identifying the source
Identifying the risk factors -> Identifying the CAUSE
Implication of control measures
what phases are involved in an outbreak investigation
steps of outbreak investigation
Distribution of cases In time are plotted on an?
epidemic curve
Distribution of cases In space are plotted on?
maps
Distribution of cases In a population are plotted in?
frequency tables
Important descriptive questions for outbreak investigations:
Characteristics of clinical syndrome
Time, spacial and demographic patterns of the disease
Herd history
Environmental history
Characteristics of clinical syndrome questions may include: (4)
Signs in diseased and dead animals
Incubation period
How long the signs are/were manifested
Prognosis – mortality, case fatality rate
What parameters might help you in approximating when an infection might have taken place?
minimum, average and maximum incubation periods
Herd history questions may include:
Keeping system, feeding and water supply
Health management of the herd (biosecurity measures, vaccinations, treatments, pest control etc.)
Production dynamics and dz situation
Contacts to other herds, animals and wildlife
Animal movements
Health situation in neighboring herds
Environmental history questions may include:
Weather conditions
Geographic location e.g. topography, soil characteristics, vegetation
Fertilizer, herbicide and pesticide use in the neighborhood
Other pollution
Data collected during the descriptive phase are used for analysis and compared with
our previous knowledge about the disease
Important questions for the analytic phase of investigation:
What kind of associations exist (- risk factors?)
What is the probable source
How the agent is transmitted
What is the cause of the disease
What is the cost of disease
What questions should be addressed/plans made during the intervention phase of investigation?
Make the plan of intervention and implication of it.
Hypothesis is tested in practice – change of management, regrouping, vaccination, treatments
Important questions:
Are the measures used thus far, adequate? What should be done better?
What are the primary measures to stop the spread of the disease?
What are measures are necessary in the long term?
What are the economic consequences of these measures?
Review this summary.
