Detecting plague: palaeodemographic characterisation of a catastrophic death assemblage.

Detecting plague: palaeodemographic characterisation of a catastrophic death assemblage. Introduction The palaeodemographic signatures of epidemics are of perennialinterest to biological anthropologists. It has been demonstrated thatpatterns of human mortality generally demonstrate a high degree ofuniformity across populations (Paine 2000:181). This is referred to asattritional mortality and is characterised by a high number of infantdeaths, low numbers of adolescent deaths and a gradual increase inmortality throughout adulthood. By contrast, an episode of catastrophicmortality refers to a short-term mortality crisis in which a high riskof death applies to all age categories. The identification ofcatastrophic as opposed to attritional mortality profiles inarchaeological samples of human skeletons clearly has important socialand palaeopathological implications (Paine 2000). A catastrophicmortality profile should mimic the age structure of the livingpopulation because all individuals have an approximately equalprobability of dying irrespective of age or sex (Keckler 1997).Catastrophic mortality is almost by definition unusual, as a populationsubjected to frequent episodes of catastrophic mortality would rapidlybecome extinct. The bubonic plague bubonic plague:see plague. bubonic plagueravages Oran, Algeria, where Dr. Rieux perseveres in his humanitarian endeavors. [Fr. Lit.: The Plague]See : Disease is an example of a disease that can causecatastrophic mortality because it is highly infectious and has a highcase-fatality rate when untreated. Currently the demographic effects ofpre-modern catastrophic events, such as the 'Black Death'plague that affected England in AD 1348-1350, are poorly understood.While the bubonic plague is strongly implicated as the cause of theBlack Death, the event occurred prior to the detailed recording ofmortality, and it is not known whether this plague episode resulted in acharacteristic demographic signature that can be detected in samples ofskeletal remains. The aim of our study was to examine the demographicstructure of a sample of human skeletal remains that represent some ofthe victims of the 1348 plague, to test whether the skeletal sampleexhibited a catastrophic age structure. The Black Death The Black Death swept from China and across Europe during thefourteenth century, causing devastating mortality. The 'GreatMortality' or 'Great Pestilence' (as it was referred tothen) reached England in AD 1348. This plague episode, like the GreatPlague of 1664, is generally believed to have been bubonic bu��bon��icadj.Of or relating to a bubo.buboniccharacterized by or pertaining to buboes.bubonic plaguea highly contagious and severe disease caused by the bacillus and possiblypneumonic plague pneumonic plaguen.A frequently fatal form of bubonic plague in which the lungs are infected and the disease is transmissible by coughing. , caused by the bacterium Yersinia pestis Yersinia pes��tisn.A bacterium that causes plague and is transmitted from rats to humans by the rat flea Xenopsylla cheopis. Also called Pasteurella pestis. . The DNA DNA:see nucleic acid. DNAor deoxyribonucleic acidOne of two types of nucleic acid (the other is RNA); a complex organic compound found in all living cells and many viruses. It is the chemical substance of genes. ofthis organism has been identified from the dental pulp dental pulpn.The soft tissue forming the inner structure of a tooth and containing nerves and blood vessels. Also called tooth pulp. cavity ofindividuals excavated from a contemporary plague pit in France(Drancourt et al. 1998; Raoult et al. 2000; but see Wood &DeWitte-Avina 2003 and Mackenzie 2003 for critiques of this evidence).In bubonic plague the bacterium has an incubation period incubation periodn.1. See latent period.2. See incubative stage.Incubation periodofapproximately 2 to 8 days. Onset is acute with high fever, prostration prostration/pros��tra��tion/ (pros-tra��shun) extreme exhaustion or lack of energy or power.heat prostration? see under exhaustion.pros��tra��tionn. and a characteristic infective lesion at the lymph nodes known as abubo bubo/bu��bo/ (bu��bo) an enlarged and inflamed lymph node, particularly in the axilla or groin, due to such infections as plague, syphilis, gonorrhea, lymphogranuloma venereum, and tuberculosis. . In the premodern pre��mod��ern?adj.Existing or coming before a modern period or time: the feudal system of premodern Japan.era there were no effective antibiotic treatmentsand the disease may have had a mortality of greater than 50 per cent(Benedictow 1987). Bubonic plague is a complex disease because it isprimarily a zoonotic ZoonoticA disease which can be spread from animals to humans.Mentioned in: Zoonosis infection transmitted from animals: humans areincidental victims. Some authors have questioned the interpretation of the Black Deathas an outbreak of bubonic plague on the grounds that Yersinia YersiniaA genus of bacteria in the Enterobacteriaceae family. The bacteria appear as gram-negative rods and share many physiological properties with related Escherichia coli. Of the 11 species of Yersinia, Y. pestis, Y. enterocolitica, and Y. infectioncould not have caused such massive and rapid mortality (Twigg 1984;Karlsson 1996; Scott & Duncan 2001; Wood et al. 2003).'Plague' was (and still is) a generic word used to describealmost any fatal outbreak of disease. Contemporary descriptions tend tobe embroidered, impressionistic im��pres��sion��is��tic?adj.1. Of, relating to, or practicing impressionism.2. Of, relating to, or predicated on impression as opposed to reason or fact: impressionistic memories of early childhood. affairs that hinder accurate diagnosis.This, coupled with the shortage of accurate mortality statistics, raisesa number of epidemiological questions (Ell 1984). What is known is thatfrom its port of entry in Dorset, the plague spread rapidly throughEngland and reached London by 1348, finally abating in 1350. Estimatesof mortality vary widely and the most often quoted figure is that onethird of the total population of Western Europe died between 1346 and1350 (McNeil 1976). Actual mortality varied greatly from place to placeand is usually estimated at 20-50 per cent (Poos 1981; Frank 1999). Epidemics due to new infectious diseases may emerge due to theintroduction of an infectious pathogen into a population with no priorimmunity or from the mutation of an existing pathogen into a morevirulent form. When new diseases arise they can initially have acatastrophic effect upon a population (Ampel 1991). Virulence tends todiminish over time, however, as a result of changes in both the pathogenand host. As a result, epidemics are frequently viewed as cyclical innature, beginning when a new disease emerges (Ampel 1991). With respectto the Black Death several authors have argued that the speed andvirulence with which the disease spread, in particular across greatexpanses of country, are not compatible with the traditional rat-fleavector model of bubonic plague (Scott & Duncan 2001). It is arguedinstead that contemporary descriptions and evidence concerning thespread of the plague are more consistent with infectious diseases passedon from person to person. Famine, typhus typhus,any of a group of infectious diseases caused by microorganisms classified between bacteria and viruses, known as rickettsias. Typhus diseases are characterized by high fever and an early onset of rash and headache. and anthrax anthrax(ăn`thrăks), acute infectious disease of animals that can be secondarily transmitted to humans. It is caused by a bacterium (Bacillus anthracis have beenpostulated as the causes of the 'Great Mortality' (Twigg 1984,1993). It is possible, and indeed quite likely, that diseases such astyphus were concurrent with the plague as famine and disease werealready ravaging England throughout the fourteenth century (Wills 1997).In the plague epidemic of 1630 in Venice, smallpox was also prevalentand accounted for a significant number of the deaths (Ell 1989). Insixteenth- and seventeenth-century England bubonic plague and typhusalternated seasonally, and the Black Death may also have consisted of asimilar alternation alternation/al��ter��na��tion/ (awl?ter-na��shun) the regular succession of two opposing or different events in turn.alternation of generations? metagenesis. (Shrewsbury 1971: 125). Clearly there are many unanswered questions and gaps in ourknowledge of the Black Death. This is exacerbated by a lack of knowledgeconcerning the demographic impact on the affected population. However,the high mortality in the Black Death, coupled with the evidence thatfew communities escaped its effects, suggests that study of acontemporary cemetery may reveal the demographic signature for thisepisode of mass mortality. This paper presents a demographic analysis ofage at death information obtained by the authors from a skeletalassemblage excavated from contexts interpreted as mass burial trenchesassociated with the Black Death in London. The archaeological data: London and the Black Death Contemporary documents state that mass graves were constructed in anumber of cities during the time of the Black Death (Creighton 1891).The only two to have been excavated in England are from HerefordCathedral (Stone & Appleton-Fox 1996) and East Smithfield in London(Hawkins 1990). It is the latter site that provides the focus of thisanalysis. The mortality of London during the time of the Black Death wasestimated at 20 000-30 000 (Creighton 1891), approximately one third toone half of the city's population. The relatively crowded livingquarters of the citizens and the thriving rat population of cities arelikely to have facilitated the spread of the disease more than in ruralareas. When the disease was at its peak the demand placed on churchyardswould have been overwhelming. In order to reduce the pressure on theseverely affected London parishes, two cemeteries were established onthe northern and eastern outskirts of the city (Grainger & Hawkins1988; Hawkins 1990: 637). One of these cemeteries, the Royal Mint site(known in the fourteenth century as the Churchyard of the Holy Trinity),was excavated in the 1980s in advance of development (Hawkins 1990). Thesite lies to the north-east of the Tower of London Tower of London,ancient fortress in London, England, just east of the City and on the north bank of the Thames, covering about 13 acres (5.3 hectares). Now used mainly as a museum, it was a royal residence in the Middle Ages. and documentarysources show that the cemetery was selected in 1349 as an emergencyburial site for plague victims. Historical records for the site arecomplete and none refer to the use of the grounds for burial of laterplague victims (Hawkins 1990: 638). Excavations revealed that only a small proportion of the total areahad been used for burial, indicating that the epidemic abated before itbecame necessary to use the total area allotted (Hawkins 1990). Theexcavated portion of the cemetery consisted of 3 mass burial trenchesand 15 grave rows; almost all burials were supine and orientatedwest-east (Figure 1). A total of 761 skeletons were excavated from thesite, of which 600 individuals were analysed in the original skeletalreport (Waldron 2001). It is estimated that the total number of burialsat the site was originally in the region of 2400 individuals: manyburials had been truncated by later activities on the site, and someburial features were only partially excavated (Hawkins 1990: 640). Therewas no noticeable segregation within the trenches by age or sex. [FIGURE 1 OMITTED] Age at death A previous study of the Royal Mint data used conventional ageingmethods to produce a mortality profile (Waldron 2001). An examination ofWaldron's data by Margerison and Kntisel (2002) showed that thisprofile exhibited more young adults than would be expected from acatastrophic skeletal assemblage. However, deriving estimates of age atdeath from adult skeletal remains has been found to be problematicbecause conventional ageing methods suffer from inherent statisticalbiases. As a result of these biases, the age distributions of samples ofarchaeological skeletal remains tend to exhibit too many adults in themiddle decades of life (Bocquet-Appel & Masset 1982, 1985; Molleson& Cox 1993). In order to identify age-related patterns from the archaeologicalfunerary fu��ner��ar��y?adj.Of or suitable for a funeral or burial.[Latin fner record, it is vital that reliable and unbiased estimates ofskeletal age at death are achieved. Recent work has demonstrated thatBayesian statistics can be used to remove reference sample bias and toobtain more reliable age estimates (Konigsberg & Frankenberg 1994;Lucy et al. 1996; Aykroyd et al. 1997, 1999; Konigsberg et al. 1997;Hoppa & Vaupel 2002). Bayesian data analysis allows us to makeinferences from data using probability models for observable quantitiesand for quantities that are unknown, but we wish to learn about (Gelmanet al. 1995: 3). It essentially provides a formal framework whereby wemay quantify and state our scientific preconceptions (Grayson 1998:331).The application of Bayesian statistics in archaeology has primarilyfocused on dating techniques (e.g. radiocarbon) (Buck et al. 1996).However, several studies over recent years have also demonstrated thevalue of a Bayesian approach for addressing statistical biasesassociated with palaeodemography (e.g. Chamberlain 2000; Gowland &Chamberlain 2002; Hoppa & Vaupel 2002). This study uses a Bayesianapproach to estimate adult skeletal age at death from the auricular auricular/au��ric��u��lar/ (aw-rik��u-lar)1. pertaining to an auricle.2. pertaining to the ear.au��ric��u��laradj.1. surface (the part of the skeleton where the base of the spine meets thepelvis) and the pubic symphysis pubic symphysisn.The firm fibrocartilaginous joint between the two pubic bones. (where the two halves of the pelvis meetat the front). These methods were then applied to two archaeologicalskeletal samples: the Royal Mint site in London and a control sample,the attritional cemetery of Blackgate in Newcastle which is of similarsize and of comparable date. Ageing method In order to produce a Bayesian ageing method, a large sample ofknown age auricular surface and pubic symphysis data is required so thatwe may produce a probability model to determine age at death given askeletal indicator state. The required data were compiled from referenceskeletons of known age of death collected from the Coimbra IdentifiedSkeletal Collection, Portugal and the Spitalfields skeletal collection,London. Known age data from the auricular surfaces of 453 individuals(Table 1) and pubic symphyses of 377 individuals (Table 2) wereobtained. The auricular surfaces and pubic symphyses were recorded usingthe stages described by Lovejoy et al. (1985) and Brooks and Suchey(1990) respectively. This information was then used as a source oflikelihoods (the probability of possessing a particular skeletalindicator stage given known age). These likelihoods were then invertedto provide probabilities of age given each skeletal indicator stageusing a Bayesian calculation.* An important component of Bayesian statistics is the use of priorprobabilities. In this context, the prior probability prior probability,n the extent of belief held by a patient and practitioner in the ability of a specific therapeutic approach to produce a positive outcome before treatment begins. represents anopinion of the probability of being a particular age before any datahave been observed. By using a prior probability we are explicitlystating our prior beliefs concerning the data. For example, whenproducing the demographic profile of a 'normal' cemetery (i.e.one used for the burial of natural deaths over a long time period) wewould expect a natural attritional mortality. We can thereforeincorporate our prior beliefs into our probability model (Gowland &Chamberlain 2002). In the use of Bayesian statistics the choice of prior is importantas it will impact upon the results, particularly so for those skeletalindicators that have a poor correlation with chronological age chron��o��log��i��cal agen. Abbr. CAThe number of years a person has lived, used especially in psychometrics as a standard against which certain variables, such as behavior and intelligence, are measured. (Aykroydet al. 1997). As a result, two different model priors were used in thisstudy, one representing natural attritional mortality and the othercatastrophic mortality (i.e. the age structure of the livingpopulation). The data for these priors were obtained from Coale andDemeny's level 5 model west life tables for stable populations withzero rate of growth (Coale & Demeny 1983). The probabilities of agegiven indicator state were then used to obtain age distributions fromthe adult archaeological skeletal remains from both the Royal Mint andBlackgate cemeteries. Our approach to the choice of prior probabilities differs from therecommendations of Hoppa and Vaupel (2002: 6). The latter authorsrecommend that prior probabilities can be inferred from the frequenciesof age indicator states in the target sample, but we avoid this.procedure because it may be unduly influenced by taphonomic effects andrecovery bias (Chamberlain 2003: 642). Instead we have chosen to baseour prior probabilities on the Coale and Demeny model life tables whichprovide good models for pre-industrial populations and have beenextensively used in palaeodemography (Chamberlain 2000: 103). Results The auricular surfaces of 180 adults from Blackgate and 132 adultsfrom the Royal Mint site were sufficiently preserved for scoring. Thepubic symphyses of only 84 individuals from Blackgate and 70 individualsfrom the Royal Mint site were preserved. The pubic symphysis tends notto be as well preserved in archaeological specimens as it is morefragile and its position when the skeleton is supine is more susceptibleto mechanical disturbance. Age distributions for the Royal Mint andBlackgate cemeteries were obtained using the method outlined above andadopting both the catastrophic and attritional priors in turn. Whenusing the catastrophic prior we see that the age distribution obtainedfrom the Royal Mint cemetery differs markedly from the Blackgatecemetery (Figures 2 and 3). Furthermore, the Royal Mint site isremarkably similar to the catastrophic age at death profile obtainedfrom the model life table, and differs substantially from the model lifetable attritional profile. By contrast, the Blackgate cemetery exhibitsan age at death distribution more akin to the attritional profile. Agedistributions were comparable when age estimations were obtained usingeither the auricular surface or pubic symphysis. To ensure that theseresults were not biased as a result of the choice of prior, the sameanalyses were undertaken using an attritional prior (Figures 4 and 5).In this instance, the Royal Mint site still demonstrated a catastrophicage-at-death profile while the Blackgate site shows an age distributionthat is almost identical to an attritional population. This similarityis remarkable given the potential biases in skeletal preservation (e.g.Walker et al. 1988; Paine & Harpending 1998) and the inherentimprecision of skeletal ageing techniques. Again, similar patterns ofage distributions were obtained when using either the auricular surfaceor pubic symphysis. [FIGURES 2-5 OMITTED] Discussion When these data are compared to the original mortality profileobtained for the site (Figure 6), the biases in the use of conventionaltechniques can clearly be seen. Waldron (2001) obtained a verypronounced peak in the original analysis between the ages of 25 and 45years. This peak is commonly observed in archaeological cemeterypopulations and, as demonstrated here, is eliminated when the biases inconventional skeletal age estimation techniques are removed usingBayesian statistics. These results strongly suggest that the Royal Mintcemetery exhibits a catastrophic mortality pattern as a result of theBlack Death plague. Results suggest that all adult age groups, bothyoung and old, were equally affected by the plague. By contrast, theBlackgate cemetery produced an attritional mortality profile, as onewould expect from a burial population during non-catastrophicconditions. [FIGURE 6 OMITTED] Although no detailed documents relating to Black Death plaguemortality exist, it is possible to make a comparison between the resultsobtained here and later plague epidemics in London. Detailed records areavailable from the parish of St Botolph in London during two plagueepisodes (Hollingsworth & Hollingsworth 1971) and in Venice in 1630(Ell 1989). These historically documented plague episodes produce asimilar catastrophic mortality profile as represented by the Coale andDemeny (1983) level 5 living age structure for zero growth rate (Figure7). St Botolph has slightly more children and fewer adults, showing aclose match with a level 5 living age structure for a population growingat 3 per cent, which is the estimated growth rate given during thistime. The mortality at the parish of St Botolph in non-plague years alsocorresponds well to the attritional mortality expected from level 5mortality (Figure 8). Therefore, the results from the Royal Mint sitealso correlate well with those expected from historical mortalityrecords of later plague episodes in the same city. [FIGURES 7-8 OMITTED] Conclusion This paper has examined the palaeodemography of a Black Deathplague cemetery excavated from the Royal Mint site in London and hascompared it to that of a contemporary attritional cemetery. We concurwith Margerison and Knusel (2002: 139) that the skeletal assemblage fromthe Royal Mint site represents an episode of catastrophic mortality, butwe believe that the discrepancies that those authors noted between themodel catastrophic profile and the skeletal age profile are attributableto biases in skeletal age estimation. We believe that by adopting aBayesian methodology we have produced a more reliable demographicprofile of the Royal Mint site, and that the age structure of theskeletal assemblage is comparable to that indicated by historicalrecords of later plague events and with living population age structurestaken from model life tables. The Royal Mint sample differed markedly tothe mortality profile of a contemporary attritional cemetery where ageat death was estimated using an identical methodology. This study offers corroboration of contemporary historical evidencefor the severe mortality of the Black Death, and our procedure has thepotential to be applied to other Black Death cemeteries and to otherhypothesised catastrophic mortality assemblages from earlier timeperiods. In broader terms, the application of this approach may enableus to assess the contribution that catastrophic mortality makes to thedemography of past societies.Table 1. Known age auricular surface data. Specimens of known age wereassigned to one of eight morphological categories ('states') accordingto the criteria described by Lovejoy et al. (1985). The data are thenumbers of individuals in each age category of the reference samplewho exhibit a particular auricular surface state StateAge 1 2 3 4 5 6 7 8 Total16-24 35 12 1 1 1 5025-34 7 21 18 21 6 6 3 8235-44 10 10 19 9 18 5 3 7445-54 3 9 15 29 17 5 4 8255-64 1 1 2 10 13 17 21 3 6865-74 12 9 13 16 5 5575-99 4 4 4 21 9 42Total 43 47 39 82 70 75 72 25 453Table 2. Known age pubic symphysis data. Specimens of known age wereassigned to one of six morphological categories ('states') accordingto the criteria described by Brooks and Suchey (1990). The data arethe numbers of individuals in each age category of the referencesample who exhibit a particular pubic symphysis state StateAge 1 2 3 4 5 6 Total16-24 33 10 2 4525-34 4 10 21 22 5 2 6435-44 4 7 31 14 6 6245-54 3 5 25 17 17 6755-64 1 11 19 24 5565-74 2 6 14 26 4875-99 1 2 9 24 36Total 37 28 38 97 78 99 377 Acknowledgements The skeletal data were collected when Rebecca Gowland was funded byan AHRB AHRB Arts and Humanities Research Board Innovations Award. Thanks are due to John Shepherd, Bill Whiteand Alan Pipe for facilitating access to the Royal Mint skeletons and toEugenia Cunha and Louise Humphrey for allowing us to use the Coimbra andSpitalfields known age skeletal collections. We are grateful to MoLAS MoLAS Museum of London Archaeology Service for providing the photograph of the Royal Mint site excavations. ChrisKnusel kindly provided unpublished information, and the contributions ofanonymous referees are appreciated. * According to Bayes' theorem: p([A.sub.i] | [I.sub.j]) = p([I.sub.j] | [A.sub.i]) x p([A.sub.i]) / p([I.sub.j]). In this equation'A' represents age and 'I' represents skeletalindicator (e.g. auricular surface). The notation p([A.sub.i] |[I.sub.j]) represents the probability of being in age category i giventhe particular skeletal indicator state j and in Bayesian statisticsthis is referred to as the 'posterior probability'. Theprobability of possessing a particular indicator state given age, shownin the equation as p([I.sub.j] | [A.sub.i]), is referred to as the'likelihood', as it is the conditional probability conditional probabilitythe probability that event A occurs, given that event B has occurred. 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The temporaldynamics of the fourteenth-century Black Death: new evidence fromEnglish ecclesiastical records. Human Biology 75: 427-48. Received: 23 October 2003; Accepted: 14April 2004; Revised: 11February 2004 R.L. Gowland (1) & A.T. Chamberlain (2) (1) St John's College, University of Cambridge, Cambridge, UK(Email: rlg31@cam.ac.uk). (2) Department of Archaeology, University of Sheffield The University of Sheffield is a research university, located in Sheffield in South Yorkshire, England. ReputationSheffield was the Sunday Times University of the Year in 2001 and has consistently appeared as their top 20 institutions. , Sheffield,UK.