1) Virology unit, Institut Pasteur de Madagascar, Antananarivo, Madagascar, 2) Director, Laboratory of Public Health, Ministry of Health 3) Director General of Primary Health care, Ministry of Health, Victoria, Seychelles

Abstract

A serosurvey was conducted in Seychelles in a non-randomized sample of 490 subjects aged 2-94 years in May-August 1997 following epidemics of an influenza-like illness in December 1996 - February 1997 and of sore throat in March-April 1997. Tests were performed for IgG and IgM antibodies by ELISA for dengue 1 and dengue 2 viruses, West Nile, Wesselsbron, Sindbis and Chikungunya. None of the participants had IgM antibodies against the different viruses tested. The presence of dengue 1 or 2 IgG antibodies was detected in 24/490 (4.9%) and 20/490 individuals (4.1%) respectively. Only 2 patients born after the last known epidemic in Seychelles (January 1978) had dengue 2 antibodies. West Nile IgG antibodies increased from 18.4% in subjects aged 0-14 to 51.7% in persons aged more than 44. Antibodies against Wesselsbron, Sindbis or Chikungunya were not recorded in the studied population. In conclusion, these data support the view that no epidemic of dengue has taken place since 1978, that West-Nile virus has been encountered in the past, and that the other viruses have not been present recently in Seychelles. (SMDJ, 1999;6:18-20.)

Introduction

Dengue and West Nile viruses are members of the Flaviviridae family and transmitted principally in a cycle involving humans and mosquitoes. Infections caused by these viruses are mostly asymptomatic. In the last 25 years, the worldwide frequency of dengue fever outbreaks has increased and dengue became the most important arthropod-borne viral disease of humans with an estimated 50 million cases annually. A severe form of the disease, the dengue hemorrhagic fever (DHF), was first recognized in the Philippines in 1953 and subsequently reported in most countries of South Asia, then appeared during the 1980s in the Americas (14). The principal domestic vector of the four types of dengue virus (dengue 1, 2, 3 and 4) is Aedes aegypti, but Ae. albopictus mosquito is claimed to be the sole vector in some areas (15).

West Nile (WN) virus, associated with Culex mosquitoes and birds, is widely disseminated in Africa, in South-western Asia, and in the Mediterranean region. The disease in humans generally is reported as an influenza-like illness but recently encephalitic forms with fatal outcome were described (11,17). Other virus indistinguishable from West Nile, the Japanese encephalitic (JE) virus is circulating in Asia. Approximately 35,000-50,000 cases of JE with 10,000 fatalities are reported annually in Asia and about half of the survivors tend to have permanent neurologic and psychiatric sequelae (5). The cycle of maintenance of JE virus also involves Culex mosquito species, birds, pigs and humans. The incidence of the disease is increasing during the last ten years in India, Nepal and Sri Lanka. Infected mosquitoes from pig breeding farms are one of the most common source of human contamination.

The Republic of Seychelles consists of 115 islands scattered over 400 000 km² in the Indian Ocean (45° to 60 ° East, 4 to 11° South) with a total land area of about 450 km², with a total population of 75 000. Mahé, the main island with Victoria, the capital, is inhabited by 80% of the total population. The wet season extends from December to March and the average temperature ranges from 26° to 35°C in April-May. Annual rainfall ranges from 2 000 to 3 000 mm. From December 1976 to September 1977, an outbreak of dengue 2 fever with a clinical picture of classical dengue was recorded, affecting approximately 75% of the population (13). Then an outbreak of dengue was reported in La Réunion in 1977-1978 (7). A second dengue-like outbreak occurred in the Seychelles during the months of December 1978 and January 1979 (6). During the epidemic, the presence of Ae. aegypti was restricted to a few areas of Victoria only, and the virus was probably transmitted by Ae. albopictus mosquitoes (13). An entomological survey conducted in Mahé in 1995 showed a high prevalence of Ae. albopictus. The Breteau index varied between 17.0 to 100.0% and the house index from 14.3 to 53.3% (3). Likewise, a low density of Ae. aegypti distribution was registered, always in limited areas. Similar situation was recorded in La Réunion where Ae. aegypti was located in a few remote areas (16).

A possible introduction of the virus in the Seychelles in 1977 from South East Asia was suspected. Relationships were found by sequence analysis between the 1977 dengue 2 strain from the Seychelles and strains isolated in 1976 from Indonesia. Likewise similarities were observed with strains isolated later in Sri Lanka in 1982, 1985 and 1990, in Burkina Faso in 1983, and in Somalia in 1984 (8,12). In the South West region of the Indian Ocean, the last epidemic of dengue (dengue 1) was reported in 1993 in the Comoros islands (4). All cases recorded in the region were reported as classical dengue fever. However, an imported fatal case of DHF was confirmed in Mauritius in November 1996. The patient was travelling from New Delhi, India, where a large outbreak occurred at that time (1). No vaccine for dengue are presently available but several candidates are on clinical trials with as yet no satisfactory results. A commercial JE vaccine (Biken Ô ) is used in hyper-endemic areas of Asia.

An influenza-like epidemic occurred in December 1996-February 1997 and several thousands of persons reported to health centers. In March –April 1997, a large number of patients reported to health centers for sore throat. These epidemics prompted to examine the seroprevalence of dengue in Seychelles.

Material and methods

A serological study was conducted by the Ministry of Health from May to August 1997 in the general population to assess any recent activity of dengue. In this study, 490 individuals (253 females and 237 males, aged 2 to 94) were selected in a non random manner and on a voluntary basis. These persons were selected mostly in health centers and among schools. Blood specimens were collected as well as clinical data and travel history. Serum samples were tested by immunocapture enzyme-linked immunosorbent assay (ELISA) for detection of IgG and IgM antibodies to dengue 1, dengue 2, and West Nile viruses in the Pasteur Institute in Madagascar. Using similar techniques, sera were tested for the presence of IgG/IgM antibodies against other arboviruses (Wesselsbron, Sindbis and Chikungunya) that may be present in this region of the Indian Ocean.
 
 

Table 1. Distribution of IgG antibodies to Dengue 1, Dengue 2 and West-Nile viruses in the human population from the Seychelles (May-Aug 1997)
Age	Tested	Female	Male	Dengue 1 IgG		Dengue 2 IgG		West Nile IgG
Years	No.	No.	No.	No.	%	No.	%	No.	%
0-14	38	18	20	0	0.0	0	0.0	7	18.4
15-29	204	102	102	6	2.9	4	2.0	70	34.3
30-44	159	82	77	11	6.9	12	7.5	67	42.1
³ 45	89	51	38	7	7.9	4	4.5	46	51.7
Total	490	253	237	24	4.9	20	8.4	190	38.8

 

Results

The influenza-like illness occurring in December 1996 - February 1997 was reported by 5.5 % of the participants to this study while 2.2% of the participants reported to have suffered from the sore throat epidemics in March – April 1997.

Dengue 1 IgG antibody was detected in 24 of 490 (4.9%); dengue 2 IgG antibody in 20 of 490 individuals (4.1%); and 12 individuals had both antibodies (Table 1). None of the 242 patients born after the 1977 epidemic had dengue 2 IgG antibodies, but two of them (0.8%) had dengue 1 IgG antibodies. Dengue 1 or 2 and West Nile IgM antibodies were not detected in any of the sera tested. West Nile IgG antibodies were found in 190 (39 %) patients, with an increasing prevalence according to the age, reaching 51.7% in the ³ 45 year age group. Twenty-five individuals had both West Nile and dengue 1 or 2 IgG antibodies. Antibodies against Wesselsbron, Sindbis or Chikungunya were not detected in the studied population.

Discussion

Negative IgM results suggest an absence of recent dengue or West Nile infection in the population and no relationship was observed between serological results and reported clinical data. Symptoms of previous dengue-like illness were registered independently of the presence of Flaviviridae antibodies in blood specimens. Mild dengue or West Nile infection could be confused with influenza. If the studied population was considered representative of the population of the island, an estimate of at least 2 900 cases of influenza like illness would have occurred in December 1996 – February 1997 in the Seychelles. Follow-up of such outbreaks would be useful in the future for an etiology identification of the disease with influenza virus detection as reported in La Reunion in 1996 (10). Within the Flaviviridae family, antigenic cross-reactions between viruses are common and may explain some of the dengue antibody positive cases. In 1977, dengue cross reacting IgG antibodies were detected in only 7.4% of the possibly estimated infected population (6). The presence of IgG dengue antibodies in three individuals of 15, 18 and 19 years of age without travel history could indicate a possible low activity of dengue virus or other cross reacting flavivirus in the area.

West Nile antibodies had been previously recorded in the Republic of Seychelles in only 3.1% of the population tested in 1979 (6). West Nile virus is associated with birds and mosquito vectors, mainly from he Culex genera and has a widespread distribution in Africa. The virus is endemic in Madagascar and was isolated from humans, birds (parrots, egrets) and from Culex, Aedes and Anopheles mosquitoes (9). A similar situation may be observed in the Seychelles. The recent encephalitic forms of WN infections were described in non endemic areas (e.g. Algeria, Tunisia, Romania). The geographic distribution of JE and WN virus are different with the exception of several states in India where both viruses are circulating. Close relationships between WN and JE viruses do not permit to differentiate them by ELISA or either by cross-neutralization test (17). The main vector of JE and WN, Cx tritaeniorhynchus, is present in the Seychelles (2). Japanese encephalitis could be introduced in the Seychelles via migrating birds.

Otherwise, an absence of Chikungunya and Sindbis infections was observed in the studied population in 1997, but evidence of few human infections had been previously found in 1979 (6). Antibodies to Wesselsbron, other flavivirus present in Africa and Madagascar and inducing generally a mild influenza-like illness in humans was not recorded in the studied population.

The last dengue epidemic in the Indian Ocean was reported in 1993 in the Grande Comore island, Federal Islamic Republic of Comoros, in relationship with a high density of Ae. aegypti mosquitoes (4). The low density of Ae. aegypti mosquitoes in the Seychelles, in La Réunion and in Madagascar (S Laventure, personal communication) may prevent dengue outbreaks in the next future. However, the circumstances of its replacement by Ae. albopictus as reported vector in 1977 are unclear. The DHF case identified in Mauritius in 1996 emphasizes the importance of etiologic investigation of suspicious cases including hemorrhagic fevers (e.g. leptospirosis, arbovirus) and encephalitis, in consideration of travel history.

The mosquito control is a continuous mass movement, and the population education on source reduction of larval mosquito breeding sites must be enforced. Further investigations on arbovirus circulation in the Seychelles are needed, including surveillance of animal and bird populations, as well as potential vectors, mosquitoes and ticks.

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