Thursday March 1 2012
Some bats carry a unique form of flu virus
“We've had bird flu and swine flu - now scientists have found BAT FLU,” says the Daily Mail. The newspaper reports that the strain “could pose a risk to humans if it mingled with more common forms of flu”.
The Mail has got in a flap over the flying mammals based on new research that found type A flu virus in fruit bats captured in Guatemala in Central America. The discovery in bats is new as the virus is typically found in winged birds, and not winged mammals.
Researchers collected 316 bats of 16 different Latin American species. Types of flu virus were found in three bats of the little yellow-shouldered species, which is a fruit eating variety common across Central and South America. After analysing the genetic code of the bat flu virus the scientists concluded it contained segments that were significantly different from those found in known influenza A viruses. They also found that some aspects of the bat flu virus could work inside human lung cells grown in the lab. This led them to conclude that the virus has the potential to mix with human flu virus, which could, in rare circumstances, lead to the creation of a new flu strain that is capable of causing a flu pandemic, like bird flu or swine flu.
Despite this warning, scientists have not been able to grow the new bat virus in chicken eggs or human cells, which is possible with existing flu strains. This suggests that the immediate risk of infection to humans is small. Rather than highlighting a danger to human health, this study is likely to guide further research that may improve the understanding of potential pandemic flu threats to humans in the future.
Where did the story come from?
The study was carried out by researchers from Centres for Disease Control and Prevention outposts in Atlanta and Guatemala, and was funded by the agency’s Global Disease Detection Program.
The study was published in the peer-reviewed science journal Proceedings of the National Academy of Sciences USA (PNAS).
The story has appeared on several online news sites and in the Daily Mail. In its headline the newspaper suggests that bat flu “could pose a threat to humans”. While the inclusion of the word “could” makes this a fair statement, the article does not make clear that the immediate risk to humans is very low. Generally, the tone of the piece emphasises a potential risk from the virus. It says there is a hypothetical risk of transmission to humans if they eat food contaminated with traces of the virus. Again, the risk of this happening seems low.
What kind of research was this?
This study was laboratory research looking at the genetics of a specific strain of type A flu virus found in bats captured in Guatemala. Originally, the bats had been examined as part of a study looking at rabies, which revealed that bats were able to carry certain forms of the flu virus.
As their names suggest, new pandemic flu strains such as the high-profile bird flu and swine flu strains often originate in animals, typically waterfowl and pigs. Usually, non-human flu strains do not cause serious harm in the original host, for instance, bird-flu does not cause death to most birds and human flu is not usually fatal to healthy humans. However, animal flu strains have the potential to swap genetic material with human strains and create a new virus strain capable of infecting and harming humans. It is the mixing of genetic material and the creation of these new viruses that represents the main danger of new flu pandemics.
The researchers say that early detection, characterisation and risk assessment of flu viruses in their animal hosts before they spread to humans is “critical” to protect public health.
What did the research involve?
Researchers collected 316 bats from 21 different species from eight locations in southern Guatemala over the course of two years.
Researchers swabbed the bats’ bottoms to gather traces of any influenza virus A. The swabs were tested in the laboratory for signs of flu genetic material using standard molecular biology techniques. Tissue samples from the bats’ mouths, livers, intestines, lungs and kidneys were also tested for flu virus.
Researchers then examined the genetic code of the viral material that had been detected in the bats and looked at how similar they were to other flu viruses that have previously been decoded.
To demonstrate ‘proof of theory’ that the bat virus could function within human cells the scientists created a mini version of the flu virus’ genetic material. They placed this into human lung cells in the laboratory and assessed whether certain functions of the bat virus could be carried out within a human cell.
The researchers attempted to grow the virus strains in a variety of mammalian cells (including bat cells and human lung cells grown in the laboratory) to study how infectious the strains were to these different types of cells.
What were the basic results?
Three of the 316 bats tested positive for influenza virus A from their swabs. All three samples were collected from little yellow-shouldered bats, which is a fruit-eating bat that is abundant throughout Central and South America.
In these three bats, all of the further samples taken from the liver, intestine, lung and kidney tissue tested positive for flu virus genetic material.
Researchers found that a specific genetic sequence within the virus, containing the code for making a vitally important flu protein called haemagglutinin, showed differences from the previously documented strains. In one of the bats the genetic material coding for a second crucially important flu protein, called neuraminidase, showed “extraordinary” differences from other known flu viruses.
In influenza A viruses the forms of haemagglutinin (H) and neuraminidase (N) proteins on the surface of each virus provide the main basis for the way it will be named and classified. For example, the combination of these proteins found in the recent swine flu outbreak meant it was known as H1N1, while the latest bird flu scare was caused by a virus known as H5N1. There are many influenza A virus subtype combinations circulating in animals in the wild. In this research the H proteins found in the samples were so different from other types of influenza that the authors say it could be classified as a new subtype, which they called “H17”. In one of the samples the researchers say they could not classify its N type as there were so many different and unusual types of N proteins.
The scientists reported that attempts to grow the virus in human cells in the laboratory and chicken embryos were unsuccessful. This suggested the virus differed from other known viruses, which can be grown under these conditions.
The researchers demonstrated that some functions of the bat flu virus had the potential to work inside laboratory-cultured human lung cells.
How did the researchers interpret the results?
The researchers conclude that “despite its divergence from known inﬂuenza A viruses, the bat virus is compatible for genetic exchange with human inﬂuenza viruses in human cells”. This leads them to suggest that there is potential for the bat virus to mix with existing human flu viruses creating a “new pandemic” virus that could pose a threat to human health.
This study of the genetic material of flu virus A in three fruit bats in Guatemala provides important new information to those involved in flu research and pandemic awareness. Previously, non-human flu strains were thought to be confined largely to birds and pigs, but this study highlights the potential for bats also to harbour flu viruses that could potentially threaten humans, given the correct sequence of rare events. The awareness this research provides may lead to a better understanding of the potential risks posed by bat flu to humans in the future.
The following points should be considered when interpreting the results of the study:
- It is important to realise that the researchers have found a new segment of genetic material in the bat flu virus that is different from other flu strains sequenced. They have not discovered a completely new virus in bats that is capable of infecting humans, and so the immediate threat to humans is likely to be minimal.
- As yet, scientists have not been able to grow the bat flu virus in chicken eggs or human cells, which is possible with all other commonly occurring flu strains. Given that they were actively trying to grow the virus and failed, this also suggests the immediate risk of infection and harm to humans is small.
- The potential threat the authors and the media warn against for the future is that the new bat virus genetic material could mix with other flu strains to create a new strain that will be capable of infecting and harming humans, like swine flu and bird flu. To date, there is no evidence that this has happened so there is no cause for immediate concern.
- Fruit bats in Guatemala do not bite people, so direct transmission of the bat virus to humans is unlikely. A suggested route of virus transmission has been if bat droppings contaminate food that is then eaten by people. This could allow the bat flu and human flu genetic material to mix, potentially creating a new strain capable of a pandemic.
This study provides no evidence to support or refute the implication that if someone were infected with the bat virus now it would be harmful, and the risk of this bat strain causing a pandemic is not known at the present time. However, a series of rare events would need to happen in sequence for a pandemic to occur. Despite the rarity, this has happened before in the case of other pandemic flu strains including swine and bird flu, although the initial transmission from these species has generally occurred through sustained close contact with livestock, such as sleeping among the droppings of chickens reared in the home.
Following the discovery of this new form of flu it will surely be explored further by agencies such as the Centers for Disease Control and Prevention, which would report any evidence of risk to the World Health Organization and its flu surveillance teams, which constantly monitor and evaluate any potential flu-based threat.
Analysis by Bazian