Excerpts from semi-organic conversations among Johns Hopkins Center for Health Security staff in their Slack #biosecurity channel (inspired by 538’s Slack Chats).
cmrivers (Caitlin Rivers): Today we’re discussing the outbreak of plague in Madagascar, which has been ongoing since August. First a little background. As of Oct 26 there have been 1,309 cases and 93 deaths. Two thirds of cases have been pneumonic (spread person to person). The case fatality rate is reported at 7%, which ProMed notes is quite low. Two cases were imported to Seychelles, both of which resulted in no secondary transmission [Update: the suspected cases in the Seychelles ultimately tested negative upon confirmatory testing]. A number of control measures have been implemented beyond the usual steps of contact tracing and isolation. The World Health Organization has released $1.5M in emergency funds, and eight specialized health centers have been opened. Public schools are closed and public gatherings are forbidden.
cmrivers: So, what do you make of this outbreak and how worried are you?
sanjana (Sanjana Ravi): My initial reaction -- I'm not super worried that this will escalate into an Ebola-level crisis, since, as an island, Madagascar is somewhat more geographically isolated and doesn't appear to have the same problem with porous borders that we saw with Guinea, Sierra Leone, and Liberia. I am a bit concerned that little has been said about vector control.
tara (Tara Kirk Sell): I don't think that this is another Ebola. Plague is endemic there (and I'm wondering if that may have a role in the low case fatality rate). It’s not good that it's in the cities but I just don't think that it's going to be as explosive.
cmrivers: I see the endemicity as a problem, actually. There's a whole dimension in the sylvatic cycle that we didn't have to deal with during Ebola.
tara: Well, I think that the endemicity means that the threat will never really go away but I also think that it means that it's not a rare event we are dealing with.
watson (Matt Watson): That said though, Ebola did show what can happen when a previously "rural" neglected tropical disease gets into cities. I don't think we know nearly enough about transmission dynamics and how to effectively intervene in those settings.
crystal (Crystal Watson): It seems to be behaving similarly to the outbreak in India in 1994. Like India, it is in a highly densely populated area with a mix of bubonic and pneumonic plague. Also, I think the close proximity of those who are infected is a huge driver. A lot of these pneumonic outbreaks occur in mine workers where they are close together in confined spaces.
cmrivers: Any guesses why this outbreak is unusually large compared to Madagascar's usual outbreaks? Is anything about the epidemiology here unusual?
crystal: I do wonder if there are plague superspreaders.
cmrivers: There are! The index case of this outbreak infected 30-some people, if memory serves.
meyerda (Diane Meyer): According to our Outbreak Observatory post, this outbreak has entered an urban and non-endemic area...perhaps that is why it is larger than normal.
michael (Michael Snyder): Also, there's a higher proportion of the more transmissible pneumonic plague variant than bubonic.
crystal: Also burials!
tara: Good point - burials are often a huge problem.
watson: Then there’s this story on families seizing plague victims’ bodies… If true, it may indicate a lack of trust in the government and international response.
cmrivers: That’s troubling. Rumors on that topic abound. One says that a “local tradition of dancing with dead bodies” is fueling transmission? Could this be true?
crystal: That would aerosolize bacteria for sure.
cmrivers: Y. pestis is non-spore forming, so I think that rumor is salacious and unfounded.
crystal: I think we chronically underestimate how long pathogens can persist in the environment. The literature suggests that it can survive for years in soil and on other surfaces.
nalexopulos (Nick Alexopulos, Communications Director): Aren’t there reindeer frozen in Siberian ice that carry transmissible plague?
cmrivers: That’s anthrax, which is spore-forming and very persistent in the environment.
nalexopulos: [goes back to writing tweets]
crystal: Plague seems to go dormant though.
cmrivers: But is it possible for transmission to happen years after death?
crystal: It says here that “a growing body of evidence suggests that Y. pestis can survive without a host for extended periods under certain environmental conditions while, in many cases, retaining infectivity.”
crystal: I wonder what the mechanism is for plague to make the switch from bubonic to pneumonic though? Is it always that superspreader?
cmrivers: One way to get at your question is to look at chains of transmission and see how many generations they last.
crystal: The superspreading issue I think needs a lot more attention. There are so many examples of this now. It seems to be a major driver of a number of outbreaks.
cmrivers: I think so too. It was a major feature in SARS and Ebola. I think this is a place where better outbreak science would be helpful. There's not much real-time effort to reconstruct transmission chains, in part because it’s hard. But it does reveal a lot about the transmission dynamics.
crystal: Definitely. If we really committed resources to understanding transmission dynamics, it would reduce a lot of uncertainty.
tara: Like Crystal said, we should understand more about why someone gets bubonic vs pneumonic plague.
michael: It’s true -- given their different transmission pathways, it's almost like having to manage two separate (but related) disease outbreaks at the same time.
sanajana: So according to WHO, human-to-human bubonic plague transmission is very rare. It's almost always the result of a flea bite. The fact that there are so many cases suggests that most are of the pneumonic variety, OR that the vector control situation is really bad.
tara: I wonder if animals are common in the home, or if it is more pest control that is the vector control issue.
michael: The WHO has cited the poor environmental and sanitation conditions as a driving factor, so I imagine vector control is playing a big role for bubonic plague transmission.
watson: There’s often talk of plague “foci” where it persists as a zoonosis, for example, this paper.
tara: So based on that paper, are climatic conditions right for this year’s Rattus rattus explosion?
cmrivers: Well three-quarters of cases are pneumonic, so I think that speaks to Michael's point about two different but related outbreaks. Even without zoonotic cases there's still a major outbreak.
crystal: I think they are synergistic.
cmrivers: What should they be doing beyond vector control?
watson: [WHO has] moved in a small stockpile of antibiotics - that's good. And actually, at 1.2 million doses, it's not all that small.
cmrivers: That is pretty sizable. Contacts need 7 days of post-exposure prophylaxis though so...it adds up.
crystal: Also, probably a lot of supportive care is needed for pneumonic cases and isolation precautions.
michael: I think Crystal’s comparison to the 1994 plague outbreak in Surat, India, is interesting. That one had about 50 case fatalities but caused an estimated 500,000 refugees.
crystal: Yes. I wonder if that slowed the epidemic in India actually? People got out of town as soon as there was a confirmed case.
cmrivers: The refugees is an interesting twist. Can you say more about that?
crystal: People got out of town as soon as there was a confirmed case. Plague is historically very scary.
michael: Panic. Made only more fascinating by the fact that they never confirmed it was plague until about 6 years later.
cmrivers: Did they take any cases with them, spreading the disease?
crystal: They did, but not hugely. Plague is less transmissible than some people think, even pneumonic. [Our founder] DA Henderson always said that.
cmrivers: That brings us back to “why is this happening now in Madagascar?”
tara: I wonder if genetic analysis at a later date will tell us if this is all one introduction or many. My money is on many. It also brings us back to my question about climate conditions! Is there a global warming component?
cmrivers: There has been some work on predicting plague emergence, e.g. here.
crystal: It's the end of dry season in Madagascar. The rodents may be hungry and are venturing into the cities to eat, whereas normally they would eat in the wild. I wonder if the condition of the soil also makes a difference?
meyerda: This article states that warmer/wetter conditions cause rodent numbers to drop, sending fleas looking elsewhere for food.
cmrivers: I think from a public health perspective it's more prudent to focus on the rat-human interface...like keeping rats out of human homes. Climate is not a modifiable risk factor, but habitation conditions are.
tara: But rats aren’t the only animal that can maintain a flea infestation, which speaks to the importance of pets [as risk factors].
cmrivers: Ok back to our roots. As a tier 1 select agent, Y pestis is considered a candidate pathogen for biocrimes. Is there anything we should be learning either from this outbreak or the response that applies to biosecurity?
crystal: It's really hard to limit accessibility of Y pestis. It's everywhere. Here is a sign from the field near my parents’ house.