#Ebola: Second, possibly third outbreak, West African outbreak growing

The number of people known or suspected to be infected with Ebola in the West African outbreak is increasing, and the rate at which it is increasing is increasing. About 40 new cases are being reported per day on average, but the number of new cases has been going up by a few a day.

However, it is still unclear that these numbers represent what is actually happening on the ground. There is little confidence that the WHO has a good idea of who is currently stricken with the disease, and efforts to contain those who are have had mixed results.

A second outbreak is now occurring in the DR Congo (formerly Zaire). This is a second separate outbreak. So, it is NOT correct to say that Ebola has spread into the Congo. It didn't. It emerged there independently.

What are the chances of that happening? I have long maintained that the conditions for Ebola spreading into human populations include factors that make the overall chance of that happening, for a large region, go up enough for multiple simultaneous epidemics to be more likely than chance might suggest. Perhaps I'll discuss my reasoning for that another time. In any event, the DR Congo outbreak, about which we know very little so far, appears to be a different strain of Ebola, so this is not the Wester African Ebola spreading to Central Africa.

There are reports of a third outbreak of an unknown disease that might be Ebola also in the DR Congo. But that could be a lot of things. Including Ebola... so we shall see.

Also, there is one new case in Nigeria, after a period of several days with no new cases.

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If the WHO reporting data are to be believed, the rate of increase is not, in fact, increasing. It is decreasing every so slightly. I will admit that the constancy of the reproduction number across the last 16 reporting periods makes me a bit suspicious and, of course, WHO has suggested that there is evidence on the ground of "massive under-reporting" but they haven't really followed up on that.

Actually, I get the rate of increase increasing at 1.6 per day. It probably depends on the exact data that are used and how it is parsed out. The reporting is for a variable number of days (each report may be 2,3, or 4 days) and that has to be taken into consideration, etc. But for the period shown in the graph above, the average additional cases is 41, for the last seven updates (going back to August 9th) it is 68. The simple slope of a linear model for the last several reporting data's per day rate is 1.6. However, you are the expert on these things; what are you basing the decrease in rate?

Given the circumstances I'm not sure why R0 would not be consistent.

I am putting this out there, please don't think of me as ignorant, but is it not possible for this strain of Ebola to have mutated into a form that is spread via aerosols? It seems to me that the rate of infection spreading across W. Africa is more consistent with an airborne infection than an infection spread through somewhat casual contact. If I am correct, the Reston strain is spread via aerosols. Would it be such a leap for Zaire to be as well, now?

By Shayna Cooke (not verified) on 26 Aug 2014 #permalink

Shayna, if Ebola in West Africa had mutated to become airborn the epidemiology would look very different, and the spread would be much more dramatic, so no, that almost certainly has not happened. The Reston case is a special circumstance, and actually, it may not be airborne in humans.

But there is nothing that I know of that excludes the possibility of it happening. On the other hand, consider the evolutionary context of Ebola. In its native setting, it has a very good way of getting around among fruit bats. So, in a sense, it may be the case that Ebola is not predisposed to develop in this way.