With Te Waihora/Lake Ellesmere just a few kilometres away, we’ve always had mosquitoes at our house. But since we installed a small pond, the mosquito population of our property has gone up (in spite of the fish, which do eat a lot of them).

Though mosquito identification is not my forte, most of the mosquitoes appear to be Culex pervigilans, the common house mosquito, or vigilant mosquito. Like most of our mosquitoes, this species is endemic to New Zealand.

Because of the isolated nature of New Zealand, few of the world’s nasty mosquito-borne diseases have ever arrived here, and no human diseases have managed to arrive and spread. But that doesn’t mean they couldn’t, if given the opportunity.

Whataroa virus is an Australian bird virus, which may also produce flu-like symptoms in people. In New Zealand, it has been found only in the area around the small West Coast community of Whataroa. It was first detected in 1962, and recent surveys show it has not increased in prevalence since then. In New Zealand, it is vectored by Culex pervigilans, which is common throughout the country.

So why hasn’t it spread?

Well, we don’t know for certain, but to get a clue, it’s worth looking at why other mosquito-borne viruses spread. Most emerging diseases come on the heels of environmental change—when the habitat changes, mosquito populations may increase (many mosquitoes do well in disturbed habitats), or the virus’ other hosts may increase or move around, spreading the virus. Environmental changes can be natural, like seasonal flooding; or human-induced, like cutting native forest (or digging a pond…).

Which brings us back to Whataroa, which hasn’t changed much at all over the past 50 years. There’s been no great increase in mosquito habitat, and little development that would favour the non-native blackbirds and thrushes that appear to be the reservoir for the virus. So the Whataroa virus has simply languished in place.

If the Whataroa virus had arrived at another location, it might have already spread far and wide (as avian malaria has done over the past 40 years). If a developer had come into Whataroa and built golf courses or fancy subdivisions, it might have spread. When the next big Alpine Fault quake happens, the resulting destruction will likely create new disturbed habitat for mosquitoes, blackbirds, and thrushes, and Whataroa virus might spread with them.

All sorts of variables determine whether and when a mosquito-borne disease becomes a problem. Some of these are known, and under our control, but many are either not understood or are out of our control. Sometimes, the only thing we can do is to react when trouble strikes.

Which is, of course, my excuse for swatting this mosquito when it landed on my arm this morning.