There has been a "significant" increase in people arriving in New Zealand carrying dangerous viruses such as dengue fever, chikungunya and Zika over the two decades, scientists say.
And the warming climate makes an outbreak here increasingly possible.
Scientists at the University of Otago looked at 17 years of reports of arboviral infections in New Zealand, from 2001 to 2017. Arboviral diseases are spread by insects like mosquitoes, ticks and fleas.
They found more than half of the infections were found in the last four years, mostly in people who'd arrived from the Pacific Islands and southeast Asia.
"This involved a substantial increase in the number of dengue fever notifications, and the emergence of chikungunya and Zika," a new paper published in journal Travel Medicine and Infectious Disease says.
Dengue is spread by mosquitoes, and there is no known effective treatment or vaccine. Chikungunya is similar to dengue, and while typically less severe there is also no known cure or vaccine. Zika too is spread by mosquitoes, and while not lethal, it can cause severe birth defects if the victim is pregnant.
"A total of 1912 overseas-acquired cases of six arboviral diseases were reported in New Zealand between 2001 and 2017," the study says.
"Most of these (81.1 percent of cases) were dengue fever, followed by Zika (9.3 percent), chikungunya (7 percent) and Ross River virus (2.4 percent).
"Five cases of Barmah Forest virus (BFV) infection and one case of Japanese encephalitis (JE) were also reported."
Ross River virus, found in Australia and Papua New Guinea and spread by mosquitoes, causes arthritis and rash, but is non-lethal. The similar Barmah Forest virus is also from Australia, spread by mosquitoes and non-lethal.
Japanese encephalitis is also spread by mosquitoes - but it's deadly, with an estimated case-fatality rate of 30 percent. There is no cure, and it kills about 17,000 people a year across southeast Asia. It's a strange disease - an infected person doesn't carry enough of the virus for a mosquito to collect and infect another person. Instead it "exists in a transmission cycle between mosquitoes, pigs and/or water birds", according to the World Health Organization.
Between 2001 and 2017 the number of people arriving in New Zealand increased on average 4 percent a year, but notifications of arboviral infections outpaced that at 8 percent a year. Most were picked up in Auckland, with Samoa, Fiji and Indonesia the most common sources for dengue, while the Cook Islands and Tonga were the main sources of Zika.
"Before 2014, only one case of Zika and five cases of chikungunya had been notified. In the final four years of the 17-year study period however, more than half of all the arboviral cases (976 cases) were recorded, with an annual average of 244 cases.
"This increase accompanied an increase in the number of traveller arrivals, especially from the Pacific region, where some countries have experienced a high burden of mosquito-borne diseases, including the emergence of Zika and chikungunya infections, since January 2012."
Luckily, there was only a single case of locally-transmitted arboviral disease detected over those 17 years - a case of Zika that was sexually transmitted. But New Zealand has an "increasingly suitable climate for new vectors to establish" - in other words, the warming climate is making life easier for mosquitoes and other disease-carrying bugs.
We also have a few species of mosquito known to be "vector-capable" - as in, able to transmit disease - and a couple of others with "vector potential".
"Most... New Zealand travellers underestimate, or are unaware of, the travel-associated risk for mosquito borne disease," the researchers said.
"Improved education and awareness of the risk related to travel to endemic destinations (mainly Asia-Pacific region) is required to encourage travellers to seek pre-travel medical advice and adopt personal protective measures against mosquito bites. This, combined with awareness among local clinicians and adequate laboratory diagnostic capability, is crucial for the early detection of travel associated notifications and for the subsequent prevention of local transmission in New Zealand in general, and in the Auckland region in particular, if competent vectors established."
The researchers also looked at the likelihood of infected mosquitoes arriving on aircraft or shipping, but said strict biosecurity measures made that an unlikely source compared to asymptomatic and pre-symptomatic people, who could potentially come across the border without any scrutiny.
"When assessing the risk of introducing infectious disease viruses to non-endemic countries with a population that is predominantly immunologically naive, like New Zealand, the main surveillance effort, besides evaluation of local competent vector status, should be in the evaluation of the extent of imported disease notified by via travellers," the study says.