Why would a Californian drought trigger an outbreak of mosquito-borne disease?

CalifornianBushfireSunset_DawnEllnerMosquitoes need water almost as much as they need blood so why is it a drought could cause an outbreak of mosquito-borne disease? Why does the drought in California mean less water but more mosquito-borne disease?

More than just water

All mosquitoes need water. It could be a teaspoon of water in a pot plant base or an expanse of wetlands inundated by tides. Following flooding, health authorities are typically quick to issue public health warnings about increased risk of mosquito-borne disease. However, more mosquitoes doesn’t always mean more mosquito-borne disease.

Mosquitoes need blood. As well as biting people, they also bite animals. Outbreaks of mosquito-borne disease typically requires the presence of wildlife, animals that act as reservoirs for the disease-causing viruses.

CaliforniaDrought_EPA

Drought is hitting California hard (Source EPA via Huffington Post)

Mosquitoes, drought and West Nile virus

West Nile virus is a mosquito-borne pathogen generally spread between birds and people by mosquitoes. Culex mosquitoes they appear to play the most important role in West Nile virus transmission in urban environments, particularly Culex pipiens.

These mosquitoes are generally not breeding in wetlands. They’re found in artificial structures ranging from backyard containers and neglected swimming pools to stormwater pipes and drains. These mosquitoes have moved out of the swamps and into the suburbs! They’ve also moved into the constructed wetlands popping up throughout the suburbs too.

Rather than water birds associated with wetland environments, the birds playing a key role in West Nile virus transmission are small songbirds common in urban areas. These birds roost in large numbers and are the target the the Culex mosquitoes that preferentially feed on birds. It is important to keep in mind that there is still a lot of learn about how the roosting behaviour of birds influences their exposure to West Nile virus.

During “dry” conditions, bird populations are concentrated in urban areas (where humans provide water and food) and mosquito populations associated with urban water-holding structures increase. During “wet” summers, bird populations may be more widely dispersed through the environment with many birds roosting and foraging well away from residential areas and reducing the contact between birds, mosquitoes and people. When the “dry” summers arrive, birds move back close to the people. People who provide water.

Culex_molestus_Photo_StephenDoggett

The Culex pipiens group of mosquitoes play an important role in the transmission of West Nile virus and are closely associated with urban environments. They like biting birds. (Photo: Stephen Doggett, NSW Health Pathology)

In the absence of rain, water stagnates in stormwater pipes and drains providing favourable conditions for mosquitoes. During “wet” summers, the mosquitoes are flushed out by increased water flows and, even if they don’t, permanent habitats are more likely to support populations of mosquito predators such as fish.

During “dry” summers, people also start storing water around the home. Once water restrictions kick in, the desire to keep the garden looking healthy can potentially pose an indirect health risk to the homeowner as they hoard water around the home that provides habitat for mosquitoes.

In short, dry conditions help concentrate mosquitoes and birds in close proximity to people and increase the risk of mosquito-borne disease outbreaks.

WestNileSign

Mosquito control in Texas in response to an outbreak of West Nile virus raised much concern within the community. It can sometimes be difficult to balance the need for mosquito control with community engagement to allay fears of insecticide -based human health risks (Source: CDC)

An outbreak in Texas in 2012

West Nile virus was first introduced into North America in 1999. Despite rapidly spreading across the continent in the subsequent years, the numbers of outbreaks steadily declined and, to some extent, it fell of the radar as a serious public health concern. There was a resurgence of the disease in 2012 with an outbreak primarily focused in Texas.

There was a substantial increase in the number of cases compared to previous years with an unusually warm spring thought to have played an important role in driving the outbreak. Health authorities were warned that outbreaks of this nature may continue.

USDroughtMonitors_7April2015West Nile virus and the Californian drought

For the past couple of years, California has been hit with one of its worst droughts in decades. It is having widespread impacts and may also be increasing mosquito-borne disease. Californian authorities have been battling potential public health risks associated with mosquitoes on many fronts. There were record numbers of deaths due to West Nile virus disease in 2014 and exotic mosquitoes were detected. This included an Australian mosquito that was found in Los Angeles.

It is relatively early in California’s mosquito season but West Nile virus has already been detected. Health authorities are warning that another bad year for West Nile virus activity could be ahead despite the ongoing drought. There is already a suggestion that the severity of the current drought may be exacerbated by climate change and that climate change may be playing a role in future West Nile virus risk internationally.

There is little doubt that prolonged drought will impact Californian residents in many ways and an increased risk of mosquito-borne disease is just one of them. Fortunately, mosquito and vector control agencies in California work closely with local health authorities to monitoring mosquito and pathogen activity to provide warnings of increased risk. However, there is responsibility for everyone to ensure that the ways in which water is conserved around the home doesn’t increase the risks associated with mosquitoes.

potplantsaucer

If you’re worried about keeping your pot plants well watered but don’t want to provide a home for mosquitoes, fill the saucer with sand. It will keep the moisture in place but there is no “free water” for mozzies to use!

If you’re not able to “dump and drain” water holding containers, make sure that they’re covered to stop mosquitoes getting in or out. If you’ve got a swimming pool that’s neglected, start chlorinating it or release fish to eat through any mosquitoes. There are also a few mosquito control products that could be used, the most appropriate would probably be the insect growth regulator methoprene, it will stop mosquitoes emerging from the water holding container.

Why not share your tips on saving water around the home while not increasing opportunities for mosquitoes on Twitter?

The photo at the top of this post is taken by Dawn Ellner (see original photo here)

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.

WNV_19992013

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.

Busting five myths of mosquito repellents

repellent_spraying_webbAfter a record year of West Nile virus activity in 2012, North American health authorities are on alert as the peak season of mosquitoes approaches. As of 16 July 2013, a total of 23 cases of human disease, including three deaths, had already been reported by CDC.

In many regions across the US, local mosquito control districts engage a range of strategies to reduce mosquito-borne disease risk. These may include broad scale insecticide use or the release of “mosquito fish” into derelict backyard pools. However, the first line of defense against biting mosquitoes remains the use of topical insect repellents.

In North America, the CDC provide detailed information on mosquito repellents but there still seems to be confusion amongst many in the community looking for alternatives to “chemical repellents”. These repellents are often perceived to be unsafe. For an Australian perspective, I’ve put together some guidelines on mosquito repellent use that provide a few more details than typically found on the websites of local health authorities.

In my experience of trying to promote the use of “known to be effective” repellents, I still find that there are many misconceptions and misunderstanding of how these products should be used. Below are the five myths I most commonly experience when it comes to the use of mosquito repellents.

Myth 1: Natural must be better

It isn’t surprising that most people associate “natural” products with better health. Many people perceive mosquito repellents derived from “natural” products, such as plant extracts, to be healthier choices. However, when it comes to mosquito repellents, there is clear evidence that these perceived “healthier” choices may not provide the best outcomes.

Studies have repeatedly shown that the most effective repellents are DEET and picaridin. DEET, in particular, has been shown to be very effective. Picaridin is pretty good too. Both products have shown to be effective in local field-based tests.

Unfortunately, many, many studies throughout the world have shown that botanical based repellents provide substantially less protection against biting mosquitoes than DEET or Picaridin. Products containing citronella, lavender, peppermint and Melaleuca oils are widespread and are often promoted as “DEET-Free” alternatives to the recommended repellents. There are many botanical-based insect repellents listed in the patent literature.

Essential oils of Australian native plants provide significantly less protection than DEET-based repellents. Expert review panels have suggested that products containing plant extracts should not be recommended in areas of endemic mosquito-borne disease or when biting mosquitoes are abundant.

It is important to remember that botanical-based repellents WILL provide some limited protection against biting mosquitoes. The biggest problem is that they will need to be reapplied 3-4 times as often as even a low dose DEET-based repellent to provide comparable protection. Botanical repellents may be fine for a quick trip to the backyard to hang the washing out but not for a long session of gardening or if you’re off for a hike.

It is also important to remember that using undiluted essential oils can also pose an important health risk, particularly in young children.

I often wonder if Koalas are bitten any less by mosquitoes than other Australian mammals given the amount of Eucalyptus leaves they consume! (Photo by DAVID ILIFF. License: CC-BY-SA 3.0)

What about “Oil of Lemon Eucalyptus”? That’s a botanical repellent and authorities recommend it against West Nile virus right? There is often some confusion regarding this product. It is not the essential oil, but rather a byproduct of the distillation process of the leaves of Corymbia citridora. Commonly known as PMD, it has been shown to be as effective as DEET (although generally requiring higher doses for comparable protection) and is recommended by the CDC in North America. The recommendations by CDC of this product should not be seen as an endorsement for other “botanical based” repellents.

Myth 2: Stronger repellent = fewer mosquitoes

This is probably the most common mistake made when choosing a repellent. The “strength” of a repellent (i.e. the concentration of active ingredients) doesn’t determine how many mosquitoes are kept at bay. It determines the duration of protection provided. It basically determines how long you are protected from biting mosquitoes.

The majority of published studies (the classic “arm in cage” style experiments) investigating the efficacy of repellents analyse the results in two ways, mean repellency rates (a comparison of how many mosquitoes land on a treated arm compared to an untreated arm) and mean protection time (for how long are all mosquito bites prevented). While the marketing companies may be interested in claims like “over 80% of mosquito bites prevented”, given that it only takes one mosquito bite for a pathogen to be transmitted, I’m hoping to prevent ALL bites! We should be far more interested in protection times than repellency.

Myth 3: Chemical repellents are dangerous

Both DEET and picaridin are considered safe. If used as directed, DEET-based repellents pose no substantial health risk. Despite being used by millions of people every year, there are few examples of reported serious adverse health impacts in the scientific literature.

There are many stories circulating about mosquito repellents having an unpleasant smell or creating an unpleasant feeling on the skin. There are also reports about damage to clothing and plastics in some instances. Some of these reports may be true but are most likely related to high concentration formulations. In the US, there are many brands available that contain over 95% DEET. In the vast majority of situations, however, most people would find that an approximate 10% DEET formulation would work perfectly well and not be associated with any of these unpleasant characteristics.

Myth 4: Apply repellent like perfume

A neighbor took great pleasure in telling me how ineffective mosquito repellents were. Repeatedly. One afternoon I saw him applying repellent to his children. The aerosol was sprayed around in the air above the kids as they jumped up and down. There was no way that the repellent was going to work.

While there is still some debate as to how DEET prevents mosquitoes bites, or how the response of mosquitoes to DEET is influenced by previous exposure or infection with a pathogen, we do know that to get the best results, the repellent should be applied as a thin covering on all exposed skin. It is for this reason I personally think creams and liquids are the best repellents to use.

Don’t apply repellent like a perfume. A spray “here and there” won’t work. Spraying it on your clothes won’t work either. Apply it in the same way as you would a sunscreen but keep in mind that you won’t need to apply it quite so often.

As summer approaches, the shelves of hardware stores and supermarkets are filled with various repellents, insecticides and traps. Some work better than others.

Myth 5: These gimmicks really work!

If it sounds too good to be true, it probably is! Gimmicks such as traps, ultrasonic devices and smartphone apps all sound very appealing if you find that putting on repellent is a bit of a hassle. Unfortunately, there is little scientific evidence that any of these will protect you from mosquito bites.

There is even a pill, recently approved by Canadian Health Authorities, that purports to only take 30 minutes to protect the swallower from mosquito bites. An excellent blog post, “Insect repellent you can eat – but does it work?”, refuting the claims of this product, is here. Also check this nice piece from 2012, “Homeopathic Insect Repellent: Is there anything the Natural Health Products Directorate won’t approve?

In short, there is nothing you can eat or drink that has been scientifically proven to prevent mosquito bites. That’s right, not even vitamin B.

In summary, the mosquito repellents widely available in North America, Australia and many other parts of the world are perfectly safe to use and can be effective in preventing mosquito bites. Many of these (and more) urban myths will persist for some time and perhaps it is time health authorities worked harder to communicate the benefits and effective use of the products available.

UPDATE: Thanks to BugGirl and Laurie Sullivan for the suggested tweaks!

Want to learn more about the amazing world of Australian mosquitoes? Check out “A Field Guide to Mosquitoes of Australia” out now through CSIRO Publishing. Over 200 pages containing a pictorial guide to almost 100 different mosquitoes along with tips on beating their bite and protecting your family from the health risks of mosquitoes. You can order online or through your favourite local bookstore or online retailer.


Read more at The Conversation: Are mosquito coils good or bad for our health?


Read more at The Conversation: What can I eat to stop mosquitoes biting me?


Read more at The Conversation: The best (and worst) ways to beat mosquito bites