Monday, 26 December 2011

FROM GREEN MONKEY DISEASE TO FRUIT BAT VIRUSES

Kenyans breathed a collective sigh of relief on Friday when it was announced that a 29 year-old woman who had bled to death in Kenyatta National Hospital the morning before was not the victim of an Ebola-like virus, as had initially been feared (for the Nation's version of these events, click here). As readers of Richard Preston's bio-thriller The Hot Zone (1994) were informed in full technicolour detail, Ebola and its congener, Marburg virus, don't take many prisoners. Typically more than half of the people who display symptoms of these highly infectious diseases die; in some outbreaks close to 90% of those infected have succumbed. The symptoms of these deadly viruses are no less terrifying: victims are apt to suffer severe hemorrhaging, bleeding from all of their bodily orifices. And they don't always go quietly: some Ebola victims die an even more horrible death, thrashing about in a uncontrollable fit as they smear and splatter blood everywhere. No wonder then that Ebola and Marburg elicit widespread fear, and panic when they are thought to be anywhere near.*

The recent scare reminded me of earlier Ebola panics. In 1980, during my first few weeks in Tanzania, overland travellers staying at the Moravian Youth Hostel in Mbeya came bearing hair-raising tales of what was then dubbed 'Green Monkey Disease', a killer infection that had led to periodic closures of the roads through the southern Sudan. Although I didn't know it then, this was a variety of Ebola, the virus named after its discovery in 1976 near the headwaters of the Ebola River in Zaire (now the Democratic Republic of the Congo, DRC). The first recorded Sudanese outbreak occurred in the same year, and killed more than 150 people. It was followed by another in 1979, and it was this recurrence (which resulted in 22 fatalities) that had affected the travel plans of my fellow hostellers. Otherwise I didn't really become aware of Ebola by name until the mid-1990s, when there were further cases in Zaire. The Kikwit outbreak in 1995, which killed 250 people, was widely reported in the press and was said to have led to tourist cancellations throughout the region, especially in countries and territories whose names also began with the letter 'Z' (like Zambia and Zimbabwe). One of these was Zanzibar, where I was living and working at the time.

The Ebola virus
(Dr. Frederick Murphy)
 The Ebola-Zaire strain is the most virulent of this family of thread-like viruses. The first of the filoviruses to be described was Marburg virus, named after an outbreak in a factory in the West German city of Marburg in 1967. The Hot Zone opens with a dramatic reconstruction of the first case in Kenya, which took place in 1980 when a Frenchman who worked at the Nzoia Sugar Factory died in Nairobi Hospital. Before dying he vomited blood over the Ugandan doctor who was treating him, and Dr. Shem Musoke was infected in turn. The presence of Marburg was detected in samples of Musoke's blood taken by a colleague, cardiologist Dr. David Silverstein, and after being close to death his life was saved. Silverstein is perhaps best known as former President Moi's personal physician, and as one of the leaders of the Jewish community in Kenya. Coincidentally I was treated by him some years later for a phantom blood clot, and it was Shem Musoke who did me the favour of diagnosing it as imaginary. But that's a story for another day.

Silverstein was also involved in the treatment and diagnosis of Kenya's second Marburg victim, a Danish boy who died in Nairobi Hospital in 1987. These have been the only confirmed incidents in Kenya to date, and it turns out that both of of the victims had visited Kitum Cave on the slopes of Mount Elgon not long before they fell ill. The Hot Zone ends with an account of author Richard Preston's foray into the cave wearing a protective suit, wondering which, if any, of its animal inhabitants or visitors might host the virus. For a time monkeys were implicated, not least because they were the immediate source of outbreaks of both Marburg and Ebola in laboratories on different continents. The first Marburg infections came from Vervets (aka Green monkeys, Chlorocebus pygerythrus, syn. Cercopithecus aethiops) imported from Uganda, and one of Preston's informants believed that there had been an earlier undocumented outbreak of Marburg among monkeys and people on the northern side of Mount Elgon. However, monkeys are evidently not the primary host of either Marburg or Ebola: like people, too many of them die too quickly during an outbreak for the viruses to spread more widely. Not that this has stopped Preston and other journalists from imagining a doomsday scenario involving international air travel...

Richard Preston preparing to enter Kitum Cave (from http://richardpreston.net/study-guide)
It turns out that the association with caves (and in some cases mines) is much more significant for an understanding of the natural reservoirs of Marburg and Ebola. In recent years a number of studies have detected the presence of these filoviruses in otherwise healthy cave-dwelling bats, suggesting that they are their main natural hosts. I've posted links to some of the relevant papers (most of them open access) in the references  below. Bats carrying Marburg and Ebola have been identified in Gabon (which has yet to suffer a recorded outbreak), DRC, and Kenya, and it seems likely that they will also be discovered in other African countries. The bats involved include a growing list of insectivorous species as well as the Egyptian fruit bat (Rousettus aegyptiacus), which has tested positive for both Marburg and Ebola. This is one of Africa's most common bats, known for roosting in large colonies in caves, unlike many other fruit bats. The good news is that health education can now be targeted at cave sites in national parks and other underground locations that receive large numbers of visitors. The bad news is that outbreaks of Marburg and Ebola are likely to occur in hitherto unsuspected places across the continent. At least last week's scare proved to be a false alarm.

Note
* For more sober information about Ebola and Marburg, including details of all the confirmed outbreaks, readers should refer to the Centers for Disease Control and Prevention website and the pages for Ebola Hemorrhagic Fever and Marburg Hemorrhagic Fever respectively.

References

Kuzmin, Ivan et al. 2010. Marburg virus in fruit bat, Kenya. Emerging Infectious Diseases 16 (2): 352-354.

Leroy, Eric M. et al. 2005. Fruit bats as reservoirs of Ebola virus. Nature 438: 575-576.

Pourrut, Xavier et al. 2005. The natural history of Ebola virus in Africa. Microbes and Infection 7: 1005-1014.

Pourrut, Xavier et al. 2009. Large serological survey showing cocirculation of Ebola and Marburg viruses in Gabonese bat populations, and a high seroprevalence of both viruses in Rousettus egyptiacus. BMC Infectious Diseases 9: 159.

Preston, Richard 1994. The Hot Zone. New York: Random House. [the link given here is to a complete typescript of the main text]

Towner, Jonathan S. et al. 2007. Marburg virus infection detected in a common African bat. PLoS ONE 2 (8): e764.

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