Abstract
Background: Although New Zealand has not yet reported local transmission of a mosquito-borne virus (arbovirus), several risk factors are present. While current literature is inconsistent on the risk status, to date, no evidence-based approach has been used to assess major risk factors and elements likely to facilitate local transmission.
Aims: 1) To identify and evaluate the current status of major risk factors and favourable conditions likely to facilitate local transmission of arbovirus infections in New Zealand
2) To outline likely scenarios and identify combinations of circumstances under which the elements of arbovirus transmission and related risk factors might coincide to trigger a local outbreak
Methods: Information on arbovirus notified cases and traveller arrivals were used to evaluate the risk of arbovirus introduction via viraemic travellers by describing trends in importation rates, seasonality and route of travel and likely origins of infection. Information on mosquito interception records and air and sea freight imports, and international passenger flight arrivals was obtained to assess the risk of exotic mosquito introduction and establishment by describing trends in the border interceptions and the role of imported goods and air travel as a mean of transport. Using occurrence records obtained from biosecurity surveillance programmes, country-scale environmental suitability maps for Aedes notoscriptus were created to evaluate the role of this species in potential local Ross River virus (RRV) transmission. Trap surveillance mosquito counts were examined for association with selected environmental factors to identify the environmental conditions favourable for local mosquito biting activity and, therefore, disease transmission.
Results: The risk of an arbovirus outbreak in New Zealand exists and is exacerbated. This is supported by the following findings:
i. The increases in imported arbovirus cases: a total of 1912 reported cases between 2001-2017, with more than half of these were reported in the final four years and dengue cases comprised 81.1%.
ii. Regular interception of exotic mosquitoes at the borders: Aedes (Stegomyia) aegypti (Linnaeus) and Ae. (Stegomyia) albopictus (Skuse) were the two most commonly intercepted species,
iii. Ongoing growth of international trade and travel with arbovirus endemic and/or mosquito vector-established countries: a 71% increase in the number of international flights to New Zealand between 2001-2017 and a ten-fold increase in the number of flights from Queensland, where Ae. aegypti is established and RRV is endemic.
iv. Ongoing expansion and widespread distribution of Ae. (Finlaya) notoscriptus (Skuse) and its occurrence in relatively high counts in different types of habitats of New Zealand.
Conclusions: Dengue and Ross River virus diseases are the most likely infections to cause local outbreaks. All requirements to initiate a local Ross River virus epidemic exist, while the absence of a competent vector is the limiting factor for dengue virus transmission. Auckland is the most likely point of virus and vector entry. The most likely mode of virus introduction is viraemic travellers entering the country through Auckland from Australia and/or the Pacific Islands, while Australia, the Pacific Islands and Japan are the most likely source of the vector. The northern North Island (where the number of adult Aedes notoscriptus collected was 40 times more than that collected from all other areas) is the most likely receptive location for introduction of RRV, while Auckland (where Ae. albopictus and Ae. aegypti were intercepted) would be receptive to dengue. Raised awareness among travellers, medical and public health authorities on arbovirus diseases, country-scale mosquito surveillance programmes, and a national early warning system would reduce the risk of a local epidemic.