Outline for a Case Study to Improve the Collaboration of Epidemiologists with Laboratory

Case study

An Outbreak of Cholera

West Bengal, India

This case study was developed by a working group led by the World Health Organization (WHO) EPR with cooperation from the Field Epidemiology Training Programme (FETP) from the National Institute of Epidemiology (NIE), Indian Council of Medical Research (ICMR), Chennai, Tamil Nadu, India.

It is inspired by an investigation conducted by Dr Tapas Kumar Sen, 2003 MAE scholar assigned to the district of 24 Parganas, West Bengal, India..

Using this case study in the classroom:

We recommend that this case study be used in conjunction with the “Laboratory Skills for Epidemiologists” module, developed by the WHO. However, it can be delivered on its own, provided key lectures are presented first.

Recommended prerequisite lectures (cf. training matrix):

Lecture 3: Taking appropriate and adequate samples safely.

Lecture 4: Transport, disinfection and biosafety

Lecture 15: Anti-microbial susceptibility testing

Lecture 16: Role of the laboratory in surveillance

Lecture 17: Quality assurance

Time required for this case study:

3hours

This case study does not come with a facilitator’s guide. The answers to all the questions for each section are provided as an introduction to the following section.

To run this case study in the classroom, we propose that it be distributed one page at a time.

Participants should take turns reading it aloud, paragraph by paragraph. Reading all paragraphs aloud and in turns has two advantages: first, everyone can quickly participate and get beyond the inhibition of having her/his voice heard in a large room; second, time is given to the whole class to understand the issue and think about the answers. The participants reading the question may try to answer it if s/he can; otherwise, it can be discussed as a group. The next participant reads the next question and so on until the end of the page. After the next part/page is distributed, the next participant continues and so on until the case study is over. Once the epilogue is read, the class should re-visit the objectives, which reinforces their learning and provides an opportunity to clarify what may not have been fully understood.

Learning objectives

At the end of the case study, the student will be able to:

1.  Understand the role of the laboratory in surveillance and outbreak detection;

2.  Identify the number of samples to take and the transport media to use during an outbreak of suspected cholera;

3.  Provide the minimum information needed to ensure appropriate laboratory testing;

4.  Take appropriate measures to prevent contamination and/or infection when collecting specimens to send to a laboratory;

5.  Identify the appropriate laboratory tests to request during an outbreak of diarrhea;

6.  Understand the key elements of quality assurance for laboratories;

7.  Interpret Vibrio cholerae biotypes and serotypes;

8.  Understand the relevance of antibiotic susceptibility testing during the investigation of an outbreak of cholera;

9.  Decide whether environmental samples are needed during an outbreak of diarrhea;

10.  Integrate epidemiological and laboratory data to formulate conclusions during cholera outbreak investigations.

Part 1. A visit to the National Institute of Cholera and Enteric Diseases, West Bengal, India

The National Institute of Cholera and Enteric Diseases (NCED) is a specialized Institute of the Indian Council for Medical Research (ICMR) located in Kolkota, West Bengal, India. Cholera is highly endemic in this region of India so while the NCED works as a reference centre for the entire country, the institute is mainly active in West Bengal.

Cholera is an infectious, acute watery diarrhea caused by Vibrio cholerae O1 and O139. Vibrio cholerae produce a powerful toxin that stimulates the secretion of water and electrolytes in the intestinal tract. Patients with cholera commonly suffer from acute dehydration.[1] In India, Vibrio cholerae O139 emerged since 1992.

In October 2004, the epidemiologist assigned to “North 24 Parganas” (a district in West Bengal) conducted a routine visit to the NCED and its laboratory. The microbiologist in charge of cholera mentioned to him that during the previous month (September), the laboratory isolated Vibrio cholerae from 65 stool samples, a substantial increase over the average isolation of Vibrio cholerae from 19 stool specimens each month between January and August, 2004 (Table 1).

Table 1: Number of stool samples from which Vibrio cholerae was isolated, National Centre for Cholera and Diarrheal Diseases, Kolkota, West Bengal, India 2004 [1]

Question 1a

What is the role of the laboratory in public health surveillance?

Question 1b

How can these data be interpreted? Is this an outbreak?

Discussion for Part 1

Laboratories play a very important role in public health surveillance when they work in collaboration with epidemiologists. For example, case definitions which include laboratory criteria are more specific – but to be able to classify someone based on laboratory confirmation, epidemiologists must collaborate with a network of laboratories to ensure prompt testing and reporting of specimens, particularly during investigations.

With laboratory results reported back to public health in a timely way, epidemiologists can then calculate incidence rates: the number of cases confirmed by the laboratory would be the numerator, and the population at risk, the denominator. For example, for poliomyelitis, health care facilities notify public health authorities when they diagnose acute flaccid paralysis, and stool samples are then sent to laboratories. When the laboratory results are reported back, surveillance officers can calculate incidence. Thus, when laboratory data are linked with epidemiological data, the quality of public health surveillance improves.

The laboratory may also take a lead role in surveillance of specific conditions for which clinical criteria are insufficient. Some countries will use laboratory-based surveillance for hepatitis A or acute hepatitis B, where identifying positive tests (IgM antibodies to hepatitis A virus or IgM antibodies to hepatitis B virus core antigen) may be more specific or meaningful than looking for cases of acute jaundice. In other cases, laboratories can play a lead role in surveillance through the identification of clusters of unusual infections for which reference laboratories may centralize biological samples (e.g., Global SalmSurv, the global surveillance system for Salmonella that uses serotyping).

In India, public health surveillance is mostly based on notification of clinical syndromes, for which laboratory confirmation is sometimes needed. The keystone of cholera surveillance is the surveillance for acute watery diarrhea[2] occurring as an isolated case or in clusters. Laboratory confirmation is sought for clinical cases of cholera. Since cholera surveillance is not solely laboratory-based, it is difficult to interpret the data in Table 1 in the absence of more information.

An outbreak is defined as an increase in incidence, but calculating incidence requires both a numerator and denominator. Table 1 includes only numerator data.

In order to interpret the data in Table 1, it is important to know the reasons behind sample collection and testing, as well as the total number of samples tested. Since West Bengal is endemic for cholera and faces seasonal recrudescence in September and October, it may be that the observed increase reflects several separate, localized outbreaks that should be investigated individually.

Part 2. Initiation of an epidemiological investigation

On 13 October 2004, with the epidemiologist back in his district office, a health care facility in Kanchrapara municipality reports a cluster of cases of acute watery diarrhea with a high proportion of cases admitted to hospital with acute dehydration. The cluster is suspected to be an outbreak of cholera.

Question 2a

What are the initial steps of the epidemiological field investigation?

Question 2b

Should a microbiologist be part of the investigation team in the field?

Discussion for Part 2

A first step of any investigation is to confirm the existence of the outbreak. In this situation, the epidemiologist reviewed available data from Kanchrapara municipality, for the number of (severe) acute watery diarrhea reported each month – which was 25 on average during the first nine months of 2004.

Further review of the surveillance data indicated that all patients lived between Kulia Road, Dharmanagar Colony and Ambagan area, in the “ward” (part of town) four and five of Kanchrapara Municipality. In the absence of any change in the reporting system and in the absence of any influx of population, the epidemiologist concluded that the current reports may likely be a true increase of incidence

The second step of the investigation is to confirm the diagnosis. A short visit to the hospital verified that patients had signs and symptoms, including dehydration, compatible with cholera. The epidemiologist decided to obtain laboratory confirmation of the diagnosis.

As a third step of the investigation, epidemiologists define a case. In this example, a case was defined as a patient aged five years or more, living in Kulia Road-Dharmanagar Colony-Ambagan area of ward four or five (Kanchrapara Muncipality since 5 October 2004. In developing a case definition, he referred to the World Health Organization (WHO) case definition in a reference surveillance document downloaded from the WHO Internet site (www.who.int).[3][2]

While it would be convenient to have a microbiologist each time an epidemiologist conducts an investigation in the field, microbiologists usually do not participate in the field. However, it is important to recognize that there will be a need to consult with the laboratory and infectious disease specialists at an early stage.

Part 3. Collection of laboratory specimens

The epidemiologist is now about to collect specimens for laboratory investigations.

Question 3a

How many case-patients should be sampled? Why? What kind of patients should the epidemiologist select to take samples?

Question 3b

What kind of infection control measures should be in place to collect the samples?

Question 3c

What kind of samples should be taken? What quantity?

Question 3d

What is a transport medium? When should it be used? What kind of transport media should be used to take stool specimens in this case?

Question 3e

When is there a need for a cold chain for the transport of specimen? Why?

Question 3f

What other practical issues need to be considered before collecting samples?

Discussion for Part 3

The rule of thumb for the number of samples during a cholera outbreak is 10 samples to confirm the diagnosis, five samples per week during the outbreak to make sure it is still cholera and to monitor the antimicrobial resistance pattern and five samples at the end of the outbreak to confirm that the outbreak is over. Using these criteria, samples were obtained from 10 patients meeting the case definition for laboratory testing and who had NOT received antibiotics. It is important to collect enough samples to avoid sampling error while not collecting too many, which may overwhelm the laboratory.

After consulting with the microbiologist and the clinician, the epidemiologist is advised to obtain rectal swabs from a sample of patients. Rectal swabs are small enough to allow easy and safe transport yet sufficient to yield Vibrio cholerae when it is present (however, they are less adapted for Salmonella). Before obtaining the specimens, hands should be washed and gloves adorned; a white lab coat should be worn when collecting biologic specimens. To obtain a proper rectal swab, a clean cotton tipped swab should be introduced in the rectum until it becomes moist. It is important to avoid cross contamination between samples during collection and handling. After completing the procedure, gloves are removed and hands should be washed.

A transport medium is designed to maintain the viability of microorganisms throughout the period of collection and shipment. Shelf lives of transport media typically range from six months to one year when stored at room temperature. It is important to check the expiry date. Such media are used when we anticipate that samples will take more than a few hours to reach a laboratory. In this specific case, the epidemiologist decided to use a Cary Blair transport medium. This media is made of agar, distilled water, Sodium Thioglycolate, NaCl and Na2HPO3. These should be stocked in districts as an emergency preparedness measure.

Cold chain is generally needed for all specimens, with a few exceptions. One of these exceptions is rectal swabs for cholera culture in Cary Blair medium that can reach the laboratory within 24 hours. In contrast, stool samples for bacterial culture are always transported under cold chain.[3] In this investigation, the specimens will reach the laboratory in Kolkota within 24 hours, so a cold chain is not being required.

Remember: The accuracy of any test procedure is dependent on the quality of the specimen. The quality of the specimen is dependent on how and when it was collected, the care given to its preservation, and how soon it reaches the laboratory.

Part 4. Labeling of specimens

While the epidemiologist prepares to send the rectal swabs to the laboratory, the physician treating the patients in the hospital suggests that the samples be sent to the School of Tropical Medicine of Kolkota, a long-standing hospital partner. [4]

To prepare the specimens for transport, the epidemiologist’s assistant asks which forms to fill out to send with the rectal swabs.

Question 4a

What kind of contact should be made with the laboratory prior to sending the samples?

Question 4b

What kind of information should be included with the samples sent to the laboratory to ensure appropriate testing?

Discussion for Part 4

Before sending any specimens, the epidemiologist should call the laboratory at the School of Tropical Medicine to speak with the microbiologist. The epidemiologist should explain what is known about the current situation, including providing initial results of the epidemiological investigation. The laboratory should confirm that they can accept the samples and that there are no logistical constraints to receive them (e.g., staff available to receive and test the specimens, no other laboratory priorities (another outbreak) or just before a week end). The epidemiologist should make note of who he/she has spoken to the laboratory contact name, title, phone number and e-mail address and the exact location to which the samples should be delivered. The microbiologist assigns an outbreak number for the specimens to ensure that all specimens received with this number would be processed and prioritized accordingly; this also facilitates reporting back of results. The laboratory contact may also want to ensure that someone from the lab would be available to receive, handle and store samples as they arrive. They also discussed the potential media interest in this cholera outbreak – so they clarified that the laboratory results would be reported directly back to the epidemiologist for appropriate interpretation and dissemination to the designated public health authority, who would subsequently be responsible for broader communication to the public.