The insect that makes dogs smell good

Every year, black fever or kala-azar kills thousands of people around the world. Among parasitic infections, this disease also called visceral leishmaniasis is nothing less than the second killer after malaria. The disease is transmitted by an insect, a small stinging fly of the sandfly family, the larvae of which then migrate into organs and tissues. As with mosquitoes, only females are dangerous and, if left untreated, the disease they carry is fatal in almost 100% of cases.

If the parasite is common in Europe, it is in Brazil that it is the most ferocious – the country accounts for nearly 95% of the infections recorded on the American continent. Why? Because the insect vector is more successful than elsewhere in laying its eggs on human hosts via another animal reservoir: dogs. How? ‘Or’ What? By modifying their smell to make them particularly irresistible to female flies. This is the main conclusion of a study conducted by Monica E. Staniek and James GC Hamilton, Lancaster University School of Medicine, UK.

Natural reservoirs of the disease

Until now, scientists have known that this parasitic manipulation can manifest itself in rodents which, once infected, change their smell and thus become more attractive to the vector. This logically led to more bites and therefore better transmission of the pathogen. But no study had yet evaluated the effect of Leishmania infantum infection on dogs, the natural reservoirs of the disease in humans to which they transmit it by licking and biting.

To do this, Hamilton and Staniek collected hair and blood from dogs in Governador Valadares, Brazil, where the parasite is endemic. Then, after finding the disease in animals, the scientists extracted the odorous chemicals from the hair of fifteen infected dogs and fifteen healthy dogs, before presenting them to male and female sand flies. As a result, if flies of both sexes were attracted to canine hair, 65.7% of females were attracted to infected samples, a difference absent in male insects.

The main thing remains to be seen: why the smell of infected dogs is so attractive to female flies, which chemicals are the most irresistible and which receptors they activate in sandflies. So much information on the biological interactions involved that can improve the fight against black fever, without having to resort to insecticides that are too harmful to the environment.

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