Richard Musgrove
That high-pitched whine in your ear signals the world’s deadliest animal – a female mosquito. Disease-causing parasites in her gut kill at least 670,000 people each year.
She brings malaria, Ross River virus, dengue fever, chikungunya, Zika, Japanese encephalitis and lymphatic filariasis, also called elephantiasis.
All in the Pacific, some on our doorstep, and some have even reached Australia’s mainland.
Dr Tanya Russell is on the frontline of the mosquito wars.
“The key is knowing your enemy, their distributions and behaviours,” says Russell, a Research Fellow, of the Australian Institute of Tropical Health and Medicine (AITHM). Russell’s foes are the 42 species of disease-carrying mosquitos in the Pacific.
Another foot soldier in the war on mozzies in the Pacific, Dr Salanieta Saketa, Senior Epidemiologist at the South Pacific Community (SPC) says mosquito-borne diseases remain a significant threat to the health and socioeconomic well-being of the region.
“The number of outbreaks is rising, with 96 reported in the Pacific region between 2012 and 2021, including 69 dengue outbreaks, 12 Zika virus and 15 chikungunya,” says Saketa.
A new weapon to counter this threat is ‘A guide to mosquitoes in the Pacific‘, released by Russell and Professor Tom Burkot, also of the AITHM.
This landmark study will help Pacific countries, including Australia, stay ahead of these highly mobile and adaptable disease-carrying species and the often-devastating outbreaks they trigger.
“We were trying to answer the questions: ‘Which mosquito species are we trying to control, and which are transmitting dengue, in particular?’” Russell says.
These questions originated from PacMOSSI, the Pacific Mosquito Surveillance Strengthening for Impact programme.
It was lead by Burkot and Russell and jointly funded by the Department of Foreign Affairs and Trade (DFAT), the European Union and Agence Francaise de Development. PacMOSSI supports Pacific Island countries to contain and control such disease carriers (called ‘vectors’).
The key is knowing your enemy, their distributions and behaviours.
Tanya Russel
Twelve months on, many questions are answered – for now. More than 250 published papers were sourced for diversity, distribution and biology data on disease-carrying mosquitos in the region. Personal contact was made with each of the 22 Pacific nation’s Health Ministries to access their routine mosquito surveillance data. After much cross-checking the mammoth task was finished, and guide completed.
The Guide has been designed to allow online updates – a response to the continually changing state of knowledge on mosquito populations within this vast area.
“The distributions of the vectors of dengue are very dynamic so we see the Guide as a living document that will be continuously updated and available online. Fortunately, the capacity for Pacific Island Countries to undertake surveillance is growing with the support of the PacMOSSI programme,” says co-author Burkot.
Twelve dengue vectors are known in the Pacific, out of the 423 mosquito species found in the region – the greatest diversity of dengue-carrying mosquitos in the world. And, at risk, are the more than 11 million people living on the thousands of remote islands and island groups across that blue-green expanse.
“PNG has about 200 mosquito species, with another 200 scattered around – some islands only have two or three species. And then the next island has a completely different suite of mosquito species,” says Russell
Know your enemy
The Guide focuses on the three mosquito genera spreading pathogens in Australia and the Pacific:
- Anopheles: malaria and lymphatic filariasis
- Aedes: dengue, chikungunya, Zika, Ross River virus, Japanese encephalitis, lymphatic filariasis, and dog heartworm.
- Culex: Japanese encephalitis, Ross River virus, lymphatic filariasis and dog heartworm.
Disease-carrying female mosquitos take blood meals from humans, picking up whatever blood parasites that person is carrying, and pass them on to their next victim. Pathogens vary with geographical location, for example, the yellow fever mosquito (Aedes aegypti) carries yellow fever in parts of Africa, Central and South America, but is the primary vector of dengue fever, chikungunya, and Zika viruses in the Pacific.
A primary vector is a mosquito responsible for most of a disease’s transmission because it can become infectious, is abundant and will readily bite humans.
A primary vector is a mosquito responsible for most of a disease’s transmission.
Blood meals nourish the female’s eggs – iron, in particular, is necessary for optimal egg development and viable offspring. Males don’t bite, but may, instead, be whining around your head, looking to mate with that female angling for your blood. Once these two mosquitos get together, the male’s seminal fluid increases female survival, number of eggs laid, and blood meals taken – improving her chances of passing on a disease-causing parasite.
A. aegypti is dengue’s primary vector in the Pacific. The species is common around humans and lays eggs in anything containing fresh water, and in some places, brackish water.
Aedes are also adaptable, particularly A. aegypti, with desiccation-resistant eggs perhaps contributing to the species’ global dispersal. Surviving out of water in truck tire treads, or on ships, anywhere relatively cool, dark and humid, these eggs easily move between islands and countries, Russell says.
Outbreak Risk
Outbreaks are caused when three major risk factors come together:
- – Connectivity between populations – an infected person arrives on an island.
- – The presence of an efficient mosquito vector. The disease won’t spread unless the parasite can survive in the mosquito’s gut and is passed on in the next blood meal.
- – A susceptible human population.
Susceptibility is about timing and general population health. A dengue strain can ‘burn’ through a human population, leaving it with some tolerance to reinfection, possibly lasting quite a few years. When that dies out, you again have a population vulnerable to another outbreak, Russell says.
The future
The Pacific region faces a challenging future given the complexities of mosquito disease spread in response to climate change, population growth and increasing travel.
Global temperature rises are predicted to lengthen annual transmission seasons by close to two months for malaria and four months for dengue over the next 50 years. The ‘epidemic belt’ for both diseases is also expected to move towards temperate areas.
The Pacific region faces a challenging future.
Dengue and Ross River fever have also been identified among a list of five climate change-sensitive diseases in the Torres Strait and Cape York regions. Cape York has 0.52% of Queensland’s population but already presents a disproportionately high percentage (2.1%) of annual dengue cases.
Some research is out of date the moment it’s published – that’s the nature of science. But now the Pacific has a baseline on which to build.
The Guide presents a crucial record of mosquito species distributions in the region, but it also recognises that some of the most comprehensive surveys used in its production were conducted more than 60 years ago, says Associate Professor Gregor Devine of the Mosquito Control Laboratory at the QIMR Berghofer Medical Research Institute.
Devine concludes that the Pacific now has a solid baseline on which to “layer further regional surveys — essential if we are to establish a current picture of mosquito-borne disease risks.”
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