Marburg virus, the founding member of the Filoviridae, is an enveloped virus with a negative-strand RNA genome. Other members of the filovirus family are the five species of ebolavirus. Filoviruses are indigenous to Africa, but the animal reservoir for the virus has not been definitively identified. The first outbreaks of Marburg hemorrhagic fever took place in laboratories in Marburg, Frankfurt, and Belgrade in 1967. The virus was believed to originate from African green monkeys that were being used for laboratory research. However, these monkeys were trapped in regions of Uganda where fruit bats are common. Other evidence suggests that bats are the natural reservoir of filoviruses. For example, two patients who developed Marburg hemorrhagic fever in 1980 and 1987 in Kenya had been in a cave inhabited by bats before they became ill. In January 2009 the first US case of Marburg hemorrhagic fever was reported in Colorado. The patient had traveled to Uganda in December 2007 and visited a python cave that houses thousands of bats.
The study was undertaken to understand why miners working in Kitaka Cave in July and September 2007 developed Marburg hemorrhagic fever. The authors captured 611 bats and found Marburg viral RNA in 31. Given the population of 100,000 bats in Kitaka Cave, at least 5,000 are likely to harbor the virus. Antibodies to Marburg virus were also detected in bat sera, and infectious virus was recovered from 4 animals, all of which were healthy.
Filovirus antigens were detected in tissues of naturally infected bats for the first time. Viral antigens were detected by immunohistochemistry in the livers of two bats from which infectious Marburg virus was isolated in cell culture (illustrated). Viral antigens were also detected in the spleen of one bat, in the cytoplasm of mononuclear cells.
Nucleotide sequence analysis revealted that in the Kitaka Cave outbreak the two miners did not infect each other, but were infected separately by two independent introductions of virus from bats to humans. Furthermore, remarkable diversity – up to 21% between virus lineages – was observed in viral RNAs from the bat colony. This diversity suggests that the virus remains for long periods in its reservoir host, and also infects large numbers of bats. R. aegyptiacus bats migrate over 300 miles to other colonies each season, providing a pool of millions of bathosts for Marburg virus.
These findings have clear implications for public health: the large numbers of bats that harbor Marburg virus have the potential to initiate epidemics of hemorrhagic disease in humans. There are many other caves throughout Africa that harbor similar colonies of bats. Given the high case fatality ratio of filovirus hemorrhagic fever – approaching 90% – it is clear that bat-infested caves should be avoided by miners and spelunkers.
Towner, J., Amman, B., Sealy, T., Carroll, S., Comer, J., Kemp, A., Swanepoel, R., Paddock, C., Balinandi, S., Khristova, M., Formenty, P., Albarino, C., Miller, D., Reed, Z., Kayiwa, J., Mills, J., Cannon, D., Greer, P., Byaruhanga, E., Farnon, E., Atimnedi, P., Okware, S., Katongole-Mbidde, E., Downing, R., Tappero, J., Zaki, S., Ksiazek, T., Nichol, S., & Rollin, P. (2009). Isolation of Genetically Diverse Marburg Viruses from Egyptian Fruit Bats PLoS Pathogens, 5 (7) DOI: 10.1371/journal.ppat.1000536