Bats as carriers of diseases
Bats have a global presence and adapt to a range of ecosystems
Ever since the outbreak of COVID -19 the focus has been drawn to the emergence of this disease also from bats amongst denials and allegations of laboratory origin of N-CoV (SARS CoV-2). The emergence and re-emergence of infectious diseases during recent years is a cause of concern. The bats are incriminated for giving Ebola, SARS, Rabies, MERS and now new coronavirus, COVID-19. Actually, the origin of diseases like Ebola virus, Marburg, SARS coronavirus (SARS-CoV), MERS coronavirus (MERS-CoV), Nipah virus (NiV) and Hendra virus (HeV) has been traced to bats. It is suspected that the Novel Coronavirus (SARS-CoV-2) also has originated from bats. Though the bats do not come in close contact with humans, spillover of viruses from bats to intermediate animal hosts, such as horses, pigs, civets, or non-human primates, seems to be the mode of transmission to human beings. Transmission to man also may occur through aerosol while intruding into bat roosting caves or via direct contact with bats, such as catching bats or been bitten by bats.
Bats belong to the order Chiroptera (Greek means Hand-Wing) and are further classified into two suborders: the Yinpterochiroptera (Megachiroptera, commonly known as megabats) and the Yangchiroptera (microchiroptera, commonly known as microbats). While megabats eat fruit, the microbats live on insects, small mammals, fish or blood. There are around 1240 recognized species of bats globally, which account for about 25% of all mammalian species, making them the second largest order of mammals. The huge diversity of bat species provides a large breeding ground for viruses. Bats are ancient species and can be traced back to about 52.5 million years ago and the long evolutionary history provides long period for a variety of viruses to co-evolve with bats to make bats their natural reservoirs.
Bats act as reservoirs for more than 60 viruses of zoonotic importance and as per latest research, host more viruses per species than even rodents. While rodents host 179 viruses, 68 of which are zoonotic; bats, on the other hand, harbor137 viruses out of which 61 are zoonotic viruses.
Features that help bats in maintenance and transmission of diseases
Bats are the only mammals that can actually fly. In order to facilitate flying, bats developed hollow bones to reduce body mass. During winter time, some bats hibernate to conserve energy. Reduced body temperature and metabolic rate suppress immune responses and delay viral clearance from bats.
Bats being social animals, millions of individuals may be found in a single cave. The large number of bats in a colony with physical proximity facilitates viral transmission among bats, and helps maintain stable circulation of viruses among bats. Several emerging viruses can be amplified in a bat colony during the breeding season. Microbats developed echolocation for navigation. Apart from ultrasonic waves, the vibration of the larynx can also generate aerosols, which can play a major role in viral transmission amongst bats.
Viral dispersal
Bats are found in all continents except the Antarctica and inhabit various ecological niches, including trees, caves, and man-made structures, such as tunnels, deserted houses, even occupied houses in rural areas. The global distribution and large range of habitats of bats pose public health risk due to bat-borne viral infections.
Bats make small flights daily for food but during seasonal migration some of bats can fly up to 2000 km. The seasonal migration of bats enables them to take viruses to distant areas like migratory birds.
The eating behavior of bats has also got a lot to do with viral transmission by bats. Fruit bats cannot take a large amount of food. To meet their energy requirements instead of swallowing, bats chew to absorb juices containing sugars and other energy components, and spit out the pulp. The fruits discarded by bats may be contaminated with viruses in saliva of bats and other animals eating such fruits get infected.
Despite their small size, bats have a relatively long life span ranging 10 to 30 years. The longer life span also increases the possibility of bats getting infected with viruses and their dispersal.
Bats survive infections without becoming sick
Fever in mammals is associated with high metabolic rate which results in beneficial immune responses. When bats fly, their body temperature and metabolic rate increases. During flight the metabolic rate of bats increases 15-16 fold over the resting metabolic rate of normothermic, otherwise active bats. Such increase in rodents is 7-fold while running to exhaustion and only 2 folds in most birds. The high temperatures of bats during flight potentiate their adaptive and innate immune defenses to fight cancer and pathogens on the other hand the lower body temperature during daily rest and hibernation may suppress the replication of many viruses. The elevated body temperature caused by high metabolic rate may help bats control infections low metabolism during the day could lead to suppression of immune response and allows better adaptation with viruses.
The deadly viruses like rabies virus, Ebola virus, Nipah virus, Hendra virus, and MERS-CoV virus etc., that may cause fatal infections in humans may not produce any symptoms in bats and they are able to survive and act as reservoirs for such viruses.
Innate immune responses in bats show notable similarity with other mammals. The Toll-like receptors (TLRs) and various types of interferons (IFN) have been described in bats. Bats express diverse IFN pathways that can play an important role in controlling viral infections. The types I IFN response to viral infections are also similar to that of other mammals though higher expression of type I IFN “IFNω” has been detected in bats. Type II IFN or IFNγ and Type III IFN are also produced in bats.
The dominance of CD8+ over CD4+ T cells in some bats indicates that bat’s adaptive immune system is better directed to control intracellular pathogens, particularly viruses compared to other mammals. Bats also have higher titres of IgG antibodies. IgG antibodies predominate maternal lacteal secretions of bats as against IgA in other mammals and have role in providing maternal antibodies to the young ones. B cells are functional and are capable of responding to LPS stimulation. Bats B cells exhibit an unusually fast response to LPS (Polyclonal B cell activator) only 5 h post-treatment.
The adaptive immune cells in bats have diverged from other mammals. The significantly higher percentage of T cells in bat spleen and peripheral blood compared to other mammals strongly suggests that bats adaptive immune system has evolved to respond effectively to intracellular pathogens.
Ecological significance of bats
But despite all the negative propaganda against bats they are important to the ecology. Despite the data showing more viruses being transmitted by bats the likelihood of bats transmitting the infections is almost same as any other mammals. Since bats are having large number of species it is not a surprise that more viruses associated with them have been detected. It is important to note the following:
Many bat species never come in contact with humans.
A number of diseases found in bats can also be transmitted by other animals. For example the likelihood of Rabies being spread by dogs, raccoons and skunks is much higher than bats. The MERS is transmitted by dromedary camels and the SARS epidemic in 2003 was associated with captive civet cats.
Despite the fact that bats spread disease, they are vital members of the ecosystem.
The knowledge about bats is not complete probably because they are nocturnal and do not entice human fancy. It is no surprise that about 25% of bat species were recognized during past two decades.
Another important fact about their value to the ecosystem is the pollination by nectar-drinking bats.
Bats eat insects that destroy crops and save billions that would have been required for pest control without bats.
Bats eat mosquitoes and help keep mosquito borne diseases under control.
A lot of research is going on about the immune system of bats and their adaptability to infections. The proper understanding of bats immunology will help find ways to prevent animal and human diseases.
Posted in #Coronavirus, #Viral zoonosis, Animal and Human Health, ZOONOSES. Tagged #Bats, #COVID-19, #Ebola, #Hendra, #Marburg, #MERS, #Nipah, #Rabies, #SARS.
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One thought on “Bats as carriers of diseases”
Dr Anupam Mital says:
Very informative and precisely composed article. Well written!
Liked by you
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