The Arab Maghreb is one of the most arid environments to host plague reservoirs. The most recent study on the area highlights the proximity of plague foci to salt water, either the Mediterranean Sea, Atlantic Ocean or importantly inland salt lakes (Malek et al, 2016). These inland salt springs, called chotts, are saltier than the ocean. They were specifically able to cultivate Y. pestis from high salt soil and isolate a high salt tolerant strain of Yersinia pestis from Algeria. Plague foci across North Africa were found at an average of 0.89 km from salt water, while the average distance from fresh water is 4.6 km.
They also note the importance of L-form Yersinia pestis in their environmental samples. L-form bacteria are an understudied cell wall deficient state that quite a few bacteria, including Yersinia pestis, use for long term survival. The L-form of Y. pestis may be important in environmental persistence. Because they are believed to have a slower reproduction rate, the L-form may also play a role in altering the molecular clock of some strains. To date, publications that focus on L-form Y. pestis have been in either Russian or Chinese. It seems clear that the L-form is found in some instances in Asia as well. Importantly, some L-form bacteria can regain their cell wall and return to active ‘normal’ growth.
Soil osmolarity is the key feature that allows (or requires) the L-form to persist. Withstanding osmotic tensions is the primary role of the cell wall. Without the cell wall, the cell loses its ‘normal’ shape, taking on a spherical shape determined by hydrophobic-hydrophilic interactions (like oil and water). As the cell membrane is primarily made of phospholipid, its the L-form shape resembles a sturdy oil globule or a liposome (B below). This was apparent by gram stain when the normal individual short rod-shaped (coccobacilli) cells transformed into clusters of completely round (cocci) cells. This was confirmed under the electron microscope where the change is very apparent.
They also isolated a strain, Algeria3, a descendant of the third pandemic, from soil containing 4% salt, that can grow in a 15% salt broth. Other Algerian isolates that were not found in high salt soils experimentally survived as well in high salt media if the salt content was ramped up in a step-wise fashion. Growth in high salt conditions altered their protein production to increase those related to osmoregulation, metabolism, outer membrane proteins and others of unknown function. Osmoregulation genes changes are a direct response to the higher salt concentration. The L-form cells are clearly still metabolically active.
Taken together these protein profiles suggest that it has adapted to survive in the salty soil with the ability to adjust its structure and function as necessary to persist. They note that other plague reservoirs are in regions of the world with salt lakes or other salty sources, but more environmental sampling will be necessary to determine if this is a universal Y. pestis capability. This all has obviously important implications for plague ecology.
Malek, M. A., Bitam, I., Levasseur, A., Terras, J., Gaudart, J., Azza, S., et al. (2016). Yersinia pestis halotolerance illuminates plague reservoirs. Scientific Reports, 7, 1–10.
For the last couple years, I have been writing about a landscape-based approach to the study of infectious disease in general and historic epidemics in particular. When I first wrote about Lambin et al.’s now classic paper “Pathogenic landscapes” nearly three years ago, I did not know then that it would be so influential in my thinking or that the Medieval Congress sessions would be so successful. In the fall of 2014, Graham Fairclough and I began talking about ways that this first congress session could be represented in the journal he edits, Landscapes. This issue is a departure from their usual approach to landscape studies so I would like to thank Graham Fairclough for entrusting me with a whole issue. It has been a challenge for both of us, and I am proud of our product.
This issue represents the wide variety of studies that can be done all contributing to an understanding of past landscapes of disease. One of the reasons why I like the phrase landscape of disease, rather than simply landscape epidemiology, is that it opens up the array of disciplines that can be involved. In the study of diseases of the past, humanistic approaches can be as valuable as scientific methods. Both are required to build a reasonably coherent reconstruction of the past. Science and the humanities need to act as a check and balance on each other, hopefully in a supportive and collegial way.
The issue was published online a couple days ago. Accessing the journal through your library will register interest in the journal with both your library and the publisher, and would be appreciated. By now the authors should (or will soon) have their codes for their free e-copies if you do not have access otherwise.
Table of Contents
Landscapes of Disease by Michelle Ziegler. An introduction to the concept of ‘landscapes of disease’ and the articles in the issue. (Open access)
Lambin, E. F., Tran, A., Vanwambeke, S. O., Linard, C., & Soti, V. (2010). Pathogenic landscapes: Interactions between land, people, disease vectors, and their animal hosts. International Journal of Health Geographics, 9(1), 54. http://doi.org/10.1186/1476-072X-9-54
The key to understanding plague — past, present, and future — has always been understanding its vector dynamics. By the latest tally, there are 269 known flea species, plus a small collection of ticks and lice, that can be infected with Yersinia pestis. With this many infected parasites, it’s not a surprise that 344 hosts have been identified(Dubyanskiy & Yeszhanov, 2016), but this list is still incomplete. (It does not include all of the minor hosts in North America.) Regardless, this is not a description of a picky pathogen! Unfortunately, it is far easier to identify infected hosts and potential vectors than to determine which of these insects are effective vectors and their transmission dynamics.
Numerous species of fleas have been identified as plague vectors in specific localities. However, only five infected potential human vectors are possibly involved in a wide distribution of human plague cases — the rat flea Xenopsylla cheopis, the cat flea Ctenocephalides felis, the human flea Pulex irritans, the human body louse Pediculus humanus humanus, and the human head louse Pediculus humanus capitis. The rat flea and the cat flea are known vectors, but they are unlikely to account for the full transmission of the massive first and second pandemics. I recently discussed the possible role of the human flea. Given the worldwide distribution of human lice, they are attractive vectors but there is still work to do before they can be considered likely primary vectors for human to human transmission during the first two pandemics.
Raoult Makes his Case…again
Didier Raoult and his team have been working on plague and their louse transmission hypothesis for a long time. It has already been ten years since they had enough information to write their first review article putting forth their human ectoparasite theory of plague transmission (Drancourt, Houhamdi, & Raoult, 2006).
At this point, their primary supporting evidence was some experiments with human louse plague transmission in rabbits (Houhamdi et al, 2006) and they thought they could associate louse transmission with an “Orientalis-like” biovar of Yersinia pestis they identified in the first two pandemics (Drancourt et al, 2004). However, later ancient DNA work showed that the first two pandemics were caused by strains of Yersinia pestis that emerged before the Orientalis biovar. Genetic reclassification of Yersinia pestis has also made the biovars largely obsolete. Tensions between groups working on ancient plague DNA developed quickly, and have been documented by historians Lester Little (2011) and Jim Bolton (2013).
This summer Didier Raoult (2016) restated his “personal view” on the role of lice in plague transmission. This essay is unusual not only as a first person narrative in science, including individual claims of discovery, but also for being so vindictive in its attack on his rivals. Again, see Little (2011) and Bolton (2013) for less biased accounts. His team has done very impressive work.
His team has continued to assemble much of the work needed to argue that human lice were instrumental in at least some of the major human outbreaks of plague during the first two pandemics. Combing primarily French medical reports in North Africa, they were able to identify observations that suggest that lice were involved in some mid-twentieth century outbreaks (Raoult, 2016; Malak, Bitam, & Drancourt, 2016). One of their most interesting findings in 2011 was the discovery of co-infection with Yersinia pestis and Bartonella quintana (trench fever) in late medieval French remains (Tran et al, 2011). Trench fever is well known to be transmitted by the human louse. Both B. quintana and Y. pestis have been found in contemporary lice taken from plague patients in regions of endemic plague in the Congo (Piarroux et al, 2013; Drali et al, 2015). Unfortunately, neither of these studies mention the presence or absence of fleas. That blood feeding lice would be infected is not a surprise, but the question of transmission stubbornly remains. There has yet to be a contemporary outbreak where all potential vectors, fleas and lice, were investigated. On a side note, the finding of widespread B. quintana is interesting, and perhaps a proxy for heavy lice infestation.
In the meantime, while Raoult’s latest summary was in press, additional evidence was beginning to be revealed. Graduate student Katharine Dean of the MedPlag project in Oslo was modeling past epidemics for transmission by rat fleas, human lice, and pneumonic transmission. In her master’s thesis, she showed that lice transmission fits the second pandemic epidemics at Givry in 1348, London in 1563-64, and Florence in 1630-31 better than rat fleas or pneumonic transmission (Dean 2015). At the most recent Yersinia meeting in October, Dean presented a poster with expanded data finding outbreaks that fit each of these three modes of transmission (pneumonic in Manchuria, rat fleas in Sydney and Hong Kong, and lice in many locations) (Dean et al, 2016). Their work is still in progress and I’m sure many will be eager to see their results in due course.
There are still a few lingering things to nail down. A modern outbreak investigation that looks at all ectoparasites, fleas and lice, in the region that suggests lice are involved. It would be good to find lice (or fleas for that matter) in a plague burial that yields Y pestis aDNA. Alternatively, detection of more coinfection of Y. pestis with a louse-transmitted infection like B. quintana would lend additional support. These findings will will require some good fortune. To differentiate between the human flea and lice, a better understanding of the pathology of a Y. pestis infection in the potential vector and its transmission dynamics is really needed. The models can’t differentiate human ectoparasites without more information.
Human ectoparasites are beginning to look much more likely especially for northern epidemics (Hufthammer & Walløe, 2013). More information is still needed to distinguish between human fleas and lice, although they may be both involved in different outbreaks. We need to be ready for yet another paradigm change in plague history. Looking at the overall plague vector dynamics of the great pandemics, from sylvatic reservoir to distant human populations, is going to get a whole lot more complicated but also more interesting.
Bolton, J. L. (2013). Looking for Yersinia pestis: Scientists, Historians, and the Black Death. The Fifteenth Century, XII, 15–38.
Drali, R., Shako, J. C., Davoust, B., Diatta, G., & Raoult, D. (2015). A New Clade of African Body and Head Lice Infected by Bartonella quintana and Yersinia pestis–Democratic Republic of the Congo. American Journal of Tropical Medicine and Hygiene, 93(5), 990–993. http://doi.org/10.4269/ajtmh.14-0686
Drancourt, M., Roux, V., Dang, L. V., Tran-Hung, L., Castex, D., Chenal-Francisque, V., Ogata, H., Fournier, P-E., Crubezy, E, and Raoult, D. (2004). Genotyping, Orientalis-like Yersinia pestis, and plague pandemics. Emerging Infectious Diseases, 10(9), 1585–1592. http://doi.org/10.3201/eid1009.030933
Dubyanskiy, V. M., & Yeszhanov, A. B. (2016). Ecology of Yersinia pestis and the Epidemiology of Plague. Yersinia Pestis: Retrospective and Perspective, 918(Chapter 5), 101–170. http://doi.org/10.1007/978-94-024-0890-4_5
Houhamdi, L., Lepidi, H., Drancourt, M., & Raoult, D. (2006). Experimental model to evaluate the human body louse as a vector of plague. The Journal of Infectious Diseases, 194(11), 1589–1596. http://doi.org/10.1086/508995
Hufthammer, A. K., & Walløe, L. (2013). Rats cannot have been intermediate hosts for Yersinia pestis during medieval plague epidemics in Northern Europe. Journal of Archaeological Science, 40(4), 1752–1759. http://doi.org/10.1016/j.jas.2012.12.007
Piarroux, R., Abedi, A. A., Shako, J. C., Kebela, B., Karhemere, S., Diatta, G., et al. (2013). Plague epidemics and lice, Democratic Republic of the Congo. Emerging Infectious Diseases, 19(3), 505–506. http://doi.org/10.3201/eid1903.121329
Tran, T.-N.-N., Forestier, C. L., Drancourt, M., Raoult, D., & Aboudharam, G. (2011). Brief communication: co-detection of Bartonella quintana and Yersinia pestis in an 11th-15th burial site in Bondy, France. American Journal of Physical Anthropology, 145(3), 489–494. http://doi.org/10.1002/ajpa.21510