Feature

When will the Mosquitoes Strike?

Diseases carried by mosquitoes are on the increase all over the world. Can we keep them out of New Zealand?

Philip Weinstein

The last decade has seen a huge resurgence in diseases caused by arboviruses (arthropod borne viruses) -- viruses which are mainly carried by mosquitoes, and which enter the bloodstream when these insects bite. Dengue fever is rampant in South America, with the continent's first urban epidemics of potentially lethal forms of the disease; yellow fever is again killing thousands in Africa, despite the availability of an effective vaccine; and Japanese encephalitis virus is invading India and parts of Asia where it has never before been seen.

Closer to home, our nearest neighbour, Australia, suffered an outbreak of over 2,000 cases of dengue fever in north Queensland in 1993, and last year, Japanese encephalitis virus transmitted locally in the Torres Strait claimed its first lives ever amongst non-travelling Australians. Not only the Australian tropics were affected. Adelaide in South Australia suffered its greatest ever outbreak of Ross River virus, with over 800 cases in 1993. Again last year, Bateman's Bay in New South Wales had an outbreak of an arbovirus which has only relatively recently been discovered to cause human disease -- the Barmah Forest virus. As both of the latter outbreaks occurred at latitudes similar to those between Auckland and North Cape, a temperate climate is clearly no obstacle to arbovirus transmission.

Surprisingly then, there are no arboviruses causing human disease in New Zealand, nor is there any evidence that exotic arboviruses imported into New Zealand have ever been locally transmitted. Why is this?

Invading Mosquitoes

Before Maori settlement in New Zealand just over 1,000 years ago, there were no humans and virtually no mammals in New Zealand, and therefore no opportunity for a primate virus like dengue virus to co-evolve with its host and local mosquito vectors. There were birds however, and the avian arbovirus Whataroa virus probably circulated then as it does now in the wild bird populations of Westland, carried by the native mosquitoes Culiseta tonnoiri and Culex pervigilans.

These mosquito vectors co-evolved with the virus and its avian host over millennia, but again, no opportunity was available for such viruses and mosquitoes to adapt to human hosts. Although not studied in detail, it is thus unlikely that any of the 12 mosquitoes native to New Zealand are very competent vectors of human arboviruses.

Unfortunately, three exotic mosquitoes are now also well established in New Zealand, and all of these are theoretically capable of transmitting human arboviruses in this country. Terry Maguire at the University of Otago has shown that New Zealand populations of the exotic Aedes notoscriptus and Aedes australis are capable of transmitting dengue virus in the laboratory, with the latter also transmitting Ross River virus. The third exotic mosquito, Culex quinquefasciatus, has been incriminated in the transmission of various arboviruses overseas.

In 1993, Marshall Laird at the University of Auckland identified a new import, Aedes albopictus, which arrived with used tyres from Japan. This mosquito is an excellent vector of dengue virus and, worse still, is also cold-resistant. Fortunately health authorities were able to eradicate the species before it became established, but this incident highlights the fact that arbovirus vectors not currently in New Zealand also need to be of concern to us.

Back to our original question then: if there are mosquitoes capable of acting as arbovirus vectors in New Zealand, and outbreaks of arboviral disease have occurred in temperate climates in Australia, why have there been no outbreaks of human arboviral disease in New Zealand?

Sore Joint Epidemics

Part of the answer lies in the size of the virus pool which is available to the vector mosquitoes. To illustrate this point, let us look at Ross River virus, one of the most likely candidate arboviruses for transmission in New Zealand. The virus is native to Australia, where it circulates naturally in a kangaroo-mosquito-kangaroo cycle. The kangaroos are known as "reservoir animals", and they show no ill effects from the virus.

Should a vector mosquito bite a person however, that person might become clinically ill with characteristically severe joint pains as well as a range of other symptoms. The victim is typically debilitated for up to six weeks, but the disease may relapse over a period of months to years.

Sporadic cases of Ross River virus are common, occurring whenever and wherever there is an appropriate interface between reservoir animals, mosquitoes and humans. When a large number of people are infected, a human-mosquito-human epidemic cycle may become established, so the presence of reservoir animals is not a prerequisite for an outbreak.

Such a situation arose in the South-West Pacific in 1979, when an infected Australian tourist brought the virus into Fiji. Local mosquitoes fed on the virus-containing blood of this tourist, thereby acquiring the infection and then passing it on to local residents. An estimated 500,000 people were infected as the disease spread to other islands, either by infected travellers or vector mosquitoes. Many infected people came or returned to New Zealand, but apparently in numbers too small to lead to the establishment of local circulation of the virus.

Although the differing vector capabilities of local mosquitoes and the different climates in the Pacific and in New Zealand may partly account for the lack of transmission in New Zealand, a small virus pool and a lot of good luck are probably more significant factors. We were lucky that time, but what of the future?

Arboviruses in the "Greenhouse"

As travel and commerce grow between New Zealand and its neighbours, the risk of a local outbreak of an arboviral disease inexorably increases. South-East Asia, Australia and some Pacific Islands have several arboviruses to export to New Zealand in infected travellers, and the more travellers visiting the country, the greater the virus pool available for local mosquitoes to ingest.

Mosquitoes themselves (some of which can carry arboviruses with them) can also be imported, and the more frequently this occurs, the greater is the probability of local establishment. Increasing urbanisation and population growth in New Zealand compound the risk because non-immune people living at high density are irresistible to arboviruses looking to cause an epidemic.

Finally, there are the predicted changes to New Zealand's climate associated with increased global warming and rainfall. By the year 2050, temperatures here may rise by between 1.5^oC and 3^oC -- enough to warm surface waters and thus speed the development of mosquito larvae. Higher temperatures also allow arboviruses to multiply more quickly in adult mosquitoes, making them more infective more quickly. And because they digest and metabolise their blood meals more quickly at higher temperatures, mosquitoes are also likely to be hungrier in 2050, biting New Zealanders more frequently.

Rainfall is also predicted to increase here, particularly in the west. The availability of standing-water larval habitats will thereby increase, further assisted by the flooding caused by rises in sea levels. Increased humidity will accompany the rain, and for adult mosquitoes this is good news. Because they are rather gangly with a high surface-to-volume ratio, they often succumb to dehydration. At higher humidities they survive longer, and have more time to spread disease. The warmer, wetter climate is also likely to enable more tropical mosquitoes, such as the infamous Aedes aegypti, to become established in New Zealand.

The foregoing paints a rather pessimistic picture of hordes of hungry and infected mosquitoes inevitably transmitting human arboviral diseases in New Zealand. In short, the local transmission of arboviruses is probably a question of "when", not "if". However, this does not suggest that arboviruses in New Zealand are likely to cause plague-like outbreaks, nor even that they will become more than a nuisance as a public health problem -- providing, of course, that we maintain effective and efficient control, as with any other infectious disease.

Control in New Zealand

From a public health perspective, one of the cornerstones of infectious disease control is disease surveillance -- if you don't know what's out there, you can't intervene. Dengue fever and Ross River virus infection are therefore notifiable diseases in New Zealand, meaning that medical practitioners are obliged to notify health authorities of any cases they may see. Such notifications are likely to be the first real indication that the diseases are being transmitted locally, and will trigger public health interventions such as education campaigns about how to avoid being bitten and, in some areas, mosquito control.

Until this happens however, there is still a lot that can be and is being done to delay the problem as long as possible. The importation of further exotic vector species is being prevented by aircraft disinsection (insecticiding) and by the fumigation of imported tyres (which can harbour mosquito eggs). Regular surveys for introduced mosquitoes monitor the efficacy of these procedures.

Travellers are advised to avoid mosquito bites -- obviously so that they don't get sick, but also so that fewer infected blood sources are brought into the country. And what about the impact of climate change? In this regard, we can only try, both as individuals and as a nation, to contribute to the global effort of reducing the emission of "greenhouse" gases.

Philip Weinstein has recently been working at the James Cook University of North Queensland. He is now with the Public Health Group at the New Zealand Ministry of Health.