-

1. INVESTIGATION OF AN
EPIDEMIC

2. OBJECTIVES
a. to define the magnitude of the epidemic outbreak or involvement in terms of
time, place and person.
b. to determine the particular conditions and factors responsible for the
occurrence of the epidemic.
c. to identify the cause, source(s) of infection, and modes of transmission to
determine measures necessary to control the epidemic; and
d. to make recommendations to prevent recurrence.

3. STEPS FOR INVESTIGATION OF EPIDEMIC

4. 1. Verification of diagnosis
• first step in an epidemic investigation
• may be spurious, and arise from misinterpretation of signs and symptoms by the
lay public.
• necessary to have on the spot verification of diagnosis.
• not necessary to examine all the cases to arrive at a diagnosis. A clinical
examination of a sample of cases will be sufficient.
• Laboratory investigations to confirm the diagnosis

5. 2 . Confirmation of the existence of an epidemic
• done by comparing the disease frequencies during the same period of previous
years.
• An epidemic is said to exist when the number of cases {observed frequency) is in
excess of the expected frequency for that population, based on past experience.
• Eg. of epidemic: common-source epidemics of cholera, food poisoning and
hepatitis A. These epidemics are easily recognized.
• modern epidemics (e.g., cancer, cardiovascular diseases). These epidemics are
not easily recognized.

6. 3 . Defining the population at-risk
(a) Obtaining a map of the area :
• Before beginning the investigation, it is necessary to have a detailed and current
map of the area.
• It should contain information concerning natural landmarks, roads and the
location of all dwelling units along each road or in isolated areas.
• The area may be divided into segments, using natural landmarks as boundaries.
• This may again be divided into smaller sections. Within each section, the dwelling
units (houses) may be designated by numbers.

7. AREA MAP

8. (b) Counting the population :
• complete census of the population by age and sex should be carried out in the
defined area by house-to-house visits.
• Needed to assess attack rates in various groups and subgroups of the population
later on.

9. 4 . Rapid search for all cases and their characteristics
(a) Medical survey :
• To be carried out in the defined area to identify all cases including those who
have not sought medical care, and those possibly exposed to risk.
• Ideally, the complete survey {screening each member of the population for the
presence of the disease in question) will pick up all affected individuals with
symptoms or signs of the disorder.
• health workers may be trained to collect relevant data.

10. (b) Epidemiological case sheet :
• The epidemiologist should have an "epidemiological case sheet" for collecting
data from cases and from persons apparently exposed but unaffected.
• The epidemiological case sheet or "case interview form" should be carefully
designed (based on the findings of a rapid preliminary inquiry) to collect relevant
information.
• This includes : name, age, sex, occupation, social class, travel, history of previous
exposure, time of onset of disease, signs and symptoms of illness, personal
contacts at home, work, school and other places; special events such as parties
attended, foods eaten and exposure to common vehicles such as water, food and
milk; visits out of the community, history of receiving injections or blood
products, attendance at large gathering, etc.
• The information collected should be relevant to the disease under study. For
example, if the disease is food-borne, detailed food histories are necessary.
• A case review form will ensure completeness and consistency of data collection.

11. (c) Searching for more cases
• The patient may be asked if he knew of other cases in the home, family,
neighbourhood, school, work place having an onset within the incubation of the index
case.
• Cases admitted to the local hospitals should also be taken into consideration.
• reveal not only additional cases but also person-to-person spread.
• The search for new cases (secondary cases) should be carried out everyday, till the area
is declared free of epidemic.
• This period is usually taken as twice the incubation period of the disease since the
occurrence of last case.

12. 5 . Data analysis
• The data collected should be analyzed on ongoing basis, using the classical
epidemiological parameters - time, place and person.
• If the disease agent is known , the characteristics of time, place and person may
be rearranged into Agent-Host-Environment model.

13. a. Time
• Prepare a chronological distribution of dates of onset and construct an "epidemic
curve" . Look for time clustering of cases.
• An epidemic curve may suggest :
(a) a time relationship with exposure to a suspected source
(b) whether it is a common-source or propagated epidemic
(c) whether it is a seasonal or cyclic pattern suggestive of a particular infection.

14. b. Place
• Prepare a "spot map" (geographic distribution) of cases, and if possible, their
relation to possible sources of infection, e.g., water supply, air pollution, foods
eaten, occupation, etc.
• Clustering of cases may indicate a common source of infection
• provide evidence of the source of disease and its mode of spread.

15. c. Person
• Analyze the data by age , sex, occupation and other possible risk factors.

17. 6. Formulation of hypotheses
(a) possible source
(b) causative agent
(c) possible modes of spread
(d) the environmental factors

18. 7. Testing of hypotheses
• All reasonable hypotheses need to be considered and weighed by comparing the
attack rates in various groups for those exposed and those not exposed to each
suspected factor.
• This will help the epidemiologist to confirm which hypothesis is consistent with
all the known facts.

19. 8 . Evaluation of ecological factors
• to prevent further transmission of the disease.
• Ecological factors which have made the epidemic possible should be investigated
such as sanitary status of eating establishments, water and milk supply;
breakdown in the water supply system; movements of the human population,
atmospheric changes such as temperature, humidity and air pollution, population
dynamics of insects and animal reservoirs.
• relate the disease to environmental factors to know the source(s) of infection,
reservoirs and modes of transmission

20. 9 . Further investigation of population at risk
• A study of the population at risk or a sample of it may be needed to obtain additional
information.
• This may involve medical examination, screening tests, examination of suspected food,
faeces or blood samples, biochemical studies, assessment of immunity status, etc.
• Healthy individuals (those who are not ill) from the same population may be studied.
This will permit classification of all members as to :
a. exposure to specific potential vehicles.
b. whether ill or not.

21. 10. Writing the report
• The report should be complete and convincing.

22. QUESTION-3
You have been posted from the department of Community
Medicine as a Health Inspector. The Medical Officer gets a letter
from community hostel authority to investigate and submit
report stating that there is a UG hostel in Gandhinagar; Since 4
months, students residing in that hostel are frequently falling ill
with fever, vomiting, diarrhea, etc. One of cook was treated for
Typhoid fever 6 months back. You were made part of the
investigating team.
a) What is your probable diagnosis and how will you proceed
with the investigation?

23. a)What is your probable diagnosis and how will you
proceed with the investigation?
• Enteric Fever Outbreak in Hostel.
( Food handler can be carrier of S.typhi)

24. b)How will you manage the situation?
Outbreak Investigation should be carried out as follows:
1. Verification of diagnosis:-
2. Confirmation of Existence of an Epidemic:
3. Defining the population at risk- obtaining the map of the area, counting the
population,
4. Rapid search for all cases with specific characteristics.- Medical survey,
Epidemiological case sheet, searching for more cases.
5. Data analysis- Time distribution, place distribution and person distribution of
cases

25. 6. Formation of Hypothesis- Possible source, causative agent, possible modes of spread
and Environmental factors.
7. Testing of Hypothesis- To confirm the source, causative agent, possible modes of
spread and Environmental factors.
8. Evaluation of Ecological factors- such as sanitary status of eating establishments and
food handlers, Water and milk supply, Breakdown in the water supply system,
Movements of human population, atmospheric changes, population dynamics of insects
of animal reservoirs.
9. Further of investigation of population at risk- this will help in classifiy the population in
to At high risk and whether ill or not.
10. Writing the Report- summary of the investigation.

26. WATER BORNE INFECTIONS

27. POLIOMYELITIS

28. • Acute viral infection
• primarily an infection of human alimentary tract
virus may infect the central nervous system (about 1 per cent) of cases resulting in
varying degrees of paralysis, and possibly death.
• India was certified as polio free since 27th March 2014.

29. Global Burden of Polio
• In the pre-vaccination era, poliomyelitis was found in all countries of the world.
• The extensive use of polio vaccines since 1954 eliminated the disease in developed countries.
• In 1988, the World Health Assembly resolved to eradicate poliomyelitis globally.
• A total of 22 cases of wild poliovirus were reported globally in 2017. Afghanistan reported 14
cases, Pakistan 8 and Nigeria reported 4 cases.
• Only 2 Polio Endemic Countries – Pakistan & Afghanistan
• In Nigeria, polio reappeared after 2 years of nil reporting. All the reported cases are of type 1
wild poliovirus (1 , 2). India was certified as polio free since 27th March 2014.

30. EPIDEMIOLOGICAL DETERMINANTS

31. AGENT FACTORS
Causative agent: Poliovirus
Picornavirus, Enterovirus family
3 serotypes – 1,2 & 3
can live in water for 4 mths & feces for 6 mths
Inactivated by- heat, U-V radiation, formalin, chlorine, drying
• Reservoir: Human , case of mild infection & subclinical case
• Infectious material: Feces & oro-pharyngeal secretions
• Period of infectivity: 7- 10 days before & after onset of symptoms

32. HOST FACTORS
• Age: Children more susceptible – 6 months to 3 years
• Sex – M:F = 3:1
• Risk factors: Fatigue, trauma, IM injections, exercise, surgical procedures.
• Immunity: Maternal antibodies protect for 6 months of infancy
No cross-immunity
Type specific
Type 2 most antigenic

33. ENVIRONMENTAL FACTORS
• More likely in rainy season
• Mostly in June to September
• Sources- contaminated food, water & flies
• Overcrowding & Poor sanitation

34. MODES OF TRANSMISSION
• Faecal-oral route: Directly- through contaminated fingers
Indirectly-through contaminated food, water, flies
More important in developing countries
• Droplet Infection: In acute phase of disease
More important in developed countries
Incubation Period : 7-14 days ( range 3- 35 days)

35. PATHOLOGY & PATHOGENESIS
• entry into host cell
• Virus multiplies in pharynx wall & Intestinal wall
• Primary viraemia
• Secondary viraemia
• To spinal cord & brain , multiplies in neurons
• Degeneration and Lesions in cells of spinal cord, brain-
stem , cerebral cortex
• Damage is permanent, asymmetrical & irreversible

36. CLINICAL SPECTRUM
• Inapparent (subclinical) Infection:
91-96%, recognized only by virus isolation / rising Antibody titres.
• Abortive polio / minor illness:
4-8%, mild self-limiting febrile illness, recovers quickly , recognized only by virus
isolation / rising Antibody titres.
• Non-paralytic Polio:
1%, Stiffness & pain in neck & back, lasts 2-10 days, Recovery is rapid

37. PARALYTIC POLIO
• Less than 1% of infections
• Asymmetrical paralysis
• H/O fever at onset of paralysis
• Malaise, anorexia, nausea, vomiting, headache, sore-throat, constipation, abdominal pain
• Reflexes diminished/absent
• No sensory loss
• Paralysis reaches its maximum in < 4 days ,
• Residual paralysis at the end of acute phase

38. PARALYTIC POLIO
- Facial asymmetry, weakness of voice, respiratory insufficiency, dysphagia , nasal
regurgitation
- Shallow irregular breathing
- BP rises then falls

39. DIAGNOSIS
• Virus isolation from stool
• Serological test:
3 types of Ab’s:
- IgG: Neutralizing antibody
- IgM: to ‘C’ antigen
- Anti-D antibodies
• Complement fixation test detects IgM & Anti-D antibodies

40. MANAGEMENT
• Isolation
• Concurrent disinfection of saliva & excreta in 10% cresol
• Absolute bed-rest
• Neutral position of limbs
• Symptomatic treatment
• Prophylactic oral antibiotics
• Massage contraindicated( for 6 weeks)
• Supportive treatment with splints
• Fluid & electrolyte balance , orally

41. • Physiotherapy:
Recommended after acute phase (6 weeks)
After residual paralysis persists
Helps weakened muscles to regain strength
To prevent contractures & deformities
Joint & paralyzed muscle moved passively through full range , for 10 min 2-3 times/
day
• Recovery:
Maximum in first 6 months , but slow recovery continued up-to 2 years

42. • Rehabilitation
Physical- metal calipers, artificial limb,
reconstructive surgeries