Bats can carry the Hendra and Nipah viruses with no ill effects: We need to know why

A coloured transmission electron micrograph of the Hendra, virus. Photo: The Electron Microscopy Unit of the Australian Animal Health Laboratory, part of the CSIRO science agency CC BY 3.0 via Wikimedia Commons
A coloured transmission electron micrograph of the Hendra, virus. Photo: The Electron Microscopy Unit of the Australian Animal Health Laboratory, part of the CSIRO science agency CC BY 3.0 via Wikimedia Commons

A review of evidence around henipaviruses, including the deadly Hendra virus in horses, suggests that a better understanding of how some species control these infections would likely provide new insights into preventing or treating cases in humans and livestock.

This was especially the case for bats, Philip Lawrence and Beatriz Escudero-Pérez wrote in the journal Viruses.

Nipah and Hendra are bat-borne henipaviruses capable of crossing the species barrier. They can cause severe disease in humans and livestock, mostly in Australia, India, Malaysia, Singapore and Bangladesh.

In humans, mortality rates can reach 60% for the Hendra virus and 92% for the Nipah virus. This makes them two of the deadliest viruses known to humans.

In their review, Lawrence and Escudero-Pérez looked at available evidence around how the viruses interact with the immune systems of different host species, including their natural hosts bats, and spillover-hosts pigs, horses, and humans, as well as in experimental animal models.

Hendra is carried by species of Australian fruit bats, with spillover events involving horses, which can act as an amplifying host. Seven people in close contact with the bodily fluids of infected horses are known to have contracted the virus since it was first identified in 1994. Three died.

The authors said the biggest changes influencing spillover events involving these viruses occur in the environment and involve increased contact between bats and livestock or humans.

The human exploitation of native habitats has increased human–bat–livestock interactions, they noted, heightening the emergence and re-emergence of high-consequence infectious diseases.

“In recent years, changing landscapes and deforestation have deeply affected bat roosting sites, thus forcing colonies to change their ecology and behavior and to look for niche expansion, often closer to human locations where foraging or agriculture takes place.”

Both viruses can be shed in the body fluids and feces of pteropid bats. The authors noted that no clinical signs have been detected in bats naturally infected with either Hendra or Nipah. However, this is not the case with other hosts.

An example is horses, where the Hendra virus has a fatality rate of 90% and infections result in fever, depression, and respiratory and neurological disease.

Since Hendra was first identified in Australia nearly 30 years ago, there have been 32 equine outbreaks, of which five have involved humans after close contact with infected horses.

The first equine experimental infection studies suggested that very close contact was necessary to transmit Hendra amongst horses and, even in such cases, it is rare.

In post-mortem examinations of infected horses, the kidneys and lungs appear to be particularly affected, with heavy viral loads.

It has been shown that the incubation period in infected horses can vary from 5 to 16 days. The first signs are rapidly followed by depression, shortness of breath, visible swelling of the lips, face, head and neck, intermittent recumbency, and loss of appetite.

In some cases, infected horses show neurological signs including muscle twitching, depression, disorientation, hypersensibility when approached, facial nerve paralysis and restlessness.

In fatally infected horses, the illness lasts about 48 hours from the first signs.

In humans, the features of the disease are similar, although horses appear to suffer greater lung damage.

In the case of Nipah virus, the natural infection of horses was first identified in a 1998 outbreak in Malaysia. Antibodies were detected in several horses and one was diagnosed with meningitis.

In an outbreak in the Philippines, several horses showed neurological signs, with some lethal cases, but it was not possible to detect Nipah by microscopic examination since samples were unavailable.

The authors traversed the effects of the viruses in other host species.

They said experimental animal models are essential in order to better understand the viruses. However, not all animals are affected equally. Thus, the multiple known animal models show different suitability for modeling different features of the viruses.

Therefore, animal models should be used according to the purpose of the study.

“In the future, further development of species-specific immunological tools will surely contribute to an increase and broader use of animal models to study different aspects of henipavirus infections.

“Importantly, while to date a few experimental studies have been performed with bats, we would like to emphasize here the importance in pursuing research studies where the immune differences between bats and other animals, including humans, affected by henipaviruses are addressed.

“The availability of bat-specific immune tools would be a considerable contribution to this field.

“An increased understanding of how some animal species, especially bats, are able to successfully control infection with highly pathogenic viruses such as the henipaviruses, will also provide new insights into how to prevent or treat infection in humans and livestock.”

A better understanding could drive the development of new therapeutic strategies and vaccine measures against these re-emerging viruses, they said.

Lawrence is with the Science and Humanities Confluence Research Centre at the Catholic University of Lyon in France; Escudero-Pérez is with the World Health Organisation’s Collaborating Centre for Arbovirus and Haemorrhagic Fever Reference and Research in Hamburg, Germany, and the German Centre for Infection Research.

Lawrence, P.; Escudero-Pérez, B. Henipavirus Immune Evasion and Pathogenesis Mechanisms: Lessons Learnt from Natural Infection and Animal Models. Viruses 2022, 14, 936.

The review, published under a Creative Commons License, can be read here

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