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December 30, 2003

Mapping Ebola

Townsend Peterson, the U of Kansas ornithologist compiling a hit list of mammal species that might serve as the Ebola virus's natural reservoir has published a new paper with the CDC detailing a projected geographic distribution of the Ebola virus and its hemorrhagic fever cousin, Marburg.

Read the paper, by all means, but the data boils down to several geographic projections of where the Filoviruses might be found, based on the natural environment of previous Ebola and Marburg outbreaks. The African projection appears on the left, but as at least one filovirus, Ebola Reston, has been traced to the Phillipines, there is also a geographic prediction of where filoviruses might occur in Southeast Asia. The darker the shade of red, the more confident Peterson and his co-authors are that Ebola or closely related filoviruses will be found there. Based on the data, Peterson & company predict that the entire tropical region of Africa is likely to harbor filoviruses

"Ebola HF in the humid rain forests of central and western Africa, and Marburg HF in the drier and more open areas of central and eastern Africa. Most of the predicted geographic extent of Ebola HF appear to have been observed; Marburg HF has the potential to occur farther south and east."

The importance of projecting the ecological range of Ebola and Marburg is that once such a map has been created, it can then be compared to the ecological ranges of possible hosts of the disease. Sadly, given the small size of the data the projected maps drew from, finding a one-to-one correlation between the virus and a host is probably too much to expect. However, there are limiting factors other than environmental type which serve to reduce the possible pool of suspects. Due to the way in which the filovirus outbreaks occur, the faunal host(s) of Ebola and Marburg is not a commonly found creature, nor is it an insect or spider.

...filovirus transmission to humans is not common, and most occurrences can be traced to a single index case (exceptions occur—e.g., the Durba Marburg outbreak appears to have involved multiple independent infections of humans from a reservoir population presumably associated with a mine). We assume that introductions to nonhuman primate populations also generally begin with single index cases, but this hypothesis is more difficult to investigate. This rarity argues against a common arthropod vector for transmission: if anthrophilic arthropod vectors were to carry filoviruses, multiple index cases would be more common, as many primates in an area would have the opportunity for infection. In addition, filoviruses generally do not replicate in arthropods or arthropod cell lines, leading several authors to speculate on more incidental modes of transfer (e.g., direct contact).

The prospective list of possible mammal hosts has been submitted to the CDC for approval, according to an email from Dr. Peterson, but it is not yet available.

We have completed the 2nd step--choosing lists of potential virus hosts based on crude geographic considerations--and it is presently awaiting clearance for submission from the CDC. It will be submitted to EID as soon as is feasible.

He also commented on the my theoretical hosts, the avian family Picathartidae, and why the initial list of suspects includes only mammals.

As far as including birds goes, well, you have to start somewhere, and we started with mammals. Doing the birds would not be too hard given the Atlases of Speciation in African Birds, and we should probably do that soon. My guess is that Picathartes would certainly come out on the list, given their distributional patterns... way too much to do, and not enough time to do it, right?

The question is, how many other animals have a similar distributional pattern? How exhaustive is the initial list of 100 mammals? I can't imagine a simple list of species to be investigated would take that long to approve--I'll post a copy when it comes out.

Posted by Bigwig at December 30, 2003 12:57 PM | TrackBack
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It would be interesting to see what similarities exist between the suspected avian retroviruses and filoviruses. If I remember correctly, retroviruses are distinguished by presence of reverse transcriptase and a double-stranded (+) RNA genome, while filoviruses have a (-) stranded RNA genome.

Of course, it's been several years since I left research for public health, so I might be a bit behind on the science.

Posted by: Captain Holly at December 30, 2003 05:43 PM
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