THE current dengue outbreak in parts of Northern Queensland threatens to develop into a major epidemic. As of the middle of this week there have been more than 240 cases of dengue notified in Cairns and Townsville, and recent heavy rains and flooding have greatly raised local concerns about the breeding of mosquitos and spread of the disease.

Disturbingly, the Townsville outbreak involves a new strain of the disease, to which the local population would have little immunity.

Dengue remains one of the major mosquito-borne threats to human health in the world today. Probably something of the order of 3 billion people worldwide remain exposed to the infection, and the World Health Organisation estimates that there are at least 50 million cases of dengue a year. As yet no cure or specific treatment for the disease exists.

A world pandemic of dengue commenced soon after the end of World War II, and by the 1980s had spread to involve much of South East Asia and the Pacific.

Dengue in Australia, however, has a much longer lineage. In Australia, dengue epidemics have been part of the public health scene for at least 130 years, and probably longer.

Most of these outbreaks have been confined to Queensland and Northern and North-Western Australia, although on a number of occasions the disease has extended down into northern NSW.

Since the end of the 19th century Australia has experienced some of the most extensive dengue epidemics in the developed world, including 10 major epidemics between 1897 and 2003-04. Many of Australia's dengue epidemics remain memorable for their impact and spatial distribution.

The epidemic of 1904-05, for example, produced more than 200 deaths in Queensland and northern NSW, and at least 100,000 cases in Brisbane. Possibly 70 per cent of Brisbane's population caught dengue during this epidemic.

The 1925-26 epidemic remains one of Australia's most significant and geographically widespread. In a little over four months it caused 147 deaths, more than half a million cases and spread to involve much of Queensland and as far south as Sydney and west to Bourke.

In parts of Queensland and northern NSW, up to 75 per cent of the population living in towns and cities caught dengue during this epidemic. This epidemic caused a major dislocation to social and economic life, produced tremendous human suffering and remains to this day, one of Australia's most significant public health crises.

Another major epidemic occurred in 1941-43 involving parts of the Northern Territory, Queensland and northern NSW. Possibly up to 85 per cent of the population was affected in some localities. Up until the late 1940s, dengue outbreaks were a regular and widespread feature of life in northern Australia.

Despite the deaths of more than 1000 Australians from dengue in this period, in terms of the impact on the general population, many of these outbreaks were fairly mild and the disease came to be seen not as a serious threat to health so much as a more-or-less ubiquitous nuisance and part of life in tropical Australia.

After 1943 Australia was thought to be largely free of dengue until the mid-1950s, when a major epidemic occurred in Townsville, producing at least 15,000 cases and extending from Rockhampton west to Barcaldine, and south to Gladstone and Biloela.

More than 25 years then elapsed before the next major outbreak in 1981-2, when approximately 3000 cases occurred in northern Queensland. In 1992-93 another epidemic broke out in Townsville and Charters Towers, and thereafter epidemics have occurred roughly every five years.

Dengue is an acute febrile disease caused by infection with a Flavivirus transmitted by the bite of an infected mosquito commonly of the Aedes aegypti species. Endemic throughout much of the tropical world, dengue is usually regarded as a debilitating illness of relatively short duration with a high attack rate but low mortality. Typical uncomplicated dengue is a two-phased illness with an incubation period of between three and 15 days, characterised by fever, headache, pain behind the eyes, pain in the muscle and joints and a characteristic red rash on the trunk and extremities.

The severe joint pains associated with the disease led to it being referred to as "breakbone fever" -- a term commonly used in the earlier part of last century. In classical dengue the acute illness last for about a week and one exposure confers immunity to the particular sub-type.

A great mimic of other diseases, dengue is often mistaken for influenza, measles, malaria or typhoid. A much more severe and life-threatening form of the disease -- dengue haemorrhagic fever -- involves internal bleeding and lowered blood pressure, sometimes culminating in clinical shock leading to death.

So far Australia has been free of this form of the disease, although it is possible that there may have been a few cases during the 1897-98 epidemic in Queensland.

The search for a dengue vaccine has preoccupied science since the 1940s, so far to no avail -- although some international research teams have recently been claiming some success, and are reportedly predicting vaccines may be available in as little as five years' time. More recent research has turned to developing a bacterial agent aimed at disrupting the life cycle of dengue mosquitos.

Dengue is not endemic in Australia although its common vector, the mosquito Aedes aegypti, is. Over the last century the disease has followed a pattern of periodic multiple invasions from outside Australia, mimicking the ebb and flow of immigration and tourism. Since the 1950s, outbreaks of the disease have come to rely on an infected tourist or traveller visiting Australia, being bitten by a local mosquito and from such an encounter, many epidemics have emerged.

Many infected people suffer only minor symptoms and are unaware that they are carrying the disease.

While dengue can also be spread by other mosquitos, particularly in northern Australia, Aedes aegypti remains the vector of most concern in Australia because of its widespread distribution and its ability to comfortably adapt to living close to humans. The mosquito is principally a container breeder and has become virtually dependent upon humans for providing larval habitats such as rainwater tanks, drums, tins, tyres, blocked roof guttering, plastic cartons, vases, plates and bird baths.

The origins of the species are open to debate. It is possible that the mosquito arrived on Australia's northern coast prior to European settlement with Malay voyagers. Equally possible is that the mosquito may have accompanied the immigration of gold miners and other immigrants to Queensland in the 1860s and 1870s.

Whatever the route, by the beginning of the 20th century the species was well established and widely distributed over northern Australia, Queensland, NSW and Western Australia. After 1950, however, the geographical distribution of the species contracted to parts of northern Queensland.

It is perhaps interesting to speculate what impact climate change might have on the current distribution of the mosquito and that perhaps its distribution may again extend south into NSW. On the other hand, the changing distribution of Aedes aegypti probably owes more to changes in technology and human behaviour than to climate.

These include the extension of reticulated water supplies and the decline of domestic water tanks; the emergence of refrigeration as an alternative to Coolgarde and Coolsafe safes as a means of keeping food fresh and cold; the related disappearance of ant pots, the replacement of steam trains with their fire buckets and water tanks by diesel trains, and the decline of coastal shipping -- allied to increased public health surveillance; and the widespread use of insecticides. These factors have helped remove potential breeding sites.

But the disease pendulum is finely balanced. The opportunity for container breeding, such as offered by rubbish in backyards and public places, blocked roof guttering and other water containers, remains high. In addition, surveys reveal a very high percentage of mosquitos breeding in indoor and greenhouse plants, in saucers of water under pot plants, in old discarded tyres, drums and bird baths.

It would appear that the plastic container and bird bath now play the role that the fire buckets, water tank and ant pot did in an earlier age. There seems little doubt that throughout its history, Aedes aegypti has followed the path of human settlement. The recent pattern of population movement to more rural blocks in Queensland and the Northern Territory, and the re-introduction of rainwater tanks may hold considerable significance for the future distribution of this mosquito and the disease it transmits.

Finally, prolonged periods of drought and/or water shortage throughout parts of eastern Australia are beginning to see the reappearance of rainwater storage tanks both as a means of garden water supply and for other purposes. Under such circumstances the potential for a reintroduction of Aedes aegypti from Queensland to NSW and perhaps further afield, remains a distinct possibility.

(The Australian, 30/01/2009)
Peter Curson is professor in population and security at the Centre for International Security Studies, the University of Sydney Sydney, and emeritus professor in medical geography at Macquarie University