Asia-Pacific Journal of Public Health http://aph.sagepub.com/ Epidemiology of Dengue/Dengue Hemorrhagic Fever in Malaysia −− A Retrospective Epidemiological Study 1973-1987. Part I: Dengue hemorrhagic fever (DHF) K Chandra Shekhar and Ong Leong Huat Asia Pac J Public Health 1992 6: 15 DOI: 10.1177/101053959300600203 The online version of this article can be found at: http://aph.sagepub.com/content/6/2/15

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Abstract Dengue hemorrhagic fever (DHF), though endemic in the sixties, emerged as a major public health problem in Malaysia from 1973 onwards. The incidence rate of DHF which was 10.1 per 100,000in 1973 has fallen dorm to 1.9 per 100,000 in 1987 with a mean case fatality rate of 6.4 per 100 persons. The Chinese appear to be more prone to DHF with the highest mean morbidity rate of 5.5 per 100,000 and case fatality rate of 6.1%. The incidence of DHF is higher in the males with a higher case fatality rate in females. Male Chinese appear to be mainly affected. The overall agespecificincidencerate is highest in two age groups, viz. 5-9 years and 10-19 years of age with a mean morbidity rate of 4.9 cases per 100,000. The mean age-specificcasefatalityrate was highest in the 0-4 years age group. Dengue hemorrhagic fever is predominantly an urban disease in Malaysia with a mean incidence rate of 5.3 cases per 100,000 as opposed to 1.2 cases per 100,000 being reported from rural areas. The mean overall incidenceof deaths in the urban area is 0.5 compared to 0.1 per 100,000 for rural areas. There is a marked seasonal correlation between D H F cases and rainfall, with a peak in August. While all four serotypes of dengue viruses are found in Malaysia, Den 2 appears to be isolated with greater frequency during all the epidemics. Key words: Dengue hemorrhagic fever, epidemiology, Malaysia.

Address for reprints: Dr K Chandra Shekhar Department of Parasitology Faculty of Medicine University of Malaya Jalan Lembah Pantai Kuala Lurnpur 59100 Malaysia

Epidemiology of Dengue/ Dengue Hemorrhagic Fever in Malaysia A Retrospective Epidemiological Study 1973-1987. Part I: Dengue hemorrhagic fever (DHF) K Chandra Shekhar MPH (Haward), P h D Department of Parasitology Faculty of Medicine University of Malaya Malaysia Ong Leong Huat Department of Social & Preventive Medicine Faculty of Medicine University of Malaysia Malaysia Introduction

Historical background Dengue fever (DF) was first recorded in Peninsular Malaysia by Skae in 1902’ who described an outbreak in Penang. The high incidence of the disease was observed in those parts of Penang inhabited by Asians. More2 found that the disease affected Province Wellesley and parts of the Federated Malay States. The distribution of outbreaks corresponded to the distribution of Aedes aegypti3. Smith3 stated that the endemicity of the dengue virus was present from 1904 onwards based on sporadic and large number of local outbreaks. A serum survey conducted by the Institute for Medical Research in 1953 indicated that, in Malaysia, dengue in man was common and widespread4. Although the disease had been recognized in Malaysia on clinical grounds for many years, the survey provided

the first laboratory confirmation of its presence. The hemorrhagic (DHF) manifestations and shock syndrome (DSS) in epidemic proportions was first reported in Peninsular Malaysia in November 1962 on the island of Penang (Pulau Pinang). From then until April 1964,6 1hospitalizedcases with five deaths were confirmed by dengue virus isolation a n d / o r serology5. Between November 1962 and July 1963, Parameswaran6*’ reported. 4 1 cases provisionally diagnosed as dengue hemorrhagic fever admitted to the pediatric unit, General Hospital, Penang. There were five deaths. Lucas’ reported 67 cases of dengue hemorrhagic fever from Penang for period between 1962-1 965. These also included cumulative cases reported by Rudnick et al’ and Parame~waran~,’. Between 1967-1969, pocket epidemics broke out in the urban areas in Peninsular Malaysia. Atotal of 192 cases were 15

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Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2

reported both from Penang and Kuala Lumpur. The number of inpatient dengue cases reported from Government hospitals between 1963-1 972 amounted to 194 cases. No dengue hemorrhagic fever cases were reported to the Ministry of Health for this period as dengue was not a notifiable disease until April 1971. While there were no epidemics, the presence of dengue activity was clearly reflected by isolations from blood samples collected from various hospitals throughout Peninsular Malaysia. However, the picture changed drastically and, as early as January 1973, an increased number ofacute viral infections (AVI) were diagnosed mainly from the southern state hospitals. Confirmations by post-mortem revealed death due to dengue hemorrhagic fever. By May 1973, an increased number of cases and deaths were noted from the states of Johor and Selangor. By the beginning of 1970, dengue and dengue hemorrhagic fever caused by various serotypes of dengue viruses had established themselves within the human populace and vectors in the Southeast Asian region including Malaysia. Since then, the disease complex (DFIDHFIDSS) has become a major public health problem. The paper describes the epidemiology of dengue hemorrhagic fever in Malaysia from 1973-1987.

Material and methods

Source of data

Definition

Dengue fever and dengue hemorrhagic fever are both notifiable diseases. All cases reported are clinically diagnosed and are confirmed after serology. Should the diagnosis be different after serological confirmation then it is changed and the case deleted from the registry. All data accumulated at the Epidemiology unit are then crosschecked with the State Directorate’s data for completeness and accuracy. All virologically confirmed cases are tabulated and analysed. The type of dengue viruses incriminated in the epidemic which areavailable from the blood results, and the trend of the disease, are then noted and the analysed data are relayed to the district for proper implementation ofthe dengue control programme.

A case here refers to an inpatient sus-

pected by the attending physician or doctor of having dengue fever/ dengue hemorrhagic fever based on the clinical criteria of the Ministry of Health. The clinical criteria are based on the World Health Organization’s9 guidelines(P1an ofAction forthecontrol of dengue and dengue hemorrhagic fever in Malaysia) and these were and are being circulated to all hospitals throughout the country to ensure uniformity in diagnosis. Typically, a suspect case of dengue hemorrhagic fever is one based on four major clinical manifestations: high fever, hemorrhagic phenomena, hepatomegaly and, often, circulatory failure. Moderate to marked thrombocytopenia with concurrent hemoconcentration is a distinctive clinical laboratory finding for differentiating it from dengue fever. The clinical features of dengue fever on the other hand, depends on the age of patients and includes undifferentiated fever with maculopapular rash to the classical incapacitating disease with abrupt onset and high fever, severe headache, muscular and joint pains and rash. The clinical criteria formulated by the Ministry of Health was instituted in 1973.

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Epidemiology: Host and Disease A. Incidence data Figure 1 shows the incidence pattern on dengue fever (DF) and dengue hemorrhagic fever (DHF) for the years 1973 till 1987. For the period 1973-1981, all these cases cumulatively recorded were from Peninsular Malaysia. However, from 1982 onwards, cases began to be reported from Sabah and Sarawak. A total of 9.2 cases per 100,000 per year were

Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2

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Figure 1. . Incidence Rates of DFand DHF in Malaysia. 1973-1987

OF

*--*OHF

-0 C

10 0

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reported for this 14-year period. Of this, 5.4 casesper 100,000personsper year manifested the milder D F and 3.8 cases per 100,000 persons presentedwith thehemorrhagicformgiving a mortality rate of 0.24 deaths per 100,000 persons. From the figuresit isevident that in the first four years (1973-1976) there were more manifestationsofthe hemorrhagic form of the disease. Then, in 1977, an equal distribution of cases was observed followed bv an increase in DF after that. The occurrence of cyclic epidemics every

The case fatality was 5.6% in 1973, reached a peak in 1985 with a rate of 10.4% and then fell down to 2.6% in

10.4 deaths per 100 in 1985. It was observed that when the incidence rates of DHF were high the case fatality rates were low and vice versa (Figure 2).

C. Distribution by persons: incidence rate, casefatality rate of DHFby ethnicity Figure 3 shows the incidence of DHF cases according to ethnicity. For the

period between 1975-1987,65.2%of the total DHF cases or an incidence rate of 5.5 cases per 100,000per year were reported from the Chinese community. The Malays formed 26.6% with an incidence rate of 1.3 cases per 100,000 per year, and 3.7% or 1.1 cases per 100,000per year were Indians. Deaths were recorded highest amongst the Indians followed by the Malays and Chinese, giving a case fatality rate of 6.8%, 6.7% and 6.1% respectively. The incidence rate among the Chinese for the periods extending from 1975-1 982 is above the average incidence rate of 5 . 5 cases per 100,000 except for the year 1982 when it recorded an incidence rate of 11.7 cases per 100,000. There was a slight difference in the rates between the Malays and Indians who recorded incidence rates of 1.3 and 1.1 cases per 100,000 for the same period. For the first six years i.e. between 19751980, incidence rates of D H F in the Malays were higher than the Indians and subsequently a progressive decline in the rates was observed for both ethnic races except for the year 1985. The case fatality rate per 100 among the Chinese was above its average value of 6.1% from 1975-1980 and 1985. The lowest rate was recorded in 1981 where it was 0.5 per 1OOcaseswithan incidencerateof4.8 cases per 100,000. The case fatality rate for the Malays was relatively higher than that for the Chinese,

Figure 2. Case Fatality of DHF in Malaysia. 1973-1987

-0

e

01

9

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Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2

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Figure 3. Incidence Rates of DHF by, *Ethnic Group in RIalaXsia. 1975-1987. 6

0

Chinese;

D. Distribution and incidence rate of DIIFand death bjp sexes

Malay; & - - . A Indian

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04

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Others are not included

ranging from a mere zero to 13.2 per 100 cases with a mean case fatality rate of 6.7%. The Indians showed a case fatality rate of 6.8% for the 12year period. There is only a slight difference in the death rates among the

three ethnic races though the incidence ratesare higher among the Chinese. One can say that for every one Malay or Indian case of DHF there is an average of four to five Chinese affected.

Figure 4. Incidence Rate of DHF by Sex in hlalaysia, 1975-1987

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Figure 4 shows the incidence rates of DHF according to sexes for the years 1975-1987. The mean incidence rate in males were 2.8 cases per 100,000 and 2.6 cases per 100,000 for females. Before 1982, the mean incidence rate in the males was 3.8 cases per 100,000 but from 1983 onwards it was 1.5 cases per 100,000, a fall by 60%. In females, the mean incidence rate for the period 1975-1 982 was 3.6 cases per 100,000. From 1983onwards, the mean incidence rate was 1.3 cases per 100,000, showing a drop of 64%, a trend similarly seen in males. The overall data on the case fatality rate for males and females was 6.0 and 6.7 cases per 100 respectively. The incidence ratio between the males and females is 1.1:l while the case fatality ratio is 0.9: 1. Therefore, there i s a slight difference in the incidence ratio between sexes and the case fatality rate is slightly higher in the females when compared to males.

E. Distribution and incidence of DHF cases by ethnic races and sexes The overall data shows that 65.1% of the total cases were reported from the Chinese community. T h e male Chinese formed 33.8% while female Chinese formed 31.3% of the total. This was followed by 26.6%ofMalays with 14.1% being males and 12.5% females. Indians constituted 3.6% of the total with 1.9% being males and 1.7% females. A male preponderance is a feature not only true for the Chinese community but for all the ethnic groups. The male:female ratio for DHF for the Chinese and Malay ethnicgroups is 1.1:l. In 1975, the Chinese males were the principal target group affected, recording an incidence rate of 10.1 cases per 100,000 persons. The rate reached a peak in the year 1982 when it was 11.6 cases per 100,000 and showed the lowest rate of 0.5 cases per 100,000 in 1985. There was a 7 1% fall in the incidence rates in the male Chinese from 1975 to 1987, with a mean rate of 5.6 cases per 100,000.

Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2 On the other hand, the female Chinese recorded a mean incidence rate of 6.9 cases per 100,000 for the period 1975-1981 and peaked in 1982 at 11.8 cases per 100,000. The mean incidence rate for the period 1983-1987 a m o n g t h e female Chinese was 2.2 cases per 100,000. These figures coincide with the figures for the males during the same time frame. Therewasan equal distribution of cases between males and females in the year 1987. The mean incidence rate for female Chinese for the period 1975-1987 was 5.2 cases per 100,000. The mean incidence rate for the Malay male was 1.3 cases per 100,000 while for the female Malay it was 1.2 cases per 100,000. In the 1982 epidemic, no significant differences were observed in the attack rates between the sexes. Forthe period 1975-1987, the incidence rate among the Malay

males had risen from 1.1 to 1.4 cases per 100,000, an increase of 21.4%. The reverse was true for the female, i.e. there was a drop from 1.5 to 1.3 cases per 100,000, a fall of 13.30/0. In the I n d i a n co m mu n it y , however, there were no significant differences in the attack rates between the sexes (1.1 per 100,000).The incidence ratio between males and females for the Chinese and Malay communities was 1.1: 1, and for the Indian community itwas 1:l.

F. Distribution of DHF cases and deaths by specific age groups For the 13-year period, a total of464 1 cases with 293 deaths were recorded from all age groups. Accumulated data shows that the age specific incidence rate was highest in the 5-9 years age group for the years 1975-1978 and then moved to the 10-19 years

Figure 5. Agespecific Hospitalization Rate for DFand DHFand Case Fatality Rate for DHF in hlalaysia, 1975-1987. 0DF Cases; A-A DHF Cases; -0

DHF Deaths

20.0

10.0

g

CJ V LL

I

a

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0,

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20 I 30 Age (years)

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Forage 50 + years: DFASHR is 0.7/100,00 population DHF ASHR is 0.5/100,000 population DHFCFRis7.7/100DHFCases

age group from 1979-1982. There was a transition of the disease from a predominantly pediatric population to the adolescent group from 1977 onwards. In 1983-1984, the highest incidence was reported in the 40-49 years age group. Another shift took place in 1985-1986, when the highest incidence was recorded in the 10-19 years age group. In 1987, however, it was in the 20-29 years age group. The overall data (Figure 5) shows that the age-specific incidence rate was highest in two age groups, viz. 5-9 years and 10-19 years, recording a mean incidence rate of 4.9 cases per 100,000. The infants and toddlers recorded a mean incidence of 2.2 cases per 100,000. The mean case fatality rate for all the age groups ranged from as low as 2.2% to 16.7%. Case fatalitywas highest in the 0-4 years age group, recording 16.7%, followed by children in the 5-9 years age group with a case fatality of 8.7%. The overall data shows that DHF is mainly a disease of children and young adults, i.e. based on the data that 63.5% of the total cases were recorded from the 0-19 years agegroup. In thecaseofdeaths, 83.3% were recorded from this same age group.

G . Distribution of cases by locality (Urban-rural) In Malaysia, there is a noted urban preference of dengue hemorrhagic fever. This distribution is usually associated with the ecological, sociological, behavioral (human and vector) and cultural practices ofthe three ethnic communities. From 1975-1987, 70.3% of DHF cases were reported from urban areas, giving a mean of 5.3 cases per 100,000 as compared to I .2 cases per 100,000 from rural areas. The ratio of urban:rural incidence was 4.4: 1. This distributional ratio confirms the urban affiliation of DHF. The distribution data shows that there has been a 67% drop in the urban cases when figures are compared between the years 1975 and 1987. On the other hand, the incidence of DHF cases in the rural areas has increased by 25% for the same period. 19

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Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2 For the 12-year period, a total of 7 1.7% of deaths were reported from urban areas. The mean overall incidence of deaths for the years 1975I987 was 0.5deaths per 100,000 compared to 0.1 death per 100,000 reported from rural areas, This would mean 80% more deaths reported from urban areas. In the latter, the incidence of deaths in 1975 was 0.9 deaths per 100,000 compared to 0.1 deaths per 100,000 in 1987, a fall of 89%. In the rural areas, the highest deaths were reported in 1977 and 1982 where it was 0.2 cases per 100,000. However, during the other years, the rate fluctuated between 0.01 to 0.03 per 100,000 with a zero incidence in 1987. The death ratio between urban and rural areas would thus be 5: 1.

continues through to September loses most ofits precipitation on the islands of Sumatra and gives rise to a dry period in Peninsular Malaysia. The endemicity of DHF is low during the period extending from January to April and then begins to rise, reaching a peak in August and then declines slowly. Figure 6 s h o w the mean monthly distribution of DF/DHF for Peninsular Malaysia for the period 1975-1 987. There is a seasonal relationship showing that cases were reported increasingly during the drought season. This could be attributed to increase in vector density or vector population due to availability of breeding sites.

H. Distribution of DHF cases by months of onset

Dengue hemorrhagic fever is caused by one of the four serotypes of dengue viruses designated Den 1, Den 2, Den 3 and Den 4. The first isolation of dengue viruses in Malaysia was carried out by Rudnick et a15in 1962who worked on 6 1 laboratory-confirmed cases. They isolated dengue virus type 2 (Den 2) from the acute phase sera of 14 patients who had DHF. In 1967-1968,95 strains of dengue vims were isolated, out of which 59 were found to be Den 4,30 Den 2, six Den 3, and four Den 1. It was in this year that Den 3 and 4 were recognized in Malaysia for the first time.

In Malaysia, epidemics of DHF are seasonally associated: south-west 'monsoon in the first half and northeast monsoon in thesecond halfofthe year. The two wet seasons are from March to May and from September to November, respectively. Between November and March, the north-east monsoon prevails and produces large amounts of rain especially on the east coast of Peninsular Malaysia, Sabah and Sarawak. The south-west monsoon which starts from May and

I. Types of dengue viruses associated with epidemics

Figure 6. Seasonal Pattern for DF and DHF, 1975-1987.

-

DF;

0 m - e - 0

DHF

150 140

130 120

110

: w-

V

100 YO

-

80

70 60

Jon Ftb Mar

Apr

Moy Jun Jul Aug S e p O c l Nor Dec Wonlhs

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In 1970, Den 2 was isolated from 82% of the confirmed cases, followed by Den 4.In 1971, Den 1 and Den 2 were the predominant serotype isolated, followed by Den 4. In 1972, Den 1 was isolated from 72% of the confirmed cases, followed by Den 2. In 1973, Den 3 and Den 2 were the predominant strains isolated fol1owed.by Den 4 and Den 1. Wallace et all0 studied the 1973 epidemic and isolated 33 virus strains and found 39% of the cases were caused by Den 2,11.6% by Den 1,9.4%by Den 3 and 8.7% by Den 4. They isolated Den 2 viruses in January-February and during every month of the epidemic except June; Den 1 viruses in JulySeptember; and Den 4 viruses in October and during every month of the epidemic except August. Den 3 viruses were isolated from May through August. They concluded that most of the strains were isolated in July. Another interesting finding was that in 10 out of 11, Den 3 isolates were recovered from patients with DHF syndromes. Two Den 2 and Den 4 strains were recovered from the same group of patients. Another feature also noted by the workers was that in 11 out of 12, Den 2 virus strains were recovered from males and all Den 3 strains were isolated from Chinese patients. Norazizah et al" isolated Den 1 and Den 3 strains of virus during the 1982 epidemic from patients admitted to the University Hospital. In the 1981 epidemic, the same group had isolated Den 1 and Den 2. There appeared to be a maximum isolation of Den 3 during the peak of the outbreak. These findings coincided with the findings of the Institute for Medical Research. It was then concluded that the 1982 epidemic was predominantly caused by Den 3 and Den 1. In 1985, the dengue virus laboratory at the Medical Faculty of the University of Malaya isolated eight strains of dengue from the acute sera of patients admitted to the University Hospital'*. Out of the eight strains isolated, one was typed Den 2, four Den 3 and three Den 2. Subsequent isolation showed a preponderance of Den 2. Results obtained from the Institute for Medical Research

Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2 confirmed. the results of the University Hospital. Interesting conclusions can be drawn regarding the strains responsible for dengue hemorrhagic fever. The serological data accumulated from 1960 up to the present shows dengue activity with all the four serotypes in Malaysia but there appears to be a much greater activity with Den 2.

Discussion Dengue has been present in Malaysia at endemic levels since 1902when the disease was detected in small outbreaks and subsequently in a major outbreak in Penang in 1962. Since then, cases have been reported yearly. In Malaysia and other parts of Southeast Asia and the Western Pacific, the clinical picture of dengue varies from a milder form, the dengue fever (DF), to a more severe form, the dengue hemorrhagic fever (DHF) with shock (Dengue shock syndrome-DSS) and a high case fatality rate. Incidence or morbidity data on dengue hemorrhagic fever from various endemic countries in South-east Asia and the Western Pacific regions varies. While an increase in incidence of the disease is mentioned in all the endemic countries, the rates per 100,000 are seldom expressed. In the Philippines, the morbidity rates ranged from 0.1 to 28 per 100,000 for theyears 1958-197513. However, Baltazar et all4 estimated that the annual incidence of DHF/DSS in Manila to be 26.3 per 100,000. In Thailand, the average morbidity rate for the whole country for periods between 1969-1973 was 65.7 per 100,000population15.The highest incidence rate was in the central region, including Bangkok where it was 95.8 per 100,000, and thelowest was in the southern region with an incidence rate of 19.0 per 100,000. It was 75.1 and 60.2 per 100,000 in the northern and north-eastern regions respectively. In SingaporeI6 the morbidity rate per 100,000 population between the years 1960and 1981 ranged from 1.1 in 1976 to 54.3 in 1973. High morbidity rates of 32.6, 41.8 and 42.2 per 100,000 were also noted for the years 1966, 1967 and 1968 respectively. The year 1965 saw one of the lowest

morbidityratesof2.5 per 100,000. In this paper, the incidence of DHF in Malaysia is presented for a 14-year period. The incidence in 1973 was 10.1 per 100,000 and had decreased to 1.9 per 100,000 in 1987 with an intermittent rise to 5.9 per 100,000 in the 1982 epidemic. While the overall morbidity rates for the entire country has decreased, the incidence rates in the epidemic prone states continues to increase and these rates appear to coincide with the rates in the neighboring countries. In Singapore, however, with the strict vector control programmes, the morbidity rates have been brought down to an almost minimal level, a fact that has not been able to be reached in this country for various reasons. The factors that have contributed to the increased incidence of DHF in Malaysia are mainly attributed to expansion of industries, movement of the population from mral to urban areas, i.e. outmigration and inmigration, and rapid urbanization with the resultant increase in urban slums and squatters without proper water supply and sanitation. Sarosa", when discussingthe epidemiology of DHF in Indonesia, quoted that the case fatality rate varied from city to city and had declined from41%in 1969to5.1%in 1976to 4.6% in 1977. Sumarmo et all8 reported a case fatality rate of 19%when studying virologically confirmed cases of dengue in Jakarta. DizonI3 reported a case fatality rate of 23% in the 1974 epidemic in the Philippines. Between 1958-1963, the case fatality rate in hospitalizedcases in BangkokThonburi, Thailand, was 6.9%". In 1973, it was 3.7%" and in 1979, 1.1%. The overallcase fatalityrate has steadily declined to around 1-2% in Bangkok and around 3-49/0 for the whole country of Thailand. The case fatality rates reported from the provinces varied greatly between different locations, ranging from less than 1% to 8% while in new outbreak areas the case fatality rates were high15. Between 1975-1 978, the case fatality rates in Burma, Indonesia and Thailand varied from 3-5%, 5-7% and 1,l-4% re~pectively'~. The case fatality rate in Singapore varied between 0

and 4.2% in 197016. On the other hand, the case fatality rates recorded in Malaysia from 1973-1987 varied from as low as 2.6% to 10.4% with a mean rate of 6.4%. The gradual fall in case fatality rate from 1973-1987 could be attributed to better disease and vector surveillance, prompt diagnosis, early notification and better management by the attending physicians. It could also be due to the fact that more dengue fevercasesarebeing reported with increasing frequency than hemorrhagic cases, a feature observed from 1982 onwards. Whether DHF has a special predilection for an ethnic race is a subject of discussion. Nimmannitya et alzo, who studied 523 patients with dengue, and Chikungunya found that differences in ethnic groups existed. Halstead2' found a high preponderance of the disease among the Thais when compared with the Chinese. In Singapore in the early years, DHF was reported to affect all the major ethnic groups in equal proportions'6. In the 1973 epidemic, the morbidity rate of Chinese was 6 I .6 per 100,000 when compared with Indians (38.9 per 100,000) and Malays (18.8 per 100,000). Between the periods 1974-1976, the morbidity rate for Chinese was 14.0 per 100,000 while in the Malays it was 13.6 per 100,000 which was seven times more than that of Indians who formed 1.9 per 100,000. Between 1977-1981, the morbidity rate for the Chinese was 36.3 per 100,000 compared to 13.1 and 14.9 per 100,000 for Malays and Indians respectively. RudnickZ2 who studied the first episode of the Penang outbreak found the disease more prevalent among the Chinese. The percentage distribution was 80.4% in Chinese, 9.8% in Indians, 8.2% in Malays and 1.6% in others. Parame~waran~.'found a high preponderance ofChinese in the 4 1 cases admitted in Penang when compared to Malays and Indians respectively (37:3:1) in the 1962-1963 epidemic. The ethnic ratio for the daily medical admissions at this period was 3:l:l for Chinese, Malaysand Indians, thus showing that Chinese were most affected. In the second episode, the Chinese comprised 50% followed by 21

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Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2 Malays who formed 25% and the In-. dians 22%, giving a ratio of 2:1:0.9. The Chinesewere the principal ethnic group affected which was consistent with the general population figures in Penang at that time. In this study, a comparison of the morbidity figures for the various ethnic groups showed a mean incidence rate of 5.5, 1.3 and 1.1 per 100,000 for Chinese, Malays and Indians respectively for the period 1975-1987 giving a ratio of 5:1.2:1. These findings when expressed in the population reflect the urban and suburban preponderance of D H F transmission where there is a larger Chinese population, with

main island in a small archipelago in the gulf of Thailand), found that out of 99 cases, 55 were females and 44 males, giving a rna1e:female ratio for all dengue infections of 1:1.2. Sumarmo et all’ found the male: female ratio for fatal infections in Jakarta to be 1 5 1 whereas the ratio for all dengue infectionswas 1:1.2. In Singapore, in 1960 and 1963/1964 more females than males were affected in the ratio of 1.7: 1 and 1.3: 1 respectively’6. In the 1973 epidemic, more males were involved (1.5:l) with marked differences in the attack rates for males and females in the 15to 19-year (males 105 per 100,000:

When the ethnic races were sexed for the period 1975-1987, the male Chinese showed a mean incidence of 5.6 per 100,000 compared to 5.2 per 100,000 for female Chinese giving a ratio of 1.1: 1. Similar findings were seen for the male Malay ethnic community though no differences were observed in the attack rates between sexes of the Indian community. One can conclude that there appears to be a predominant male preponderance to DHF in the Chinese and Malay communities. A possible explanation could be that most ofthe Chinese and Malay community members are engaged in business activities near their

0 t h e 3 ~ The . results of this study also appear to concur with the findings in Singapore. While the incidence rate among the Chinese was the highest, the case fatality was the lowest when compared with the three ethniccommunities and this was vice versa for the Indian community who recorded the lowest incidence rate but showed the highest case fatality rate. There were no significant differences between the case fatality and incidence rates between the Malay community and the Indians. This means that while the Chinese are more prone to the infection or disease, mortality rates are higher among the Indians and Malays. This can be attributed to ignorance of the disease or its sequelae by these community members as they do not prefer to seek medical attention immediately or prefer to resort to the help of traditional medicine men or traditional healers to alleviate their problems. Halstead” found that the crude hospitalization data failed to show significant differences in the attack rates for males and females in Thailand. Serologic surveys conducted in Bangkok also showed no differences in the infection rate. A study by Halstead2’later showed that the male: female ratio for DHF below the age of threeyearstobe 1:0.9anddeathstobe in the ratio of 1:l while in the 4-14 yearsagegroupit was l:1.2and 1:1.3. Girls slightly outnumbered boys in secondary dengue infections. Winter et a124, working in Koh Samui (the

24-year (males 119 per 100,000 and females 47.9 per 100,000)agegroups. For the years 1974-1976, males continued to predominate over females in the ratio of 1.3:l and between 1977-198 1,the ratio was 1.4: 1. In the 61 Penang cases reported between 1962- 1964, the ma1e:female ratio was 1:1.75. P a r a m e ~ w a r a n found, ~.~ amongthe41 cases,asexratioof 1:2.1 meaning that there was a higher proportion of females affected between the periods 1962-1964. Lucas’ again reported a higher proportion of females affected for the periods 1962-1965 with a ma1e:female ratio of 1: 1.5. Between 1966-1968,areversal was seen with a higher proportion of males being affected, giving a ratio of 1.8:l. Between 1975-1987, there was no significant difference in the attack rates between the sexes (only a 0.2 or 0.7 percent increase) giving a ratioof 1.1:l.Itcanbesaidthat inthe early sixties DHF was reported more in the female sex, then this trend was reversed between 1966-1 968 t o affect males and subsequently after the 14-year (1973-1987) period the proportions showed only a slight difference in the attack rates between males and females. When the death rates were compared, the reverse was true with more females dying of DHF than males (ratio 1:1.2). It can be concluded that while the incidence rateofDHFis higheramongthemales the case fatality rate is higher in the females. This feature appears to fit the Singapore scene.

ent or indiscrete sanitation, vectors breed in close proximity and are thus are able to feed on them and disseminate the disease. DHF/DSS has been considered a children’s disease. DHF, unlike classical dengue, attacks mostly children under the age of 14 years and causes significant mortality among them. Halstead” found DHF/DSSattack rates to be lower in the one-yearolds than in any other groups of children in Thailand and Thonburi. Nimmannitya”, on the other hand, found a change in the age distribution of DHF in Bangkok during the period 1958-1968. She concluded that the median age of D H F patients had shifted progressively from three years and ten months (1962-1965) to five yearsandsevenmonths(1971-1973). She further concluded that the percentageofpatients above the ageof 12 years has changed very little although there had been an increasing number of cases in young adults in Bangkok and the older age group outside Bangkok. Winter et a124found that 53.5% of his population were in the 5- to 9-year age group and 28.3% in the 0to 4-year age group. They concluded that the children who were predominantly affected by DHF had a median age of 7.0years. Ramakrishnan et a12’ found hemorrhagic manifestation in 28.9% in the 5- to 9-year age group followed by 13.3% in the 1- to 4-year age group and 10- to 14-year age group. Sumarmo et all8 found a majority of the fatal cases in the 5- to 9-year age group. Saroso” showed

houses in close proximity to each

-

females 57.3 per 100,000) and 20- to

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homes and, because of their inappar-

Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2 that for periods between 1975-1977, children under the age of 15 years were affected with the highest rate among the 5- to 10-year age group. DizonI3 quoted that in the early years from 1954-1958,approximately95% of the cases were children below the 15-year age group, but after 1966 there was a corresponding increase in adult cases. In 1974, 78.5% of the cases were in children. In the younger age groups, about 32% of the cases wereintheagegroup5-9yearsY27.2% in the age group 1-4 years, 14.9% in the age group 10-14 years and only 4.5% were infant cases. Hayeset a126working in the Philippines found that 78%ofhispatients were less than 15 years of age. The median age for male and female patients were 9.9 and 9.6 years with a ma1e:female ratio of 1:1.4. In Singapore, on the other hand, older children and young adults were affected in 196016. In the 1973 outbreak, the attackrateswere high in both children and young adults. The age-specific rates per 100,000 for the different age groups were 84.3 (1-4 years), 83.5 (5-9 years), 81.8 (10-14 years), 81.7 (1 5-1 9 years) and 84.2 (20-24 years). The mortality rates were high in children below ten years of age, being 4.7 per 100,000 in the 1- to 4-year age group and 4.2 per 100,000 in the 5- to 9-year age group. During the period 1974-1976, the age-specific morbidity rate was high in older children and young adults (5-24 years) but low in the children less than five years of ageI6. Between 1977-1981, the same distribution pattern was observed. In the early sixties, Parame~waran~,’ had reported that out of the 41 cases, 85.3% were in the 5- to 9-year age group and none were less than three years old. The five deaths recorded then occurred in the 6-7 years age group. Rudnick5 describing the 61 cases found the medianage tobe7.5years.Lucass,on the other hand, found a median age of 7.3 years in the 67 cases he described. In this study three interesting features wereobserved: viz that the DHF/DSS attack rates were lower in the 0-4 years age group thus confirming the findings of Halstead2’. F r o m 1975-1978, the age-specific attack

rates were seen predominantly in the 5- to 9-year age group showing that there was no change in the age groups attacked. The third interesting feature was a n “age-shift” after 1979-1982 to the 10- to 19-year age group and back to the 5- to 9-year age group in 1983-84 and then back to the 10- to 19-year age group from 1985 up to 1987. This age shift has been explained to be a useful indicator of the movement of communicable diseases, showing decreasing virus transmission rates inpopulationssupporting endemic infection. Thus the mean age-specific rates per 100,000 for the different age groups in Malaysiaare2.2(0-4years), 4.9(5-9years), 4.9 ( 10-19 years), 3.4 (20-29 years), 2.4 (30-39 years), 1.6 (40-49 years) and 0.5 (> 50 years). Thus the target groups principally involved are those in the age group 5-19 years. These rates, however, are lower when compared to the surrounding South-east Asian countries. It can also be concluded here that DHFis a disease that affects children and young adults between the ages of 0-1 9 years considering the fact that they form 63.5% ofall cases recorded. The age-specific mortality rates were highest for infants and toddlers (16.7%). This can be interpreted as DHF having a fatal sequelae in infants between the ages of 0-4 years. Halstead2’ states that the highest attack rates were seen in the age group with the largest number of susceptibles and this was certainly true in the Malaysian context where 37% of the Malaysian population are in the 0- to 14-year age group. Outbreaks of DHF disease have occurred and spread to involve most of the major urban areas as well as some of the rural areas. Halstead” found that scattered cases of DHF were first recognized in major towns and the number of cases increased and there is a centrifugal spread of disease to smaller urban and rural communities. In the Philippines, major outbreaks occurred principally in Metro Manila or in thepoblacioizesof localities and in the populous and overcrowded areas where environmental sanitation was badI3. In Thailand, most of the early cases were reported from Bangkok and smaller

towns15and even villagesout ofBangkok. In Singapore, D H F occurred in densely populated urban areas which had high population densities of Ae. aegypti and Ae. albopictiis16. In the 1973 outbreak in Malaysia, it was concentrated in urban and suburban housing estates but more widely scattered in rural areas. Rudnick2’ originally stated that the geographical distribution of the cases based on residence showed that the majority of the cases were from outer urban and suburban areas of the city with relatively few in the central, densely populated areas of the Penang city. He also observed that in the 1962 epidemic, the majority of the cases were clustered in the crowded, poorer, central districts of the city. This study shows that D H F has an urban preference and is usually associated with the ecological, sociological, behavioral and cultural practices of the population. The storage of water in earthenware jars in rural areas without piped water supply was the major factor responsible for the breeding ofAe. aegypti and the transmission of DHF in rural areasz3. A detailed analysis in this paper shows that almost 70% of the cases were reported from cities and major towns. The mean incidence data clearly shows that 5.3 casesper 100,000were reported from urban areas compared to 1.2 cases per 100,000 from rural areas giving a ratio of 4.4:l. Deaths continued to be reported from urban localities with as much as 72% of deaths being reported from urban areas. From 1975-1987, themeanmortalitydatawas0.5 deathsper 100,000 compared to only 0.1 deaths per 100,000 from rural areas with a ratio of5:l. A variety of relationships between disease incidence and rainfall has been suggested. There is a positive correlation between the two variables in B u r m a , T h a i l a n d , a n d t h e Philippines28all of which have a wet season and prolonged dry period. The same authors found that there was an increase in DHF cases following the March to May wet season and that the size of the increase was positively related to the size of the moisture 23

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Asia-Pacific Journal of Public Health 1992/1993 Vol. 6 No. 2 surplus. Winter et alZ4who studied a small epidemic of DHF in Koh Samui, found that cases began to be reported from the months of July to OctoberandpeakedinJuly. This peak has been clearly exemplified by Wellmeg9 in northern Thailand, central plain and southern Thailand. In all these areas, peaks were observed in July or August. On the other hand, Hayes et alz6 showed that cases increased steadily from May until the peak in November with 69% of the infections occurring from October through December. The results of this study show that DHF incidence in Malaysia has a specific seasonal pattern with a peak in August. The biting activity and vector density appears to correlate well with the peak in DHF cases. It has been documented that the vast majority ofsevere and fatal cases of hemorrhagic fevers in South-east Asia and the Pacific are due to all serotypes of dengue viruses. Specific isolates have been identified with epidemics in various countries some associating dengue virus type 2 to 'the hemorrhagic type of dengue. Over the years a higher proportion of Den-2 seems to be seen in patients with a secondary dengue infection in Malaysia. Overall, DHF, a sinister disease which emerged as a major public health problem in the seventies continues to ravage the nation inspite of the organized and concerted efforts by the Ministry of Health, viz. source reduction, law enforcement and healtheducation. It isevident from all these data that DHF outbreaks are related to disturbancesin humanecology; exodus of susceptibles moving from non-endemic to endemic areas; rapid urbanization with creation of slums and squatters; and standards of living and climatic conditions that have perpetuated vector breeding, distribution, dissemination and ultimately the transmission of the disease.

Acknowledgements The author is indebted to the Director and staff of the vector borne disease control 'programme, Ministry of Health, Government of Malaysia, for

all the data on dengue outbreaks. The Director-General of Health is specially thanked for granting permission to publish this paper. Finally, my sincere thanks are to Mrs Agatha Chow and Ms Saina Kasbullah of the Department of Parasitology and to Encik Latiff and members of the Department of Medical Illustration for their invaluable technical assistance.

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