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Breakthrough in identifying biting midges

October 8, 2011

Scientists at the Antwerp Institute of Tropical Medicine have developed a molecular technique to easily and dependably identify the biting midges that spread bluetongue disease.

No-see-ums are very small. In this picture, one is compared to the common mosquito. © ITG
The same types of biting midges are also able to spread deadly African horse sickness, a disease seen as posing a growing potential threat to the European horse industry.

Until now, identification of biting midges was a problem.

The technology developed by the scientists also helps to understand how bluetongue spreads, and how to control it. They report in the journal Medical and Veterinary Entomology.

Bluetongue primarily affects sheep, but cattle and other ruminants are also affected.

It is not dangerous to people, but it causes great economical damage. Until a few years ago, the disease did not hit northern Europe.

People assumed only tropical midges could transfer the responsible virus. Also, the virus only replicated in the midges over 15 degrees Celsius. But in 2006 The Netherlands and Belgium were hit.

Once the virus was present, local midges were showed to be capable of transmitting it.

There are a lot of species called 'biting midge' (Cullicoides to biologists), and with classical tools it is almost unfeasible to differentiate them - there is a reason why they are popularly called no-see-ums.

They must be placed under a microscope and measured in minuscule detail, even down to the size of their genitals, to identify them. It was work for specialists.

The Antwerp researchers noted that if scientists did not know which midges transmit the disease, they would not know where it will spread, or where to fight the insects.

The researchers developed a simple and cheap molecular identification technology that identifies cullicoides species in the lab with 100 per cent certainty.

They concentrated on the most important species in northern Europe, Culicoides obsoletus, C. scoticus, C. chiopterus and C. dewulfi, but they can reliably identify more than 20 species.

Tests on larvae are also promising. Until now, there has been no way to differentiate larvae.

The scientists produced a "gene chip", or micro-array - a glass slide to which short pieces of DNA are attached - that are characteristic for each species.

Such a slide can contain several dozens of different pieces of DNA, on well documented positions.

When the gene chip is washed with DNA from an unknown midge, identical pieces of DNA will recognise and stick to each other. This causes a blue colour reaction at that spot.

With the naked eye one can recognise the characteristic pattern of spots for each species.

Contrary to other molecular identification tests, this one is very specific, and able to recognise several species at the same time. It helps to determinate midges that are difficult to recognise under the microscope.

And it works. When the French CIRAD (Centre for International cooperation and Research for Agronomy and Development) sent out a ring test to see which labs could identify four species of biting midges with molecular methods, ITG was the only institute to identify all samples correctly, three times in a row.



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