The fact that bats act as reservoirs of human viruses was recognized in the first half of the twentieth century when rabies was found in South and Central America. The hypothesis that the bat could act as a reservoir of viruses causing EIDs in humans is in the second half of the twentieth century, a few decades later. Most genotypes of rabies or rabies-associated virus within the genus Lyssavirus of the Rhabdoviridae family have been documented in bats. In recent years, bats have gained notoriety after being involved in a large number of EIDs. The bat-borne viruses that can affect humans and cause EIDs fall into different families and these are:
• Paramyxoviruses, including Baptist viruses.
• Nipah viruses,
Ebola hemorrhagic fever filoviruses,
Marburg hemorrhagic fever filoviruses,
• Sudden acute respiratory syndrome-like coronaviruses (SARS-CoV),
Their list is probably far from complete, and interestingly, the bats’ ability to host strong retrovirals likely contributed to the shaping of mammalian retro viruses. Also, sialic acid receptors for avian and human influenza virus are found in small brown bats in North America. This could potentially facilitate the emergence of new zoonotic strains. In this context, it becomes urgent to solve three basic problems as soon as possible.
Do bats help serve as a source of pathogenic viruses for animals and humans regarding the pathogens that cause EIDs in humans? Are bats reservoirs for viruses that have yet to infect humans but could be the source of EIDs in the future? Can bats be thought of as living test tubes through which new viruses can be developed through genomic change and genetic drift? To answer questions, it is important to monitor bat populations and analyze the diversity of viruses circulating in these populations. Although informative, the study of a few samples and viruses circulating in a particular ecosystem cannot explain the global dynamics of viral populations found in different bat families around the world. The isolation and sequencing of viruses is an important step, but has not performed enough to capture the scope of the phenomenon. Polymerase chain reaction (PCR) has also contributed to better characterization of bat-borne viruses in relation to viruses that have already produced EIDs in humans when primers are present.
More recently, high-throughput sequencing and metagenomic approaches have led to a quantum leap in surveillance and information seeking. However, a global vision remains indispensable, and various laboratories in different countries meet the cataloging of bats in a comprehensive manner. When looking at the results, it should be noted that there is a lot of variation from one country to another. Asia leads largely in data accumulation ahead of North America and Africa, and later Europe and South America. The superiority of Chinese results for Asia’s contribution is even more impressive. Almost 60% (58.9%) of articles in Asia come from China, followed by Vietnam with 16.8%. All other contributing countries are below 7%, so 6.5% for both Thailand and Cambodia is interesting to highlight the relationship between the number of publications and the geographic origins of the scientific teams that published them. Because Asia / Southeast Asia is considered one of the hotspots on the planet for the emergence of new viruses.
The Co-Evolution of Bats and Viruses
The biological interaction of viruses and their hosts is often antagonistic, with a delicate balance of action and counter actions between the host immune system and virus escape mechanisms. The parasite-induced reduction in host condition increases the selection for host resistance mechanisms. On the other hand, new host defenses increase selection on the parasite. A tight genetic interaction between host and pathogens can lead to ongoing host-parasite coevolution, defined as the reciprocal evolution of interacting hosts and parasites. The antagonistic co-evolutionary arms race of parasite infectiousness and host resistance leads to adaptations and counter-adaptations in coevolution. It also plays a central role in the evolution of host-parasite relationships in the microbial world. An important consequence of coevolution is its impact on the genetic diversity of host and parasite populations.
It is widely assumed that host-parasite coevolution exerts a high selective pressure on both host and virus, exerting a great influence on biological evolution. Selected traits, related genes, and underlying selection dynamics represent main areas of interest for understanding host-parasite coevolution. The evolution of bats is a very successful singular history among mammals adapted to a wide range of environments, producing an enormous variety of species with high mobility and longevity. Although there are many more known rodent species, bats harbor more zoonotic viruses and more total viruses per species than rodents. Moreover, bats contain significantly higher rates of zoonotic viruses than any other mammalian order. The antagonistic co-evolutionary arms race of parasite infectiousness and host resistance leads to adaptations and counter-adaptations in coevolution. It also plays a central role in the evolution of host-parasite relationships in the microbial world. Its origin is estimated to be around 64 million years ago, or to trace the Cretaceous-Tertiary border.
Millions of years of bat evolution may have led to the coevolution between the host and the pathogen. The antagonistic coevolution between the infectiousness of viruses and the resistance of bats is still not fully known. The ability of bats to harbor highly lethal viruses to humans without apparent morbidity and mortality has long been debated. The abnormal lack of ethology observed in infected bats may be due to the choice of resistance mechanisms. The evolution of flight in bats is accompanied by genetic changes in their immune systems to adapt to high metabolic rates. During their flight, their increased metabolism and higher body temperatures may have strengthened their immune systems, increased resistance and thus increased the variety of viruses they harbor. This increase in metabolic rate in bats is estimated to be 15 to 16 times when it is only seven times for running rodents and two times for birds.
The Marburg, Angola, Ebola, and Makona-WPGC07 viruses have been shown to reproduce efficiently at bat’s flight temperature, i.e. 37 and 41 ° C. This indicates that the flight-related temporal rise in temperature does not affect filovirus replication. Moreover, many strains exhibit a daily drowsiness with decreasing body temperature, which could be a virus resistance strategy that inhibits optimal virus replication. Also, there is a unique interferon system (IFNs) that can explain bats’ ability to coexist with viruses. Mammals have a large IFN locus that contains a family of IFN-α genes that are expressed following infection. Conversely, bats only have three functional IFNs – & # 945; with an IFN locus, but is structurally and persistently expressed. This constitutive expression can turn into a highly effective system for controlling viral replication and explain the resistance of bats to viruses. Differences have also been observed in the immune response between bat species against the same virus.
Significant differences in the percentage of seroconversion against European bat lyssavirus type 1 (EBLV-1) were observed between two species from two different families. The percentage of seropositive Rhinolophus ferrumequinum was much lower than that of Myotis myotis and different seroconversion rates were observed. Significant differences in seroconversion rates were found among bats, depending on whether they were previously infected. It has been suggested that long-term recurrent infections of bats can gain an important immunological memory and decrease susceptibility to rabies infection. Immune competence in bats may vary depending on body condition (through nutritional status and stress) and reproductive activity. As a result, it may lead to a lower seroprevalence of rabies among or within bat species.
Author: Ozlem Guvenc Agaoglu