Hantaviruses are not new. They have circulated for decades in rodent populations, particularly in rats and mice. Humans can become infected if they are bitten or scratched by a rodent or by inhaling aerosolised particles. These are tiny bits of rodent urine, faeces or saliva floating through the air that are contaminated by the virus.
There are many different hantaviruses but only one can spread from person to person: the Andes hantavirus from South America. This is the strain that recently killed several cruise ship passengers.
Infections between humans can be prevented by closely observing people who were exposed and isolating those who are sick. This limits the risk of further spread, as transmission generally requires close contact.
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However, as an interdisciplinary group of scientists working on emerging infectious diseases, we argue that hantaviruses might pose a much bigger threat to African countries than currently known. We are concerned for three reasons.
Firstly, diagnostic testing capacity across much of the African continent remains limited. This is a real issue. In many rural settings, under-resourced diagnostic services may overlook sporadic cases. This may allow hantaviruses to spread without anyone noticing. Our medical expertise tells us that larger outbreaks are likely to be recognised eventually. But these delays in diagnosing the cases will slow down effective control measures.
Secondly, monitoring systems are lacking and likely to miss infections in wildlife and in human beings.
Thirdly, climate change and accelerating changes to the way land is used could increase the risk of spread of hantaviruses from animals to people. This is because global change may increase rodent populations and bring rats and mice into closer contact with humans.
For example, modelling studies in the Americas found broad zones with enzootic circulation (where an animal community always carries a certain disease). This is because many rodent species tend to live across a wide variety of environments where humans are also found. As human and rodent populations increase, the likelihood of encounters also increases. Some rodent species flourish in habitats shaped by humans or even in buildings. This poses a high risk for transmission of pathogens.
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As a typical zoonosis (animal disease that spreads to humans), hantaviruses must be seen as a One Health issue. One Health is an approach that understands and takes into account the close connection between human, animal and ecosystem health. Hantaviruses cannot simply be seen as a clinical management or infection control issue.
It is really important that African governments set up better monitoring of wildlife so that they can detect when and where animal viruses like this are likely to spill over into the human population. This will help stop larger outbreaks of hantavirus, which can be deadly.
Weak surveillance may be allowing hantaviruses to spread unnoticed
In Africa, scientists have discovered several hantaviruses, including Sangassou virus in Guinea in small mammal species, such as rodents. More recently, hantaviruses were found in shrews and bats too - not just in rats and mice as previously thought.
The fact that hantaviruses may circulate in a much wider range of animals and environments than scientists originally realised makes their ecology and potential spillover risk into humans more complex.
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One of the current problems facing Africa is that there hasn't been enough research into the ecology of hantaviruses and which animals host them. There are very few genetic sequences available that would allow scientists to analyse interactions between viruses and hosts and the possible risk this poses to humans.
Combined with limited monitoring of the disease, Africa is experiencing a hantavirus surveillance gap. This gap needs to be closed because hantavirus infections and disease may be more widespread than many health systems assume.
Climate change and land use
Climate and land-use change influence rodent populations which host hantaviruses, and increase human-rodent contact. Hantavirus boomed in the US between 1993 and 1995 because El Niño brought very heavy rains and warmer winters, which led to a bumper crop in seeds that rodents eat. This improved nutrition led to a massive increase in rodent numbers. Outbreaks elsewhere have likewise been linked to weather phenomena.
More rodents means more of them seeking food and shelter in the vicinity of humans. More competition for resources leads to more aggressive behaviour between animals and biting transmits the virus. Because El Niño episodes are predicted to become more frequent and intense in future, hantaviruses are likely to affect African countries more and more.
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In Africa, land-use change is likely to play an increasingly important role in hantavirus ecology and emergence, as was the case with Lassa fever (another virus spread by rodents) in Nigeria and Guinea. Deforestation, agricultural expansion, mining activities, road construction and urban growth are transforming natural habitats across many regions of the continent. These environmental changes can force populations of rodents, shrews and bats to move into farms, villages, peri-urban settings and water sources used by people.
When humans expand into previously undisturbed habitats in search of land, food, or economic opportunity, this also creates a new opportunity (known as an ecological interface) where hantaviruses and other zoonotic pathogens may circulate more easily between wildlife reservoirs and humans.
What needs to happen next
When people and wildlife come into close contact, viruses like Andes can jump from animals and begin transmitting between humans. Hantaviruses can cause severe human disease and this is likely far more widespread than currently recognised.
Fortunately, the risk of Andes hantavirus spreading beyond the cruise ship passengers and crew and their close contacts is small. But Sars coronavirus and monkeypox virus are recent examples that some zoonotic viruses have the potential to spread rapidly and widely among humans.
Virological and ecological studies of wildlife reservoirs and surveillance of possible hantavirus infection and disease in humans in endemic regions are needed. This requires specialised diagnostic tools combined with samples from rodents in areas where humans have disturbed their habitat and have since experienced unexplained febrile illness (acute high fevers).
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Once there is firm evidence of human disease, scientists and medical professionals will be able to argue for the widespread use of diagnostic tests. The results of these tests will determine how much of a threat the virus poses to human health.
Genetic sequencing and data-sharing partnerships can then help connect animal, environmental and human signals into a clearer picture of risk.
The greatest gap currently may be the failure to identify where, how, and under which environmental conditions spillover events occur before outbreaks emerge.
Strengthening surveillance to identify high-risk interfaces, emerging transmission zones, and drivers of spillover is therefore essential to anticipate potentially pathogenic African hantaviruses before larger outbreaks occur.
PhD candidate Maambele Khosa co-authored this article.
Wolfgang Preiser, Head: Division of Medical Virology, Stellenbosch University
Carla Mavian, Assistant Professor, University of Florida
Cheryl Baxter, Head Scientific Research Support, Stellenbosch University
Richard Lessells, Senior Lecturer, University of KwaZulu-Natal
Tulio de Oliveira, Director of the Centre for Epidemic Response and Innovation (CERI) at Stellenbosch University and the KwaZulu-Natal Research Innovation and Sequencing Plaform (KRISP)., Stellenbosch University
