Great News From Liberia
The news from Liberia is wonderful: the last known Ebola patient has been discharged from treatment! [NYT 3/6/15] This doesn’t make Liberia “Ebola free” – that requires 42 days (twice the incubation period) with no new cases. Many contacts are still being monitored, and others may not have been identified, so Liberia may still have to deal with a few more local outbreaks before it can declare victory. Also, so long as Ebola continues in neighboring countries the disease may be brought back to Liberia by travelers (though note this heartening item about an effective Liberian border control operation: [IOM 2/4/15]). And the risk of an unrelated jump to a human from an animal reservoir will continue indefinitely. Nevertheless, this is a moment to be savored!
Update 3/24/15: A new patient has been identified in Liberia, who probably was infected by her boyfriend, an Ebola survivor. [NYT 3/24/15]. This is disappointing, but the fact that only one instance has come to light suggests that this mode of transmission is rare.
Ongoing Transmission in Sierra Leone and Guinea
Sierra Leone initially showed a similar pattern of precipitous decline in patients, but since the end of January weekly cases have stabilized between 60 and 100. [WHO 3/4/15] This is better than exponential growth, but it’s not on track to ending the epidemic. Guinea didn’t have as sharp a peak, but it also hasn’t experienced such a dramatic improvement: about 50 cases are continuing to arise each week. In both countries the steady rate of new cases implies an Effective Reproduction Rate of about 1. Pushing this down just a little would eventually end the epidemic, while letting it rise would allow exponential growth to resume. Liberia has shown that it’s possible to snuff out the epidemic, but Sierra Leone and Guinea are not yet following in Liberia’s footsteps.
I don’t claim to know how Liberia did it, but somehow the Effective Reproduction Rate dropped to about 1 in September, then fell much lower from October on. Sierra Leone’s peak and fall were similar, but occurred about two months later, and the decline has now leveled out. While the opening of new treatment centers certainly helped in recent months, my own best guess is that the main factor in both countries is that people simply “got religion.” That is, a large enough fraction of Liberians came to accept that Ebola is caused by a virus, not a curse; that people who fall ill need to be cared for in safe settings, rather than at home; and that corpses need to be handled with extreme care. I suppose that a comparable fraction of Sierra Leone’s population reached similar conclusions, but the epidemic is now continuing there and in Guinea within sub groups that haven’t fully accepted the message.
The last Ebola patient in Liberia, Beatrice Yardolo, came from a worrisome cluster of cases “in a community called St. Paul’s Bridge, located in what was one of the biggest Ebola hot spots in Monrovia, New Kru Town.” [NYT 3/6] The New York Times reported that, “[t]he outbreak was unusually violent and far-reaching, in part because it involved criminal gangs that did not cooperate with the authorities and fears that it could be spread through a knife fight with one member nicknamed Time Bomb.” Even more troubling was the fact that known contacts on several occasions left Monrovia for other parts of the country. [FrontPageAfrica 2/3/15] One of the escaped contacts died in Margibi County, and required the tracking of 256 contacts there. [GNN Liberia 2/4/15]
Ebola came into the Yardolo family from Beatrice’s cousin Steve, who probably became infected at an Ebola treatment center where he worked. [AllAfrica 3/6/15] Steve infected several other members of the family, eventually including his daughter Amanda. Beatrice took responsibility for Amanda’s care, but again and again Amanda was mis-diagnosed and rejected from Ebola treatment centers. Beatrice was left responsible for bathing and otherwise caring for her. After Amanda died Beatrice went immediately to a treatment center started showing symptoms, which probably saved her life.
Ebola didn’t end in Liberia because people behaved well, but because the strategies of isolation and contact tracing worked despite much bad behavior and many mistakes.
Sierra Leone’s experience is vividly described in another New York Times piece. [NYT 2/18/15] The epidemic was re-ignited by three sick fishermen who landed in a poor area of Freetown in early February. Two dozen people in the area ended up contracting Ebola from this initial group before transmission was brought under control. A quarantine was imposed but was frequently broken. One shopkeeper fell ill but was afraid to go to a hospital in Freetown. Instead he broke quarantine and traveled three hours to his home village, where he visited a traditional healer, then died. He caused the infection of some 42 people in the district.
Ebola is still being actively transmitted in Sierra Leone and Guinea, and may yet recur in Liberia, or spread to other countries. But the Liberian experience gives every reason to hope that the techniques that worked there can eventually end this epidemic altogether. The possibility must be acknowledged, however, that the epidemic will continue indefinitely, neither growing exponentially nor definitively ending. In this case Ebola would become “endemic” to West Africa — just one of the typical diseases that people get there. I can’t disprove this — indeed it seems to be happening now in Guinea — but I have several reasons for doubting that this will happen with the current strain of Ebola:
- It’s too deadly. A cluster of a less lethal disease might go relatively unnoticed, but this can’t happen with Ebola. The deaths of family and friends, one after another, cannot be ignored. The presence of Ebola in a community will not be tolerated for long.
- Its incubation period is too short. A disease that takes months or years to emerge (e.g. AIDS, leprosy, tuberculosis) can hide in a human population, but Ebola emerges quickly, or not at all.
- It’s only transmitted from person to person. Aside from the wild animal reservoir (discussed below) Ebola appears to be transmitted only from person to person. A disease that could also infect some more familiar animal, like fleas or rats, could hide in the animal reservoir and reemerge unpredictably. Fortunately, Ebola shows no sign of doing this.
- It’s only transmitted by direct contact. Fortunately, Ebola isn’t airborne; it’s transmitted only by direct contact with bodily fluids or contaminated surfaces. This makes contact tracing and isolation so effective. It’s much more difficult to contain a disease like the flu that spreads through the air.
- People are infectious only when they are ill. Update 3/24/15: (except for the semen of male survivors). There is no indication that there are “Typhoid Marys” who can transmit Ebola without themselves falling ill, except for one instance in which a male survivor appears to have infected his partner, presumably through semen, which is known to contain Ebola virus for several months after recovery. [NYT 3/24/15] This will make the disease a bit more difficult to eradicate, but is not cause for panic. As noted above, the fact that just one infection of this type has come to light suggests that this mode of transmission is infrequent. Additionally, now that an example exists one might expect many “discordant couples” (in which the male is a survivor and the female is not) to use condoms, which should further reduce the risk. One question that calls for further research is how long the virus can persist in the semen of a male survivor, and whether a negative semen test is sufficient to make unprotected intercourse safe for the survivor’s partners.
For these reasons I think it’s highly unlikely that Ebola will become endemic based on human-to-human transmission (absent mutation). So long as we don’t lose focus I expect that Sierra Leone, and eventually Guinea, will be able to end this epidemic using the standard techniques of contact tracing, isolation, funeral management and education.
The Wild Animal Reservoir
In one important respect Ebola already is endemic to East Africa: For forty years it has jumped every few years from its wild animal reservoir (probably fruit bats) into a human population. Just this year an unrelated Ebola outbreak in a rural area of Uganda flared briefly and was extinguished. [CDC] There is every reason to expect this pattern to continue, since:
- even when we have a vaccine it will never be feasible to vaccinate the entire relevant population,
- contact with infected animals such as bush meat may be reduced through education but can’t be prevented altogether, and
- there is no conceivable way to eliminate Ebola from the animal reservoir.
This is too bad, but it’s a bearable situation, since most outbreaks start in remote rural areas, where they can be quickly snuffed out so long as they are detected early and the response is competent and decisive.
So far as we know the West African epidemic arose from a single jump between a wild animal reservoir and a human. There might be some reason why this was an isolated incident that won’t recur. But it seems much more likely that Ebola is now established in a West African animal reservoir, from which it will now jump periodically into humans, the same way it has for decades in East Africa. After the current epidemic is brought under control it will be essential for all West African countries to establish systems for early detection of new Ebola outbreaks, as has long been the case in East Africa.
We’re lucky that infective contacts between humans and the wild animal reservoir are so rare. If Ebola became established in an animal population that lived in closer contact with humans — rats, pigs, or even dogs — animal-human transmissions could be much more frequent. There is no evidence, however, that this is happening. Genetic evidence continues to indicate that the entire West African epidemic was sparked by a single animal-human transmission in Guinea, from a bat to a 2-year-old boy.