Fruit bats carry more deadly viruses than all of those stored in the world’s high security laboratories put together. But why do other animals and humans die when the hosts themselves remain healthy? Could the key to fighting deadly diseases be found in their genes?

HENIPAVIRUS – This family of two related viruses, the Hendra and Nipah virus, proves fatal in up to 70% of cases.

LAGOS BAT VIRUS – Around a third of bats carry this pathogen, which can trigger rabies in humans.

MARBURG VIRUS – This causes fever in humans and can lead to internal bleeding and death within days.

EBOLA VIRUS – This pathogen has devastating effects: it is highly contagious and no cure currently exists.

The stumble seemed so inconsequential that Sonja Metesch* will only it remember several weeks later. During a tour of a cave in the middle of the Ugandan rainforest, the 40-year-old Dutch woman loses her footing and injures her hand supporting herself on some rocks. Just a scratch, she thinks, as she rejoins the group. It’s a mistake that will prove fatal…

HOW DO YOU GET INFECTED WITHOUT REALISING? Back in the Netherlands, Metesch develops a raging fever. Then things spiral out of control – fast. She is rushed to hospital suffering from multiple organ failure and, just days later, dies. Metesch has fallen victim to the Marburg virus, a disease transmitted by wild animals – even though she never came into direct contact with one. How could that have happened? The virus must have entered her body when she cut her hand on the rocks.

The open wound must have come into contact with some faeces in the bat-infested cave. Fruit bat faeces to be precise. Droppings from a species that carries more viruses inside its body than all of those stored in the world’s high security laboratories… So do fruit bats really pose such a risk to humans or was the Dutch woman’s case an extremely unlucky one-off? Humans have been afraid of bats for centuries.

In recent times, this fear has soared as scientists have proved that the animals carry viruses that cause deadly epidemics like Ebola – even though it is very rare for them to transmit the virus directly to humans. “It only happens if you are bitten or eat the meat of an infected fruit bat,” explains Tony Schountz from Colorado State University.

Or – as in Metesch’s case – when an open wound comes into contact with the animal’s saliva, blood or excrement. Which begs the question: if fruit bats carry deadly pathogens like the Ebola and Marburg viruses, why do they never fall ill themselves? Researchers have now discovered that while other animals and people die from these viruses, fruit bats won’t suffer so much as a fever.

On the contrary, their lifespan is ten times that of other similar-sized mammals and they almost never develop cancer. So does the key to healing also lie inside the fruit bats’ deadly cargo? Researchers are convinced it does and are now trying to uncover the secret of the fruit bat immune system. They hope that we may soon be able to thank fruit bats for improving our health – and even our life expectancies.

HOW DO YOU LIVE WITH A DEADLY ENEMY IN YOUR BODY? For that to become a reality, scientists first need to understand how fruit bats can live seemingly healthy lives unharmed by the pathogens lurking in their cells. The virologist Linfa Wang from Duke-NUS Medical School in Singapore has researched just that.

He spent almost two decades in Australia studying fruit bats and how they transmit the deadly Hendra virus to horses. Wang discovered that, unlike other mammals, the fruit bats could control the virus. They displayed neither a fever nor a raised level of white blood cells in their blood. But how did their immune systems somehow render the virus harmless?

Scientists believe the answer lies in the animals’ fast metabolism. To help them fly, fruit bats require up to 20% more calories than a non-flying mammal. This speedy metabolism raises the level of free radicals in their blood, which have the potential to damage their DNA.

To prevent this from happening, a bat’s immune system is constantly in overdrive. It cleans up the dangerous molecules – including potentially deadly viruses. Since their immune system functions so well, the animals don’t get sick. The virus multiplies in their bodies and they pass it onto their offspring – but they never develop symptoms.

FRUIT BATS – Like all bats, fruit bats belong to the Chiroptera order. After rodents, these are the most diverse species of mammals in existence. They can weigh up to one kilo and have a maximum wingspan of 1.7 metres. Unlike other types of bats, most fruit bats do not navigate using echolocation. Instead, they have outstanding vision and an excellent sense of smell. Their diet consists mainly of fruit, flowers, pollen, nectar and insects.

The African straw-coloured fruit bat shown here has a maximum head-to-toe length of just 20 centimetres and a maximum wingspan of 75 centimetres. It has a dog-like head and a good sense of smell with which to sniff out ripe fruits. The bat also feeds on nectar, and thus plays an important role in the pollination of plants – 40% of trees in the world’s rainforests would be unable to produce fruit without their help.

Its favourite food sources are the Borassus and date palm trees. Thanks to the strong, hooked claw on its thumbs the bat is a good climber and can defend itself against attackers. It can use its feet like hands to open the shells or skins of fruits. The bat bites into hard shells with its sharp canines and mashes the fruit between its teeth, sucking out the nectar.

Unfortunately, however, this is by no means the case for humans. In the event of a similar viral invasion, our immune system would react by exhibiting an extremely powerful inflammatory response. Wang explains: “Contrary to popular perception, few viruses kill humans. Humans actually kill themselves because of the excessive inflammation.” In other words, unlike fruit bats, which have a constantly activated immune firewall, our operating system ends in a short circuit – and it collapses.

“IT’S ONLY A MATTER OF TIME UNTIL WE WILL BE ABLE TO UNLOCK THIS SECRET” Scientists are now trying to identify the proteins that fruit bats use to control inflammation and prevent tumours forming. These proteins -or modifications thereof – could then be used to treat conditions characterised by life-limiting inflammation such as arthritis, rheumatism and heart disease.

The findings could also contribute to a cure for deadly viruses like SARS and Ebola. Many human lives could be saved. Zoologist Emma Teeling from the University of Dublin is convinced: “It’s only a matter of time until we will be able to unlock this secret once and for all. Mother Nature has the answer.”

Eradicating fruit bats for fear of contagion could have catastrophic consequences, says Wang. Studies have shown that culling populations can actually increase the risk of an epidemic – moving or destroying colonies increases stress levels in the bats, which raises their viral load and the risk of spillover. The contamination of vast areas from injured and dead bats also raises the risk of the virus spreading.

This in turn raises the risk of other animals being infected. Moreover, bats are crucial to the planet’s ecological balance because they eat insects that damage crops. In fact, a colony of bats can end a pest outbreak in a single night, not thanks to the viruses they carry but simply by deploying their excellent hunting instinct.