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Showing posts with label Mosquito. Show all posts
Showing posts with label Mosquito. Show all posts

West Nile Virus - WNV

The West Nile Fever is a disease caused by the West Nile Virus (WNV), which is part of the viral genus Flavivirus – like the Japanese encephalitis virus, dengue fever virus and more. The West Nile Virus infects mostly birds, but may infect mammals (like people) and even some reptilians, when bitten by an infected mosquito. The most common vector of WNV are mosquitoes from the Culex genus.

The virus and the disease got their name “West Nile Virus/Fever” as they were first identified and reported to the scientific world in West Nile District of Uganda in 1937. However, the disease is probably among us long before that. It is suspected to be the cause of death of Alexander the Great.

West Nile Virus

Most people infected with the West Nile Virus will probably not know it, as in most cases (nearly 80%) the infection is asymptomatic. The majority of the humans that do catch the disease (nearly 97% of the sick people) will experience relatively mild fever that we call West Nile Fever.

Only a minority of the infected people (less than 1% of the total infected people and about 3% of the sick ones) will experience West Nile Meningitis or Encephalitis, which in many cases has severe consequences, including death.

West Nile Fever symptoms usually appear 2 to 8 days after infection (i.e. by an infected mosquito bite, see ahead). The symptoms are normally fever, weakness, headache, chills, sweating, swollen lymph nodes, drowsiness, pain in the joints and other flu symptoms. Most symptoms will last 1 week to 10 days. Some symptoms like swollen lymph nodes, may last longer.

The West Nile Virus is an Arbovirus, which means that it belongs to a group of viruses that are transmitted only by arthropod vectors like mosquitoes. In order for a person to get infected with West Nile Virus, he/she must get bitten first by an infected mosquito female (since only the females are biting).

The best West Nile Virus carrier pool is the bird population. Mosquitoes are transferring West Nile Viruses among birds and thus maintaining the pool. The mosquito itself will get infected only if it will bite an infected bird first. Infected birds have high West Nile Virus density in their blood, enough to infect the biting mosquito.

When biting an infected mammal, like a human for instance, the mosquito will absorb West Nile Viruses with the blood meal. However, since the West Nile Virus density in an infected human blood is low relative to birds, the mosquito female will absorb only a small dosage of viruses, small enough for its immune system to get over it. Therefore, viruses that infect animals other than birds are going into a dead-end.

Two to three days after the mosquito female had bitten a WNV infected bird, it will oviposit (lay eggs). Couple of days more and it is ready to bite again – searching for the second victim. If by the time it will bite again the West Nile Virus has spread into its salivary glands, the new victim (that can be an unfortunate human) will be infected with the virus.

Read more valuable info on mosquito life cycle and how to get rid of the mosquito bites to avoid WNV infection and its risky outcomes.

WNV and the Right Bird

In order that a mosquito female will become a vector of West Nile Virus it has to bite an infected bird first. Infected birds have high West Nile Virus density in their blood, enough to infect the biting mosquito. Biting an infected mammal, like humans or horses for instance, will not infect the mosquito female and it will not become a vector.

Which bird is more likely to infect mosquitoes with West Nile Virus? Scientists often ask themselves that question. A bird that just recently got bitten by a West Nile Virus carrying mosquito has not accumulated enough viruses in its blood to infect the second biting mosquito. On the other hand, a bird that got bitten by such infected mosquito long ago is either over the fever and does not have enough viruses to infect again, or is already dead by now and will not attract any biting mosquito.

WNV and the Right Bird

Accordingly, only a bird that got bitten by a West Nile Virus carrying mosquito early enough to rich the virus density pick, but not too long ago to be over the pick (immune and getting better or dead) can actually infect another mosquito to become the next WNV vector.

For years crows thought to be one of the leading candidate bird species to carry West Nile Virus. However, researchers now think that at least in North East USA – American Robins, being favorite host for Culexmosquitoes, are the main carrier of this recently spread virus.

Beside the question regarding its immune system, a question regarding the bird attractiveness for mosquito bites should be asked as well when wondering what makes a bird a preferred vector of West Nile Virus. So what makes a bird more attractive to mosquito bites? Mosquitoes are attracted mainly by odors for their blood meal host. Probably Robins got the most attractive odor to the mosquito females.

Behavioral patterns may also be a reason. If the bird can be found in the same location as the mosquitoes, it is more likely to get bitten. For this reason I believe albatrosses, for example, are rarely bitten as they spend much of their time in the middle of the ocean. I also believe that day active birds (like Robins) will get bitten more frequently by night active mosquitoes (like Culex), as they are more likely to get bitten while asleep. Moreover, for a very active bird like the swallow for instance, this should be even truer.

Finally, different studies have demonstrated that infected mosquito females change their behavior – especially biting and flying behavioral modes. Thus, I often wonder if a West Nile virus infected mosquito female is attracted to birds in a similar manner as a healthy mosquito?

Think of a bird species that sleeps up high on particularly high trees. Imagine also that a certain mosquito species is in the habit of biting these birds while they are sleeping. Will a West Nile Virus infected (and probably weak) mosquito female be likely to make the effort to fly so high for a blood meal, or will it change its feeding behavior and search for a biting victim closer to ground?

In summary, numerous factors are joined together to determine whether a certain bird species would be involved with West Nile Virus cycle or not. See herein more details on West Nile Virus :)

Mosquito Nets

A mosquito net is one of the best protection means against mosquito bites. You can use any net which has a mesh size small enough to prevent the mosquitoes from passing through and it actually isolates you from the mosquito threat.

A mesh size of about one square millimeter is good enough for most, if not all, mosquito species netting. Sometimes smaller pest, such as some species of sand flies, requires even smaller mesh size, so you best identify all the threats before deciding on the right netting for your house protection.

The most common use of mosquito nets is to cover our house windows and doors. In areas infested with mosquitoes (or any other insect/pest) where people still want to keep air circulation going, screens on the windows and doors are a reasonable compromise. Air can flow in and out of the house, though not as good as with a completely open window.

Mosquito Nets

Mosquitoes, on the other hand, stay mostly out. Mostly I said? Whenever the window or door gets open for a brief moment, few mosquitoes will manage to go in! Additionally, any little hole in the mosquito net will be discovered by the mosquitoes to find their way inside. Thus, even in houses well protected with mosquito nets, few hungry mosquitoes will find their way in.

For that reason, some people cover their beds with mosquito nets too, especially those of the infant family members that cannot be protected using insect repellents, due to health restrictions. The downside is that bed net holds the air even more than a window screen and the air circulation is almost gone. Fortunately, many places are air conditioned these days, especially in warm environments.

Mosquito nets are also very useful while camping or staying outdoors. Most of the tents and many of the gazebos these days are equipped with embedded mosquito nets.

Furthermore, in really badly infested areas mosquito nets are needed for protection not only to the resting people, but also during vacation activities (i.e. hiking, fishing and boating) or working activities (farming, gardening, etc.). For these cases, there are mosquito nets that come down around the shoulders of a wide hat, similar to beekeepers’ hat. By wearing long sleeves clothing and if necessary gloves, adding mosquito net over the face will cover all the exposed body surface and do a great isolation job.

One important tip regarding netting that not many of you are aware of: If the mosquito net is touching you or is too close to your skin, you may still get bitten through the net. A minimum distance of 1 cm (half an inch) between the mosquito net and your skin is necessary to insure yourself.

You are welcome to read more interesting facts on the secrete behind mosquito bites and on mosquito repellents.

Anopheles Mosquito

The mosquito family (Culicidae) can be divided to three sub families: Culicinae, Toxorhynchitinae, and Anophelinae – better known as the Anopheles mosquito.

In its adult stage of his life cycle, the Anopheles mosquito can be easily identified while resting even to the unprofessional eye. The adult rests with its body, head and proboscis (the sting) in an almost straight line. Furthermore, the straight line is positioned at an angle to the substrate, with some Anopheles mosquito species appear to be standing on their heads… This adult resting manner is typical only to the Anopheles mosquito.

Anopheles Mosquito

The Anopheles mosquito’s larvae is also distinct. Like all mosquito larvae they are aquatic and occur in various types of pools (from very slow running water to stagnant water). Mosquito larvae breathe atmospheric air at the water surface. However, while most mosquito species have breathing siphon, Anopheles mosquito larvae do not.  They spend most of their time parallel to the water surface, breathing through opening located on the 8th abdominal segment.

Single small eggs (no more than 200) are laid on the water surface by the Anopheles mosquito female. About two to three days later the egg hatches. On cold weather, hatching maybe delayed for more than a week.

Anopheles mosquitoes are famous as some of the Anopheles species can transfer Malaria. There are nearly 500 Anopheles mosquito species known to science, but only about 50 can actually transfer Malaria. When an Anopheles mosquito bites an infected person, it gets infected with Malaria itself and then bites a healthy person – infecting him/her too. Studies have demonstrated that most Anopheles mosquitoes prefer human for blood meal, bite mostly indoors and prefer the night with peak biting between 11 pm and 2 am.

The Malaria parasites are Protist called Plasmodium. Four species of Plasmodium can infect and be transmitted to humans by the Anopheles mosquito. They have 3 cycles during the development of the disease: one cycle in the infected mosquito and two cycles in the infected person. During these cycles the parasites transform through 3 phases:

When an Anopheles mosquito bites an infected person, it sucks the parasite along with the blood. The parasites enter the mosquito blood meal as phase 1 in its life cycle (gametocytes). Then it develops to the 2nd phase within the mosquito gut (sporozoites). This 2nd phase fills the salivary glands of the infested mosquito.

When this female Anopheles mosquito bites another person for a blood meal, phase 2 (sporozoites) are injected into the blood stream of the fresh victim, and Malaria have been transferred. The sporozoites migrate in the bloodstream to the liver and infect liver cells (hepatocytes), where they multiply into a 3rd phase (merozoites). After a while they rupture the liver cells, and escape back into the infected victim’s bloodstream.

The merozoites in the bloodstream infect red blood cells, where they develop into ring forms, trophozoites and schizonts which in turn produce further merozoites. Sexual forms (gametocytes – back to phase 1) are also produced, which, if taken up by a mosquito, will infect the Anopheles mosquito female and continue the life cycle!

To summarize, the anopheles mosquito also known as the Malaria mosquito should be carefully controlled to avoid his global distribution and his ability to easily transfer the Malaria desease and infect us humans. Different mosquito control methods are available and are well established. For personal mosquito bites protection read more on our best mosquito repellent recommendation.

Natural Mosquito Repellent

The natural/organic fashion penetrates quickly into our lives as many people prefer to eat organic food, use natural deodorants, natural tooth paste and so on. The insect repellents market was not excluded: natural mosquito repellent products are gaining much more popularity and are available from several vendors in the market.

Add to this the recognition that the most popular mosquito repellent Diethyl-meta-Toluamide (known as DEET) is not that good for our health, and no wonder you see the natural mosquito repellent alternative being more popular than ever.

But is the popularity of natural mosquito repellents justified? For start, DEET is a highly effective repellent that, according to the US Environmental Protection Agency (EPA) stays active for up to 8 hours. Can a natural mosquito repellent achieve that?  Well, few studies demonstrated for some of the natural mosquito repellents even longer effectiveness than DEET. Still, other studies showed that the same repellents do not even come near the DEET active duration.

Natural Mosquito Repellent

The EPA publishes a list of substances they accept as mosquito repellent. Though authorities in countries around the world are not obliged to accept the American ruling regarding mosquito repellents, in many countries the EPA decisions are respected. As for today, the EPA recognizes the following natural mosquito repellents, but according to the American agency none of them is approved for 8 hours as DEET:

  • Catnip Oil – also known as catmint (7 hours duration!)
  • Oil of Lemon Eucalyptus – (6 hours duration)
  • 2-undecanone – which is derived from wild tomato (4.5 hours)
  • P-Menthane-3,8-Diol – also known as PMD, made of oil of lemon eucalyptus (2 hours)
  • Citronella – (1 hour)

Other natural mosquito repellent compounds that are commonly mentioned (but not by the EPA) are neem oil, garlic extract, thyme essential oil, celery seed oil, mint oil, spearmint oil, rosemary oil, lavender oil and geranium oil. Furtheremore, people from many parts of the world use natural mosquito repellents from other plants that are unfamiliar to most western society.

Most studies with the aforementioned natural repellents showed the mosquito repelling efficiency close to 100% but others showed less than that. Furthermore, many of the natural mosquito repellents are highly efficient at the beginning (sometimes even less than an hour) but their efficiency degrades quickly over time.

Out of the natural mosquito repellents I reviewed, the ones recognized by the EPA (see above) are the most promising ones and also Neem, Rosemary, Lavender and Geranium oils. Since some studies on these natural mosquito repellents demonstrated higher efficiency than others, it appears the natural repellents have different efficiency for different mosquito species, at different parts of the world and under different personal conditions (running/resting, before/after shower, etc.).

To summarize, using natural mosquito repellents is a very wise decision how to avoid mosquito bites and should be selected per its effeciveness and durability over time. Catnip oil is currently the most long lasting one, but Lemon Eucalipus oil is a very good choice too. One best consider his odor preferences too before purchasing his personal repellent.

Mosquito Traps

In contrast to repellents, mosquito traps base their protection on attracting mosquitoes instead of repelling them. Most trappers are pleased to watch how many mosquitoes they catched but overlook the number of mosquitoes that were attracted but managed to escape…

Furthermore, we humans will always be more attractive to mosquitoes than any mosquito trap, as they have evolved to detect and be attracted mainly to us! As good as mosquito traps will be, they will never be more than a mimic of us… For this reason, many entomologists reject the idea of using mosquito traps (and other nuisance insects trapping) as a mean to locally reduce mosquito population.

Mosquito Traps

Nevertheless, mosquito traps can be quite useful as a complementary measure together with a more efficient local control method. For example, use of mosquito repellents in the target area and a mosquito trap somewhat far-off, as an attractant away from the target area.

The most common mosquito trap is a light trap, based on the fact that Nocturnal (night active) insects are attracted to light. Many mosquito traps are using ultraviolet light as it can be detected by insects, but does not disturb our darkness. These mosquito traps are almost useless for day-biting mosquitoes like the Asian tiger mosquito. Furthermore, they will normally kill other nocturnal insects like many species of moth, more efficiently than the mosquitoes themselves. Such light traps for home use are normally surrounded with an electric coil to zap the insects to death.

Carbon-dioxide (CO2) is a major attractant for the biting mosquito females, a fact that did not elude mosquito trap designers. With the addition of dry-ice, CO2 gas tank or a device that generates CO2, such advanced light traps will increase the catch of mosquitoes, sometimes even 100 times more. Some mosquito traps with CO2 generation even skip the usage of light completely.

The optimal CO2 concentration around a mosquito trap should be similar to the concentration of CO2 around a breathing person. The more sophisticated mosquito traps also mimic our body temperature, generating temperature of 37ºc (99ºf) around the trapping point. Modern mosquito traps even add additional odors that attract mosquito females, in their search for making the mosquito trap even more attractive. Such odors can be of lactic acid and even human sweat…

We focused here on mosquito traps for the adult biting mosquitoes. Other kinds are aimed to attract the mosquito females when searching for a place to oviposit (lay eggs). Since the mosquito oviposits only after getting her blood meal (i.e. biting us), the oviposition mosquito traps are of no real interest to the private consumer and are used mainly by researchers or as part of a large mosquito control campaigns.

To summarize, the best domestic mosquito traps should include as many attractants as possible and should be located only in the surrounding area. Such a trap will be fairly effective as it will attract and catch as many mosquitoes as possible away from us. Putting the mosquito trap too close to us can actually have negative impact, since it will summon mosquitoes to our vicinity that had no intention to be there in the first place…

Mosquito Control

The main target of mosquito control is to minimize the mosquito population in a given area to control their nuisance and risk for public health.

There are few pratical methods to obtain mosquito control. Still, the general recommendation for effective control is to operate in several types of action. In most cases, only a combination of few methods will significantly reduce the mosquito population size. Here is a review of the most common methods, divided according to the mosquito life cycle stage  affected:

Mosquito Control

1) Adult mosquito control 

Controlling adult (flying) mosquitoes is done by spraying insecticides. Most of the sprays commercially available are chemical agents; however more and more insecticides of vegetal extracts appear in the market lately. The control agent should be replaced from time to time to avoid resistance to appear among the treated mosquito population.

Generally speaking, adult mosquito control is not sufficient to obtain efficient results in the long run. It helps with the immediate relief only. For longer and sustainable impact, the efforts should target the younger stages too: eggs, larvae and pupae.

2) Eggs prevention mosquito control

The possible oviposition (eggs laying) sites should be identified and treated so they would not suit for mosquito larvae. This mosquito control type of action relies on the mosquito female’s high sensitivity to correct oviposition habitat selection. First, dry any possible water source of ponds, etc. to minimize the available larvae locations. If drying is not an option: Clean the water area from local vegetation, exposing the water source to a direct sunlight.

The direct sunlight will overheat the water, making them unsuitable for the larvae. It will also help the natural predators to prey the larvae which will become more detectable to them in the clear (plant free) water. Finally, make sure to seal any opening of water tanks to prevent the mosquitoes to reach the water.

3) Larvae and Pupae mosquito control

This type is covered by three different methods in general: chemical control, physical control and biological control:

  • Chemical mosquito control uses chemical pesticides in the water, which are toxin to the mosquito larvae and pupae (and usually other organism as well). In big water bodies it should be sprayed mostly near the shores. The most common pesticides are: Carbon-chlorides, Pyrethroids, Carbamates and organic Phosphorous.
  • Physical mosquito control is also called MLO (Mosquito Larvicide Oil). This special oil easily spreads all over the water surface and prevents from the larvae and pupae to come up and breathe air, which is essential for their existence.
  • Biological mosquito control is based on using the mosquito natural enemies. A bacteria called BTI is an example of a parasite that kills mosquitoes larvae and do not harm any vertebrate. Predation of the larvae is another example for biological control: Such predator can be invertebrates like back-swimmers, dragonfly nymphs, diving beetles and others. Vertabrate predators can be different species of fish (Gambusia – the mosquito fish is the most famous) and tadpoles of salamanders and newts.

To summarize, remember that the best mosquito control results shall be obtained when using a mixture of the above methods. West Nile Virus and other mosquito transferred diseases can be avoided by consistent control plan in your area!
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