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Flashcards in Epidemiology and Disease Deck (82)
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What are the steps in disease investigation (Thrusfield 2007)?

- Observation and recording of natural occurrence of disease (basic parameters e.g. morbidity, mortality)
- Presentation and analysis of the observations
- Determine the cause of the disease (causal principles and approaches)
- Understand transmission, clinical signs, diagnosis ("disease ecology")
- Efficiency of different techniques to combat disease


Define endemic

Commonly found in, or restricted to a particular region, community or group of people. R0<1


Define epidemic

Level of disease in a population significantly greater than usual R0>1


What is R0?

The average number of cases infected by a primary case i.e. R0>1 means that for every infected person, 1 or more people will get infected


How can R0 be used to assess degree of control over a disease?

When R0 reaches 1 or lower, this is a sign that we are gaining control over the disease and controlling its spread


What factors are considered in the observation and recording of the natural occurrence of disease?

- Infectious or contagious disease
- Prevalence and incidence
- Morbidity and mortality
- Sensitivity, specificity, positive predictive value, negative predictive value of diagnostic tests
- Gold standard of testing, accuracy
- Potential methods of disease control, prevention and eradication


What is included in the presentation and analysis of the observations in a disease outbreak?

- Number of cases and how this relates to the normal number seen
- Number/proportion of individuals at risk
- Distribution of the disease
- Rate of disease spread, timing of infection (calves vs adults)


What are Hill's criteria for establishing cause of disease?

- Time sequence (does cause precede effect?)
- Strength of association (apply percentages to potential causative factors)
- Biological gradient (dose-response relationship?)
- Consistency
- Coherence and plausibility


What approaches can be used to determine the cause of a disease in an outbreak?

- Case control studies
- Cohort studies


What are case control studies?

Comparison between cases and non-cases, looking retrospectively at the differences in exposure to risk factors


What are cohort studies?

Retrospective or prospective analysis of individuals exposed and not-exposed to risk factors, and see if they develop disease


What is an advantage of cohort studies over case control studies?

Larger population of all individuals that have potentially been exposed to the cause are assessed, providing a denominator to work with


How are cohort studies carried out?

- Start with a population, identify all those that have had contact with risk factors
- Observe outcome


How are case control studies carried out?

- Start with the outcome (and control group)
- Work backwards to identify possible risk factors
- Often then go onto cohort studies to get more precise information


What methods can be used to identify the strength of association of a risk factor, and this identify who is at risk in a disease outbreak?

- Odds ratio
- Relative risk ratio


What is an Odds Ratio (OR)? When is this mostly used?

- Odds of disease in an exposed group vs odds of disease in an unexposed group
- Commonly used in case-control studies


What is a Relative Risk ratio (RF)? When is this mostly used?

- Ratio of risk of disease in exposed group vs the risk of disease in unexposed group
- Commonly used in cohort studies


What is signified by an OR or RF >1?

Risk factor is associated with disease


What is signified by an OR or RF <1?

Risk factor is not associated with disease


What is the advantage of RF over OR?

Can be used to narrow down potential risk factors to the most likely


What information needs to be gathered initially in a disease outbreak scenario?

- Number of individuals affected
- Clinical signs
- Time to onset of signs
- Common activities
- Morbidity/mortality
- Potential causes


Why is sub-typing of pathogenic microorgansism for investigation of disease outbreaks important?

- To be able to determine the source of the disease
- Confirmation if strain of the suspected cause and strain causing the disease are the same and therefore whether the suspected cause is the true cause


What methods can be used for subtyping genetic material for pathogenic identification?

- Fingerprinting
DNA sequencing


Outline fingerprinting in subtyping of genetic material for pathogenic identification

- Comparison of "fingerprint" from case and potential source
- If they are the same, then this identifies the cause
- Done by trace comparison


How do molecular methods aid identification of pathogen?

- Amplify target pieces of pathogen DNA from case and source
- Can determine speciation (if strain unknown)
- Or strain identification (if known)


What are the advantages of sub-typing for pathogen identification in disease outbreaks?

- Increased sensitivity and specificity of diagnosis
- Faster
- Good for organisms typically difficult to culture
- Outbreaks detected and controlled earlier
- Can provide evidence for causation in combination with epidemiological evidence
- Influence prevention or control programmes


Describe the framework for disease control

- Triad of agent, host and environment
- Knowledge of all factors is required in order to prevent , control and eradicate diseases
- Can identify actions in each part to control disease


Define disease prevention

- Avoiding disease occurrence
- Reduces the incidence and prevalence of disease
- Applies to all diseases


Define disease control

- Reducing the disease frequency to a tolerable level
- Reduces the prevalence of disease
- Mainly applies to diseases that are commonly found or seen


Define disease eradication

- The complete elimination of disease, or disease agent from a region
- Mainly applies to exotic diseases, or those commonly found undergoing eradication programmes (BVD in Scotland, TB in UK)


What methods can be undertaken at each point of the disease triad in order to establish disease control?

- Host: increased resistance/decreased susceptibility of host
- Environment: decrease pathogen transmission
- Pathogen: decrease pathogen population


Describe methods of disease control at host level

- Improve host resistance e.g. National Scrapie plan, breeding up resistant genotypes
- Vaccination: strategic or blanket


Describe methods of disease control at environment level

- Restriction of host movement e.g. limited travel in Mexico during swine fever outbreak
- Wuarantine/isolation
- Control of vectors e.g. Ixodes ticks and Lyme disease
- Biosecurity e.g. fomite control, management systems


Describe disease control methods at pathogen level

- Decrease pathogen load using therapeutics (not always effective)
- However expensive, poor at regional/national level, concerns over antimicrobial and anthelmintic resistance


What are the different methods of disease eradication?

- Test and cull
- Pre-emptive culling
- Blanket culling
- Stamping out


Outline the test and cull method in disease eradication

- Animals testing positive are removed and slaughtered
- E.g. bTB
- Some more accurate than others, consider specificity/sensitivity


Outline the pre-emptive cull method in disease eradication

Animals exposed to infection are slaughtered


Outline the blanket-cull method in disease eradication

- Animals on contiguous premises surrounding an infected farm/premises are culled
- E.g. killing domestic pigs in Estonia to stop African swine fever spread 2015


Outline stamping out in disease eradication

- Combination of all disease eradication measures (test and cull, pre-eptive and blanket)
- Additional preventative measures e.g. disinfection, burning of carcasses, dangerous contacts culled


Discuss the notification system for food-borne diseases

- Many food-borne infections are notifiable in people
- Voluntary and statutory reporting by laboratories
- Repors from local authroities and environmental health
- Collated by PHE or equivalent
- Under reporting likely as most food poisoning/infection with food-borne pathogens remains undiagnosed


What is DALY?

Disability-Adjusted LIfe Year
- Attempt to measure more than just morbidity and mortality rates
- Mostly used in humans


What is YLL?

Mortality x life expectancy at death
- Higher if there is high mortality rate, and people die young leads to higher DALY


What is YLD?

Incidence x duration of disease x "disability weighting"
- higher if there is high incidence, and/or disease is chronic and/or disease is severe
- Higher YLD increases DALY


Who is responsible for disease outbreak investigations?

Statutory responsibility of the LA, in practice it is the local Consultant in Communicable Disease Control (CCDC)


Outline the features of the preliminary phase of outbreak investigations

- Collate and assess information (consider common factors)
- Establish tentative diagnosis
- Agree case definition
- Collect relevant specimens
- Conduct in-depth interviews with initial cases
- Conduct on site investigations at implicated premises
- Form preliminary hypotheses
- Consider likelihood of continuing public health risk
- Initiate immediate control measures proportionate to PH risk
- Identify need to convene formal Outbreak Control Team and activation of outbreak control plan


Explain what is included in Outbreak Control Plans

- Should already exist at local level
- System of communications and liaison between stakeholders locally and nationally
- Clear arrangement for media liaison
- Arrangement for the care of patients including reducing secondary person-person spread
- Arrangement for the provision of the necessary equipment and facilities including staff OOH
- Clear statement of purpose of plan in control of food related incidents


Who should form part of an Outbreak Control Team?

- Local Authority of Environmental Health
- PHE (regionally and nationally and NHS)
- FSA (nationally and internationally)
- +/- veterinary input


What factors need to be considered with regards to communication in a disease outbreak investigation?

- Media responsibility assigned
- Identification of all parties that need outbreak information
- Identification of most effective routes of communicaion
- Accuracy and timeliness of communication while complying with relevant legislation
- Constructive media use
- Ensure relevant material collected to inform final written report and consider distribution


What features are included in the descriptive epidemiology in a disease outbreak investigation?

- List of those at risk
- Identification of individuals posing risk to further spread
- Establishment of case definition
- Identification of as many cases as possible
- Collection of data from affected individuals on standardised questionnaires


What analysis and interpretation is required in a disease outbreak investigation?

- Attack rates
- Confirmation of common factors
- Categorisation by "time, place or person" associations
- Construction of epidemic curve
- Review of existing data
- Review of preliminary hypotheses, consider need for further investigations
- Collect necessary specimens
- Cohort or case control studies
- Further microbiological studies e.g. typing
- Ascertain source and mode of spread


What are the aims of control measures employed in a disease outbreak?

- Control the source
- Control the mode of spread
- Protect individuals at risk
- Monitoring of effectiveness and maintenance of disease surveillance must continue


Describe the final phase of disease outbreak investigations

- Identify end of outbreak (when number of new cases returns to background levels)
- Produce outbreak report


Define the term One Health

Recognition of the link between human, domestic animal and wildlife health and the threat disease poses to people, their food supplies and economies, and the biodiversity essential to maintaining the health environments and functioning ecosystems we all require


What is meant by ecosystem services?

The services provided by the ecosystem to human life: supporting, provisioning, regulating and cultural


Give examples of supporting ecosystem services

- Nutrient cycling
- Soil formation
- Primary production


Give examples of provisioning ecosystem services

- Food
- Fresh water
- Wood and fibre
- Fuel


Give examples of regulating ecosystem services

- Climate regulation
- Flood regulation
- Disease regulation
- Water purification


Give examples of cultural ecosystem services

- Aesthetic
- Spiritual
- Education
- Recreational


What is meant by the terms zoonotic and zoonosis?

- Zoonotic: transmission from one species to another
- Zoonosis: disease transmissible between vertebrate animals and humans (but when this is spread between humans, is this still a zoonosis?)


How can zoonoses be classified?

- Generally grouped according to a combination of pathogen taxonomy, species of origin, mechanism of transmission
- Can be classified in many different ways


Discuss how a vector for disease may be the true host

- Mosquito/tick/flea often considered as vectors, but are likely to be the true hosts which other animals including humans acting as vectors
- Considering them as hosts may improve understanding of epidemiology and thus prevention


Define vector

Method of transmission of disease between living organisms


Discuss the term reservoir with regards to epidemiology

- Endemic, liaison and amplification hosts
- Host in which the pathogen is able to subside until the true host is found


Define community

Collection of species/populations living in the same space and interacting


Discuss the term pathogen

- An agent that is causing disease
- However may not cause disease in all species
- Most infectious agents are not pathogens


Outline the principles of the evolution of pathogenicity

- Virulence may allow increased transmission
- However may also be irrelevant to transmission
- May be essential to life cycle of virus or competition with less virulent strains


Outline how virulence may be required for transmission

- May increase amount of virus/pathogen spread e.g. diarrhoea, sneezing
- Contact rates with susceptible individuals may be increased through behavioural changes e.g. rabies
- Through differential virulence may promote apparent competition e.g. squirrel pox affects red but not grey squirrels, thus beneficial to grey squirrel population


Outline how pathogenesis may be irrelevant to transmission

- No selection against morbidity after transmission has occurred e.g. most acute diseases of childhood
- No selection against virulence in dead-end hosts e.g. zoonoses


Give examples of how pathogenesis may be essential to life cycle of virus or competition with less virulent strains

- In-host local evolution e.g. LCMV, hepatitis B or poliovirus
- Trade off between transmissibility and infectious period e.g. myxomatosis, if host killed too quickly will not be able to spread disease


How is R0 of a disease calculated?

= betaxSxL
- Beta = the transmissibility (product of infectiousness, susceptibility and contact rates)
- S = susceptibles
- L = the infectiuos period, roughly equivalent of 1/mortality rate for a fatal infectious disease


Explain the possible connection between Emerging Infectious Diseases and zoonoses

- May be old zoonoses e.g. measles, rubella
- recent zoonoses e.g. HIV
- Present zoonoses e.g. ebola
- Future zoonoses e.g. EID including animal diseases
- Para-zoonoses i.e. parts of agent transmitted across e.g AMR (genetic recombination, genetic reassortment)


Give potential reasons for Emerging Infectious Diseases

- Globalisation
- Population growth and spread
- Environmental changes
- Behavioural changes


Discuss the control of Emerging Infectious Diseases

- Need to consider evolution and ecology
- Extermination often not a good option


Describe the APCD cycle in disease outbreak investigation

- Plan: describe problem, focus on questions, equipment selection, expertise, budget
- Do: collect samples, documents and photos, communication, precautionary measures e.g. containment
- Check: formulate hypothesis, data supporting, data undermining, data presentation, communication
- Act: decide on course of action, control measures, public information, co-ordination, case review


What equipment might be needed to investigate a disease outbreak?

- Swabs
- Gloves
- Faecal containers
- Drag swabs


Give examples of sample types that may be used in disease outbreak investigations

- Animals (and people): faeces, urine, blood/sera, feed, hides, litter, bedding
- Environment: water, dust, environmental swabs
- Food samples
- Swabs from vehicles


How many samples should be taken in a disease outbreak investigation for a disease prevalence of 5%?



Describe the transport of samples in disease outbreak investigations

- Need to ensure do not dry out
- Kept at 4degreesC
- Taken aseptically, processed within 48 hours
- Use enriched samples where do not know the number of the sample that are infected


Describe the laboratory tests that can be done in disease outbreak investigations

- Direct plating: bacteria and fungi, robust pathogen in large numbers, established protocols (ISO), strongly selective media available
- Enrichment: all microorganisms, low numbers/fragile organisms, high b/g, pre-enrichment may be necessary before selective
- Tissue culture: viruses or intracellular pathogens, may require significant upstream processing
- Molecular: all microorganisms, usually rapid, may detect pathogen DNA when microorganism cannot be recovered, good for live or dead tissue


Describe how to isolate foodborne zoonotic pathogens from abattoir samples

- E.g. chicken skin, water, environmental swab
- Stomaching of skin and swab before performing serial dilutions
- Then perform serial dilutions from all samples and plate (do duplicate plates)
- Reverse pipetting
- Leave to dry then incubate


Describe the processes of "stomaching" of samples

- Place sample in bag
- Add 10ml PBS
- Place into stomacher
- Punches the sample and PBS together to get pathogen into the liquid


How can the presence of Salmonella be confirmed on plates?

- Take sample from suspected colony, make solution on clean glass slide with 10ul of PBS, add drop of poly-O Salmonella antiserum If positive will become granular
- On brilliant green agar, will appear red