What’s the evidence: Do chickens prevent malaria and Zika infections?

It’s been four weeks since this paper was published, but I’ve been on holiday and received a lot of insect bites, so my interest in a chicken mosquito-deterrent hasn’t waned.

The Telegraph had the headline:

“Suspending a chicken over your bed could protect against Zika virus and malaria”.

They also had some fairly confident remarks from one of the authors about the size of the effect

“can cut populations by up to 95 per cent throughout an entire house, so it’s very efficient”

and possible utility of their tested compounds

“I think it should [prevent Zika]. We haven’t tested it on other mosquitoes but there are lots of varieties which won’t feed on chickens and so would be repelled.”

Malaria kills hundreds of thousands of people each year. Zika is disabling and afflicting thousands this year. Headlines and quotes like this generate a huge amount of hope. But is it false hope? What’s the evidence?

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Photo by Muhammad Madhi Karim

This paper included many different useful and interesting pieces of work. The researchers:

  • collected data on the populations of human and domestic animal species in three villages
  • collected blood-fed mosquitoes from 10 houses and 5 pit shelters in three villages
  • identified mosquito species and source of last blood meal
  • collected headspace samples from 5 individuals from each species in 1 village
  • reared an Anopheles arabiensis colony
  • analysed chemicals present in the headspace of different species
  • measured electrophysiological response of antennae to samples and isolated compounds
  • counted mosquitoes captured in CDC suction traps baited with control solvent, synthetic compounds or a live chicken

 

This is a huge amount of work, so I’m going to focus on the question that seems to be of most interest:

 

Can chickens, or compounds extracted from chickens, reduce malaria or Zika infections in humans better than current mosquito repellents?

 

If this is the question of interest (although we may be more interested in whether we prevent deaths, or disability, rather than reducing the number of cases) then the key evidence is that generated in the final point. From the other aspects of this paper we can see that they started off well. They identified villages with mosquitoes. They identified the species of mosquito that would be present, and that these mosquitoes might be likely to bites humans indoors. They identified compounds that showed activity in a laboratory setting. Then they carried out a randomised controlled trial to see whether these compounds would reduce the number of mosquitoes captured in a trap near sleeping humans.

 

They found that compared to solvent only, the four identifiable chicken compounds, two host compounds, and a live chicken, significantly reduced the number of mosquitoes captured in a suction trap. This sounds really promising – but there are problems.

 

Surrogate outcome

They’re not measuring how many cases of malaria or Zika were prevented, and they’re not counting the number of bites people received. They are only counting the number of mosquitoes were captured in a trap near a person – which might be a very poor surrogate for measuring how many cases of malaria or Zika were prevented.

 

Study site

They chose one of the three villages they studied in Ethiopia. Is this relevant to Zika transmission in cities in Brazil? Is it representative of the other populations in which mosquito repellents might be used?

 

Mosquitoes

The species of mosquito examined is one that transmits malaria in some areas, but does not transmit Zika – how certain are we that these findings would be replicated in other mosquito species?

 

Control group

The control they used was solvent – the same solvent that some of the tested compounds were solubilised in. Perhaps it would have been more useful to know how the test compounds compared to existing repellents. I’m not sure what a good control for a live chicken would be…

 

Statistical analysis

The statistical analysis was carried out post-hoc (with no mention of the analysts being blinded to the treatment groups) – the data was examined, then a method for analysing it was devised – this is known to lead to bias giving more positive findings.

 

Other sources of bias

Randomisation method, allocation concealment, blinding of personnel and outcome assessment, missing data and selective reporting – none were adequately described by the paper, but none seemed likely to be the source of a high risk of bias.

 

In summary, I think the findings of the RCT are likely to be fairly accurate. But they don’t justify the hype and the hope. This is not evidence that chickens prevent cases of Zika or malaria. When we have an RCT that looks at the mosquito species relevant to both diseases, in a variety of settings in which a repellent would be used, examining the effect of the repellent compared to the current best repellent, and examining the number of deaths, disabilities and infections, and finding at least one of the first two significantly reduced, then we can get really excited.

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