Diseases Environment- · Communicable Diseases Surveillance in Singapore 2004 31 Table 2.6 Dengue clusters identified, 1990 – 2004 *A cluster is defined as two or more cases epidemiologically

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26

Communicable Diseases Surveillance in Singapore 2004

27

IIVECTOR-BORNE DISEASES

Vector-borne and zoonotic diseases are diseases

transmitted to humans by insects or animals. Vectors

may transmit infectious diseases to humans by the

blood-feeding arthropods such as mosquitoes and ticks

or through contaminated urine, tissues or bites of infected

animals such as rats or dogs. The causative organism

may be viral, bacterial, fungal, protozoan or parasitic and

the transmission could be direct or via food and water.

DENGUE FEVER/DENGUE HAEMORRHAGIC FEVER (DF/DHF)

Dengue fever is an acute febrile viral disease

characterised by sudden onset of fever for 3 - 5–days,

intense headache, myalgia, arthralgia, retro-orbital pain,

anorexia, gastrointestinal disturbances and rash. Early

generalised erythema may occur in some cases. The

infectious agents are flaviviruses comprising four

serotypes (dengue-1, 2, 3 and 4) and are transmitted

by the Aedes mosquito.

A total of 9,459 laboratory confirmed cases of DF/DHF

[comprising 9,291 cases of dengue fever (DF) and 168

cases of dengue haemorrhagic fever (DHF)] were

reported in 2004, an increase of 97.6% from the 4,788

dengue cases reported in 2003. Of these, 9,358 were

local residents, comprising 61 imported and 9,297

indigenous cases. The remaining 101 cases were

foreigners not residing in Singapore and who reportedly

acquired the infection overseas, including 91 who came

to Singapore for medical treatment. The majority of the

indigenous cases (83%) received inpatient treatment.

There were nine reported dengue deaths, including a

foreigner from overseas who sought medical treatment

in Singapore. The incidence increased sharply from April

to August and remained high for the rest of the year

(Figure. 2.1).

Figure 2.1

E-weekly distribution of DF/DHF cases, 2003 – 2004

0

50

100

150

200

250

300

350

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53

Week

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2003 2004

The incidence rate among indigenous cases was highest

in the 15 – 24 year age group with a male to female

ratio of 1.6:1 (Table 2.1). Among the three major ethnic

groups, Chinese had the highest incidence rate, followed

by Malays and Indians. Foreigners comprised 23.9% of

the indigenous cases (Table 2.2) with the majority

employed as labourers or related workers in the

construction and manufacturing industry.

28

Table 2.1

Age-gender distribution and age-specific incidence rates of indigenous# DF/DHF cases, 2004

#Cases acquired locally among Singaporeans, permanent and temporary residents.

*Rates are based on 2004 estimated mid-year population.

(Source: Singapore Department of Statistics)

Table 2.2

Ethnic-gender distribution and ethnic-specific incidence rates of

indigenous# DF/DHF cases, 2004

#Cases acquired locally among Singaporeans, permanent and temporary residents.



The economic/occupational profile of indigenous DF/

DHF cases is given in Table 2.3. Students accounted

for 18.7%, 12.6% were professionals and 9.0% were

cleaners, labourers and related workers.


29

Table 2.3

Economic/occupational profile of indigenous DF/DHF cases, 2004

There were 61 (0.7%) imported cases, defined as local

residents with a history of travel to endemic areas seven

days prior to the onset of illness. The majority of these

cases (78.7%) were from Southeast Asian countries: 22

from Malaysia and 17 from Indonesia, 4 from Cambodia,

3 from Thailand, 1 from Myanmar and 1 from the

Philippines (Table 2.4).

Table 2.4

Imported DF/DHF cases, 2000 - 2004

30

Residents in Housing & Development Board (HDB) flats,

compound houses and condominiums constituted

64.0%, 20.2% and 9.1% of the cases, respectively.

However, the incidence rate of residents of compound

houses (587 per 100,000) was three times higher than

that of HDB residents (192 per 100,000) (Table 2.5).

Table 2.5

Incidence rates of reported indigenous DF/DHF cases by housing type, 2004

*Rates are based on census of population 2000.


Most of the cases were concentrated in the central

(30.8%) and south-eastern (29.7%) parts of Singapore

[according to the boundary demarcated by the

Community Development Council / National

Environment Agency (NEA) Regional Office] (Figure 2.2).

Figure 2.2

Geographical distribution of dengue cases, 2004

A total of 559 clusters involving 2,434 epidemiologically

linked cases were identified. The median number of

cases in each cluster was 3 (range 2 to 57) and the

median duration of transmission was 4 days (range 1 to

56) (Table 2.6). The number of clusters increased by

more than three-fold compared to the previous year.


31

Table 2.6

Dengue clusters identified, 1990 – 2004

*A cluster is defined as two or more cases epidemiologically linked by place [within 150m (200m till 2002)] and time (within 14 days)

Of the 559 clusters identified, there were 34 clusters

(6.1%) having 10 cases or more. They were in the areas

listed in Table 2.7. The median number of cases in these

34 clusters was 16.5 (range 10 to 57) and the median

duration of transmission was 24 days (range 10 to 56).

Table 2.7

Dengue clusters identified, 2004 (10 or more cases)

32

DHF Deaths

A total of nine fatal cases of DF/DHF,comprising two

cases of DF and seven of DHF were reported in 2004.

Of these, eight cases were indigenous infections in local

residents. The remaining case was a non resident

foreigner who sought treatment in Singapore after

acquiring the infection overseas.

Laboratory Surveillance

All reported cases of DF/DHF were confirmed by one or

more laboratory tests; viz. anti-dengue IgM antibody,

enzyme linked immunosorbent assay (ELISA), and

polymerase chain reactions (PCR).

A total of 560 blood samples obtained from both

inpatients and outpatients were tested for dengue virus

by polymerase chain reactions (PCR) at the Department

of Pathology, Singapore General Hospital, KK Women’s

and Children’s Hospital’s laboratory and Environmental

Health Institute, NEA.

Of these, 215 (38.4%) were positive. All four dengue

serotypes were detected, comprising DEN-1 (67%),

DEN-2 (27.6%), DEN-3 (2.4%), and DEN-4 (3.0%)

(Figures 2.3 & 2.4).

DEN-1 had emerged as the predominant circulating

serotype in the second half of 2004 whereas in previous

years (2001 - 2003), DEN-2 was the predominant

serotype (Figure 2.4).

Table 2.7 (cont’d)

Dengue clusters identified, 2004 (10 or more cases)


33

Figure 2.3

Surveillance of dengue virus serotypes, 2004

0

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Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

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Figure 2.4

Surveillance of dengue virus serotypes, 1992 – 2004

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34

Aedes Surveillance

Aedes surveillance and source reduction are routinely

carried out daily by NEA officers. The focus is primarily

on areas which has historically high rates of dengue.

Every month, a total of 40,000 residential premises and

900 non-residential premises are inspected. A

geographical information system is used for clustering

and analysis of mosquito breeding sites and dengue

cases. In addition, 2,000 ovitraps are placed around

Singapore for Aedes surveillance (Figure 2.5).

The distribution of dengue cases was more closely

associated with Aedes aegypti than Aedes albopictus

(Figure 2.6). The overall Aedes premises index was

around 2%, with the highest percentage detected in

compound houses (Figure 2.7). The top five breeding

habitats for Aedes aegypti were ornamental containers

(47%), domestic containers (34%), bamboo pole holders

(6%), roof gutters (5%), and other habitats (8%) (Figure

2.8). For Aedes albopictus, the most common breeding

habitats were ornamental containers (39%), domestic

containers (31%), discarded receptacles (8%), canvas/

plastic sheets (7%), and other habitats (15%) (Figure

2.9).

Figure 2.5

Locations of ovitraps used for Aedes surveillance


35

Figure 2.6

Geographical distribution of Aedes albopictus, Aedes aegypti and dengue cases, 2004

Figure 2.7

Percentage of premises breeding Aedes mosquitoes, 1998-2004

0

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36

Figure 2.8

Distribution of Aedes aegpyti by top 5 breeding habitats, 2004

Figure 2.9

Distribution of Aedes albopictus by top 5 breeding habitats, 2004


37

Outbreaks of Dengue

Eunos Crescent

On 18th August 2004, the Ministry of Health was notified

of a case of dengue fever involving a construction worker

residing in Eunos Crescent. Within the next week,

another three cases involving residents in the same area

were reported. As soon as the cluster was identified,

epidemiological investigations and vector control

measures were carried out.

A total of 56 laboratory confirmed dengue cases were

identified in the outbreak. (Figure 2.10) All had their onset

of illness between 11th August and 6th October, 2004.

Hospitalisation was required for 47 (84%) of the cases.

The cases comprised 22 working adults, 17 students, 7

housewives, 3 retirees, 2 unemployed and 1 construction

worker. Four cases were uncontactable. The majority of

the cases were in the 25 - 34 years age group, with a

male to female ratio of 1.4:1.

All cases were clustered by residential or workplace

address within a 250-metre radius from the initial focus

of transmission. As part of vector control operations,

mosquito breeding habitats were identified in 62 (2.4%)

of the 2,614 premises inspected. Abundant breeding

habitats were found in domestic containers (32.3%) and

ornamental containers (28%). Aedes aegypti and Aedes

albopictus accounted for 69.7% and 23.2% of the

breeding respectively.

Figure 2.10

Geographical distribution of 56 DF/DHF cases at Eunos Crescent area,

August – October 2004

38

Dengue cluster alerts to General Practitioners

Since 2004, medical clinics within a 500-meter radius of

a newly identified dengue clusters have been alerted to

the ongoing transmission of the disease in their locality

through advisory letters. These alerts inform General

Practitioners of the location and transmission status of

clusters, and urge them to maintain a high index of

clinical suspicion when treating patients with fever and/

or rash. They are encouraged to notify suspected cases

immediately without waiting for laboratory results. A total

of 1,559 advisory letters were issued during 2004.

Bedok North Ave 2

On 2nd November 2004, two cases of dengue fever

involving residents of Bedok North Ave 2 were notified.

Within the next three days, another three cases were

reported from the same area. Epidemiological

investigations and vector control were immediately

instituted.

A total of 57 laboratory confirmed cases were eventually

reported. All had onset of their illness between 26th

October and 5th December 2004. Hospitalisation was

required for 52 (91%) of the cases.

The cases comprised 21 working adults, 17 students, 5

housewives, 3 retirees, 1 construction worker and 10

undetermined. The majority of cases were in the 25 –

34 years age group, and male to female ratio was 1.6:1.

All cases were clustered by residential or workplace

addresses within a 250-metre radius from the initial focus

of transmission. (Figure 2.11) A total of 4,503 premises

were checked. 88 (2%) were found to be breeding

mosquitoes. The main breeding habitats were

ornamental containers (29.8%), domestic containers

(18.3%) and bamboo pole holders (18.3%). Aedes

aegypti and Aedes albopictus accounted for 68.8% and

29.7% of the breeding, respectively.

Figure 2.11

Geographical distribution of 57 DF/DHF cases at Bedok North Ave 2 area,

November – December 2004


39

LEPTOSPIROSIS

Leptospirosis is a zoonotic bacterial disease of variable

clinical manifestations. The common presenting features

are fever, headache, chills, severe myalgia and

conjunctival suffusion. The etiologic agent is a spiral

organism, leptospires, a member of the order

Spirochaetales found mainly in infected wild and

domestic animals. The mode of transmission is through

direct contact of the skin (especially if broken) or mucous

membranes with the urine or tissues of infected animals.

Soil or vegetation contaminated by infected animals may

also result in infection. Occasionally leptospirosis has

occurred following the ingestion of food contaminated

by the urine of infected rats.

In 2004, a total of nine laboratory-confirmed cases of

leptospirosis were reported, compared with 29 in 2003.

Of these five were local residents, comprising three

imported and two indigenous cases. The remaining four

cases were foreigners who came to Singapore for

medical treatment (Table 2.8 and 2.9).

Table 2.8

Age-gender distribution and age-specific incidence rates of reported

leptospirosis cases^, 2004

Êxcludes four foreigners seeking medical treatment in Singapore

*Rates are based on 2004 estimated mid-Year population


Table 2.9

Ethnic-gender distribution and ethnic-specific incidence rates of reported

leptospirosis cases^, 2004

Êxcludes four foreigners seeking medical treatment in Singapore

*Rates are based on 2004 estimated mid-Year population


40

MALARIA

Malaria is a parasitic disease characterised by fever and

chills. Most serious malarial infections may present with

cough, diarrhoea, respiratory distress and headache.

The infectious agent is a protozoan parasite,

Plasmodium, and there are four different species namely,

P. vivax, P.malariae, P. faciparum and P. ovale. The mode

of transmission is via a bite of an infective female

Anopheles mosquito.

In 2004, a total of 152 laboratory-confirmed cases were

reported, an increase of 28.8% from the 118 cases

reported in 2003 (Figure 2.12). However, 150 (98.6%)

cases were reportedly acquired overseas. Among the

92 local residents affected, 90 were classified as imported

cases, one was indigenous and one was classified as

cryptic. The remaining were tourists (15) and foreigners

seeking medical treatment in Singapore (45).

Figure 2.12

E-weekly distribution of reported malaria cases, 2003 – 2004

0

1

2

3

4

5

6

7

8

9

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53

Week

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Among the 92 reported cases of malaria in local

residents, the age-specific incidence rate was highest

in the 25 – 34 years age group. The male to female ratio

was 4.4:1 (Table 2.10). Among the three major ethnic

groups, the incidence rate was highest for Indian,

followed by Malay and Chinese (Table 2.11).

Table 2.10

Age-gender distribution and age-specific incidence rates of

reported malaria cases, 2004^

êxcluding 45 foreigners seeking medical treatment in Singapore and 15 tourists




41

Table 2.11

Ethnic-gender distribution and ethnic-specific incidence rates of

reported malaria cases^, 2004

êxcluding 45 foreigners seeking medical treatment in Singapore and 15 tourists



Malaria parasite species

The distribution of the cases by parasite species was P.

vivax (71.1%), P. falciparum (27.0%), P. malariae (1.3%)

Table 2.12

Classification of reported malaria cases by parasite species, 2004

P.v. - Plasmodium vivax P.m. - Plasmodium malariae P.f. - Plasmodium falciparum

*Including relapsed and induced cases that were imported.

Overseas-acquired malaria

The majority of the malaria cases acquired overseas was

infected in India (42.7%) and Indonesia (38.7%). P. vivax

accounted for 84.4% of the infections acquired in India

(Table 2.13).

and mixed infection (0.7%) (Table 2.12).

42

Table 2.13

Imported malaria cases by country of origin and by parasite species, 2004

P.v. - Plasmodium vivax P.f. - Plasmodium falciparum P.m. - Plasmodium malariae

Most of the cases (79.3%) had onset of fever within three

weeks of entry into Singapore (Table 2.14). For P. vivax

malaria, 16.7% did not develop symptoms until more

than six months after entry while falciparum malaria

accounted for 0.7%.

Table 2.14

Imported malaria cases by interval between period of entry and onset of illness and

by parasite species, 2004


43

P.v. - Plasmodium vivax P.f. - Plasmodium falciparum P.m. - Plasmodium malariae

The overseas-acquired cases comprised 43 (28.7%)

Singapore residents, 36 (24.0%) work permit/

employment pass holders, 3 (2.0%) foreign expatriates,

8 (5.3%) other foreigners, 45 (30.0%) foreigners seeking

medical treatment in Singapore and 15 (10.0%) tourists

(Table 2.15).

Table 2.15

Classification of imported malaria cases by population group, 2003 - 2004

The majority of Singapore residents who contracted

malaria whilst travelling overseas were on social visits

or holidays. All but one admitted to taking no or

inadequate chemoprophylaxis (Table 2.16 and 2.17).

Table 2.16

Purpose of travel for local residents who contracted malaria overseas, 2000 - 2004

Table 2.14 (cont’d)

Imported malaria cases by interval between period of entry and onset of illness and

by parasite species, 2004

44

Blood film examination for malaria parasites

A total of 1,366 blood films were collected during routine

epidemiological investigations and examined for malaria

parasites. None of the blood films tested positive for

malaria parasite (Table 2.18).

Table 2.17

History of chemoprophylaxis for local residents who contracted malaria overseas,

2000 – 2004

Indigenous/cryptic malaria

There was one indigenous and one cryptic case of

malaria.

The indigenous case was notified on 11th February 2004.

This case was a SAF trainee, enlisted in June 2003 at

Pulau Tekong Camp. In December 2003, he was posted

to Stagmont Ring Camp where he had onset of fever

and chills on 1st February 2004 and admitted to NUH on

9th February. Blood test revealed Plasmodium vivax with

a parasite load of 0.45%.

The case had last travelled out of Singapore in June

2002 to Malaysia. Whilst undergoing BMT in Pulau

Tekong he was on maloprim chemoprophylaxis. Prior to

his onset of fever, his movements had been confined to

Stagmont Ring Camp and his residence in Hillview

Avenue.

Active case detection was carried out in Stagmont Ring

Camp and Hillview Avenue. Forty-four residents and

foreign workers were screened and were found to be

negative for malaria parasites. An arrangement between

MINDEF and construction companies ensures all foreign

workers working in Pulau Tekong and Pulau Tekong

Kechil are screened for malaria. A total of 400 blood

films had been examined but none were found to be

positive. MINDEF also carried out fever surveillance

amongst the 800 SAF trainees who had left Pulau Tekong

at the same time as the case in December 2003. None

of the SAF trainees were found to have fever.

NEA and MINDEF also conducted a joint mosquito larval

survey, adult trapping and thermal fogging at Pulau

Tekong and Pulau Tekong Kechil. Two anopheles

breeding sites were detected. NEA conducted mosquito

larval survey and fogging within the vicinity of Hillview

Avenue, and no anopheles breeding was detected.

The cryptic case was notified on 27th September 2004.

He was a self-employed delivery man of no fixed abode.

He had onset of fever, chills, rigors, headache and body

weakness on 18th September 2004 and consulted a GP

twice before he was admitted to CGH on 24th September

2004. Blood testing showed Plasmodium falciparum with

a parasite load of 10%.

The case did not give any recent record of travel prior to

his onset of fever. His movements were confined to a

carpark in East Coast Park and Pasir Ris/Tampines/

Bedok areas where he carried out deliveries during the

day. Active case detection was carried out on 40 National

Park Board officers. None were positive for malaria

parasites.


45

Table 2.18

Malaria Surveillance, 2004

MURINE TYPHUS

Murine typhus is a rickettsial disease whose course

resembles that of louseborne typhus. The infectious

agents are Rickettsia typhi (Rickettsia mooseri) and

Rickettsia felis. Mode of transmission is by infective rat

fleas that defecate ricketsiae while sucking blood from

its host. This contaminates the bite site and other fresh

skin wounds. Occasionally cases occur following the

inhalation of dried infective flea faeces.

In 2004, a total of 27 laboratory confirmed cases of

murine typhus were reported, an increase of 68.8% from

16 cases reported in 2003. One (3.7%) was a Singapore

resident, 24 (88.9%) were foreign workers and 2 (7.4%)

cases were foreigners seeking medical treatment in

Singapore. All were male. Of the 25 local residents

infected, one case was imported and 24 were

indigenous. The incidence rate was highest for foreigners

in the 25 – 34 years age group (Tables 2.19 and 2.20).

Table 2.19

Age-gender distribution and age-specific incidence rates of reported

murine typhus cases, 2004^

êxcluding 2 foreigners seeking medical treatment in Singapore

* Rates are based on 2004-estimated mid-year population.


46

Rodent surveillance

Rodent surveillance and control are regularly carried out

by NEA officers. Food establishments are routinely

inspected for rodent infestation. In addition, wet markets,

food centres and bin centres in Town Council estates

are assessed on a six-monthly basis. Feedbacks and

complaints from public are also used to identify locations

infested with rodents. When burrows are detected, they

are treated with rodenticide and subsequently sealed

up after all the rodents are eradicated. In addition,

respective agencies managing the area are advised to

step up on cleansing maintenance and improve on

housekeeping and refuse management to deprive

rodents of food and habitats.

In 2004, a total of 1,374 sites were inspected. Rodent

burrows were detected in 423 sites (31%) (Figure 2.13).

Of the infested sites, 86% had low level of infestation (<

6 burrows), 9% had medium level of infestation (6 - 10

burrows) and the remaining 5% had high level of

infestation (> 10 burrows) (Figure 2.14). The common

habitats were: planted turf areas in Town Council areas,

around bin centres/bin chutes, and cracks and crevices

along roadside drains. The common causes identified

for these infestations were: uncovered refuse bins, refuse

spillage, unkempt bin centres and defective structures.

Table 2.20

Ethnic-gender distribution and ethnic-specific incidence rates of reported

murine typhus cases, 2004

êxcluding 2 foreigners seeking medical treatment in Singapore

* Rates are based on 2004-estimated mid-year population.



47

Figure 2.13

Geographical distribution of rodent burrows detected, 2004

Figure 2.14

Rodent infestation levels in premises inspected, 2004

48

Rat Attack programme

In March 2004, NEA launched Rat Attack programme -

an eight-month pilot project in five Town Councils

(Aljunied, Marine Parade, Sembawang, Tanjong Pagar

and West Coast/Ayer Rajah). The objective was to

identify a sustainable model for rodent control in public

housing estates island-wide, involving all stakeholders

in the community.

A Rat Attack Task Force was formed in each project area,

led by Heads of NEA Regional Offices. Representatives

from Town Councils, pest management companies

(PMCs), Hawkers Association, Shop Merchants

Association, Coffeeshop Association, refuse collection/

cleaning contractors and grassroots organisations

[Citizens Consultative Committees (CCCs) and

Constituency SARS Task Force (CSTF)] were invited to

the Task Force to drive the project. The Task Force

worked with the other stakeholders (trade operators,

conservancy cleaners and residents) to tighten up refuse

management and adopt better housekeeping to deprive

rodents of food and habitats.

The pilot project consisted of an Attack Phase and a

Maintenance Phase. In the Attack Phase, the PMCs

carried out a baseline survey to assess the current level

of infestation and to identify irregularities in environmental

sanitation/refuse management practices that were

contributing to the problem. The rodent population was

then reduced through intensive rat baiting and poisoning

as well as various measures to improve environmental

sanitation. This was followed by the Maintenance Phase

to maintain the pilot sites in a rodent-free state.

The results from the pilot project showed that sustainable

rodent control is achievable using Rat Attack model. NEA

plans to replicate this model across all 16 Town Councils

in Singapore over a two-year period.

Documents

Diseases Environment- · Communicable Diseases Surveillance in Singapore 2004 31 Table 2.6 Dengue clusters identified, 1990 – 2004 *A cluster is defined as two or more cases epidemiologically