In June 2005, two groups of researchers published papers in Science suggesting that fungi that kill mosquitoes could help control the spread of malaria (see Fungus could be next weapon in war on malaria).

Simon Blanford's team from the United Kingdom, and Ernst-Jan Scholte's group from the Netherlands, both showed how spraying walls or cloth with spores of two fungal species — Beauveria bassiana and Metarhizium anisopliae — killed 80 per cent of mosquitoes.

But researchers writing in this week's issue of Science (7 October) warn that the method could have harmful side-effects on human health and the environment.

Marsha Ward and MaryJane Selgrade from the US Environmental Protection Agency say research shows that mice exposed to Metarhizium anisopliae have symptoms similar to human asthma.

Oliver Hutchinson and Andrew Cunningham from the Institute of Zoology, United Kingdom, say that because these fungi also kill other insects, they could create a new breed of resistant insects. This could happen because only resistant insects would survive the fungi, and has already been observed among locusts, ants and termites.

They say the fungi could also kill other larger animals such as fish and reptiles. 

Both groups welcome new weapons against malaria but advise caution in their use.

Blanford's and Scholte's teams respond to these warnings in the same issue of the journal. They say that as the fungal spores would be applied in oil, they would not add to the existing airborne particles that could cause allergic reactions.

Furthermore, they say, the number of different species a fungus can infect depends on individual 'samples' of the fungus. The study that showed the fungi could affect fish was done with much higher doses of spores and with a different strain, they add.

Link to letters in Science