Parasites may make dogs smell good to disease-spreading sandflies

Parasites that cause the disease visceral leishmaniasis, also known as kala-azar, may make dogs smell more attractive to female sandflies, which feed on a dog’s blood and can pick up the parasite and transfer it to humans through a bite.

“The idea is that it’s a parasite manipulation,” says Gordon Hamilton at Lancaster University in the UK. “The parasite needs to be able to ensure that it is transmitted to the next host. It has to be able to promote itself in some way.”

Leishmania infantum is one of a family of parasites that cause an infection that is fatal if left untreated. While the parasite is widespread in Europe and North Africa, many infections occur in Brazil, which accounts for 95 per cent of all cases in the Americas, according to the World Health Organization. That may be because sandflies there efficiently spread the parasite, says Hamilton.

To study how it spreads, Hamilton and Monica Staniek, also at Lancaster University, gathered samples from dogs in Governador Valadares, Brazil, by walking through neighbourhoods and asking dog owners if they could use blood and hair samples from their pets.

The team extracted odour-causing chemicals from the hair of 15 infected and 15 uninfected dogs, and then presented them to male and female sandflies. They placed odour samples from an infected and uninfected dog into separated chambers of a Y-shaped tube and monitored which chamber the flies chose to enter.

Female sandflies feed on blood, while the males don’t. Both sexes were generally attracted to the dog hairs, but 65.7 per cent of the female sandflies were attracted to the infected samples while the males were equally attracted to samples from infected and uninfected dogs.

“They showed convincingly that infected dogs do attract the sandflies,” says Shaden Kamhawi at the US National Institute of Allergy and Infectious Disease. “But they have yet to identify why, and what is the odour, and what are the receptors in the sandflies that are responding to the odours.”

Understanding the biological interactions at play could help efforts to control the disease without having to do widespread damage with insecticides that can kill a population of the insects and harm the environment, she says.