Fruit bats have an almost supernatural ability to harbor some of the planet’s most deadly viruses without getting sick themselves. Credit: Howard Lipin | TNS

The U.S. military has a long history of enlisting the help of animals in warfare. The bottlenose dolphin’s sophisticated bio sonar enabled the Navy to detect and clear underwater bombs during the Iraq War, and homing pigeons played a vital role as secret messengers during both world wars, with some awarded medals for bravery.

But there is one animal that the military has had significantly less success in conscripting, and that is the bat.

In the wake of the Pearl Harbor bombing in 1941, hundreds of Mexican free-tailed bats were recruited as part of a harebrained scheme to blow up Japanese cities by arming the flying insectivores with bombs and releasing them from planes. The idea was that the bats would roost in buildings and explode, killing the enemy as they slept. What could possibly go wrong?

Well, quite a lot. The plot was riddled with flaws. No one had invented a bomb smaller than a can of beans, which would be impossible for an animal the size of a mouse to carry. And, most crucially, bats — unlike dolphins and pigeons — cannot be trained to follow orders.

Despite its imperfections, the batty plot was nevertheless given the green light. Its creator, a maverick Pennsylvania dentist and inventor named Lytle Adams, had some friends in high places. He had persuaded first lady Eleanor Roosevelt to check out one of his earlier ideas — a plane that delivered mail without landing. So when he detailed the bat proposal in a letter to President Franklin D. Roosevelt, it didn’t immediately wind up in the trash. Instead, it was forwarded to the National Research Defense Committee — the group from which the Manhattan Project was spun off — with a presidential note of recommendation.

“This man is not a nut,” Roosevelt wrote. “It sounds like a perfectly wild idea but is worth looking into.”

The bat bomb plan was stamped “top secret” and assigned the suitably sci-fi code name Project X-Ray. A crack team of senior Army types, arsenal experts, engineers and biologists was assembled. Together, they set about vaulting the scheme’s more vertiginous hurdles.

The first stage was to capture thousands of Mexican free-tailed bats from caves in the Southwest, where they roosted in the tens of millions. Then a bomb had to be developed that was light enough for half-ounce bats to transport. In a quintessentially American twist, parts for the diminutive bomb were manufactured in a factory owned by crooner Bing Crosby.

With bats and bombs sorted, it was time to conjoin them. The miniature explosives were to be attached to the bats with twine, the presumption being that the bats would gnaw through it and leave the bombs behind. Then came the issue of controlling the bats. They were placed in refrigerators, forcing them into torpor for easy handling and transportation. But timing their thaw proved tricky. Several early tests with dummy bombs were a dud because the bats woke too late (causing them to plummet ingloriously to the ground once released) or too soon (before their cargo had been attached and allowing them to escape the base).

Undeterred, the scientists ran a test using real incendiary devices in June 1943. Things did not go as planned. A report on the experiment stated somewhat evasively that “testing was concluded … when a fire destroyed a large portion of the test material.” It failed to mention that the barracks, control tower and several other buildings at the auxiliary field station in Carlsbad, New Mexico, were set spectacularly ablaze by escapee bat bombers. The need to maintain military secrecy prevented civilian firefighters from entering the scene, and fire leaped from building to building, incinerating most of the base. As a final insult, a couple of winged missiles went AWOL, taking up roost under a general’s car before exploding.

The project never recovered from this ignominious retreat, and it was canceled in 1944. Having set up some 30 tests and spending a couple million dollars, the United States put its focus behind developing a bomb that exploited the power of atoms. This proved to be easier to control than bats.

Today, the U.S. military is again interested in bats not as front-line attackers but as defenders against a potentially devastating threat: Russian bioweapons.

Fruit bats have an almost supernatural ability to harbor some of the planet’s most deadly viruses without getting sick themselves. Inject an Egyptian fruit bat with the Marburg virus — a hemorrhagic relative of the infamous Ebola virus — and nothing happens. Do the same thing to a human, and within a week, the patient could be bleeding to death.

These bats’ extraordinary super-immunity has long fascinated virologists, and new research has shed light on how these flying frugivores achieve their supreme skill. Unpacking the mystery involved some cunning detective work from a coalition of scientists at Boston University and the U.S. Army Medical Research Institute of Infectious Diseases. Their work was published in the journal Cell.

“What we are trying to do is to study bat immunology, but that turned out to be a very difficult thing to do when starting from scratch,” Thomas Kepler, a professor of microbiology at Boston University, said. It took decades to create the reactive substances necessary to study human or mouse antibodies. With bats, he explained, they were starting from zero.

So Kepler’s team jump-started its work by examining the whole genome of the Egyptian fruit bat, chosen because it is a known reservoir for the lethal Marburg virus. It took two years just to assemble the genome. Once done, they compared it with other mammals’ genomes to hunt for idiosyncrasies, in particular an increase in size in any gene families that control the production of defensive proteins involved with immunity. They found significantly large interferon genes.

“These are interesting and very important, as they serve as the front line of antiviral defense,” Kepler said. Once a cell has become infected by a virus, the interferons alert the surrounding cells. “They are basically a warning saying, ‘I’ve just been infected,’” he said. Neighboring cells then start shoring themselves up for a viral invasion.

The other supersize set of genes in the fruit bat controlled the receptors on “natural killer,” or NK, cells. These are essentially the body’s police system for identifying infected cells. Typically, these receptors are activating, which means they trigger the NK cell to kill the damaged cell. But the fruit bat’s NK receptor genes appear to activate and inhibit NK cell function.

This suggested to Kepler and his team that the bat immune system may respond in a unique way to viral infection, offering what he calls “soft protection.” Instead of attacking and killing an infected cell, which leads to a cascade of inflammatory responses in the host, their NK cells might have a more nuanced response. They might, for instance, effectively starve the virus by turning down the host’s cellular metabolism.

The bat’s unique approach to viral infection could also explain why viruses that transfer from bats to humans, including Ebola, are so severe. “A virus that has co-evolved with the bat’s antiviral system is completely out of its element in the human,” Kepler said. “That’s why it is so deadly — the human immune system is overwhelmed by the inflammatory response.”

Kepler believes that this insight into the fruit bat’s super-immunity could eventually lead to a cure for Marburg. “It’s possible that we could develop drugs that dampen down inflammation and arrest the virus by depriving it of what it needs to grow rather than trying to kill it outright,” he said.

So where do bioweapons come in? Natural outbreaks of Marburg virus infection have occurred in African countries and are rare but extremely deadly, with a fatality rate of up to 90 percent. There is no antidote — and that has made the Marburg virus a prime candidate for biological warfare.

The Soviets had a keen interest in the Marburg virus in the 1980s and managed to develop an especially lethal strain after an accident at the Vector Institute, their germ warfare center in Siberia. The chief scientist there, Nikolai Ustinov, accidentally injected his thumb with the virus, which was intended for a guinea pig he was holding.

Ustinov suffered a devastating death, but the Soviets managed to profit from the mistake by harvesting Ustinov’s organs for fresh samples of the virus. These proved to be even more powerful than the original strain. According to a former institute insider who wrote a book on his experience, Ken Alibek, the Soviets named it “Variant U” and sent it to be approved for use by the Soviet Defense Ministry in early 1990.

The Marburg virus is classed as a Category A bioterrorism agent by the Centers for Disease Control and Prevention, and Kepler’s study was supported by the Defense Threat Reduction Agency, a Defense Department division established during the Manhattan Project era to combat weapons of mass destruction.

If the virus is ever deployed as biological warfare, the fruit bat’s super-immunity may hold the answer to preventing its spread. But it may also go some way toward redeeming the bat in the eyes of the U.S. military — and could even make the animal an unlikely hero.

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