403 - Bats, a reservoir of resurgent viruses
May 2012
Bats carry numerous diseases that have the potential for resurgence in humans and other animals. A study carried out in collaboration with IRD(
1) researchers and published in Nature communications has revealed the planetary threat they pose. Bats are the source of paramyxoviruses, which cause measles, mumps and numerous respiratory illnesses, some of the primary causes of infant mortality worldwide. These small flying mammals are reputed to have spread these infections throughout the animal kingdom.
Researchers have discovered 60 new types of paramyxovirus. But most importantly, they have recently observed forms of paramyxovirus in bats that are genetically highly similar to forms that were thought to be specific to humans. The existence of such an animal reservoir compromises the quest for eradication of certain human diseases such as measles. Other viruses such as the ravaging Hendra and Nipah viruses, rife in Asia and Australia, have also been observed in latent forms in Africa. Bats need to be placed under a high level of surveillance immediately.
Measles, mumps, pneumonia, influenza and encephalitis in man, Carré's disease in dogs, Ovine Rinderpest (PPR)… all of these diseases are caused by viruses from the same family: Paramyxoviridae . A vast international study( 1), carried out in collaboration with IRD researchers and published in Nature communications has led to the discovery of more than 60 new species of these dangerous infectious agents, almost double the number previously recorded. This family of highly diverse pathogens affects all animals, from canines to fowl, cattle and humans. As a result, it is not always easy to determine which host is responsible for these viruses. Thanks to testing carried across the globe, the research team has recently discovered their source: bats.
All indices agree
Virologists have collected over 10,000 animal samples, including more than 90 Chiroptera( 2) species from Africa, Latin America, Asia and Europe. As a result of blood and organ analysis, researchers have observed a large genetic diversity of paramyxoviruses in these small mammals. This suggests that these infectious agents have had enough time to evolve in bats over the course of history. They have thus been present for a very long time in this order of animals. In addition, scientists have found them in all known species of bat worldwide. This planetary spread signifies that it is the result of movement from continent to continent from a common ancestor and that these flying hosts have been carriers for millennia.
Lastly, biologists have found nearly all genera from the paramyxovirus in bats, which has not been the case with any other animal. Such viral representation confirms that they are at the origin of all infection across the animal kingdom. To provide the final proof, researchers investigated the probability that each order - bats, rodents, birds, humans, canines or bovines - could be the source of contamination. Using paramyxovirus phylogeny - the family tree, so to speak -, the probability of transfer is highest from bats to other animals.
The threat is still hovering
Researchers have also made a worrying discovery. Chiroptera might also be a reservoir of certain paramyxoviruses that were thought to be specific to humans. Scientists have found evidence among these small animals of paramyxoviruses that are genetically very similar to those observed in man and which could cause infection in humans once again. Childhood diseases such as measles or mumps, which the WHO considers as having been practically eradicated, in developed countries at least, could re-emerge. Any eradication hypothesis( 3) requires all animal reservoirs to be eliminated.
Continents on the brink
Another worrying finding from the study is that certain highly dangerous viruses have been discovered in regions of the world where they were thought to be absent. This is the case for the Hendra and Nipah viruses, two emerging pathogens which have recently been the cause of fatal encephalitis( 4) epidemics in Asia and Australia. No other cases have been detected in the world until now. And yet, researchers have found the viruses in the organs of African bats. In Gabon and Ghana, where the study has focused, two infectious agents seem to be highly present, which raises fears for possible emergence on the African continent.
Bats are already recognised as carrying diseases such as Ebola and rabies, notorious for devastating outbreaks, although these are rare and geographically contained. We are now learning that they are reservoirs of a multitude of infections that affect humans and animals worldwide. All epidemiological study on paramyxoviruses should now take into account the ecological data available for these airborne animals.
(1) This research has been carried out in collaboration with the universities of Bonn, Hanover, Marburg, Cologne and Ulm, the Noctalis centre, the Bernhard Nocht Institute for Tropical Medicine, the Charité Medical School and the Institute for Novel and Emerging Infectious Diseases in Germany, CIRMF in Gabon, the Czech Republic Academy of Sciences, Strandja national park in Bulgaria, Kumasi University in Ghana, Lubumbashi Univesity in DRC, Bahia University in Brazil and Stellenbosch University in South Africa, Chumakov Institute of Poliomyelitis and Viral Encephalitides in Russia, the Smithsonian Tropical Research Institute in Panama, KCCR in Ghana, the Institut Pasteur in Bangui, Central African Republic, the Netherlands Center for Infectious Disease Control, the Muséum National d’Histoire Naturelle and the CNRS.
(2) Bats belong to the order Chiroptera.
(3) The WHO announced a new strategic plan in April 2012 aiming to eliminate measles in at least six WHO Regions by 2020.
(4) Encephalitis is a swelling of the brain
No immunisation for the South
Among the most common paramyxoviruses in humans is the measles virus, one of the most contagious diseases in existence. It remains fatal in developing countries, particularly for infants and young children. Sub-Saharan Africa has made enormous progress in the last decade, thanks to large-scale systematic vaccination campaigns. According to the WHO, the number of deaths annually fell by 85% between 2000 and 2010, from 396,000 to 60,000. But in southern Asia, where measles morbidity is still the highest in the world, hundreds of children under the age of five are still dying every day. High fever, rashes, blindness, diarrhoea, pneumonia... malnutrition and HIV infection are also risk factors that may lead to complications and death. There is no specific treatment for the disease. Pneumonia, usually caused by several paramyxoviruses, is the leading cause of death in children under the age of 5 worldwide. The WHO estimates that 1.4 million children die of pneumonia each year, more than AIDS, malaria and measles combined. 85% of deaths occur in Southern Asia and Sub-Saharan Africa.
By Gaëlle Courcoux
