Globally, a small proportion of the approximately 55,000-year-old human deaths caused by the rabies virus are the result of infection by variants or viruses associated with bats. Human rabies caused by the bat rabies virus has been reported regularly in South and North America, Africa, Europe and Australia. These viruses are as follows:
Genotype 1: rabies virus,
Genotype 2: Lagos bat virus,
• Genotype 4: Duvenhage virus,
• Genotype 5: European bat lyssavirus type 1, EBLV-1,
• Genotype 6: European bat lyssavirus type 2, EBLV-2,
Genotype 7: Australian bat lyssavirus),
In 1994, when an acute respiratory disease outbreak occurred in a human and 14 horses in the Brisbane suburb of Margaret, Australia. These EIDs eventually affected 2 humans and 22 horses. Four additional outbreaks were observed in 1994, 1999 and 2004, infecting two people and five horses and killing all but one human. A virus of the Paramyxoviridae family, genus Henipavirus, pteropus bats and named HENDRA virus have been shown to be the etiological agent of this disease. Nipah virus (NiV), another member of the family of Pteropus encephalitis in the family of Paramyxoviridae, was discovered in Malaysia in 1998 in an outbreak that affected 283 people and caused 109 deaths, with a case fatality rate of 39%. Direct contact with infected pigs has been defined as the predominant mode of human infection. Subsequently, NiV outbreaks were observed almost every year in Bangladesh and sometimes in India.
Bangladesh outbreaks have been shown to be linked to the consumption of fresh palm tree sap contaminated with the secretions and faeces of Pteropus bats containing NiV. In 1997, another member of bat-derived Paramyxoviridae, Menangle virus was identified. It has been isolated from stillborn piglets from this virus in Australia. Two out of 250 people living in contact with the infected animal showed febrile illness with a measles-like rash and were found to have high titer anti-Menangle virus antibodies. These people never come into contact with flying bats, suggesting that the bat-borne virus was transmitted to humans after the infection of their pig or their young. Prior to 2002, although coronaviruses (CoV) were known to be agents of respiratory infections in humans (e.g. common winter fever), they received little attention. Human CoV (HCoV) has gained a strong reputation after being identified as responsible for the severe acute respiratory syndrome (SRAS) outbreak in humans.
SRAS originated in China in 2002, then spread to 29 different countries and caused more than 8000 infected patients and nearly 800 deaths worldwide, with a case fatality rate of about 10%. Serological analysis of samples collected from healthy people collected in Hong Kong in 2001 revealed a prevalence of 1.8%. And it suggested that the circulation of SARS-related viruses occurred before the 2003 pandemic. In fact, the SARS-like CoV circulating in Chinese horseshoe bats has spread and has adapted to the wild Himalayan palm civet cat that is often sold as food in Chinese markets. After the mutation, this CoV has adapted to humans and spread from person to person. During the SARS outbreaks in Toronto and Taiwan, some individuals have been very effective in transmitting SARS-CoV and these have been termed Superpreaders.
A few years later, the emerging Middle Eastern Bat Respiratory Syndrome (MERS) -CoV was reported in Saudi Arabia in 2012. Once again, human MERS-CoV was likely caused by a virus associated with bat-CoV and was likely transmitted through camel-human contact. MESR outbreaks have had a limited spread to other countries in the Middle East, but have not been found in people returning from the Middle East. So far, 2081 people have been infected with MERS-CoV and 722 of these have died from the disease, with a case fatality rate of 34.7%. It should be noted that during the MERS-CoV epidemic in 2012-2014, the apparently super-efficient person-to-person transmission did not occur. However, the MERS-CoV outbreak that affected the Republic of Korea in 2015 was caused by a single 68-year-old person who developed fever in the Middle East 2 weeks after returning from 2 weeks of travel. On his return to Seoul, this person visited the medical center on May 17 and was placed in isolation the next day on suspicion of MESR before being diagnosed with MERS on 20 May.
Of the 36 deaths, a total of 186 people were infected, 44.1% of the cases were hospitalized patients, 32.8% were caregivers and 13.4% were healthcare professionals. Interestingly, a total of 83.2% of transmission events are epidemiologically linked to five superpreaders, all of which have pneumonia characterized at the first medical consultation. In August 2015, 1413 laboratory confirmed cases of MERS were reported worldwide, 502 of which died. The cause of superspreading events is still unclear and may be the result of virus mutation, high viremia due to higher levels of virus shedding, environmental factors such as co-infection, or host altered immune status. A recent study on a virus closely related to the Middle East respiratory syndrome coronavirus (MERS-CoV) found in a Pipistrellus bat supports the bat-borne origin of MERS-Cov.
Ebola haemorrhagic fever is caused by a zoonotic virus that was discovered in 1976 during an epidemic epidemic affecting approximately 300 people infected in the Democratic Republic of Congo and Sudan. Ebola virus is a fulminant septic shock following a non-specific viral syndrome. And it is responsible for a serious and often fatal disease characterized by coagulopathy resulting in multiple organ failure and severe bleeding complications. Although the Ebola virus remained silent for several years, it continued to circulate in these areas, with 34 infected people resurfacing in Sudan in 1979 and more than 350 infected people in Gabon and DRC in 1994–1995. Between 1996 and 2014, several outbreaks were reported in different African countries. It is known that each epidemic affected thousands of people from a few for the 2014 epidemic, with a case fatality rate of 52%.
Although the recent emergence of viruses known to carry by bats has not led to very large outbreaks (several hundred to several thousand infected people), the ability of some of these viruses to adapt to person-to-person spread and the high mortality rate associated with these infections (case fatality rate of over 30% of infected people ) contributed to their view as an important public health risk by international medical authorities. This partly explains that, in the face of the threat, for example, SARS, MERS and Ebola outbreaks, after a period of relative disorganization, each occurrence causes a rapid response by health authorities. In some cases, treatment of the disease is largely limited to supportive therapy and requires appropriate control measures. This is true for the Ebola epidemic in West Africa, the largest in history in 2014.
Ebola haemorrhagic fever was diagnosed in Guinea in December 2013, and outbreaks later occurred in Liberia, Nigeria, Senegal and Mali. As of September 18, 2014, WHO had reported 5,335 cases with 2,622 deaths, and the case fatality rate was about 50%. In early 2015, there were additional cases in Mali and Sierra Leone. In April 2015, Ebolavirus outbreaks resulted in more than 10,880 deaths among 26,277 cases. In March 2016, WHO reported a total of 11,323 deaths among 28,646 cases, indicating a decrease in the human spread of the virus. There is no direct evidence that the bat is a reservoir for the disease that triggers ebolavirus in humans. Still, Ebola-related viruses have been found in tissues of a few bats and experimental infections. Some of the bats with the virus are as follows:
• Hammer-headed fruit bat: Hypsignathus monstrosus,
• Franquet’in apoletli yarasa: Epomops franqueti,
Small collar fruit bat: Myonycteris torquata,
Angolan free-tailed bat (Mops condylurus), small free-tailed bat (Chaerephon pumilus) and Wahlberg’s epaulette fruit bat (epomophorus wahlbergi) caused viral replication in these bats with the Zaire strain of the Ebola virus. Widespread infection of cave-dwelling bats by the Crimean-Congo hemorrhagic fever virus (CCHFV) has also been reported. This suggests the role of bats in the life cycle and geographic distribution of this virus. It is generally accepted that bats are a source of high viral diversity that can directly or indirectly cause a new epidemic. Over the past 20 years, a great deal of international effort has been made to identify viruses in different bat families.
The total number of bat-associated sequences on GenBank has increased exponentially over the past 20 years. A review of articles citing bat-borne viruses shows that rabies (55,000 people infected each year, case fatality about 100%), with 2792 articles (33%). Surprisingly, the virus family in second place is Coronaviridae, with 2622 articles (31%) accumulating different episodes, the total number of cases remains relatively low, with cumulative cases of about 8000 people and an average case fatality rate of about 10%. In addition, the number of scientific reports on the virus family ranks first in terms of publications on the Corona virus, MeSH terms viruses and Rhinolophidae bats. And the MeSH terms show that it comes second when it comes to viruses and fruit-eating bats.
The number of articles published on the bat-borne virus is not related to the number of infected people and case deaths. However, it argues that it reflects a risk perception felt by public authorities, health authorities and financial institutions according to the society. Even if the knowledge accumulation through research studies is likely affected by these risk perception issues, the coronavirus sample continues to be very interesting to tackle the emergence phenomenon. The increased risk of pathogen transmission among bats, animals and humans in Southeast Asia is a result of the increasing human population and the humanization of the environment (deforestation, agriculture) that has drastically changed the landscape.
Author: Ozlem Guvenc Agaoglu