Wetenschap - 2 augustus 2016

Less malaria with mosquito trap

tekst:
Albert Sikkema

Eradicating malaria seems to have come a big step closer thanks to a simple mosquito trap and a carefully designed bait. But we are not there yet, caution the researchers.

©Melchert Meijer zu Schlochtern

Wageningen entomologists created a sensation last month with the results of their Solarmal project on the Kenyan island of Rusinga. Newspaper headlines around the world talked of a ‘breakthrough’, ‘mosquitoes driven to despair’ and the ‘eradication’ of malaria. Although there is no immediate prospect of eradication of the disease, the excitement is certainly justified. During their four-year period of trials with odour traps on the island in Lake Victoria, the Wageningen researchers succeeded in cutting back the number of malaria mosquitoes by 70 percent. In two years the prevalence of malaria – the percentage of the 25,000 islanders with the malaria parasite in their blood – went down by 30 percent. ‘That is a very big decrease in such a short time,’ says entomologist and project leader Willem Takken. The results of the malaria project were presented in Kenya last year and were published in the scientific journal The Lancet at the beginning of August.

New cases

For the study all 4300 households on the island were issued with a mosquito trap containing an odour mixture created in Wageningen. Because the trap runs on electricity, the households were also issued with a solar panel, a battery and a mobile charger. Before and after the odour traps were installed an estimate was made of the number of malaria mosquitoes on the island. The residents were examined for the presence of malaria and the number of new malaria cases – the incidence – was monitored.

Incidence

The incidence dropped during the project period by as much as 93 percent. But that figure needs a bit of adjusting, says Tobias Homan, who obtained his PhD on the malaria project in April. ‘The drop in the number of new cases could be seen all over the island, even in places where there weren’t any odour traps yet, so it cannot entirely be put down to the odour-baited traps.’ In the villages with the traps there were an average of 40 percent fewer new cases of malaria than in villages without traps.

The number of new malaria cases started going down before the odour-baited traps were installed because the islanders had started using impregnated mosquito nets and were getting bitten less frequently. Homan reckons the Wageningen study missed some new malaria cases too. ‘Sick people go to the clinic. We went from door to door to record new cases, and we didn’t call in at the clinic.’

Marvellous

Neverthelss, the results are marvellous, says Homan. ‘At first 35 percent of the islanders had malaria in their systems. That went down to 24 percent in two years. And that group is still walking around with malaria, but there are hardly any new infections because the number of mosquitoes has gone down by 70 percent.’ Without such new infections the malaria parasite will naturally disappear from the human body in 12 months. So the odour-baited traps could be the last little push needed to get rid of malaria completely.

Electricity

The residents of Rusinga have now wholeheartedly adopted this new form of malaria prevention, says Willem Takken. They have formed 82 local saving groups which collect money monthly for the maintenance of the malaria traps and the solar panels. ‘Those savings enable the staff who installed the traps and panels for the project to go on doing maintenance so the mosquito traps stay in working order.’

The complete set of trap and solar panel costs 150 euros per household.  This time it was paid for by the project but even if a household had to pay for it itself, it is not an impossible sum, thinks Takken. After all, the odour-baited trap is proving effective and people in Africa tend to spend about 10 percent of their income on fighting malaria.

Bed nets

Malaria has been on the wane in Africa for the past ten years thanks to the distribution of bed nets impregnated with insecticides. ‘This anti-malaria programme funded by the Global Fund with money from the rich industrial countries has been highly successful,’ says Takken. ‘The number of malaria cases worldwide has gone down by 50 percent. And the number of deaths from malaria has gone down from two million in 2005 to half a million people per year in 2015.’

What is worrying, however, is that the drop in the number of Africans with malaria – now at 30 percent – is not continuing. Takken sees two reasons for this. Firstly, mosquitoes are biting people more outside the house, where they are not protected by a net. Secondly, there is evidence that malaria mosquitoes are becoming resistant to the insecticides. Takken: ‘That always happens sooner or later, as we saw with DDT as well. That is why we have opted for non-chemical measures using odour-baited traps. Our bait mixture imitates the smell of humans. The mosquitoes cannot cope without the bait of those odours, which they need in order to find their food. As a result, this approach could be very sustainable and could be a solution to the resistance problem.’

Positive responses

Other malaria researchers are impressed by the results of the Solarmal project. ‘This is tremendously good news because it proves that odour-baited traps are effective in the battle against malaria,’ says Remco Suer, co-founder of the Wageningen company In2Care. ‘This demonstrates that controlling vectors can make a real contribution to combatting a disease.’ In2Care makes mosquito traps with insecticide for combatting malaria and zika. An interesting detail: In2Care too worked for a while on odour mixtures that imitate human odours in order to attract malaria mosquitoes. ‘But we abandoned this line of research because it was too tricky,’ says Suer.

Leading British malaria researcher James Logan is enthusiastic too. ‘This study demonstrates a significant effect of trapping on malaria and malaria mosquitoes,’ responds Logan, a researcher at the London School of Hygiene and Tropical Medicine. ‘This highlights the fact that novel interventions could play an important role in the control of mosquito-borne diseases, alongside conventional methods like bed nets and insecticides.’

Limitations

Yet the odour traps do have limitations which call for clarification or improvements. They caught the commonest mosquito on the island, Anopheles funestus, but had no demonstrable effect on the reduction in numbers of Anopheles gambiae and A. arabiensis, two other carriers of malaria in Africa. ‘That was because their numbers were already very low on Rusinga as a result of the use of bed nets,’ says Alexandra Hiscox, who coordinated the malaria project in Kenya. ‘For that reason we could not demonstrate a significant effect.’ Nevertheless, the odour trap indisputably attracts the other malaria mosquitoes too, according to Hiscox. ‘We developed the odour mixture in Wageningen using A. gambiae, and now on Rusinga it is obviously effective for A. funestus too.’

Follow-up research

Hiscox and Takken are keen to do follow-up research on the Kenyan island to find out whether the traps remain effective and whether malaria can be completely eradicated there in the coming years. ‘I am curious as to whether this will lead to the elimination of malaria or whether the prevalence will get stuck at, say, 10 percent.’

One issue to address in trying to keep the traps effective is that the odour mixtures need replenishing every three to four months. Hiscox wants to find out whether that works out in practice. Meanwhile the Wageningen entomologists and the manufacturer of the odour traps, the German company Biogents, is looking at whether they can develop a longer-lasting odour mixture.

Donors

Takken and Hiscox are also looking for possibilities for rolling out the project over a larger area. A few donors have now expressed interest. And individual Africans have got in touch to say they would like to install the odour traps in their own villages.  Hiscox: ‘We are going to take a serious look at that, but it must be done properly if you are to get the desired effect.’

KADER 1

Complex logistical operation

Tobias Homan got his PhD in April for a study of Wageningen UR’s Solarmal project in Kenya. Together with his colleague Alexandra Hiscox, the Swiss Tropical and Public Health Institute and the Kenyan institute Icipe, he had to think up and set up a sound experimental procedure – on an island without electricity or infrastructure.

For the 25,000 islanders, 4300 traps had to be installed in villages and at isolated houses. The island was divided into 81 clusters of about 51 households each, all of which received an odour trap, a solar panel and a battery over two years. But which cluster should the team start with? They had to establish a protocol for that, so the measurements in clusters with an odour trap could be compared with those of clusters without a trap.

All the information had to be registered at three levels: cluster, household and individual. Homan: ‘Then you act like a local council. The residents’ codes have to be right and you need to keep track of things like relocations during the period. Setting up this data system took a year.’

In the course of the project the entomologists worked with an entirely digital data collection system for the first time in a developing country. All the households on the island, which is 16 kilometres long and 5 kilometres wide, were documented with a GPS, making all the residents’ medical, entomological and social data available per location. The digital data system also enabled researchers all around the world to access the latest research data.

KADER 2

Do the mosquito traps work against zika too?

If you can beat back malaria mosquitoes such as Anopheles funestus with the Wageningen odour-baited trap, can you also use it to trap the transmitters of the zika virus, Aedes aegypti and Aedes albopictus? Probably, says Wageningen entomologist Alexandra Hiscox. During a small-scale test in Latin America this year, the odour trap caught zika mosquitoes as well, but more thorough research is needed to establish exactly how it works.

Dutch PvdA (labour party) MP Roelof van Laar is interested in this. He has asked minister of Public Health Schippers whether the Wageningen odour trap could be used to combat zika on the Dutch Antilles. Van Laar wonders whether one of the Antillean islands would be a suitable test location.

 


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