When it comes to keeping track of Ebola, there are three categories of cases: confirmed, probable, and suspected. A case is confirmed when a biological sample tests positive for the virus. Probable cases have either been seen by a clinician, or have had close contact with a confirmed Ebola patient. Suspected cases have the signs and symptoms of Ebola, but have not been evaluated by a clinician nor had a sample tested.
Keeping accurate records of case counts is important to outbreak control. We need to know how many people are sick in order to plan for how many hospital beds, healthcare workers, PPE kits, etc. will be needed. Without a solid understanding of the disease burden, it's difficult to mount an effective response.
The symptoms of Ebola virus disease are fairly nonspecific, especially in the beginning: fever, headache, muscle aches, malaise. So suspected cases may not have Ebola after all, but may instead be suffering malaria, Lassa Fever, or some other viral illness. However, the World Health Organization cautions that "a substantial proportion of these suspected cases are most probably genuine cases of EVD.".
In order to keep those accurate records, samples from suspected and probable cases must be collected properly, and transported to the laboratories capable of doing the testing. The laboratories must then process the samples, record the results, and return those results to the relevant public health department. Each step is an opportunity for attrition. Sierra Leone and Liberia provide data on the number of cases in each category.
The case confirmation pipeline seems to be functioning very differently by region during this outbreak. Liberia has a very high number of unconfirmed cases, but they also have a very high number of cases overall. The number of unconfirmed cases seems to be growing over time, while the number of confirmed cases is holding steady. Sierra Leone seems to be doing a better job of confirmation.
If we look at the proportion of cases confirmed instead of the raw numbers, we get a better idea of where things stand. Sierra Leone has been able to confirm around 90% of the cases they think are Ebola, and that has been holding pretty steady over time. Liberia on the other hand was not doing great to begin with, and the situation has deteriorated as the outbreak has progressed. Between 20% and 30% of cumulative cases are confirmed in recent weeks.
A more granular look at the data reveals some interesting trends. (It's difficult to make sense of plots displaying all 16 counties in Liberia, so I split them up into three separate scatter plots. The x axes are the same across each of the subplots.) The capacity to confirm cases varies widely at the county level. Most counties are somewhere around the 40% mark. River Gee, Maryland, and Gbarpolu Counties though are at an alarming 0% - these counties have 19, 8 and 1 total cases, respectively though, so that's not too bad. Margibi County though has a 564 cases, of which just 41 are confirmed. Montserrado County, a huge hotspot, has just 358 of 1,635 cases confirmed, and its confirmation proportion has been declining over time.
Counties in Sierra Leone seem to be doing well on the whole, with the odd exceptions of Bonthe and Kono Counties. Those two localities have 3 and 62 total cases, respectively.
There are several reasons I can think of that could explain the variation in case confirmations. It could be that clinicians and burial teams in those regions are not trained to collect samples, so nothing is entering the confirmation pipeline. Alternatively, perhaps certain places do not have the infrastructure to support the process. For example, perhaps they do not have a driver to transport samples to the lab, or maybe they don't have someone in charge of coordinating lab results. Or maybe there are just so many new probable and suspected cases that it's not feasible to move any significant fraction of them through the pipeline.
The World Health Organization's roadmap complicates things further. (Here's the source for the above figure.) It takes a bit of patience to read this map, but focus your attention on the boxes with L, meaning laboratory. A red box means no or inadequate capacity; yellow means pending deployment, and green means functional and meeting demand.
I showed about how Margibi and Montserrado Counties have particularly low confirmation rates. The WHO's map suggests that both of those counties' laboratory testing capacity is functional and meeting demand. The incongruity suggests to me that the bottle neck is not in the testing itself, but earlier in the pipeline - specimen collection or mobilization, for example.
Given the importance of confirming cases, I think it would be very useful for outbreak responders to learn more about the confirmation pipeline, and identify the bottlenecks. Hopefully in coming weeks and months as the international community continues to build a stronger response, we see an improvement in these numbers.
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