Watch your bats
Authors: Bernard Dixon
Journal: The Lancet Infectious Diseases (2006)

Because bats are nocturnal and we human beings are mostly active during daylight, it is tempting to assume that these gloomy flying mammals have only a very minor role in transmitting communicable diseases among human beings. Tempting but perhaps wrong. The evidence of my bulging file on lyssaviruses, rabies, and other maladies indicates that the past decade has seen a considerable growth in both evidence and speculation linking bats with human virus infections. The trend seems to have become more pronounced since the news about a year ago that health authorities in northern Brazil were struggling to cope with a wave of attacks on people by vampire bats carrying rabies virus. The entire scenario could have been crafted specifically for media hype, and was covered in predictably colourful terms. One report warned of an imminent intercontinental migration of rabied bats. Disagreement over key "facts" enhanced the mood of helplessness. Some experts blamed the attacks on deforestation in the Amazon, which was consuming more and more of the bats' natural habitat. Others insisted that the population of bats had exploded as increased cattle farming boosted their food supply. Whatever the truth, the reality was scary, with 23 rabies deaths in the human population in just 2 months. But rabies is not the entire story. Malcolm Bennett recently drew attention to the fact that two independent groups have described severe acute respiratory syndrome (SARS)-like coronaviruses in insectivorous bats (Epidemiol Infect 2006; 134: 905). The evidence adduced, while not conclusive, deserves to be taken seriously since searches for the source of SARS after its emergence in China in 2002 have not yielded fully satisfactory answers. Himalayan palm civets, initially suspected to be the key carriers, have not been found to be infected widely in nature and might (like human beings) have been accidental hosts for the virus. In Australia and Asia, fruit bats are now known to harbour at least three zoonotic paramyxoviruses: Hendra, Menangle, and Nipah viruses. The same type of bats, though of different species and genera, are also under suspicion for their capacity to serve as reservoirs of Ebola virus. Bennett suggests it is not altogether surprising to find that bats could be the source of many potentially zoonotic infections. "Based simply on counting the number of species, bats represent almost 20% of all mammalian diversity, second only to rodents (50% of all mammalian species), and, like rodents, they are found on every continent apart from Antarctica", he writes. "Furthermore, many bats live in large colonies with close contact between individuals, life-history traits that might select for endemic infections". For the moment, and certainly as far as the UK is concerned, rabies remains the only tangible threat associated with bats. A few years ago naturalist David McRae became the first person in Britain for a century to die from a rabies-like virus. Working in Angus, Scotland, McRae acquired European bat lyssavirus (EBLV) type 2, probably from a tiny Daubenton's bat, while working for the wildlife agency Scottish Natural Heritage. Careful watch is now being maintained in efforts to ensure that the virus does not become endemic in Britain as it is on the continent. Reporting an isolation of the same virus, in Lancashire, UK, Nick Johnson and colleagues commented on its zoonotic potential: "Four cases of EBLVs being transmitted to humans have been recorded, and there have also been cases of their being transferred to domestic and wild animals", they wrote in the Veterinary Record (2003; 152: 383). "Medical advice should be sought following a rat bite and vaccination is highly recommended for those who handle bats". In light of Malcolm Bennett's concerns, it might be timely to sample the bat population for a wider range of potential pathogens. Perhaps we also need to pay more attention to "liaison hosts", illustrated by the palm civets that probably amplified the bat SARS viruses before they were ferried to humans.