Bats are members of the order Chiroptera, which is the second-largest order of living mammals. The geographic distribution of these flying mammals is tremendous as well, ranging from the limit of trees in the Northern Hemisphere to the southern tips of Africa, South America, and New Zealand. Within such a wide distribution, bats frequently encounter human populations. However, close contact between bats and humans is problematic with regard to disease ecology and epidemiology because bats are natural reservoirs (primary hosts) or intermediate hosts for numerous zoonotic pathogens—that is, infectious disease agents that are transmitted from animals to humans. Specifically, bats harbor more than 60 pathogenic viruses that can infect humans, including Ebola, Marburg, Nipah, Hendra, and rabies viruses. Bats also harbor various coronaviruses, including those responsible for severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and coronavirus disease 2019 (COVID-19). See also: Chiroptera; Coronavirus; Disease ecology; Epidemiology; Exotic viral diseases; Infectious disease; Mammalia; Pathogen; Rabies; Virus; Zoonoses
Intriguingly, bats are highly resistant to many of the pathogenic agents that bats harbor. Because bats are, for the most part, unaffected detrimentally by zoonotic viruses and rarely show symptoms of illness, scientists surmise that the bat immune system has undergone a series of adaptations that suppress inflammation. In a number of studies, researchers noted the absence of various genes that typically encode molecules responsible for triggering inflammatory responses and cell death. In particular, bats lack AIM2-like protein receptors. In humans and other mammals, these immune proteins identify pathogenic DNA within cells and prompt cellular inflammation reactions that, if successful, would eliminate dangerous agents. However, the presence of these inflammation-inducing proteins typically leads to excessive inflammation that contributes to the damage (especially in older affected individuals) that infectious pathogens cause. In addition, other studies found reduced levels of NLRP3 proteins and the enzyme caspase-1 in bats. Normally, mammalian NLRP3 proteins and caspase-1 trigger inflammation in response to pathogenic attack and infection. However, in bats, the genetic processes leading to the creation of NLRP3 proteins and caspase-1 are suppressed, diminishing any inflammatory responses. Through these adaptive measures and others that curtail excessive or inappropriate inflammation, the immune system of bats has evolved to the point where it can tolerate the presence of numerous potentially lethal viruses. See also: Cellular immunology; Gene; Immunology; Infection; Inflammation; Protein
Researchers have also noted that internal temperatures of bats are elevated to approximately 40°C (104°F) during flight. This high temperature in bats is similar to the temperature of a high fever in humans. Typically, fever regularly accompanies inflammation, and increased body temperature may accelerate certain biochemical reactions of use to the host in combating infection. Therefore, the high temperature engendered by the daily flying routines of bats may disable the effects of pathogenic viruses in bats, although the viral load remains high enough that the virus can be transmitted to other organisms. See also: Animal flight; Temperature
