Category Archives: Mosquitoes

Know Your Mosquitoes

Recently we have seen some online discussion about mosquitoes biting 24/7, and while this is true, it is not all species of mosquitoes that bite all the time – only that anytime during the day/night one might be bitten, but by different types of mosquitoes, carrying different diseases at different times. Below is a chart that tries to draw some of the distinctions among the different types of mosquitoes.  It is not all inclusive. Some references are listed at the end. Finally there is an abstract about possible changes in malaria mosquito biting behaviors, although we should use caution in that this has not been verified universally.


Reference Links

  • How Mosquitoes Work.
  • Be vigilant to different mosquito breeding grounds.
  • Biological Notes on Mosquitoes.
  • Mosquito. From Wikipedia, the free encyclopedia.
  • Anopheles Mosquitoes.
  • Differentiate Culex, Anopheles and Aedes Mosquitoes.
  • Flight performance of the malaria vectors Anopheles gambiae and Anopheles atroparvus.

Effects of changing mosquito host searching behaviour on the cost effectiveness of a mass distribution of long-lasting, insecticidal nets: a modelling study. Malaria Journal 2013, 12:215 doi:10.1186/1475-2875-12-215. Olivier JT Briët ( Nakul Chitnis (

Abstract: Background The effectiveness of long-lasting, insecticidal nets (LLINs) in preventing malaria is threatened by the changing biting behaviour of mosquitoes, from nocturnal and endophagic to crepuscular and exophagic, and by their increasing resistance to insecticides. \

Methods: Using epidemiological stochastic simulation models, we studied the impact of a mass LLIN distribution on Plasmodium falciparum malaria. Specifically, we looked at impact in terms of episodes prevented during the effective life of the batch and in terms of net health benefits (NHB) expressed in disability adjusted life years (DALYs) averted, depending on biting behaviour, resistance (as measured in experimental hut studies), and on pre-intervention transmission levels.

Results: Results were very sensitive to assumptions about the probabilistic nature of host searching behaviour. With a shift towards crepuscular biting, under the assumption that individual mosquitoes repeat their behaviour each gonotrophic cycle, LLIN effectiveness was far less than when individual mosquitoes were assumed to vary their behaviour between gonotrophic cycles. LLIN effectiveness was equally sensitive to variations in host-searching behaviour (if repeated) and to variations in resistance. LLIN effectiveness was most sensitive to preintervention transmission level, with LLINs being least effective at both very low and very
high transmission levels, and most effective at around four infectious bites per adult per year. A single LLIN distribution round remained cost effective, except in transmission settings with a pre-intervention inoculation rate of over 128 bites per year and with resistant mosquitoes that displayed a high proportion (over 40%) of determined crepuscular host searching, where some model variants showed negative NHB.

Conclusions: Shifts towards crepuscular host searching behaviour can be as important in reducing LLIN effectiveness and cost effectiveness as resistance to pyrethroids. As resistance to insecticides is likely to slow down the development of behavioural resistance and vice versa, the two types of resistance are unlikely to occur within the same mosquito population. LLINs are likely cost effective interventions against malaria, even in areas with strong resistance to pyrethroids or where a large proportion of host-mosquito contact occurs during times when LLIN users are not under their nets.


Finally please note that one malaria intervention alone will not solve our problems so we need to apply a mix that includes Nets, Indoor Residual Spraying, Diagnosis with mRDTs, Appropriate treatment with Artemisinin-based Combination Therapy, Intermittent Preventive Treatment, one day a vaccine and others …

Malaria Vector Bionomics During the Dry Season in Nchelenge District, Zambia

Smita Das and Douglas E Norris of the Johns Hopkins Bloomberg School of Public Health Department of Molecular Microbiology and Immunology and Johns Hopkins Malaria Research Institute have written our guest blog posting based on a poster they presented at the recent JHU Global Health Day.

picture1-smita-das-and-douglas-norris-jhmri-sm.jpgAs part of the International Centers of Excellence in Malaria Research (ICEMR) in Southern Africa project, mosquito collections are being conducted in Nchelenge District in Luapula Province, Zambia. Nchelenge experiences hyperendemic malaria despite continued implementation of indoor residual spraying (IRS) and long-lasting insecticide nets (LLINs) as control measures.

Center for Disease Control light trap (CDC LT) and pyrethroid spray catch (PSC) collections performed during the wet season in April 2012 revealed the presence of both Anopheles gambiae s.s. and An. funestus s.s. Both species were highly anthropophilic and the Plasmodium falciparum sporozoite infection rate in An. funestus was higher compared to An. gambiae.

In the dry season collections, An. funestus continued to be the dominant species with even fewer An. gambiae caught compared to the wet season.  Due to the abundance of An. funestus and high human malaria infection rates in Nchelenge, it is predicted that the human blood index and entomological inoculation rate for An. funestus is higher than that of An. gambiae in both seasons.

The multiple blood feeding behavior and insecticide resistance status of both malaria vectors will also be explored as this can give us an idea of estimating the transmission potential of these mosquitoes. The vector data in Nchelenge present unique opportunities to further our understanding of malaria transmission and the implications for malaria control in high-risk areas.

Modeling Malaria – getting a handle on vectors

Models represent reality but the closer they come to reality, they better they are at helping us plan.  A session at the American Society of Tropical Medicine and Hygiene yesterday addressed the modeling process for vector control.

VECNet is developing the capacity to take data from multiple sources to tailor vector populations and behavior to local situations. Such models need to consider vector bionomics/population variables, weather/climate/environment, and effectiveness of deployed vector control strategies.

a-stephensi-map-project-2.jpgModelers encourage us to think beyond existing malaria control strategies and consider a varierty of mosquitoe behaviors beyond direct feeding on humans and immediate resting thereafter. Such understandings can lead us to ask whether new interventions could be directed at other vector bevahiors such as …

  • laying eggs (oviposition)
  • feeding on sugars
  • seeking hosts
  • mating
  • resting generally

In short, we were challenged to look at aspects of vector biology that have been ignored or unknown in the past.

nga_gambiae_ss-sm.pngThe MAP project out of Oxford is also beginning detailed mapping of vectors by region and utlimately my country.  Globally there are 41 dominant vector species, so the work ahead is immense, but some mapping has started with three in a program called Risk Mapper.

The session also included product impact estimation. This should help program planners decide on hypothetical outcomes of investments in different existing interventions and even consider possible outcomes were new interventions developed to address the other aspects of mosquito behavior outlined above – e.g. traps, repellents.

The modeling process requires a lot of data that needs to be updated as control interventions proceed. Such data requires a strong corps of entomologists and health information systems staff that many countries lack.  Hopefully modeling efforts will also include these elements of human resource development.

Ivermectin against malaria: novel idea, but can it be scaled

Several news sources have picked up on a new article in the American Journal of Tropical Medicine and Hygiene that reported when communities take annual ivermectin doses for controlling onchocerciasis and lymphatic filariasis, they may also be protecting themselves or their neighbors from malaria. Specifically the researcheds reported that ivermectin Mass Drug Administration (DA) “reduced the proportion of Plasmodium falciparum infectious Anopheles gambiae sensu stricto (s.s.) in treated villages in southeastern Senegal.”

The process works the same way that ivermectin treats head lice in that when insects take a bloodmeal from someone who has swallowed the drug, the medicine kills the insect. Sarah Boseley points out that some of the attractions of ivermectin are that it has been safely used in humans for 30 years and that it is inexpensive.

The Merck company has been supplying ivermectin (under its brand name Mectizan) free through the Mectizan Donation Program (MDP) to the African Program for Onchocerciasis Control for 16 years.  Over time coverage has reached over 100,000 villages in 18 African countries. Annual distribution in Africa contrasts with more frequent distribution in the smaller focal transmission points in Latin America where the diseases has almost been completely eliminated.

MDP reports that, “Currently, more than 70 million treatments are approved for onchocerciasis in Africa and Latin America and 80 million for lymphatic filariasis in Africa and Yemen each year.” That is a lot of free medicine and one of the largest and far reaching corporate social responsibility programs known.

Back to Senegal – the researchers found that the effect of ivermectin on mosquitoes lasted up to two weeks. They also raise the question of whether more frequent ivermectin distribution in onchocerciasis or filariasis MDA communities during the main malaria transmission season would be feasible.  Possibly a small scale operations research proposal could be submitted to the Mectizan Expert Committee.

cdi_report_08.jpgOf importance is the fact that the Community Directed Treatment with Ivermectin (CDTI) approach utilized by APOC projects has been tested and found quite accommodating to the addition of other Community Directed Interventions (CDI) such as community case management of malaria, ITN delivery and use monitoring and Vitamin A distribution among other basic health services.

The availability of tens of millions of exra ivermectin doese in communities where MDA is already occurring is unknown at present, let alone the feasibility of starting free ivermectin in malarious areas that have no onchocerciasis or filariasis.  In addition, for onchocerciasis, the idea time for distribution is before the rainy season so that microfilariae loads are seriously reduced before the black fly vectors emerge. This timing may not benefit malaria control fully.

Regardless of the unknowns, it is encouraging that people are thinking of synergistic ways to control the various endemic diseases that inflict suffering on poor communities.

Mosquito Mis-Match

dscn3027sm.jpgVisiting a clinic in Binji Local Government Area of Sokoto State Nigeria today we saw the attached poster.  In fact there were three copies posted on walls in the public waiting areas. It was not clear whether this poster was intended as a job aid (reminder) for the health staff or a public education poster. If the latter, there is too much happening on the poster for it to be a good BCC material.

The main concern about this poster is that it implies that all mosquitoes are responsible for malaria. The pictures in fact implicate other mosquitoes such as the culicine species and Aedes aegypti.  Covering pots and getting rid of old cans and bottles will prevent mosquito breeding, but more than likely will prevent yellow fever, not malaria.

Clearing dirty gutters can prevent breeding of culex and the various viral diseases they carry, but not malaria. Yes, we want people to prevent mosquito breeding generally, as there appear to be no ‘good’ mosquitoes, but promising that preventing breeding of non-anophelene species will prevent malaria is misinformation.

We can be straightforward with the public and say there are many types of mosquitoes, and they carry many different diseases. It helps to prevent the breeding of all, but as different ones breed in different places, we can not expect every action will result in the prevention of malaria. We also need to stress that not all fevers are malaria, and we need testing to ensure the right medicine is given, not ACTs for dengue or west nile virus (which are carried by the other mosquitoes – adding to the public’s confusion).
At present there are two major vector control measures recommended by the RBM community – insecticide treated bednets and indoor residual spraying.  A few places have been successful with limited larviciding, but since breeding sites are ubiquitous, this is an almost never ending activity.  Lets focus on what is feasible.

Finally we need to recall that when we talk to local people about mosquito breeding, they may not understand what we mean by mosquito larvae.  Among the Yoruba, for example, these larvae are called tanwiji and are thought to be a different animal from actual mosquitoes themselves.  People have been confused by attempts at BCC on larval control thinking that health workers are saying that swallowing the tanwiji causes malaria.

Again, for now lets make headway with our treated nets and appropriately targeted IRS.  If we can achieve those coverage targets, the 2015 Millennium Development Goals will be achievable.

Mosquitoes also do not want to be infected

When the small copepod or cyclops swallows a guinea worm larva, seeing it as food, several things may happen as the larva develops – either someone swallows the cyclops when drinking the pond water, continuing the guinea worm cycle, or the larva grows so large that the cyclops is destroyed. Control measures have included putting temephos in ponds to kill the cyclops. All of these mean death for the cyclops. None of these alternatives bode well for the cyclops.

dscn0333-sm.jpgIt is not surprising to learn, as reported in PLoS Biology that disease vectors or intermediate hosts themselves are not very ‘happy’ to get infected with disease organizms that are later passed to humans. In essence Anophleles gambiae mosquitoes have genes that encode ‘essential components of the mosquito immune defense against malaria parasites.’

Furthermore, these genes are not static. Rottschaefer and colleagues report that, “Our data indicate that functionally variable APL1 alleles are evolutionarily maintained to combat diverse pathogens, perhaps including but probably not restricted to Plasmodium species.”

Most malaria control measures to date that involve the vector are aimed at killing, repelling or sterilizing.  These range from the newest, a toxic sugar bait, to the widely used instecticide treated bednets. There is exploration into a human vaccine that would prevent the parasite from developing in the mosquito.

While mosquitoes are certainly a nuissance in their own right, they are not necessarily the main enemy in the fight against malaria. We should certainly continue using bednets and indoor spraying and in appropriate cases larviciding, as major gains have been made from these. It would be hard though to completely eliminate the vectors.  Therefore continued research is needed on vaccines and genetic modifications that make mosquitoes a hostile host to plasmodium species.

The great outdoors … for mosquitoes

The press has had a field day over the publication of data in Science that identifies a new population subgroup of the African malaria vector Anopheles gambiae. The mosquito variant in adult form is hard to catch and in the laboratory, very susceptible to malaria infection.

The authors clearly point out that there is no evidence yet that the GOUNDRY mosquito, named after a village in Burkina Faso where the researchers worked, is highly attracted to humans. The press has rounded up experts like William Black, a medical entomologist at Colorado State University to speculate on the matter. Black was quoted in the LA Times thus …

“We’ve got egg on our face. We’ve been working with this mosquito for so long … and right under our noses, here’s this other form of mosquito,” he said — one that could force researchers “to start thinking about what’s going on outside of those huts.”

Again Science itself accurately headlines an editorial on the article by saying that these findings could have ‘unexpected’ results.

Because the new mosquitoes were found in the same larval pool collections and were genetically indistinguishable from indoor-resting adult mosquitoes, the options for control point to larviciding should ‘Goundry’ prove a threat to humans.  This is a challenging control measure as not all larval pools are easily visible and in fact may be multitudinous. Of course prompt and appropriate treatment and IPTp have to be part of the mix, despite health systems weaknesses in delivering medicines that may be more of a threat right now than the mosquitoes.

The key lesson is not that an immediate, previously unknown threat lurks outside our huts, but that nature and malaria can continue to surprise us.  We have surveillance in Southeast Asia for parasite resistance and the growing potential for human-primate malaria transmission, to name a few of the upcoming challenges to eliminating this ancient disease. Vigilence is the key.

Two for One – ivermectin and mosquitoes

Suppose that when mosquitoes bit you, they died. The possibility that a human blood meal can kill mosquitoes sounds far fetched, but has been observed as a by-product of mass community mass drug administration (MDA) of ivermectin for lymphatic filariasis in Senegal.

Researchers in Senegal compared villages where MDA was performed with a control set of villages and concluded that, “Ivermectin MDA significantly reduced the survivorship of An. gambiae s.s. for six days past the date of the MDA, which is sufficient to temporarily reduce malaria transmission. Repeated IVM MDAs could be a novel and integrative malaria control tool in areas with seasonal transmission, and which would have simultaneous impacts on neglected tropical diseases in the same villages.”

This is not the only time links have been made between malaria and lymphatic filariasis. A good example was community distribution of insecticide treated nets were provided in Nigeria, which curtailed the mosquitoes that carried both diseases.

In MDA programs for filariasis control “The goal is to treat 80% of the eligible, at risk population yearly, for at least 5 years, in order to interrupt transmission and prevent children from becoming infected.” The window of opportunity for collaboration between MDA and malaria control programs is therefor, very focused.

A smaller scale study reported in July of this year found that, “In mosquitoes feeding on volunteers given ivermectin the previous day, mean survival was 2.3 days, compared with 5.5 days in the control group (P < .001, by log-lank test). Mosquito mortality was 73%, 84%, and 89% on days 2, 3, and 4 in the ivermectin group." Since ivermectin started as a drug for veterinary parasites, it is useful to note that similar results on mosquitoes were found after cattle were given ivermectin. Researchers from Michigan State University* reported that, "Most (90%) of the An. gambiae s.s. that fed on the ivermectin-treated cattle within 2 weeks of treatment failed to survive more than 10 days post-bloodmeal. No eggs were deposited by An. gambiae s.s. that fed on ivermectin-treated cattle within 10 days of treatment." The authors concluded that, "Treatment of cattle with ivermectin could be used, as part of an integrated control programme, to reduce the zoophilic vector populations that contribute to the transmission of the parasites responsible for human malaria." community-ivermectin-distribution-in-buea-cameroon.jpgIvermectin distribution of course has been the major strategy of the the African Program for Onchocerciasis Control (APOC) for the past 15 years.  APOC’s efforts will continue much longer than those of lymphatic filariasis in over 100,000 communities throughout the continent.

More attention to joint planning and coordinating of malaria and other disease control efforts should be synergistic and mutually beneficial for the populations, who according to APOC, live beyond the end of the road.


Update on Malaria Research in Mozambique

Arsenio Manhice, a journalist from Mozambique, provides us an update on malaria research at a leading institute in his country. A version of this report appeared in Portuguese in the newspaper “notícias“:

cism_logo.gifA series of scientific initiatives are underway at the Center for Research in Health Manhica (CISM) aiming to provide solutions to tackle the problem of resistance to drugs and insecticides used against malaria.

According to Eusébio Macete, Director of the biomedical research institute, among other initiatives, scientists are collecting mosquitoes that transmit the malaria parasite. The exercise includes an analysis of the different episodes of illness in people who arrive at clinics in the district of Manhica.

“We hope to have a block of information that can monitor the trend of malaria in its most complex context. That is why we consider the clinical aspects and impact of various measures are being introduced to control the disease as spraying, use of mosquito nets and medication,” the Director said.

For the purpose of study and possible solutions, the researchers began to distribute mosquito nets in the province of Sofala. Districts were chosen Inhaminga, Mwanza, Nhamatanda and Gorongosa.

dscn8015-sm.JPGThis is a joint initiative between the Centre for Health Research Manhiça, US President’s Malaria Initiative (PMI) of the United States, PSI and the National Malaria Control Program. PSI is involved in the local distribution of mosquito nets.

In Manhica CISM will monitor the transmission of malaria to know how it varies. “We do what are called cross-sectional studies that look at aspects such as the number of people who were infected and number of mosquito nets, houses fumigated and malaria cases registered in hospitals,” the Director said. It is an annual activity.

Studying malaria in pregnant women is another component of research that is being seen by scientists. This arises because one of the guidelines of the National Malaria Control is using intermittent preventive treatment with sulphadoxine-pyrimethamine (SP).

Due to the resistance of parasites to SP in other countries, the CISM is preparing a new alternative to save pregnant women. The initiative is from Mozambique and four other African countries. Having started in March 2009, the study ends in the middle of next year. “The goal is to see if mefloquine might have the same effect as SP in terms of preventing malaria in pregnancy.”

New solutions are not enough. Macete encourages people to use the tools to combat malaria are available. “Certainly there is a complexity that is the durability of the nets during the rainy season. The technicians who do the spraying must find the balance needed for example to do more patrols and use insecticides that last longer,” the Director stressed.

For now, the scientist believes that much work must be done to adjust the conditions of the country versus the available financial resources, characteristics of transmitters and type of insecticides available in the market.

Breweries should become malaria partners

PLoS One has just published a study from Burkina Faso entitled, “Beer Consumption Increases Human Attractiveness to Malaria Mosquitoes.” Beer and water consumers were compared and “Water consumption had no effect on human attractiveness to An. gambiae mosquitoes, but beer consumption increased volunteer attractiveness.”

african-beers-sm.jpgSpecifically, “Body odours of volunteers who consumed beer increased mosquito activation (proportion of mosquitoes engaging in take-off and up-wind flight) and orientation (proportion of mosquitoes flying towards volunteers’ odours).”  The authors therefore concluded that, “beer consumption is a risk factor for malaria and needs to be integrated into public health policies for the design of control measures.”

This is not the first study to look at what attracts mosquitoes to human beings. For example in 2003 Mukabana and colleagues found that “… mosquitoes preferred certain individuals despite being presented with emanations of three persons simultaneously.”

A year later BBC reported on another study the found that, “A key chemical found in
sweat is what draws the mosquito that spreads malaria in Africa to bite its human victims.” The researchers from Yale and Vanderbilt indicated that, “The chemical, or odorant, in sweat responsible for this attraction is called 4-methylphenol.”

Researchers from the University of Florida also explained that, “… the process of attraction begins long before the landing. Mosquitoes can smell their dinner from an impressive distance of up to 50 meters … This doesn’t bode well for people who emit large quantities of carbon dioxide.”

So back to beer drinking – the smells and chemical attractants emitted by the beer drinkers put them at risk. What can be done?  During these days of counting malaria out we need all the partners we can get.  We ask whether breweries are contributing their fair share to protecting their customers and the customers’ families from mosquitoes? Shouldn’t breweries contribute a certain portion of the price of each bottle to the national malaria control program or an appropriate NGO?

Breweries are known for having contests and give aways at local pubs in order to increase sales – instead of giving away caps and t-shirts with the beer logos, maybe they should now give out insecticide treated bednets with their logos.  The role for corporate responsibility by the breweries could not be more clear.