When tsetse flies fall for a host, they keep coming back for more

Tsetse flies, which transmit the trypanosome that causes sleeping sickness in humans and its equivalent in animals, primarily feed on the blood of various animals, such as ruminants, reptiles and humans. However, flies that fed on a given species first time around tend to return to the same species over the next couple of days, rather than changing hosts.

To achieve this result, the researchers worked in a laboratory in Burkina Faso with swarms of 125 flies, each taken from a population of some 100 000 flies reared at CIRDES. Those swarms were then offered a menu with a single dish: one ruminant or reptile species. A few days after that single dish, they were given a choice of two species. Once they had fed, they were dissected to see where the blood they had eaten came from.

Over the next two days, the flies chose the blood of the same species

The results were statistically processed, and showed that the flies that had fed on a ruminant first preferred to feed on a ruminant second time around, rather than on a reptile. Likewise, flies that had fed on a reptile first preferred a reptile second time around. However, while this was the case if the interval between meals was less than two days, it was not so for longer intervals. The experiment was repeated for a three-day interval, and the flies, who were short of food by then, fed indiscriminately on ruminant or reptile blood irrespective of their original host: one's always less difficult when one's hungry!

These results could help us to understand, and even control, trypanosomes and how they are transmitted between species. The stakes are high: of the 42 poorest countries in the world, 32 are in Africa and are home to tsetse flies. Trypanosomiases are found throughout two thirds of the continent of Africa, and cause the deaths of three million head of livestock and the loss of 500 000 tonnes of meat and 1 million tonnes of milk each year. More than 60 million people are at risk of catching the parasite, which kills 100 patients a day.