Outbreak investigation

Question 1

Objectives of outbreak investigation (4)

Answer 1

1. determine the extent of the problem,
2. identify probable cause(s) and source of the problem
3. identify important risk factors for the disease
4. make recommendations for control, treatment and prevention of future cases

Question 2

Steps in an outbreak investigation

Answer 2

1. Confirm existance of outbreak
2. Establish a provisional diagnosis and develop case definition
3. Determine mangnitude of problem - count cases, establish denominator, calculate attack rate
4. Analyse data in terms of when, where, who
5. Formulate a working hypothesis in an attempt to identify the type/source of epidemic
6. Conduct studies to evaluate hypotheses
7. Undertake intensive follow-up investigations as needed (to identify high risk groups and possible further outbreaks)
8. Implement control and preventive measures as soon as reasonably possible
9. Report the findings with recommendations for dealing with future outbreaks

Question 3

Case definition

Answer 3

Set of standard criteria for deciding if an animal/unit has disease of interest. Cases may be individual animals, pens, cages, ponds, mobs, whole farms, villages etc Subject to random (lack of precision) and non-random (false positives/negatives) errors. Trade off between sensitivity and specificity. Useful to have case definitions for suspect (based on field data) and confirmed (based on lab data) cases. Use broad case definition for unrecognized/serious syndromes (high sensitivity/low specificity) e.g. exotic disease.

Question 4

Establishing a diagnosis

Answer 4

May be known or unknown disease

• Objective if disease is known is to identify factors contributing to the occurrence and extend of outbreak
• Objective if disease is unknown is to identify aetiology and contributing factors

Where possible tests should be used to verify diagnosis however implementation of control measures may occur before results are available. Take specimens from a number of animals at different stages of disease (including healthy animals - from affected and non-affected farms). Sample 10-30 or more cases and non-cases.

Question 5

Animal - characteristics

Answer 5

Do attack rates vary for animals with different characteristics? Which groups have the highest and lowest AR? Calculate AR by age, sex, geographical origin, genotype, etc. Note that these often relate to space/time patterns due to husbandry practices. AR = # animals affected/# animals at risk at beginning of outbreak

Question 6

Time - epidemic curve - utility (4), features (7)

Answer 6

Utility:

1. What is the exact period of the outbreak?
2. Given the diagnosis, what is the probable period of exposure?
3. Given exposure window, what is the likely pathogen(s)?
4. Is the outbreak most likely point source, continuous source, propagating source?

Features:

1. Index case - first case
2. Endemic level
3. Ascending branch - steep means short incubation/exposure window
4. Peak
5. Descending branch
6. +/- Secondary peak - due to introduction of susceptibles
7. Time interval should be approx 1/4 length of incubation period

Question 7

Place - spatial distribution

Answer 7

Draw cases/non-cases on map. What are the significant features of the geographical distribution of cases? What are the relevant attack rates?

Question 8

Trace forward and trace back

Answer 8

A

Question 9

Intensive follow up

Answer 9

Includes additional laboratory analyses and epidemiological studies - searching for additional cases on other premises, documenting movement of animals and feedstuffs, feeding tirals and transmission experiements to refine hypotheses

Question 10

Reporting

Answer 10

Small investigations - brief discussion with farm manager outlining important features and actions required to prevent future occurrences. Include written report. Large investigations - published in scientific literature