Category Archives: Fleas

Roundtable on Campbell’s Climate, Disease, and Society in the Late Medieval World

by Michelle Ziegler

Bruce Campbell. The Great Transition: Climate, Disease, and Society in the Late Medieval World. Cambridge University Press, 2016.


When I first learned that Bruce Campbell was working on this book, I wondered if it would be the first grand synthesis of the new paradigm. Although there have been some very good regional books in the last couple of years, Campbell has indeed brought forward the first analysis of the Black Death and 14th-century crisis using global evidence. Although not entirely clear from the publisher’s description, this is an economic history that draws on interdisciplinary evidence.

I proposed this session and recruited participants without ever seeing the book (though I had seen his Ellen McArthur Lectures). I got very lucky that the panel matched up so well with the book. The five panelists who were able to attend were (from left to right below) Mongolian historian Christopher Atwood, Historian of Medicine Wendy Turner, Evolutionary Biologist Boris Schmid, Archaeologist Carenza Lewis, and Economic/Environmental Historian Philip Slavin.

roundtable 2017
Great Transition Roundtable: Christopher Atwood, Wendy Turner, Boris Schmid, Carenza Lewis, and Phillip Slavin. (Photo by Nükhet Varlik, used with permission)

Everyone agreed that Campbell’s book will become the foundation upon which the new synthesis of plague history will be built. Campbell synthesized a vast amount of data with a particular appreciation for the integration of climate and disease data. Most agreed that this was a very high-level view of the crisis, an aerial view if you will, that leaves many details to be filled in. Some missed an analysis of the relationship with cascading levels of analysis down to the level of individuals. On the other hand, Atwood remarked that this is far more detailed than would be possible in Asian studies today. Perhaps not surprisingly, this interdisciplinary panel would have liked to see more evidence from other fields such as archaeology and social history used.  As Lewis noted, archaeology, in particular, could have given more support to the economic and environmental arguments without pulling away from the flow of the book.

The global evidence is primarily limited to climate data. Several panelists remarked that it is still very Eurocentric view, and Anglo-centric on top of that. There is more data that could have been gathered particularly from the Mediterranean. War as a syndemic factor and as a result of climate or disease weakened societies was not given much space in Campbell’s analysis. One effect of such a high-level regional treatment is that causes of local mortality from war (including the environmental destruction of war) can be overlooked because it doesn’t effect a large enough piece of territory. Slavin has also pointed out that Campbell’s interpretation tends to come across as somewhat deterministic, here and there. Thus, in discussing the Great European Famine of the early 14th century, Campbell provided an engaging analysis of the environmental context of the famine as its causation, without considering various intermediate links, demographic and institutional. As a result, Slavin noted that Campbell’s interpretation of the Great Famine as an exogenous disaster stands out as unilateral; famines, across space and time, are incredibly complex phenomena.

Developing a historical paradigm based heavily on scientific data is like building a house on shifting sand. Eventually, the sand will swallow the house. The best you can hope for is to be precariously perched on the ridge of a dune.  Most biological data is out of date by the time it is printed in a book. While there were a few misunderstandings, most of the discrepancies between Campbell’s portrait of the plague and other diseases is simply out of date even though he incorporates information up to about 2015. For example, ancient DNA studies have found evidence for at least one, and probably several, (still unlocated) local reservoirs of plague in or near Europe, so the idea that plague was frequently imported from the East no longer holds (but Boris disagrees on this view).

Some other hypotheses on plague transmission, though proposed several years ago, have failed to gain much traction. While evidence continues to mount that the soil plays a role in Yersinia pestis’ survival and that human ectoparasites could be the primary vector at the pandemic level, these hypotheses are not proven yet. This doesn’t mean that they won’t eventually be accepted, just that we are not yet there.

Figure 4.05 REV 6
Stages in the plague cycle (Figure 3.27, Campbell 2016)

I am reproducing figure 3.27 here because I think this will prove to be popular with teachers. With that being said there are a few comments that need to be made about it. The role of the soil in the enzootic environment of the rodent’s burrow is poorly understood at the moment. However, teachers could just explain that level 1 simply represents the environment of the rodent burrow.  Level 5 is where the real debates are going on now among those who study transmission. Campbell does leave open which ectoparasites are involved, human fleas or lice, but there is not yet general acceptance of human ectoparasites as major vectors. It may yet come, but we aren’t there yet. While local cases of pneumonic plague will occur any time there is bubonic plague, it is unlikely to be a major driver in a pandemic. The red box that I have added to the figure is where the really critical events are happening for human epidemics and pandemics. While I do believe that humans should be considered hosts in pandemic level transition, a variety of other hosts, always including rodents, will continue to be instrumental in the amplification and must be involved for the endurance of an outbreak in a locality.

While working with scientific detail is challenging for historians, after Campbell’s book I think it will be necessary to address scientific information at least to the level where there is a consensus. As long as historians stay with information that has been confirmed by a second study or that has obviously gained scientific consensus, the risks of using scientific information really are manageable. Finding a scientist who has your trust to comment on drafts is a good practice (and the reverse for scientists writing history!).

There were some concerns. There was a feeling that correlation does not necessarily equal causation. Schmid and Slavin would have liked to see more evidence of statistical analysis to support the conclusions drawn. They had a sense that the patterns that Campbell noted in a number of his overlapping time series might prove to be coincidental, rather than significant when tested with robust statistics.

Wendy Turner addressed pedological uses of the book. She found that, at least for a history of medicine course, it could not be used alone as a textbook. It does not have enough social history to address the complete impact of the Black Death. I don’t think that was its purpose, as much as some of us hoped it would be.  “The” Black Death book has yet to be written. When it is it will have to address all the social, medical, scientific, economic, and political impacts — a tall order. It is likely that aDNA studies will have more to contribute to shore up the transmission routes of such a project. Campbell’s book could be a major text (if not the only one) for an economic or environmental history course if it is supplemented by other texts. Archaeology as done by Carenza Lewis or Per Lagerås would support Campbell’s overall argument.  Turner and others agreed that it is not written for introductory students and they wondered how even upper-level students would respond to the density of the material. It should be required reading for graduate students who focus on the 14th century or any of the infectious/famine crises.

Atwood observed that historians tend to recognize a “crisis” about every couple of centuries and wondered if these mostly European events/crises over millennia were not tied to changes that had swept across all of Eurasia. In effect, Campbell’s book lays the supportive groundwork for arguing that the Eurasian land mass should be considered as a whole rather than European only or Asian only.  I think we could make an argument that the Afro-Eurasian landmass is one historic unit. The Indian Ocean is still an underappreciated communication avenue.

The most lively discussion with the audience concerned the terminology for the 14th-century events — transformation, crisis, collapse, etc. Positions seemed to be based at least partially on training, with some rejecting the term collapse under any circumstances, while others were more open to its use in areas like “population collapse”.  For me, this is an internal matter for historians to resolve. Terminology can be a fickle thing, but data is always preeminent. And that is a good place to leave this post. Campbell has done us all a service by compiling a huge amount of data that will be the foundation of a new era of analysis of the 14th century and the Black Death. For this above all else, we must be grateful.


Presentations on the Plague from the European Association of Archaeologists, Vilnius, Lithuania, 2016

I just discovered that most of the presentations from the “Plague in Diachronic and Interdisciplinary Perspective” session of the Europan Association of Archaeologists meeting in Vilnius, Lithuania on 2 September 2016 are now on YouTube.  I think I have collected them all here. Enjoy 3 hours of plague talks!

Introduction-Plague in diachronic and Interdisciplinary perspective by Marcel Keller

From Mild to Murderous: How Yersinia pestis Evolved to Cause Pneumonic Plague by Wyndham Lathem (30 min)

Reconstructing ancient pathogens – discovery of Yersinia pestis in Eurasia 5,000 Years Ago by Simon Rasmussen (15 min)

Plague in the eastern Mediterranean region 1200-1000 BC? by Lars Walloe (15 min)

Placing the Plague of Justinian in the Yersinia pestis phylogenetic context by Jennifer Klunk (15 min)

A demographic history of the plague bacillus revealed through ancient Yersinia pestis genomes by Maria Spyrou (15 min)

Analysis of a High-coverage Yersinia pestis Genome from a 6th Century Justinianic Plague Victim by Michal Feldman (15 min)

Early medieval burials of plague victims: examples from Aschheim and Altenerding (Bavaria, Germany) by Doris Gutsmiedl-Schumann (15 min)

Fleas, rats and other stories – The palaeoecology of the Black Death by Eva Panagiotakopulu (15 min)

Plague in Valencia, 546: A Case Study of the Integration of Texts and Archaeology by Henry Gruber (15 min)

Germany and the Black Death: a zooarchaeological approach by M.A. Paxinos

The Promiscuous Human Flea

Female Pulex irritans, the human flea, from the Katja ZSM collection (CC3.0)
Female Pulex irritans, the human flea, from the Katja ZSM collection (CC3.0)

by Michelle Ziegler

The human flea seems like a misnomer today. We are not its current primary host, but that doesn’t mean that it once wasn’t our primary flea.  Pulex irritans was first described by Carl Linnaeus as the “house flea” in 1758 (Krasnov 2012:4) and it is still found in homes in many parts of the world.

For the most part, the human flea is a nuisance, an irritant as its name implies. Except when it isn’t, when it occasionally transmits Yersinia pestis, the plague, to people. Pulex irritans has been in homes with human plague cases from Arizona to Madagascar (Archibald & Kunitz, 1971; Ratovonjato et al, 2014). In 2006, Drancourt, Houhamdi, and Raoult argued that either the human flea or louse played a major role in human plague epidemics.

Human fleas have been found in the homes in several areas where plague occurs. P. irritans infected with Y. pestis were found on a dog in a home of a plague victim on Navajo land in Arizona in 1968. They also report knowledge of  Y. pestis being isolated from  P. irritans fleas on dogs in the home of an infected child in Kayneta in 1968 (Archibald & Kunitz, 1971).   A recent survey of plague regions in Tanzania found 50% of the fleas in homes were P. irritans (Haule et al, 2013). A recent survey of fleas in Madagascar found that 98% of the fleas found inside control homes in the control region of the study were Pulex irritans (Miarinjara et al, 2016). The fact that they did not find them in the homes within the area of the plague outbreak a month earlier may be due to extensive spraying of insecticide to end the epidemic. Human fleas are suspected of being the vectors for a variety of zoonotic diseases in Iran today (Rahbari, Nabian, & Nourolahi, 2008).

The human flea, Pulex irritans, has had a very interesting and convoluted history. All of the Pulex fleas are thought to have evolved in South America, perhaps on guinea pigs or peccary. P. irritans is the only member of its genus that has left the Americas.  It made it to Eurasia long before the “Columbian Exchange”.  So it crossed a land bridge at some point to begin spreading in Eurasia, and it need not have crossed on a human.  Ötzi the 5000-year-old ice mummy from the Italian Alps yielded two human fleas from his artifacts (Schedl, 2000). P. irritans has also been found Egypt from 3500 BC  (Bain 2004) and 1350-1323 BC (Panagiotakopulu, 2001) showing that it does well in warm, dry climates also. So not only where they present for the entire known period of plague but they have been specifically found in warm and cold regions. Pulex irritans has been found in floor debris of uncovered sites from Roman Britain (Kenward, 1998). They were common inhabitants of early medieval Irish homes (O’Sullivan, 2008).  They are fairly common finds in Norse Greenland settlements. Unfortunately flea surveys have not been done on most continental archaeological sites (or at least I haven’t found them).

So why is P. irritans called a promiscuous flea? It has nothing to do with sex! In this case promiscuous means that it will feed off of a wide variety of host species. It has a truly impressive host range beyond humans including pigs, dogs, cats, goats and sheep, cattle, chickens, porcupines, multiple species of foxes, wolves, coyotes,  golden jackel of Iran,  badgers, prairie dogs,  rabbits, wild cats,  and mice. There are undoubtably more species that could be added. It seems to be very common on foxes in North America and Europe. These are, of course, primarily predators of rodents.  Given its wide range of hosts, its distribution and frequency among hosts has probably fluctuated wildly due to environmental and biodiversity changes over the last millennia.

Such a wide host range also makes it a potential bridging vector, one that can move disease between a wild reservoir to a domestic space transmitting it to domestic rodents, pets, and humans. Importantly, bridging vectors work in both directions, meaning that it could be instrumental in developing a wildlife reservoir after a human epidemic in a new region.

General flea life cycle (CDC). Adults are only 5% of flea biomass.

flea-pyramid-1P. irritans has a life cycle that is well suited to thriving in buildings like houses, barns, sheds, and animal nests or dens. Most of their biomass is in the egg stage. Small white eggs are often laid on the host but almost always fall off on to the floor. They do particularly well on the floor of stables and animal sheds where fermenting manure and debris keeps the eggs warm and moist.  They also do well in human homes where it is usually warmer and more humid than outdoors. They breed all year around. The eggs will hatch into larvae that resemble maggots within 4-6 days. The very active larvae will feed on organic debris including feces of the adult flea and other animals. After three molts it will develop a cocooned pupae where it will undergo a metamorphosis to the adult flea. It can remain in the pupa for several months if necessary until the conditions are suitable. So although human fleas are usually not present in stables or sheds during the coldest months the pupa can easily span the winter to emerge as adults in the early spring. This may explain why they are often the most abundant in the spring when all of the pupae from the late fall and winter emerge. It is unclear if the lifecycle pauses inside a heated human home. A well fed adult can live up to 513 days and even starved can last 135 days (Krasnov, 2012: 54). It is unclear how long they live after being infected by Yersinia pestis (or other pathogens). Fleas only feed on blood as adults so this is their only phase that can be infected by Yersinia pestis.

Modern infestations of P. irritans in Greece and Iran can give a few insights into its disease ecology. Sheep and goats are consistently the most heavily infested animals with P. irritans in modern Iran and Greece. In parts of Iran, P. irritans is the most common flea captured from humans or domestic livestock: goats, sheep, cattle and chickens (Moemenbellah-Fard et al, 2014; Rahbari,  Nabian, & Nourolahi, 2008;  Rafinejad et al, 2013). In some modern surveys, P. irritans is over 90% of the fleas collected in rural areas, found on sheep, goats, cattle, humans and chickens — “wherever the animal infestation was high the fleas easily transmitted to humans” (Rahbari et al, 2008:44). In Greece, Christodoulopoulos et al. (2006) made a very important observation:

“fleas accumulated in the goat environment with each successive generation leading to an increase in their number. This conclusion could be corroborated by the observation that the most successful flea control measure was the change of barn location with movement of the goats to another far away new-constructed barn.” (p. 142-143)

So even with modern insecticides, sheep dips, and building techniques available, the infestation of the building could not be controlled. This has implications for human housing. Observations of flea ecology in Iran back this up, albeit without addressing methods of eliminating infestations.

The Iranian reports discuss human flea bites more. Noting that men who worked with animals had a higher bite rate. Bites are primarily around the ankles and lower legs, often multiple bites in a row.  In Iran they noted that human reactions to the flea bites varied from highly allergic to no sensitivity at all (Rahbari, Nabian, & Nourolahi, 2008). This is a difference in human immunology to the fleas and sensitivity is likely to alter the immune response to not only the bite but also bacteria in the bite. There is also likely to be heterogeneity in which humans and animals are bitten.

As we begin to take Pulex irritans more seriously as a plague vector, there is a lot of basic biology that needs to be done yet. How long can they survive infected? How does it effect their feeding behavior? Some studies showed that a small percentage of P. irritans can block, so what effect does that have on transmission in that small percent of fleas?


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