An integrated response to the In December 1998, the southern saltmarsh mosquito, establishment of the exotic Aedes camptorhynchus, was identified for the first time in New Zealand and an established population mosquito Aedes (Ochlerotatus) confirmed at Napier. Nationwide surveillance has camptorhynchus in Napier detected larvae at other sites on the east coast of the North Island and in Northland. Eradication has been The southern saltmarsh mosquito, Aedes (Ochlerotatus) successful at Napier, and has commenced camptorhynchus, is an aggressive biting mosquito and therefore a at the other east coast sites. A treatment significant public nuisance. In Australia it is a vector of three programme has commenced in pathogenic alphaviruses: Ross River virus(1), Barmah Forest virus(2) and Sindbis (Okelbo) virus. It has also been found carrying the Northland to contain, but not eradicate, flaviviruses Kokobera virus(3) and Murray Valley encephalitis virus. the mosquito. Ross River virus causes a significant, non-fatal but potentially Graham Mackereth, Co-author chronic and relapsing disease in humans(4), and its vertebrate host range also includes marsupials(5), dogs, horses, cattle, pigs and Eradication fruit bats(6). S-methoprene, which prevents the emergence of adult mosquitoes In 1999, after its detection at Napier, a serological survey of from the pupae, was the principal method used to eradicate the 353 animals, including horses, cattle and possums living near the mosquitoes at Napier. S-methoprene granules were applied by air at mosquito infested area, found no antibody to alphavirus or 6 kg per hectare, and pellets were used in deeper drains. Because flavivirus(7). The Ministry of Health surveyed 53 mosquito-bitten eggs can survive for long periods, a chemical with prolonged people from the Hawke’s Bay region for antibody to Ross River activity was required. S-methoprene was chosen because it is virus, and re-tested 20 later to detect any subsequent released slowly from granules and pellets. Effective residual levels of seroconversion. There is no evidence that the exotic mosquito active ingredient were maintained for a 20-month period by introduced any pathogenic viruses. application of granules every 21 days, and every 30 days for the pellets. Sites subject to variable water levels were monitored to In the presence of an established southern saltmarsh mosquito ensure that residual S-methoprene levels were maintained, and that population, one viraemic person bitten by a mosquito could result dry areas were not over-dosed. in an epidemic of Ross River virus, as is thought to have occurred in Fiji from a viraemic traveller(8). Failure to control the mosquito in The bacterial insecticide Bacillus thuringiensis var israeliensis (Bti) New Zealand could eventually result in Ross River virus becoming was used for initial containment. In the eradication programme, endemic, with recurring epidemics of disease. ground teams applied it in situations where small-scale hand-held spray-pack applications were required. The route of entry of the mosquito into New Zealand is not known. It may have resulted from windborne dispersal from coastal Where possible, potential habitat was modified to make it Australian habitats, been introduced in the cabin or cargo space of unsuitable for larval development by filling depressions, clearing an international aircraft, or in association with soft-top container- blocked or ponded drainage channels, and maintaining water levels loaded ships and live sheep ships from Australia. in other channels. Habitat modification and Bti were used on an organic farm as an alternative to S-methoprene. Response The Ministry of Health managed the Napier mosquito incursion Ecological monitoring with advice from a Technical Advisory Group (TAG) and invited Several non-target aquatic species, including damsel flies, experts. The field response was managed by Healthcare Hawke’s Bay chironomids, mysids, seed shrimps and trout, were tested for at a Mosquito Response Centre in Napier. The main components of susceptibility to S-methoprene. Acute mortality tests were carried the response were mosquito surveillance and mosquito eradication. out using the standard field dose and 10 times the field dose. The standard dose caused no mortality in the monitored non-target The life cycle of Ae camptorhynchus is not completely described(9). species(10). Chironomids were susceptible at 10 times the standard The eggs, which are laid on moist mud above the water level, are dose, which was expected as S-methoprene is used overseas to resistant and survive desiccation for a long period. Flooding triggers control chironomids(11). Chironomids are present only in fringe the eggs to hatch. Larvae have been found in fresh (in Australia), areas of habitat in Hawke’s Bay(10). brackish and salt water. There are four larval instars before the pupal stage. Development from eggs to adults can occur in two to An expert review committee, which met in Napier to review eight weeks. When adults emerge from pupae they fly to find eradication progress in February 2000, concluded that since the shelter in vegetation, which can be some distance (5-8 km) from start of the programme there had been a 99.96% reduction in the breeding sites. adult saltmarsh mosquito population. Larval populations showed a page 14 Surveillance 28(3) 2001 during routine surveillance at Muriwai, south of Gisborne. Intensive surveillance was undertaken of the east coast from Wellington to Hick’s Bay. In October 2000, further infestation was found at the Wherowhero lagoon 20 km south of Gisborne and at a small site at Sponge Bay just north of Gisborne. An infested site was identified at the Maungawhio Lagoon, Mahia and 80 km south of Napier at Porangahau. Nationwide surveillance detected the mosquito at Kaipara in February 2001 and Mangawai in April 2001. The map shows the known locations of the mosquito. Eradication measures at Napier, Gisborne, Mahia and Porangahau are continuing. Containment and control has commenced at Kaipara and Mangawhai. Acknowledgements The authors would like to acknowledge Sally Gilbert and Andrew Kilometres Forsyth, Ministry of Health, Steve Garner, Healthcare Hawke’s Bay, and Bryn Gradwell, Southern Monitoring Services. Location and date of detection of Aedes camptorhynchus as at April 2001. References similar decline in both numbers and positive sites. Adult and larval (1) Ballard JWO, Marshall ID. An investigation of the potential of Aedes camptorhynchus (Thom.) as a vector of Ross River virus. Australian Journal of numbers dropped rapidly in the first five months of the Experimental Biology and Medical Science 64,197-200, 1986. programme. Flooding 10 months into the programme resulted in (2) Lindsay MD, Johansen CA, Smith DW, Wallace MJ, Mackenzie JS. An outbreak of Barmah Forest virus disease in the south-west of Western Australia. the hatching of eggs (presumably on higher ground than Medical Journal of Australia 162, 291-4, 1995. previously) and a small increase in trapped adults and larvae that (3) Russell RC. Vectors vs. Humans in Australia – Who is on top down under? An month. The last adults were detected in April 2000, and the last update on the vector-borne disease and research in Australia. Journal of Vector Ecology 23,1-46, 1998. larvae in July 2000. (4) Boughton CR. Australian Arboviruses of Medical Importance. A Handbook for General Practitioners and Other Clinicians. Royal Australian College of General Surveillance Practitioners Services, Melbourne, 1996. (5) Azuolas JK. Arboviral diseases of horses and possums. Arbovirus Research in Surveillance was used to delimit the extent of the incursion and Australia 7, 5-7,1997. evaluate the success of eradication. Surveillance for exotic (6) Kay BH, Aaskov JG. Ross River virus (epidemic polyarthritis). In: Monath TP mosquitoes had previously focused on detecting freshwater (ed). The Arboviruses: Epidemiology and Ecology, Vol IV, pp 93-112. CRC Press, Boca Raton, Florida, 1989. (12) container-breeding species at major seaports and airports . (7) Mackereth G. A survey for arbovirus infection in New Zealand livestock Nationwide surveillance for the southern saltmarsh mosquito was exposed to the exotic mosquito Aedes camptorhychus. Surveillance 26(4), 10-1,1999. undertaken by the regional public health services. Landcare (8) Marshall ID, Miles AR. Ross river virus and epidemic polyarthritis. Current Research NZ Ltd identified 619 zones with habitat suitable for the Topics in Vector Research 2, 31-56, 1984. saltmarsh mosquito, involving a total area of 86,900 hectares. (9) Noiton D, Brady H, Shulmeister J. Southern Saltmarsh mosquito literature review on the mosquito ecology and the transmission of Ross River Virus. A further 356 zones were identified as having evidence of some A report for the New Zealand Ministry of Health. August 2000. suitable habitat. Zones for which there is evidence of suitable (10) Browne GN, Taylor JF, Dunford PR. Acute Mortality of Non Target Organisms Exposed to S-methoprene (Prolink XR-G). Journal of American Mosquito habitat within 5 km of an airport or port exist at Kaitaia, Control Association (in press). Whangarei, Auckland, Whitianga, Tauranga, Gisborne, Napier, (11) Lothrop BB, Mulla MS. Field evaluation of controlled release pellet formulation Paraparaumu, Hokitika and Invercargill. of Methoprene against chironomid midges in man made lakes. Journal of American Mosquito Control Association 14(3), 335-9, 1998. Detailed ground surveys were required to delimit suitable habitat, (12) Kay BH. Review of New Zealand Programme for Exclusion and Surveillance of Exotic Mosquitoes of Public Health Significance. Report to the Ministry of which can change depending on factors such as rainfall, tidal Health, Wellington, 1997. changes, irrigation systems and ground water levels. (13) Anon. Surveillance for the Southern Saltmarsh Mosquito. Guidelines for Public Health Services. Ministry of Health, 1999. Surveillance sampling methods include the trapping of adult G F Mackereth mosquitoes in light traps and dipping for larval mosquitoes in National Centre for Disease Investigation ponds(13), although the sensitivity of these methods has not been Email: [email protected] quantified.