In 1998, members of the public living in Hawke’s Bay found themselves being bitten by a particularly aggressive mosquito that attacked during the day. Health officials collected specimens and soon identified a species new to New Zealand, Ochlerotatus camptorhynchus, commonly known as the southern salt marsh mosquito or campto, as the culprit.
The species is endemic to Australia, Papua New Guinea, the Solomon Islands and some Pacific Islands and is one of several mosquitoes that can carry the Ross River virus (RRV). In Australia, Ross River virus infection is a notifiable disease with five thousand cases costing tens of millions of dollars each year in lost work time and for treatments. The disease—also known as epidemic polyarthritis— manifests with a range of unpleasant and sometimes debilitating symptoms including aching muscles and joints, fever, chills, sweating, headaches, skin rashes and chronic fatigue. While most people recover within a month, in ten per cent of cases symptoms persist for years. At present, there is no cure and symptoms can only be treated with painkillers, anti-inflammatories and rest. An infestation can adversely affect both domestic and commercial activities such as tourism, fishing, forestry and horticulture. It is believed to be primarily a disease of kangaroos and wallabies transferred to humans by mosquito, but mosquitoes are now thought to be transferring it directly from person to person.
To date, there have been no known cases of RRV being contracted in New Zealand, but we represent a naïve population, and it is not a disease we want.
Campto’s preferred habitat is ephemeral ponds around the high tide line in salt marshes or in low-lying coastal farmland and estuaries. It was around large areas of land uplifted by the Napier earthquake that the Hawke’s Bay incursion was most prevalent. Mosquitoes lay eggs that hatch into water-borne larvae, but whereas most mosquitoes produce rafts of floating eggs—hatching perhaps a few hundred at a time—Campto lays individual eggs a few inches above the surface of the water where inundation by a high spring tide or heavy rain triggers hatching. After each high-water event, vast numbers of the mosquito appear simultaneously.
Following the Napier discovery, Hawke’s Bay Health launched a project to establish the extent of their campto problem and deal with it accordingly. At the same time, other public health units were warned by the Ministry of Health to look out for the mosquito. As more and more mosquitoes were found and the need for coordinated action grew, the ministry decided to appoint Southern Monitoring Services Ltd (SMS), a large privately-owned biosecurity consultancy company, to take over the project. SMS set up a dedicated division called New Zealand BioSecure to handle the programme.
When campto is found in a new area—usually during routine exotic mosquito surveillance activity—NZ BioSecure now surveys the area to identify potential habitat. Experience has taught the survey teams what to look for. Campto prefers a salinity level of three to nine parts salt in every thousand of fresh water so salinity levels are checked regularly. Nature also offers a hand. The easy-to-spot yellow daisy, Cotula coronopifolia, a plant of southern African origin known as bachelor’s button or brass buttons, has been found to grow where campto lays. White cups on sticks are used to take samples or “dips” from pools that might contain larvae and special light traps are placed nearby.
These are more than just lights, replicating conditions to which campto is naturally attracted—the exhaled breath of cattle. Carbon dioxide is fed to the trap via a small tube from a gas bottle, and a phial containing the chemical octinol, which smells like cattle breath, is attached close by. Mosquitoes, attracted by the light and smell, are pulled down into a container by a fan. Run from twelve-volt batteries, traps can be placed at remote locations and left for up to a week.
Once an area has been identified as mosquito territory, it is treated, especially over the wet winter months and after rain and high spring tides, when eggs will have hatched. NZ Biosecure use S-methoprene (an insect growth regulator) which they spread in pellet form in waterways. Once dissolved, it prevents the pupae from hatching. At the application rate used, the chemical seems to affect only campto mosquitoes.
Since the first mosquito was found in Napier in 1998, the species has been discovered in Tairawhiti, Mahia, Porangahau, Whitford and more recently around the Kaipara Harbour, where there are large areas of reclaimed farmland as well as drainage ditches and salt marshes.
When exotic mosquitoes were found in Whitford in 2000, the company acted quickly. Mosquitoes were identified positively as campto on Monday, a team assembled on Tuesday, surveillance was done by Thursday and the approval to treat gained on Friday—aided by the Hon Marian Hobbs’ power as Associate Minister of Biosecurity to approve treatment without resource consent.
In contrast to the six hectares treated at Whitford, the scale of the Kaipara project is huge, with two thousand seven hundred hectares of potential campto habitat. Government approval to eradicate the insect in the area came in June 2002 and treatment began four months later. By March 2003, thirty-five light traps were strategically deployed across the area and twenty thousand dips were being taken each month. Over sixteen hundred hectares of high-risk and difficult to access land has been treated with methoprene once or twice a month, spread aerially by helicopter or using quad bikes and knapsacks on the ground.
So far, treatment has been successful. The World Health Organisation advise that surveillance for Ochlerotatus camptorhynchus should continue for two years after the last treatment occurred (9 months after the last adult found) before an area can be declared clear. In Napier, treatment ended in April 2001 and routine trapping and dipping finally ended in August 2003. Surveillance in Tairawhiti, Mahia, Porangahau and Whitford ended in the first half of this year with the end of the Kaipara project forecast for December 2005. SMS believe the Kaipara eradication project will have been the largest of its kind in the world.
However, it is not the end of the line for campto in New Zealand. This year fresh infestations have been found at Whangaparaoa north of Auckland and the Wairau River mouth near Blenheim.