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Bats act as reservoirs for fulminant diseases
More than half of a century ago, pioneering research at the CDC linked the transmission of rabies to bats and their cave-dwelling environments. Even today, human acquisition of rabies from bat bites occasionally occurs. The long delay between bat contact and the onset of unusual neurologic symptoms in human victims has made diagnosis challenging.
Fast-forward to the present decade when three baffling and occasionally fatal illnesses have attracted world headlines: Marburg/Ebola viruses (Filoviridiae) in sub-Saharan Africa, Nipah virus (similar to Hendra virus) in Southeast Asia and SARS-coronavirus in China. Bats of varying species and host trophism have been implicated as the reservoir for these terrifying diseases that have focused worldwide attention.
Perhaps the most notorious, Marburg/Ebola, has been associated with high mortality and transmission of fatal hemorrhagic fever from African patients to the very medical personnel who have sought to contain the disease and alleviate human suffering. Although person-to-person transmission through body fluids and fomites appears clear, an air-borne route of transmission was also documented in the original Marburg outbreak that took place in Germany decades ago. The tantalizing question about Marburg/Ebola disease has been the actual reservoir for the virus in nature, since outbreaks occur sporadically and at long geographical distances apart. When fatal outbreaks have occurred in sub-Saharan Africa, investigative teams arrived days or weeks after the epidemic had subsided. Despite efforts to culture literally tens of thousands of insect and mammalian species, no living virus has been isolated from any source.
Nonetheless, insect-devouring and fruit-consuming bats now appear to be linked to Marburg transmission and disease. Although no isolate of the virus has been made from a bat, prevalence of antibody in bat populations is now well documented. It is possible that the surviving bats develop an antibody response, and those who succumb from infection rapidly do so, such that viral isolation has not been possible.
A link to SARS?
In Southeast Asia, the evidence is now compelling that the original source for the SARS-coronavirus — the masked palm civet (Paguma xarvata) — was merely an intermediary host, probably the source of virus for market-workers and butchers processing these wild animals (which were considered a gourmet delicacy in Chinese cuisine).
High antibody titers in workers handling masked palm civets led to culling of thousands of such animals, thus depriving gourmands in China of one of their prized delicacies.
Nonetheless, it now appears that horseshoe bats isolated from Southern China are the definitive reservoir for the SARS-coronavirus and that the civet was only an intermediary host. Even more intriguing are results of studies in Colorado caves that have shown that a coronavirus different from a SARS-coronavirus can be isolated in low frequency from bats. Fortunately, no outbreak and transmission of coronavirus capable of epidemic and fatal respiratory disease has occurred, but it should be obvious that the potential is there, should key mutations develop in North American coronavirus. The conclusion from a Colorado investigation is that bats may be the original hosts from which all coronavirus lineages are derived.
Nipah/Hendra virus originally caused fatal disease in abattoir workers in Malaysia and was originally diagnosed as equine encephalitis. Evidence suggests that bats, one of the most numerous of all mammalian species, meet energy requirements by ingesting fruits and spitting out partially digested fruits and seeds. This bat saliva contains residual virus. The animal pens of pig farms in Malaysia where Nipah outbreaks first occurred were littered with partially digested fruits regurgitated from bats. Similar observations were made with Hendra virus outbreaks in Australia. In Bangladesh, Nipah virus has been shown to be directly transmitted from bats to humans.
Clearly, elimination of bat populations is not possible. Improved understanding of transmission in nature and how humans become an “accidental” or “overflow” target from animal sources is the essential lesson. With Marburg/Ebola, the lethality of disease has triggered promising efforts toward a vaccine development.
African workers in underground mines where outbreaks have occurred might be an appropriate target for vaccination. When the SARS-coronavirus was first isolated, the rapid response of the pharmaceutical industry was gratifying in that research readily identified molecular biologic approaches (viral receptor antagonists) that could impede transmission of virus. Epidemiologic measures quickly controlled the SARS outbreak, but it is gratifying that such interventions are available “on reserve.”
For Nipah/Hendra virus, one approach to control may also be vaccination. Thus, better understanding of the virus and the phenomenon of “overflow” into human population as civilization encroaches on nature, as well as devising efficient means to induce an immune response, are the best hopes for protecting humans from the onset of some of the most terrifying viral diseases known.
For more information:
- Lowell S. Young, MD, is Director of the Kuzell Institute for Arthritis and Infectious Diseases at the California Pacific Medical Center in San Francisco and Clinical Professor of Medicine at the University of California, San Francisco. Dr. Young is also a member of the Infectious Disease News editorial advisory board.
- Leroy EM, Kumulungui B, Pourrut X, et al. Fruit bats as reservoirs of Ebola virus. Nature. 2005;438:575-576.
- Pourrut X, Délicat A, Rollin PE, et al. Spatial and temporal patterns of Zaire ebolavirus antibody prevalence in the possible reservoir bat species. J Infect Dis. 2007;196(Supp 2):S176-S183.
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