In recent years Australia, including NSW, has seen increasing notifications of Dengue and Chikungunya virus infections(1). In addition, there have been significant Zika virus outbreaks in our region in early 2014. Dengue, Chikungunya and Zika viruses are spread by the same mosquito vectors and infections with these viruses present with similar signs and symptoms. The global increases in these infections, along with the significant potential for the introduction and spread of their mosquito vectors in Australia, make dengue, Chikungunya and Zika significant emerging viral threats to Australia, with the potential for outbreaks involving local transmission and the possibility of establishment in Australia.
Dengue, Chikungunya and Zika viruses result in infections with a broadly similar range of signs and symptoms including fever, maculo-papular rash and muscle, back and/or joint pain(2),(3). This means these infections are often difficult to distinguish from one another and highlights the importance of serological tests to confirm putative clinical diagnoses. There are however distinguishing features that are unique to each infection. Around one in twenty dengue cases develop into potentially fatal severe dengue which is primarily characterised by plasma leakage with or without haemorrhage. Chikungunya is associated with arthralgia symptoms, similar to those seen in other alphavirus infections like Ross River virus, which can last for months or years. Zika infections often involve conjunctivitis and are also associated with the development of severe autoimmune complications including Guillain-Barré syndrome.
No antiviral treatments are available for these infections, therefore treatment is supportive. With appropriate detection, monitoring and management even severe dengue has a fatality rate of less than one per cent(4). Prevention is therefore the most effective intervention, primarily via vector control and the avoidance of mosquito bites.
These viruses share common mosquito vectors, the most important of these being Aedes aegypti and Aedes albopictusii(2). Therefore the concerns around local transmission and increased vector range resulting in greater potential for the spread of outbreaks are common to all three viruses. Aedes aegypti is currently found in north Queensland, but historically Aedes aegypti was found as far south as Perth in Western Australia and the NSW/Victorian border(5). The global range of Aedes albopictus has been increasing and this invasive mosquito became established in the Torres Strait in 2005(6). The spread of Aedes aegypti and the potential introduction of Aedes albopictus to mainland Australia may create the circumstances where significant outbreaks of dengue, Chikungunya and Zika may be seen outside of north Queensland, including all the major population centres on the east coast.
Dengue is the most widespread of these viruses with an estimated 2.5 billion people living in countries with local dengue transmission. Around 50 million dengue cases occur each year with 20,000 deaths(4).
In the last decade the global range of Chikungunya has increased dramatically with the virus becoming established across much of Asia and parts of the Pacific(7). The first cases of local transmission of the virus in the Americas were reported in December 2013(8), and by the end of June 2014 almost 260,000 cases had been reported in 22 countries(9).
Zika virus came to prominence in 2007 with a major outbreak on the island of Yap(10). Although this outbreak was not associated with any deaths, subsequent analysis estimated that 73% of the population of Yap were infected in the course of the outbreak. In 2014, significant Zika outbreaks have occurred in French Polynesia, New Caledonia, the Cook Islands and Easter Island(11).
Dengue outbreaks were recorded in NSW, Queensland and Western Australia before 1955 with locally-acquired cases recorded as far south as Gosford in NSW(5). There were no dengue cases reported in Australia between 1955 and 1981. Dengue re-emerged in Cairns in 1981 and there has been dengue activity in north Queensland in most years since 1990. In recent years, the number of dengue notifications has increased, with the highest number of dengue cases recorded in 2013 (1,840 Australia wide, with 299 of those in NSW (acquired in Queensland or overseas); Figure 1a) (1). There has also been an increasing trend in Chikungunya notifications with the highest numbers recorded in 2013 (133 Australia wide, with 24 of those in NSW; Figure 1b) (1), although to date all have been imported cases. Zika virus infections are reported nationally within the Arbovirus infections (Not Elsewhere Classified) category. Imported Zika virus infections have been confirmed in Victoria, NSW and Queensland in the last 12 months(12),(13),(14).
Figure 1: Number of notified cases of selected arboviruses, Australia and NSW, 1991-2013.
Source: National Notifiable Diseases Surveillance System (NNDSS) (1).
a) Dengue notifications in Australia and NSW, 1991-2013
b) Chikungunya notifications in Australia and NSW, 2002-2013
The control of these emerging arboviruses will involve surveillance both of cases and mosquito vectors. Additionally, support to establish and maintain disease surveillance efforts in our region, such as the World Health Organization’s Asia Pacific Strategy for Emerging Diseases(15), will assist to identify and manage outbreaks that could be sources of imported cases to Australia.