杏吧原创

MOSQUITOES that are resistant to insecticides might turn out to be friends,
not foes, in the war against killer diseases such as malaria and yellow fever.
Researchers in Cardiff have turned received wisdom on its head with their
discovery that resistant mosquitoes are the least likely to transmit disease
when they bite someone.

鈥淓veryone has always assumed that insecticide resistance is a bad thing
because you get more insects that live longer, and so transmit more disease,鈥�
Janet Hemingway from Cardiff University in Wales told New 杏吧原创.
But her research in Sri Lanka suggests otherwise.

Hemingway discovered that Culex quinquefasciatus mosquitoes resistant
to organo-phosphate insecticides are less able to transmit lymphatic filariasis
than ordinary flies. Caused by Wuchereria bancrofti鈥攁 parasitic
worm spread by mosquitoes when they feed鈥攖his disfiguring disease leaves
victims with huge swellings in their limbs and genitals, and afflicts around 120
million people worldwide.

Hemingway is looking to see if the same phenomenon occurs in mosquito species
that spread malaria. If it does, health authorities might have to rethink their
strategies for tackling these and other mosquito-borne diseases such as dengue,
yellow fever and Japanese encephalitis. 鈥淭he research shows we may be able to
live with resistance because there may be benefits,鈥� she says.

In Sri Lanka, Hemingway and her colleagues from local universities visited
patients with filariasis in seven districts. Arriving each morning, they
captured mosquitoes which had been feeding during the night and crushed them for
analysis.

They found that the most resistant insects were virtually free of parasites,
as measured by the amount of detectable parasite DNA. By contrast, 80 per cent
of the non-resistant mosquitoes were infected with W. bancrofti.

Next, she fed insects in the lab with infected blood and analysed them two
weeks later. She found that in three quarters of the 250 non-resistant insects,
parasitic larvae had reached the salivary glands. But she found no worms at all
in the salivary glands of the 200 resistant insects. This suggests that the
larvae perish in the gut.

Hemingway is now unravelling how this might happen. One possibility is that
the parasite larvae are killed in the gut by an enzyme called esterase, which
usually enables resistant flies to destroy organo-phosphate pesticides. But she
has found that high levels of esterase can alter the chemical composition of gut
cells. She wonders whether this prevents the worms penetrating the gut wall and
entering the insect鈥檚 blood system and salivary glands.

Chris Curtis of the London School of Hygiene and Tropical Medicine, says
Hemingway鈥檚 findings contrast with his own in Dar es Salaam, Tanzania 20 years
ago. Over 80 per cent of resistant Culex insects he dissected had
larvae in their salivary glands. 鈥淧erhaps only some amplified esterases have
this fortunate side effect of destroying developing filariae,鈥� he says.