The movement of mosquitoes around the world is more likely to drive the spread of mosquito-borne disease outbreaks than a changing climate. Health authorities received yet another reminder of how easily pest mosquitoes can move about with human activity. They can sneak in and set up home before we even know they’re there.
Mosquitoes deserve more credit. We’re impressed with the adaptation of birds, fish and frogs to narrow ecological niches or extreme environments. How about showing the same respect for these insects that have left behind their life in tree holes or leaf axils and moved into our cities. They’ve switched tastes from primates to people and their pets. They may only fly short distances but we help move them around the world with increasing frequency accompanying globalisation and discounted airfares.
Exotic mosquitoes and international travel
Long before planes made the planet a much smaller place, we were already moving mosquitoes massive distances. It could probably be argued that the brown house mosquito, Culex quinquefasciatus, first came to Australia in water filled barrels with the first fleet and the movement of the Yellow Fever mosquito, Aedes aegypti, and associated pathogens from the Caribbean to Philadelphia and triggered a catastrophic outbreak of disease in 1793.
In more recent times, the spread of the Asian tiger mosquito, Aedes albopictus, has been well documented and has raised concerns amongst international health authorities with regard to outbreaks of disease caused by dengue and chikungunya viruses.
This week saw the announcement that a mosquito, Aedes notoscriptus, had made its way from (most likely) Australia to California. The news attracted plenty of attention from media in Australia and North America. The mosquito discovery was due to the diligence of two agencies in Los Angeles, The Greater Los Angeles County Vector Control District and San Gabriel Valley Mosquito and Vector Control District. The joint media release from these two agencies described the discovery and identification:
“During an expanded search this summer for the invasive Asian tiger mosquito, staff from the San Gabriel Valley Mosquito and Vector Control District (SGVMVCD) and the Greater Los Angeles County Vector Control District (GLACVCD) collected unusual specimens from a couple of homes. After someinitial research, photographs of the mosquito were sent to Dr. Cameron Webb and John Clancy with theMarie Bashir Institute of Infectious Diseases and Biosecurity at the University of Sydney, Australia and they confirmed it to be Aedes notoscriptus.”
This was a great example of international collaboration. There was also little doubt that without the availability of high quality photographs we wouldn’t have been able to offer such a rapid identification. Not so long ago we would have had to ship specimens back and forth to confirm identification.
What does this mean for California?
Apart from questions regarding how this mosquito made it half way around the world, the critical issue now is to assess what pest or public health risk this mosquitoes poses to Los Angeles, California and North America more generally.
Aedes notoscriptus (commonly referred to as the ‘backyard mosquito’) is widespread in Australia. From the cold climates of Tasmania to the tropical north of the country. The mosquito is also found in New Zealand, Western Pacific and Indonesia.
The mosquito is closely associated with urban areas. Eggs are laid in a wide range of natural and artificial water-holding containers. There are very few Australian’s who haven’t been bitten by this mosquito. It is a constant companion at summer BBQs and is considered a nuisance-biting pest, mostly biting in the afternoon and early evening. However, it does have broad tastes when it comes to blood feeding. The abundance of this mosquito is relatively low compared to some of our “wetland” mosquitoes (such as Aedes vigilax and Culex annulirostris) and the mosquito doesn’t fly far, generally less than 200m.
The low natural dispersal of the mosquito is thought to have contributed to the emergence of subpopulations in some regions of Australia. More recent studies have found that phylogenetic analyses of DNA sequence data from mitochondrial markers indicate the mosquito is a complex of divergent genetic lineages, some geographically restricted, others widespread. There is, however, no doubt assisted movement of mosquitoes is also occurring around the country. As with any container-inhabiting mosquito, human movement will drive the spread of this mosquito in North America.
From a public health persepctive, Aedes notoscriptus is a less significant nuisance-biting pest than Aedes albopictus and Aedes aegypti. However, it is a vector of arboviruses and parasites. Aedes notoscriptus is an effective vector of Ross River and Barmah Forest viruses. A range arboviruses have also been isolated from field collected specimens in many parts of the country and the mosquito has been implicated in Ross River virus transmission in major cities including Sydney and Brisbane. Neither Ross River virus or Barmah Forest virus are known to exist in California.
Fortunately, the mosquito is generally not considered an effective vector of dengue viruses, West Nile virus, Yellow Fever virus or, chikungunya virus but it has been shown to be susceptible to Rift Valley fever virus. Aedes notoscriptus is an effective vector of dog heartworm, Dirofilaria immitis.
How did it get there?
We may never know. Mosquitoes can be moved about in lots of different ways, from cargo holds in aircraft and ships to baggage and belongings of individuals. Industrial, mining and agricultural equipment may provide a route of movement too.
Given that established populations have been discovered, it may be more likely that personal belongings may have been moved to California from Australia or New Zealand with travellers or family relocating for work. Perhaps surveys of the local community, combined with genetic analysis of the specimens will help answer this question.
The take home message for local authorities, both in California and Australia, is that the mosquito did make the trip, slipped through the cracks of quarantine and become established. While this introduction may not pose a significant impact to California, the introduction of Aedes albopictus to Australia could have far more substantial impacts. If we can export our mosquitoes, we should remain vigilant of the potential for exotic mosquitoes to find their way to us from Asia, Europe or the Americas.
What should authorities do?
There are already strategies in place for the surveillance and control of container-inhabiting mosquitoes in California. These strategies would prove effective in tracking and controlling Aedes notoscriptus. The one advantage authorities have is that Aedes notoscriptus is readily collected in carbon dioxide baited light traps whereas Aedes albopictus and Aedes aegypti are generally not collected as often by these traps.
In summary, the mosquito should not be considered a major pest or public health risk. The priority for local authorities should remain the other container-inhabiting species such as Aedes albopictus and Aedes aegypti. Notwithstanding these species, other mosquitoes associated with urban environments, particularly Culex spp., are currently playing an important role in one of the largest outbreaks of West Nile virus. In fact, the “proportion of mosquitoes infected with West Nile virus is at the highest level ever detected in California“.
Californian authorities have enough on their plate without the extra worry of an Australian hitchhiker turning up and moving in!
The wonderful photo of Aedes notoscriptus at the top of this post is provided by Jared Dever, Direcror of Communications, at Orange County Vector Control District.