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Bats: The virus hosts
Published on: Sunday, July 16, 2017

BATS are interesting because they are the only mammals in the world that can fly like a bird.

They have a high metabolism rate which explains why they play host to viruses (or virus host).

In other words, they can carry a large number of viruses without becoming ill. These viruses cannot metabolise on their own and thus rely on other living things (in this case, bats) to reproduce.

Most importantly, bats have been identified as a "major natural reservoir" for an increasing number of emerging zoonotic viruses (being transmitted from animals to humans). Humans are mainly exposed to viruses from bats via the animals' faeces. Such viruses include henipaviruses, hepaciviruses and pegiviruses as well as variants of rabies viruses and bat lyssaviruses.

In an interview with the Secretary-General of the Asia-Pacific Society of Clinical Microbiology & Infection, Professor Paul Ananth Tambyah (who is a Professor of Medicine based in Singapore) said the concern is with bats because they are very strong and have a high metabolic rate so there are many viruses that can live inside these flying mammals for a prolonged period, and yet don't cause any harm to them (bats).

The Senior Consultant Infectious Disease Physician said bat droppings contain a fungus called Histoplasma which can affect the lungs as well as other parts of the body, including the spleen and bone marrow.

This, he said, is especially so in people with weakened immune systems such as those who have had transplants such as kidney transplants.

Sharing his experience, he said he has come across patients who had visited Sarawak's caves and come back with anaemia. "The anaemia in patients with Histoplasma is usually from bone marrow involvement in patients with weakened immunity. It can be quite serious."

Histoplasmosis can sometimes present with no symptoms apart from perhaps fever.

"Whenever we have patients who say they have visited Sarawak, we would ask them whether they had gone to the caves because bats find shelter in caves. Once it is diagnosed, Histoplasmosis can be treated relatively easily with anti-fungal drugs such as itraconazole and amphotericin," said Prof Paul.

By now, we know that the outbreaks of SARS (or severe acute respiratory syndrome) during 2002-2003 and 2003-2004 were caused by a newly emerged coronavirus, now known as the SARS coronavirus (SARS-CoV).

Research into SARS showed that the virus responsible for the outbreaks came from bats.

It was only in 2013, a decade after the SARS pandemic that scientists confirmed that it originated in bats.

In retrospect, Prof Paul said the first SARS pandemic started in the southern Province of Guangdong in the People's Republic of China in November 2002-2003, and spread quickly around the world.

It was brought under control in July 2003, only after it had spread to 33 countries on five continents.

"It was a global spread. Singapore was not spared. SARS spread rapidly throughout Singapore from a single traveller who returned to Singapore after holidaying in Hong Kong where she caught the virus from an infected hotel guest," he recalled.

Globally, SARS infected 8,096 people and resulted in 774 deaths, with China, Hong Kong and Taiwan being the three worst-hit areas.

Prof Paul opined: "When SARS occurred, it was like a perfect storm because they were keeping the civet cats and the bats very close to each other." He was referring to the situation in Guangdong Province both in 2002 when SARS first appeared and again in 2004 when it re-emerged.

SARS first broke out in the animal markets where a large number of live animals (civets, bats, etc) kept in cages were being traded in the southern People's Republic of China. Civets are mammals with a catlike appearance.

There are farms in China which breed the mammals as pets.

Asked on the lessons drawn from SARS, Prof Paul said good epidemiology is very important and likewise surveillance capacity as well as using evidence-based infection control. Epidemiology is the branch of medicine that deals with the incidence, distribution, possible control of diseases and other factors relating to health.

Australia's Professor Edward Holmes, a virus specialist who had investigated viruses in bats across the world, had said that in most cases, bats must pass the virus on to another species that lives closely to humans, before the virus is passed to humans. "For example, in the case of SARS, the virus was passed from bats to civets and then to humans," he was quoted as saying.

Based on the "Review of Bats and SARS", the first evidence of SARS-CoV infection in animals came from a study conducted in a live animal market in early 2003. From the 25 animals sampled, viruses closely related to SARS-CoV were detected in three masked palm civets (civet species native to Southeast Asia).

This initial study indicated that at least three different animal species in the Shenzhen market (masked palm civets, racoon dog and Chinese ferret badgers) were infected by coronaviruses that are closely related to SARS-CoV.

However, in 2004, bats became the prime suspect as the "virus host" when an international team of scientists reported evidence of a SARS-like virus in wild Chinese horseshoe bats from Yunnan Province '(meaning to say, a virus that looks like the SARS virus was found in the horseshoe bats in China).

Bats were highly likely to be the original source of the deadly SARS virus, according to the scientists, as published in the journal "Science" in 2005.

Civet cats are not the original source of SARS, said the "Science". Previous research had suggested that the civet cats, a food eaten in Southern China, were the original source of the virus.

Singapore's Dr Linfa Wang, a member of the World Health Organisation SARS Scientific Investigative Committee and Animal Reservoir Group, contended that while the 2003 research supported the view that civets were responsible for transmitting the virus to humans, it was wrongly interpreted by the public and some government officials as proof that the civet was the natural host.

Hence, studies were conducted to determine whether civets (involved in the transmission of SARS-CoV to humans) were an intermediate host or the natural reservoir host of SARS-CoV.

By 2013, "Science" had published that bats were confirmed as the original source of SARS.

So those civets in the 2002-2003 outbreaks were the intermediate host and not the natural reservoir host.

Globally, there has been "NO SARS" for more than a decade since its outbreak in China in 2002, but nobody knows why, Prof Paul observed. "Touch wood," he said. "The reason why people like me are worried about bats as the source of viral infections in the future is because of the recent history of infections such as Nipah virus (NiV), SARS and Ebola which have all come from bats. Bats are the only flying mammals so it is possible that they carry many viruses with the potential to spread among humans."

It was reported that the Guangdong Government was taking public health measures to reduce bat droppings following the outbreaks of SARS.

As for bird flu, a contributory factor is the global consumption of chicken meat. "When the consumption goes up, the number of birds also goes up, and likewise, the number of sick birds will also go up."

The Nipah virus outbreak in Malaysia (September 1998 to May 1999) had killed 105 people in Malaysia and destroyed the country's pig industry. Humans contracted the infection from close contact with infected pigs.

Over a million pigs in Perak, Negri Sembilan and Selangor were culled. Nipah is the home of many of the country's pig farms. It also spread to Singapore.

By 2000, Malaysian researchers had traced the Nipah virus to a native fruit bat species.

It was discovered that Malaysian Island flying foxes were the natural reservoir hosts of NiV.

Based on a medical publication, the initial transmission of NiV from bats to pigs occurred through contamination of pig swill by bat excretions, as a result of migration of these forest fruit bats to cultivated orchards and pig farms.

Against this scenario, in concluding his paper on "Emerging Infectious Disease Threats in Asia" at the 3rd Borneo Tropical Medicine and Infectious Disease Congress here, Prof Paul emphasised the need to learn about bats in terms of their reservoir distribution and transmission in order to prevent future outbreaks. - Mary Chin

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