The return of the La Niña weather pattern will result in wetter spring and summer in many parts of Australia.
We know mosquitoes need water to complete their life cycle. So does this mean Australia can expect a bumper mozzie season? How about an increase in mosquito-borne disease?
While we have seen more mosquitoes in the past La Niña events, and we may see more mosquitoes this year, that doesn’t necessarily mean we will see more related diseases.
This depends on a number of other factors, including local wildlife, which are essential to the life cycle of disease-carrying mosquitoes.
What is La Niña?
La Niña is a phase of the El Niño-Southern Oscillation, a pattern of ocean and atmospheric circulations over the Pacific.
While El Niño is generally associated with hot and dry conditions, La Niña is the opposite. La Niña brings somewhat cooler, but wetter conditions to many parts of Australia. During this period, northern and eastern Australia are particularly likely to experience wetter spring and summer.
Australia’s most recent major La Niña events were in 2010-11 and 2011-12.
When there is a La Nina, the walker circulation intensifies, bringing wet and warm conditions to Australia. (Supplied: Bureau of Meteorology)
Why is wet weather important for mosquitoes?
Mosquitoes lay their eggs on or around standing or still water. This can be water in ponds, backyard plant containers, clogged gutters, floodplains, or wetlands. Mosquito larvae (or “wrigglers”) hatch and spend the next week in the water before emerging as adults looking for blood.
When the water dries up, they die. But the more rain we get, the more opportunities for mosquitoes to multiply.
Mosquitoes are more than just a nuisance. If they bite, they can transfer viruses or bacteria into our blood to make us sick.
While Australia is free from major outbreaks of major international diseases such as dengue fever and malaria, mosquitoes still cause debilitating diseases every year.
These include transmission of the Ross River virus, Barmah Forest virus, and the potentially fatal Murray Valley encephalitis virus.
La Nina systems increase the chance of cyclones – and mosquitos. (Supplied: parts list)
What if it rains more?
We have long known that floods provide plenty of water to increase mosquito abundance. With more mosquitoes, there is a higher risk of mosquito-borne diseases.
The amount of rainfall each summer is also a key indicator of seasonal outbreaks of mosquito-borne diseases, particularly the Ross River virus.
The inland regions of Queensland, New South Wales and Victoria, particularly the Murray Darling Basin, are particularly vulnerable to mosquito boom and bust cycles and mosquito-borne disease.
In these regions, the El Niño South Oscillation is believed to play an important role in controlling the risks of mosquito-borne diseases.
El Niño’s hot and dry conditions are usually not ideal for mosquitos.
However, in the past, large outbreaks of mosquito-borne diseases have been associated with widespread inland flooding. This inundation is typically associated with the prevailing La Niña conditions.
For example, Murray Valley encephalitis outbreaks had significant human health effects in the 1950s and 1970s and occurred at a time of moderate to severe La Niña events.
Mosquito-borne disease outbreaks have also occurred in the past decade, when La Niña was causing above-average rainfall and flooding.
- Victoria’s record breaking Ross River virus epidemic in 2016-17 following extensive inland flooding
- The 2014-15 Ross River virus outbreak in southeast Queensland is due in part to an increase in mosquitoes associated with freshwater habitats following seasonal rains
- Eastern Australia’s major mosquito-borne disease outbreaks, associated with widespread flooding during two record breaking La Niñas between 2010 and 2012. These included Murray Valley capaphalitis and mosquito-borne diseases in horses caused by the closely related West Nile virus (Kunjin strain).
We cannot say for sure that there will be more diseases
The history and understanding of mosquito biology mean that with the prospect of more rain, we should expect more mosquitos. But even during floods, it is not always easy to predict outbreaks of mosquito-borne disease.
This is because of the role that wildlife plays in the transmission cycles of the Ross River virus and Murray Valley encephalitis virus.
In these cases, mosquitoes that carry viruses and are ready to bite people do not hatch from the flood. These mosquitoes must first bite wildlife where they ingest the virus. Then they bite people.
How native animals such as kangaroos, wallabies, and waterfowl react to rainfall and flooding plays a role in determining the risk of a mosquito-borne disease. In some cases, inland wetlands flooding can lead to an explosion of local waterfowl populations.
How can we reduce the risks?
There isn’t much we can do to change the weather, but there are steps we can take to reduce the effects of mosquitos.
Wearing insect repellant outdoors will reduce the risk of mosquito bites. But it is also important to dispose of, cover or throw away drinking water containers in our garden at least once a week.
In many parts of Australia, local authorities also monitor mosquitoes and mosquito-borne pathogens. This gives an early warning of the risk of a mosquito-borne disease.
Cameron Webb is a Clinical Associate Professor and Chief Hospital Scientist at the University of Sydney. This article originally appeared on The Conversation.