Could a “skeeternado” really happen?

skeeternadoWhile promoting the idea that the world needed a “Mosquito Week” to help raise awareness of mosquito-borne disease, Bill Gates also managed to plant the seed of an idea for Hollywood filmmakers. Forget Sharknado, how about Skeeternado?

While the idea may have been laughed off, there is a possibility that a “skeeternado” may actually occur. Perhaps not a “Hollywood” styled ento-storm event but we know mosquitoes swarm for various reasons and wind dispersal of mosquitoes could well be responsible for triggering outbreaks of mosquito-borne disease.

More like midges than mosquitoes

Is this a 1000 ft high swarm of mosquitoes in Leziria Grande at Vila Franca de Xira in Portugal? (Photo Ana Filipa Scarpa)

A great shot has recently emerged from Portugal of what is purported to be a “mosquito twister”, a swarm of hungry mosquitoes. Unsurprisingly, it created a bit of buzz on news websites! Unfortunately, it was probably more likely to be a swarm of midges than mosquitoes (I would love to know if it turned out to be mosquitoes). It is not unusual to get large swarms of non-biting Chironomid midges around wetlands, particularly in spring and autumn.

I’m often called out to investigate “mosquito problems” that actually turn out to be midge problems. Midges are a common component of Australia’s wetland ecosystem and although they don’t bite, they can be pests and there are even reports of sensitization and allergic reactions in humans exposed to the insects.

In some regions, chironomid midges can cause potentially significant nuisance problems and working groups on midge control have been established. This is particularly the case in new urban developments where wetlands provide habitats for the midges and the brightly lit, pale coloured buildings attract the insects at night.

Midges can often be seen in large numbers around lights and, in severe instances, dead insects can be a problem when they pile up against windows, doorways and on balconies. These problems are a particular concern of property developers trying to sell newly constructed dwellings! Some chironomid species have also been associated with crop damage in newly flooded rice paddies and control is often required.

Swarms of chironomid midges around freshwater wetlands at Sydney Olympic Park

Swarms of chironomid midges around freshwater wetlands at Sydney Olympic Park

The hunger swarms

Mosquitoes are known to swarm though. Hungry swarms. Huge numbers of mosquitoes have been documented in phenomenal numbers in Alaska. Many accounts from field stations, such as Toolik Field Station, have provided “nightmare” scenarios (for the record, I would LOVE to have the chance to get up there and do some mosquito research!).

While perhaps not as extreme as those mosquito populations in Alaska, there have been plenty of instances in coastal regions of Australia when huge numbers of saltmarsh mosquitoes chase me out of the wetlands. It does feel like you’re caught up in a swarm but I’m not aware of female mosquitoes hunting down a bloodmeal in a swarm!

Swarms of mosquitoes engulf a researcher at Toolik Field Station, Alaska (Photo: Alaska Dispatch)

Swarms of mosquitoes engulf a researcher at Toolik Field Station, Alaska (Photo: Alaska Dispatch)

The sexy swarms

It appears that sex is a primary motivator for mosquitoes to swarm. A wonderful account of swarming mosquitoes from 1969 documented male Culex pipiens swarming above vegetation with females darting into the swarm to find a mate. Similar observations have been made of Culex quinquefasciatus. Video analysis revealed that male and virgin female mosquitoes swarmed above vegetation (or some other physical structure that acted as a “swarm marker”) and that individual mosquitoes didn’t seem to have preferred positions within the swarm, but drifted at random until the male mosquitoes detected the sound of the female’s wing beats. Once they hear the female, the males slow their flight speed and alter their flight patterns until a female is found. Similarly, swarming behaviour of the malaria mosquito Anopheles gambiae has been documented and suggests there is even flight-path coordination between swarming male mosquitoes.

The pregnancy swarms

Swarming behaviour of saltmarsh mosquitoes in Florida has also be documented. I love this description from a 1953 paper, “Field Observations on the habits of Aedes taeniorhynchus“, published in Ecology describing the swarming behaviour of gravid female mosquitoes:

“They were flying around one another in a cloud from the ground up to a height of 7-8 m…The flight of the individual mosquito was slow and without sudden changes in direction,
graceful but with the same majestic calm which is seen in a female which has just finished a blood meal, and which in both cases is probably caused by the heavy weight of the abdomen.”

Interestingly, this paper did not offer an explanation as to why the gravid female mosquitoes were swarming in this manner. At first it was thought it was related to egg laying behaviour, perhaps assisting in finding suitable habitats, but when followed, no egg laying was observed. Why were they swarming?

The swimming swarms


Aquatic swarms of saltmarsh mosquito (Aedes vigilax) larvae in the estuarine wetlands near Newcastle

It isn’t just adult mosquitoes that swarm. The saltmarsh mosquito, Aedes vigilax, is famous for the high density aggregations of their immature stages that occur in the estuarine wetlands in coastal Australia. The behaviour is generally only observed in the late development stages just before pupation occurs. There can often be hundreds of thousands of larvae in these aggregations and the dark “swarms” of larvae can even be seen from the air (I haven’t check Google Earth but it wouldn’t surprise me). If you ever see something like this, best to head inland for the next week or so because these mosquitoes are serious nuisance-biting pests and can transmit Ross River virus.

The cyclonic swarms

One of the most interesting developments in insect-borne disease research recently has been the role of long distance wind-blown insect dispersal. Much of the recent research and modelling has been done with biting midges that transmit pathogens of veterinary importance (e.g. Bluetongue virus) and has shown that infected insects may introduce new pathogens into northern Australia from neighbouring regions or may distribute pathogens long distances within mainland Australia.

Similarly, windblown dispersal of infected mosquitoes associated with low pressure systems has been proposed as an explanation for the introduction of Japanese encephalitis virus into northern Australia in 1995 and 1998.

Kite traps” have been used, up to 310m, to trap windblown mosquitoes and to provide circumstantial evidence that aerial dispersal of mosquitoes from PNG to Cape York peninsula (about 200km) is possible. The results of these studies have been used as evidence to support the theory that major weather events could play a role in moving mosquitoes infected with Murray Valley encephalitis virus or Kunjin virus from endemic to non-endemic regions within Australia.

Were the meteorological events associated with the La Nina in 2010-2012 responsible for the 2011 outbreak of Kunjin virus amongst horses in southeast Australia?

I wonder how an increase in extreme tropical weather events in association with a changing climate influence outbreaks of mosquito-borne disease?

Perhaps a real life “skeeternado” is just around the corner…..

Why not join the conversation on Twitter by following me at @mozziebites?





What do you need to know about West Nile virus?

With the arrival of mosquito season in North America, health authorities have started issuing warnings about prevention of potentially fatal West Nile virus. My latest coauthored publication reviews the epidemiological and clinical aspects of this mosquito-borne pathogen.

West Nile virus is a pathogen generally spread by mosquitoes from birds to humans. While only about one in five people infected develop symptoms (inc fever, headache, body aches, nausea, vomiting, swollen lymph glands or a skin rash.), for those over 50, there can be more serious implications and the illness may be fatal. There is currently no vaccine, avoiding mosquito bites is the only way to prevent disease.

The virus was first detected in North America in 1999 and has since spread from coast to coast, having a significant impact on the health of both people (resulting in an economic burden of around $56 million per year) and wildlife (particularly birds). Interestingly, from 2007 there was a steady decline in the activity of the virus and many thought that major outbreaks would be a thing of the past but 2012 saw a one of the largest outbreaks in almost a decade. During this time, Texas was particularly hard hit with 1,868 cases and estimated costs of around US$47.6 million.

The activity of the virus in 2013 wasn’t insignificant either.


Annual total numbers of West Nile virus disease cases and deaths reported by CDC 1999-2013.

It hasn’t only been North America that has been impacted by West Nile virus. Outbreaks of human and animal illness have also been reported in Europe. In fact, cases of West Nile virus were reported from France in the 1960s. However, there wasn’t a major outbreak until 1996-1997; prompting warnings from health authorities about the future risks associated with this pathogen in Europe. The activity of the virus in North American, Europe and Africa provides interesting opportunities to research the genetic differences between regions and potential implications for surveillance and disease control. Europe has developed an extensive surveillance program to assess activity of endemic and exotic mosquitoes and activity of the virus.

Human illness in Europe resulting from West Nile virus infection during 2012 (European Centre for Disease Prevention and Control)

It is interesting to note that one of the key factors linking outbreaks of West Nile virus in both North America and Europe is the presence of closely related mosquitoes. Unlike dengue, chikungunya or yellow fever viruses that are spread by Aedes mosquitoes, and malaria parasites by Anopheles mosquitoes, West Nile virus is primarily spread by Culex mosquitoes. In particular, the bird-feeding mosquitoes within the Culex pipiens group.

The Culex pipiens group, particularly Culex pipiens, Culex quinquefasciatus and Culex molestus, are closely associated with urban environments. With mosquitoes found in close contact with humans, there is greater risk associated with potential outbreaks.

Our recent review article in the International Journal of General Medicine provides an overview of the clinical and epidemiological aspects of West Nile virus and is a good starting point for anyone interested in this pathogen and the factors that drive outbreaks in North America and Europe.

The abstract of our paper is here:

The reurgence of West Nile virus (WNV) in North America and Europe in recent years has raised the concerns of local authorities and highlighted that mosquito-borne disease is not restricted to tropical regions of the world. WNV is maintained in enzootic cycles involving, primarily, Culex spp. mosquitoes and avian hosts, with epizootic spread to mammals, including horses and humans. Human infection results in symptomatic illness in approximately one-fifth of cases and neuroinvasive disease in less than 1% of infected persons. The most consistently recognized risk factor for neuroinvasive disease is older age, although diabetes mellitus, alcohol excess, and a history of cancer may also increase risk. Despite the increasing public health concern, the current WNV treatments are inadequate. Current evidence supporting the use of ribavirin, interferon α, and WNV-specific immunoglobulin are reviewed. Nucleic acid detection has been an important diagnostic development, which is particularly important for the protection of the donated blood supply. While effective WNV vaccines are widely available for horses, no human vaccine has been registered. Uncertainty surrounds the magnitude of future risk posed by WNV, and predictive models are limited by the heterogeneity of environmental, vector, and host factors, even in neighboring regions. However, recent history has demonstrated that for regions where suitable mosquito vectors and reservoir hosts are present, there will be a risk of major epidemics. Given the potential for these outbreaks to include severe neuroinvasive disease, strategies should be implemented to monitor for, and respond to, outbreak risk. While broadscale mosquito control programs will assist in reducing the abundance of mosquito populations and subsequently reduce the risks of disease, for many individuals, the use of topical insect repellents and other personal protective strategies will remain the first line of defense against infection.

The full paper can be downloaded for free here.

You can also read more background to West Nile virus and the 2012 outbreak in my piece for The Conversation. For a comprehensive look at how the pathogen is managed in North America, download the CDC publication “West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control“.

Why not join the conversation on Twitter by following me at @mozziebites?

The image at the top of this piece is taken from Mother Jones.