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Cholera outbreak in Nancowry group of islands

  
 

In October – November 2002, an outbreak of severe watery diarrhoea appeared in three islands in Nancowry group of islands viz. Kamorta, Nancowry and Trinket. The outbreak spread to many villages in these three islands and affected a large number of tribal population. An investigation was conducted during this outbreak.

 Clinical and epidemiological data was obtained from Kamorta CHC and by interviewing the patients and relatives. Stool samples/rctal swabs were collected from all the patients admitted to CHC, Kamorta after the beginning of the investigations. Samples were collected from different water sources in the villages visited. Sea water samples from coastal areas were also collected. Samples were processed for bacterial enteric pathogens following standard procedure at the temporary laboratory set up at CHC, Kamorta. Isolated bacteria were serotyped using commercially available anti-sera. Water samples were processed for isolation of E. coli and other enteric pathogens.

  
 
 

Fig 1. Cases of severe diarrhoea treated at CHC, Kamorta during the period January 2001—November 2002

Fig. 1 shows the number of cases of severe watery diarhoea treated at Nancowry CHC during the period January 2001 to November 2003. Till October 2002, the usual number of cases were around ten per month. In October 2002, this became more than 250, which is clearly in excess of the number of cases expected. Hence the presence of outbreak was confirmed.

 

 

 
 
 
 

The outbreak started at Tapong village in Nancowry island on 5 October 2002. The index case was an 18 year old girl. She developed diarrhoea and vomiting on the morning of 5 October and took treatment from the local subcentre. She did not seek medical assistance at CHC, Kamorta and died on the same night. During the next few days several persons in Tapong village were affected. By this time awareness about early hospitalization was spread among the people and patients were reporting early. Hence there were no further fatalities at Tapong village. Eight cases occurred during 5 – 7 October. Then there was a gap of three days and another four cases occurred on 11 and 12 October. This probably indicates secondary cases.

  
   
 

Fig 2. Date of onset of outbreak in different villages of Kamorta, Nancowry and Trinket islands

Fig. 2 shows the dates of onset of outbreak in different villages in the three islands viz. Nancowry, Kamorta and Trinket. Outbreak started at Tapong on 5 October. Within the next few days it spread to the villages on the northern part of Nancowry island and then to Kamorta and other villages on the southern edge of Kamorta Island. Then it spread northwards on both the eastern and western costs of Kamorta island. During this spread, villages in Trinket island lying west of Kamorta were also affected. There were a few exceptions to the general pattern of spread. For example the village Bunder Khadi that lies further north than the villages Ramzo, Payuha, Munak, Changuwa etc. was affected earlier. Apparently Kamorta, the headquarters, was affected soon after the outbreak appeared in Tapong on 5 October. All the patients from Tapong were treated at Kamorta and it is possible that these patients were the source of infection at Kamorta.

 

 
 

Fig 3. Distribution of cases of severe diarrhoea from all villages of Nancowry, Kamorta and Trinket treated at CHC, Kamorta by date of reporting

Fig 4. Distribution of cases of severe diarrhoea treated at CHC, Kamorta during October—November 2002 by date of reporting and village
 
 

Distribution of cases on date of reporting for all cases occurred in Nancowry, Kamorta and Trinket islands is shown in Fig. 3. It shows multiple peaks indicating occurrence of secondary cases. The overall trend showed an increase in the number of cases reaching a peak by first week of November and then declining. The epidemic curve shown in Fig. 3 is a combination of multiple epidemic curves in different villages in the three islands. Each of these epidemic curves showed multiple peaks. Fig. 4 shows the break up of the cases from some of the villages.

  
     
 
 

No

Village

Attack rate

No

Village

Attack rate

  
 

1

Bunder Khadi

82.2%

9

Chota Inak

18.8%

  
 

2

Munak

46.2%

10

Hitui

16.9%

  
 

3

Trinket

36.5%

11

Masala Tappu

15.0%

  
 

4

Bada Inak

34.6%

12

Safed Balu

8.9%

  
 

5

Cnampin

32.8%

13

Tapong

7.3%

  
 

6

Changuwa

31.3%

14

Kakana

4.1%

  
 

7

Darring

24.9%

15

Kamorta

3.7%

  
 

8

Ramzao

20.2%

16

Pilopilow

0.9%

  
 

Table 1. Attack rates in different villages during the cholera outbreak in Nancowry

  

In some villages the outbreak reappeared after being absent for many days. In Tapong, the initial outbreak ended on 12 October, but reappeared on 2 November after a gap of 20 days. In another village, Derring, outbreak started on 14 October and continued up to 27 October. Then after a gap of five days it started again on 3 November. On 3 and 5 November four cases occurred. Another case occurred on 8 November. Then after a gap of four days  another six cases occurred on 13 and 14 November.

There are 45 inhabited villages in the three islands and 16 among them were affected. As per the residents list maintained by the Andaman and Nicobar Administration there are 3,806 persons residing in the three islands. Four-hundred and sixty eight cases were reported to CHC, Kamorta during October 5 and November 20 giving an attack rate of 12.3%. Attack rates ranged between 0.9% and 82.2% (Table 1). All age groups were affected. Attack rate was highest among infants and those aged 20 – 24 years.

  
 

Fig 4. Age-group specific attack rates during cholera outbreak in Nancowry

Fig. 4  shows the age specific attack rates. Attack rate among males was 12.8% and that among females was 12.0%. The difference was not statistically significant. There were three deaths in the hospital. Besides another three persons including the index cases died at their home. Thus the case fatality rate was 1.3%. All the deaths occurred among adults.

 

 
 

Age group

No

+ve

 (%)

Age group

No

+ve

 (%)

  
 

0-1

5

1

20.0

25-29

9

2

22.2

  
 

1-4

2

0

0.0

30-34

7

1

14.3

  
 

5-9

9

4

44.4

35-39

5

2

40.0

  
 

10-14

4

0

0.0

40-44

5

1

20.0

  
 

15-19

3

3

100.0

45-49

4

1

25.0

  
 

20-24

10

5

50.0

50+

4

1

25.0

  
 

Total

 

 

 

 

67

21

31.3

  
 

Table 2. Age group wise samples, isolates and positivity rates during cholera outbreak in Nancowry, October—November 2002

  
 

Stool samples/rectal swabs were collected from 67 patients. Vibrio cholerae was isolated from 21 cases. Eighteen of them were O1 Ogawa El Tor biotype and three were non-O1 non-O139. V. cholerae was isolated from patients from 11 of the 16 affected villages and from all the three islands. Isolates of V. cholerae were recovered from stool samples of patients in all age groups except in 1 – 4 and 10 – 14 year age groups. Age-group wise samples processed, isolates and positivity rate is shown in table 2

The investigations identified the first reported outbreak of cholera in Andaman & Nicobar islands.  The mode of spread of the outbreak from village to village was not fully understood However, movement of the tribes has contributed to this. The source from where V. cholerae was introduced into these environments is not clear. The possibilities are that effluents from ships that makes both mainland-island and inter-island trips or some non-tribal carrier residing in the tribal villages  introduced it in the islands. The possibility of poachers from neighboring countries introducing the organism also can not be ruled out.