Climate change: NZ's mozzie-borne disease risk to rise

A species such as Aedes albopictus - or Asian tiger mosquito - becoming established in New Zealand could be a worst case scenario. Photo / File

It's a familiar New Zealand story, the havoc wreaked by all the pest species we've brought in. But in the case of mosquitoes, the worst might be yet to come. There were 12 mostly bird-nibbling mosquito species in pre-European times. Since our arrival, three new species have become permanently established here. We've stopped dozens more at our ports, and eradicated one. Climate change will make that border control more difficult, says University of Auckland Senior Research Fellow Jose Derraik. And if new species arrive, an outbreak of mosquito-borne disease in New Zealand becomes ever more likely.

What's a worst case scenario of mosquito-borne disease in New Zealand?

If a species such as Aedes albopictus - the Asian tiger mosquito - or Aedes aegypti - known as the yellow fever mosquito - became established here.

These are two of the most important mosquito vectors of human disease worldwide.

Under the climatic models that have been done in the past, climate change would make Aedes aegypti more likely to become established.

However, due to temperature limitations it would probably be restricted to the northern tip of New Zealand.

But Aedes albopictus is one of the world's worst invasive species, and it could establish under our temperate climate.

Importantly, it's an efficient vector of a number of important human pathogens, such as dengue fever viruses or Zika virus, and it can breed in a wide range of microhabitats in close association with humans (eg used tyres, buckets).

This is a species that would be of major concern, because it would greatly magnify the likelihood of an outbreak occurring in our country.

Aedes albopictus is one of the reasons why we need one of the strictest biosecurity controls in the world at our borders.

In the modern world we now have rapid movement of people from one country to another.

New Zealand has a constant inflow of arrivals - New Zealanders returning from overseas as well as foreign visitors - and invariably some of these travellers enter the country infected with a virus acquired overseas.

If one of these people is then "bitten" by a suitable mosquito vector an outbreak could eventuate.

This isn't a fictitious scenario, as this is exactly what happened with a major outbreak of Ross River Virus in Fiji in 1979.

Hardly anyone in New Zealand has been exposed to mosquito-borne viruses, so our population would be highly susceptible to infection, and a disease could spread rather rapidly under the right conditions.

It is important to point out that New Zealand is a very unique place.

If one disregards Antarctica, we are the only country in the planet where there has never been a single confirmed case of disease transmission from mosquitoes to humans within national borders.

If this is possible, why hasn't it happened already?

The key issue is that we don't have very efficient mosquito vectors established in New Zealand, yet.

Things have changed a lot in the past century.

Aedes albopictus is one of the world's worst invasive species. Photo / File

If you think about New Zealand before humans arrived, the only land mammals that we had were bats (which happen to eat a lot of mosquitoes).

There were only 12 native species of mosquito - that's a very low number compared to similar-sized temperate countries - and although some will feed on most warm-blooded animals, they had evolved primarily to feed on birds.

When humans came we completely changed New Zealand's environment: we brought in a lot of land mammals, we wiped most of our forests out, and we accidentally brought in three exotic species of mosquito.

Culex quinquefasciatus and Aedes notoscriptus are very good mammalian feeders which could become disease vectors in New Zealand.

Aedes notoscriptus in particular, is a very aggressive feeder.

It'll breed in any object that can harbour stagnant water, so is very well established in urban areas around Auckland.

Currently, this is probably the mosquito that could most likely lead to an outbreak in New Zealand.

It has been shown to play a role in vectoring diseases in Australia.

But it's not a very efficient vector of viruses to humans.

This issue, combined with a relatively low level of virus-infected travelers, means this scenario hasn't played out in New Zealand yet.

Our biosecurity measures have successfully stopped more than 30 other species from establishing in the country by intercepting them at port entry points - including both the Asian tiger mosquito and Aedes aegypti.

The southern saltmarsh mosquito Aedes camptorhynchus, which can transmit the Ross River virus, was found in 1998 and became established in certain areas of the North Island. Fortunately, it seems that the species was successfully eradicated in 2010.

How will climate change increase these risks?

When I did my Masters degree in Dunedin, one of my mentors, Professor Alan Mark, commented that ideally the main entry port into New Zealand should be Dunedin.

If all goods and most of the people arrived in New Zealand down south, we'd have a much lower number of exotic species becoming established here.

This is because the colder climate would act as a barrier, and likely prevent many exotic species from getting a foothold here.

Climate change and warmer temperatures will create conditions that are more suitable for important mosquito vectors to become established in New Zealand.

Aedes albopictus is one of the reasons why we need one of the strictest biosecurity controls in the world at our borders. Photo / File

With warmer temperatures "moving" southwards, it's going to make it easier for new species to establish around other ports of entry further south, so that maintaining stringent border controls becomes more complicated and expensive.

In addition, because of the temperate climate of New Zealand, mosquito vectors have a relatively narrow window period to reach higher densities, and this would likely be extended with climate change.

Further, warmer temperatures would also improve conditions for virus replication and transmission in New Zealand soil.

I believe most researchers in this area would say it's not a matter of "if", but "when" an outbreak will happen here.

There's a term "invasional meltdown", describing how animals like possums actually make this situation worse too. Can you explain that?

Yes, if you think of the scenario of how New Zealand land cover has changed, most of our forests have been replaced by urban areas and farms.

Native New Zealand mosquitoes were mostly adapted to live in a forest environment.

When I did my PhD, for example, it was very clear that if you went further into the Waitakere Ranges, you would not find a single exotic mosquito species.

But when you go to highly modified environments there is marked a shift towards exotic species.

So, possums, cattle and humans are favoured by exotic mosquitoes as sources of mammalian blood, which is generally preferred by mosquito vectors that could establish in New Zealand.

Further, animals such as possums have a marked environmental impact, and if left unchecked they could also aid exotic mosquito invaders through the degradation of our forest habitats.

In some cases, these introduced mammals could also act as reservoir hosts for human pathogens.

Possums, for example, are potential hosts of Ross River virus.

If exotic species are taking over from the natives, how are our native mosquitoes faring?

Well there is at least one native species that may actually be threatened.

Maorigoeldia argyropus is a very large (and rather beautiful) mosquito that is particularly interesting because it seems to have lost its ability to feed on blood.

There are no records of this species doing so in the wild, and all laboratory attempts to induce feeding on blood failed.

This is one native mosquito that is well adapted to a forest environment, and there is some evidence that the species appears to be restricted to relatively large and mostly pristine areas of native forest.

It seems that due to its strong association with such environments, Maorigoeldia argyropus has been disappearing from many areas throughout the country as the original native forest has been progressively removed.

• This interview by Lynley Hargreaves first appeared on Sciblogs.co.nz and was supported by Royal Society Te Aparangi.