How could mosquitoes cause a disease outbreak?

Factors involved in the potential for a disease outbreak spread by mosquitoes to occur in New Zealand include increased shipping movements (risk of exotic mosquitoes becoming established here), international travel (people bringing disease in which may then be spread by mosquitoes), whether local mosquito species could become carriers of disease, a non-immune human population, and suitable mosquito breeding environment (which may also increase due to global warming effects).

Two main scenarios

An outbreak of a mosquito-borne viral disease could occur in two main ways:

Exotic vector mosquitoes
Mosquitoes carrying the virus (vectors) could arrive at a New Zealand port, escape, infect the human population in that area and breed - with subsequent generations of mosquitoes of the imported, and possibly local, species continuing to pass the infection from one person to another.

Infected people
A person infected by the viral disease arrives from overseas to New Zealand, is bitten by a "native" or imported (uninfected) mosquito species, which then becomes a vector (carrier) to other people. People on holidays overseas may also have been infected (without realising it) and return home before symptoms have developed. Dengue fever is an example of a disease people can contract from mosquitoes while overseas. From 2006 to 2010 there were 436 cases of dengue fever in New Zealand (NZ Public Health Observatory/ ESR surveillance database figures.)

Whether these possible scenarios would turn into a large outbreak depends on a large enough pool of infected people and sufficient vector mosquitoes being present in the same area.

Vector species and breeding environments
The ability of a particular species to act as a vector and whether a favourable breeding environment is present are two other factors. New Zealand is generally thought to be a very favourable, and untapped, breeding ground for mosquitoes. 

Factors increasing the risk of mosquito-borne viral disease in New Zealand

• Increased travel and shipping movements means more risk of mosquitoes arriving
• People carry the virus for days before symptoms develop, allowing for international travel
• New Zealand habitats are underexploited so imported mosquitoes could establish quickly
• Population growth and urbanisation mean a greater concentration of susceptible people
• Global warming could reduce mosquito development time and increase mosquito numbers, and increase the ability of already established mosquitoes to transmit disease.  

Biosecurity measures

MAF Biosecurity New Zealand is the government authority "responsible for maintaining effective biosecurity standards for the movement of goods to and from New Zealand". Standards and regulations relate to the importing and exporting of plants, animals and other materials which may represent risk. Regulations also cover requirements for transportation, including containers and cargo, ships, aircraft and transitional facilities.

From 1 July 2010, MAF Biosecurity New Zealand assumed responsibility for the surveillance of mosquitoes that may be found in salt marshes around the country. The NZ Ministry of Health remains responsible for mosquito surveillance at New Zealand's ports and airports.

MAF Biosecurity NZ requests the public to report any suspected exotic disease or pests in animals, plants, fish or bees by phoning 0800 80 99 66.

Related topics

See also Mosquitoes - the threat of new disease in NZ

Mosquitoes - types that are a cause for concern

Original material provided by New Zealand Doctor magazine, February 2004. Reviewed and edited by everybody, November 2011.