Review of vector-borne diseases in Hong Kong

doi:10.1016/j.tmaid.2010.01.004

Travel Medicine and Infectious Disease

Volume 9, Issue 3, May 2011, Pages 95-105

https://doi.org/10.1016/j.tmaid.2010.01.004 Get rights and content

Summary

The epidemiology of vector-borne diseases in Hong Kong has changed over the past decade but still poses a significant public health risk. We provided a comprehensive review of the epidemiological information and analysed the trends of major vector-borne diseases, including the vector situation in Hong Kong. The incidence of malaria has dropped dramatically in the past few decades and is now mainly an imported disease acquired from malaria endemic countries. Locally acquired dengue fever occurred in 2002 and 2003, and thereafter all cases were imported, mainly from Southeast Asia areas. Only a few local cases of Japanese encephalitis were reported in the past decade. In contrast, there is a notable increase in scrub typhus and spotted fever cases. The emergence of chikungunya fever in Asia and Indian Ocean countries also resulted in importation of human cases. Given the heavy traffic between this international city and other parts of the world, as well as the presence of vectors in this densely populated area, vigilance should be maintained against these infections. Comprehensive public health measures encompassing disease surveillance, vector surveillance and control measures with support from all sectors of the community are required to combat the old and newly emerging vector-borne diseases in Hong Kong.

Introduction

Vector-borne diseases have become increasingly important in recent decades. Malaria is a public health burden for many countries and control of malaria was listed as one of the Millennium Developmental Goals by the United Nations in 2000.¹ Dengue fever is a regional concern in Southeast Asia where high activity is reported cyclically.² Emerging infections such as West Nile virus infections and chikungunya have caused large outbreaks in both developed and developing countries in recent years. The complexity of controlling vector-borne diseases is due to the variety of micro-organisms causing the diseases including viruses, bacteria and parasites, intermingled relationship between the agents, vectors, environment and human population.

In Hong Kong, there are 47 notifiable diseases under the Prevention and Control of Disease Ordinance and more than one-fifth (10/47) are vector-borne diseases. These include chikungunya, dengue fever, Japanese encephalitis (JE), malaria, plague, relapsing fever, typhus and other rickettsial diseases, viral haemorrhagic fever, West Nile virus infection and yellow fever. No case of plague, yellow fever or relapsing fever has been reported for decades. No case of West Nile virus infection or viral haemorrhagic fever, other than dengue haemorrhagic fever, has ever been reported to the Department of Health. This article therefore reviews the current and recent vector-borne disease situation in Hong Kong and focuses on dengue fever, malaria, JE, chikungunya, typhus and other rickettsial diseases.

Section snippets

Dengue fever

Dengue fever has been a statutory notifiable disease in Hong Kong since 1994. The annual number of notifications in Hong Kong ranged from 3 to 58 (Fig. 1). In 2007, many Southeast Asian countries experienced an unusual rise in dengue activity. Hong Kong also recorded the highest number of cases (58) since it became a notifiable disease. From 1994 to 2008, there were a total of 358 notifications, affecting 193 males and 165 females. Most of the cases were adults and only 12 cases were aged below

Malaria

Malaria was endemic in Hong Kong before as well as soon after the World War II. At that time, malaria accounted for a substantial proportion of the morbidity and mortality in Hong Kong. During the 1960s and 1970s, the number of malaria cases declined significantly and the drop was attributed to the change in vector ecology that was brought about by urbanisation, together with enhanced surveillance and strengthened vector control measures in the territory. In 1983, a cluster of 19 malaria cases

Japanese encephalitis

JE became a statutory notifiable disease in Hong Kong on 16 July 2004. Before this, the number of cases reported to Department of Health voluntarily by doctors ranged from zero to five cases each year (Fig. 7). Before the 1980s, almost all reported JE cases were acquired locally. Most cases were thought to have contracted the disease in the New Territories where migratory bird roosts, local pig farms and rice fields were found.

In the past decade (1999–2008), 13 cases were reported affecting

Chikungunya fever

In the past two decades, chikungunya outbreaks have re-emerged in some African countries such as Democratic Republic of Congo, Ivory Coast and Senegal, and in Asian countries including India, Indonesia, Malaysia, Sri Lanka and the islands of the Indian Ocean.⁷ Local outbreaks were reported for the first time in Italy in 2007 and in Singapore in 2008.8, 9, 10 Hong Kong has reported several imported cases but no locally acquired cases.

The vector is present in Hong Kong and there is a risk of

Typhus and other rickettsial diseases

Rickettsial diseases are caused by Gram-negative obligate intracellular bacteria in the family Rickettsiaceae. Patients present with a systemic febrile illness and may have a rash. These vector-borne diseases are transmitted by the bite or faeces of infected arthropod vectors. Scrub typhus (Orientia tsutsugamushi) is transmitted by the larval stage (chigger) of trombiculid mites and the geographic distribution comprises Asia and the Pacific region. Spotted fever may be caused by over 30 species

Discussion

Comprehensive public health prevention and control measures including disease surveillance, laboratory capacity, vector surveillance and control, public health education and preventative measures for travellers contribute to a successful vector-borne disease control programme in Hong Kong:

Conclusion

The epidemiology of vector-borne diseases in Hong Kong has changed over the past decades. Many cases are now imported from endemic areas. Persistent vigilance against vector-borne diseases is essential since multiple factors favour the spread of communicable diseases in this densely populated international city. In Hong Kong we rely on effective surveillance systems, early detection and diagnosis of cases and stringent vector surveillance and control. Sustained efforts from, and partnering

Conflict of interest

The authors have no conflict of interest to declare. None of the authors received funds for this review article.

Acknowledgements

The authors thank the Centre for Health Protection for organising this consensus meeting and for the excellent secretariat support. We also thank Dr. PY Lam, Director, Department of Health, Hong Kong, and Dr. Thomas Tsang, Controller, Centre for Health Protection, Department of Health, Hong Kong for granting approval to release the material for publication.

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Chikungunya virus (CHIKV) and Zika virus (ZIKV) have been present in Southeast Asia and South Asia since the 1950s. Before 2005, CHIKV in Asia was characterized by sporadic outbreaks followed by years of apparent inactivity. Since 2005, Asia has experienced widespread outbreaks, which expanded the known distribution of CHIKV to new countries and new regions, such as the Middle East and the Pacific. The epidemic East/Central/South African genotype replaced the endemic Asian genotype in many countries; however, it was the Asian strains that spread to the Americas via the Pacific, a route that ZIKV would also later take. ZIKV remained neglected until the first recorded outbreak in the Federated States of Micronesia in 2007. Clusters and sporadic cases of ZIKV are now reported throughout Southeast Asia. New and existing arboviruses continue to have major public health impact on Asia, where ideal conditions exist for transmission by the main vectors Aedes aegypti and Ae. albopictus. The history and epidemiology of these arboviruses in Asia will be reviewed in this chapter.
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Over the years, Hong Kong has been non-endemic for DENV, with almost all documented cases having been imported. Two self-limiting indigenous outbreaks occurred between 2000 and 2014, affecting 20 residents.5,6 However, the risk of progression should not be underestimated, because of the close geographic proximity of Hong Kong to hyperendemic countries, as well as the extensive spread of Aedes albopictus mosquitoes, the only major vector for DENV found locally.6
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Past dengue exposure of the general population was examined by dengue virus (DENV) IgG testing of archived samples from voluntary blood donors. Vector intensities were determined by local ovitrap index (OI). Analyses were made in the context of population statistics at both the district and sub-district level.
The overall prevalence of DENV IgG was low at 2.25%. Positive donors were more likely to be older, non-Chinese, and female. Neither the OI nor the location of residence was associated with DENV serology. The sub-district level OI was clustered, but no correlation could be confirmed with the location of residence of positive blood donors.
The cumulative exposure of Hong Kong residents to dengue has so far been low. Coupled with the lack of a spatial relationship between exposed cases and vector intensities, a high risk of local transmission of DENV is not supported. The apparently higher exposure likelihood of females could be explained by past infection in workers from dengue endemic countries, while frequent travel could have exposed older adults to DENV. Continued surveillance, risk assessment, and intensive vector control remain essential to prevent the transformation of a non-endemic to an endemic city.
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The large fluxes of visitors and cargoes are susceptible to the risks of importing infected vectors and humans. The geographical invasion of pathogens can infect local mosquito populations and induce secondary local transmission [53]. A local university campus was selected as a representative study area (Fig. 1).
Urbanization engenders habitats with distinctive and often adverse microclimates. Green spaces are actively incorporated in urban designs to make cities resilient against climate-change, disease spread and other environmental challenges. Adoption of green roof as an innovative urban green infrastructure has been hindered by the perceived disservice of attracting urban pests like mosquitoes. This study evaluated important microclimatic attributes that explain vector abundance differences between urban extensive green roofs and two control site types in humid-subtropical Hong Kong. Mosquito-trapping data were collected fortnightly for a year (March 2015 to 2016) from seven sites representing three experimental treatment groups: green roofs (GR), low-elevation blue-green spaces as positive controls (PC), and bare roofs as negative controls (NC). Concurrent ambient microclimate parameters were measured on site. Human-biting female mosquitoes of species known to transmit infectious diseases locally and globally constituted the vector abundance data. Generalized linear models evaluated the effects of site-specific microclimatic factors on abundance. Time-lagged microclimatic measurements were analyzed to reveal latent effects. The models consistently identified temperature and wind speed as significant factors. Wind speed accounted for differences in abundance across site types, while temperature exerted its effect on abundance chiefly through seasonal variation. Contrary to common preconception, urban green roofs had significantly lower vector abundance relative to controls. High wind exposure on elevated buildings, which acted as a deterrent of mosquito flight, was found to be the underlying microclimatic factor. Our findings could inform urban greening design and policy, as well as disease control and transmission risk assessment.
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Epidemiological investigations were performed for all reported laboratory-confirmed JE cases, and the cumulative incidence was calculated for the surveillance years 2007–2010.
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Review of vector-borne diseases in Hong Kong

Summary

Introduction

Section snippets

Dengue fever

Malaria

Japanese encephalitis

Chikungunya fever

Typhus and other rickettsial diseases

Discussion

Conclusion

Conflict of interest

Acknowledgements

Med Maladies Infect

Health in the millennium development goals

Press release “WHO ALERT: prompt action needed on dengue”

Local dengue fever cases in 2002

Public health Epidemiol Bull

Review of dengue fever cases in Hong Kong during 1998 to 2005

Hong Kong Med J

Centre for health protection, Department of Health, Hong Kong. Human JE vaccination – considerations for special groups

Centre for health protection, Department of Health, Hong Kong. Japanese encephalitis in Hong Kong situation update, prevention and control strategy

Changing patterns of chikungunya virus: re-emergence of a zoonotic arbovirus

J Gen Virol

Joint ECDC/WHO European risk assessment on chikungunya fever in Italy: a mission report

Press release on February 6, 2008: update on chikungunya fever cases (5)

Press release on August 2, 2008: chikungunya fever cases detected at Kranji Way

Department of health, Hong Kong. Press release on statutory reporting of chikungunya fever and EV 71 infection

Outbreak and spread of chikungunya

Wkly Epidemiol Rec