In the wild, many bat species are contending with declines in their food supply, likely caused by humans destroying their natural habitat. These decreases in food can adversely impact the bat's immune system, making them susceptible to viruses that they naturally harbour. Humans are also increasingly coming into direct contact with bats, which can lead to viruses being transmitted from bats to humans. However, little research has determined how the amount of virus shed by bats is impacted by changes in their diet. This led Caylee Falvo and Daniel Crowley from Cornell University, USA, and colleagues from Montana State University, the National Institute of Health, Mississippi State University, University of Missouri and Colorado State University (all in the USA) to determine whether poor diets impact the Jamaican fruit bat's (Artibeus jamaicensis) metabolism, immune system and ability to shed an influenza virus (H18N11-IAV), which naturally resides in the bats’ gut.
First, the team transported 26 healthy male Jamaican fruit bats from a colony housed at Colorado State University to their new home in Montana State University. After the bats rested for three weeks, the team separated them into groups to be fed different diets for 21 days. The first group of nine bats was fed fruit supplemented with protein (a standard, well balanced diet); the second group of nine bats was fed only fruit (a poor diet with high sugar and low protein) and the third group of eight bats was fed fruit with coconut oil (a poor diet that was energy rich with high fat and low protein). Then, the researchers exposed 6 or 7 bats from each group to the virus by spraying it into their mouth and noses, before continuing to feed the bats the same diet for another 20 days. Throughout the entire experiment, the researchers collected blood samples, swabbed around the bats’ rectum and at the end of the experiment they collected liver samples, all to measure changes in the molecules produced by the bat's metabolism. Additionally, at periodic intervals, the researchers measured the bats’ body masses, to identify any changes, and collected extra samples from around the bats’ rectum to measure how much virus the bats were producing.
After analysing their measurements, the researchers found that the bats on the poor – high-sugar, low-protein – diet produced the most virus and for the longest time, even compared to bats on the standard well balanced diet. The bats on the high-sugar, low-protein diet also had the lowest levels of gut metabolites (such as arginine, citrulline and ornithine), which may have impaired their immune system and ability to fight the virus. In contrast, the bats on the high-fat, low-protein diet shed the least amount of virus and for the shortest time. This was likely due to the high levels of arginine, citrulline and ornithine that they produced, which may have helped their immune system to fight the virus. This led the researchers to conclude that unbalanced diets can impact the metabolism and immune system of bats, but in different ways. When the researchers looked at how much food the bats consumed, the bats on the two poor diets ate more overall, compared to the bats on the balanced diet, but gained little additional body mass.
Overall, Falvo, Crowley and colleagues have shown the intricate and complex dynamics between a bat's diet, immune system and metabolism, which can impact the amount of virus bats release into the environment. These findings can help researchers to understand better, and hopefully prevent, future situations where viruses could be transmitted from bats to humans.
