The way we make and think about retrospective diagnosis is changing. Over the last decade, laboratory results have become the preferred (maybe even mandatory) method of making a retrospective diagnosis . To extrapolate a few positive laboratory results to cover an entire epidemic, it must correlate with reported signs and symptoms and ideally epidemiology. There are pitfalls at every step of the process.
Today, laboratory results focus on identifying species specific biomolecules, aDNA and non-DNA molecules such as specific capsule lipids. I previously posted more detail on the non-DNA biomolecules used in paleomicrobiology. Each of these methods has its critics and proponents. Laboratory results should always be logical with the osteological analysis of the skeletal remains and with at least the majority of the written record. We have to remember that just because an infection was detected, it doesn’t necessarily mean that they died from it. If a skull has a sword wounds, it is unlikely that they died from tuberculosis. There is a significant difference between dying with a disease and dying from it.
For these science reports to be helpful to historians and anthropologists, they need to provide better context on the skeletal remains including how the remains were dated and associated with a particular epidemic. To give one example, a recent letter to Emerging Infectious Diseases reports Yersinia pestis in unerupted teeth from tombs under Sant’ Andrea church in Barletta, Italy . They suggest this identifies the 1656-1657 epidemic as the plague. Although the tombs were said to have been used “since the 14th century” and have been hypothesized to have been used for the 1656-1657 epidemic, they do not give any indication how these five youths were selected for study. They report that their negative control from the same tomb complex dated to before the epidemic, so presumably this could be easily corrected by giving the rationale an archaeologist used to select these youths.
Assessing signs and symptoms and the epidemiology of past disease requires correctly using ancient and medieval documents. Medical treatises are not necessarily as helpful as they may seem. Medieval authors often copied classical texts that were centuries old and their new observations were heavily influenced by contemporary philosophical paradigms . Instead, a variety of documents that can be broadly classified under government records (tax and manor rolls, wills, court records, etc); biographies and hagiographies; chronicles, annals and medieval histories can all be very useful in part because the disease is often not the main focus of the account .
Using such an array of ancient and medieval works has plenty of pitfalls even for historians, much less those from other disciplines. Mitchell has outlined a number of pitfalls to retrospective diagnosis from medieval texts that I will try to summarize with a few notes of my own .
- Not enough information in the source for an accurate diagnosis.
- An apparent eye-witness account is actually copied from an older source.
- Translation is not representative enough of the original language for diagnosis.
- Inadequate knowledge (of the researcher) of disease signs and symptoms and how they are represented in this cultural context.
- Cherry picking symptoms to fit a theory.
- Failure to understand that a source is not representative of the body of contemporary sources.
- Not realizing that more than one disease is present. Likewise, assuming that two or more diseases are present when they are not.
- Assumption that the antique or medieval disease is one that still exists today.
- Assumption that the characteristics of the disease have not changed from then until now, including transmission methods. Likewise, assuming that characteristics of the microbe must have significantly changed over time.
- Failure to understand how the environment and living conditions change the epidemiological characteristics of the epidemic.
Epidemiology is the most difficult parameter to assess. For ancient and medieval epidemics, I find it very difficult to consider epidemiology part of the retrospective diagnostic process. There are simply too many variables in the environment and human cultures of the past. Even diseases like malaria and influenza that we think we know so well still throw us surprises today . Not even seasonality is set in stone as the 2009 H1N1 influenza pandemic recently showed us. After now centuries of study, we still can be hard pressed to explain the rise and fall of malaria. By it’s very definition, a pandemic is an extraordinary situation compounded by historical distance for study of historic outbreaks.
Even removing it from the diagnostic process, epidemiology is still one of the most important fields of study for past epidemics. If we can answer the important epidemiological questions on past epidemics then I think we can say that we understand at least the science of the epidemic. For many well-known pathogens like influenza and plague, we have had many outbreaks but very few pandemics to learn from to prepare for future threats.
 Little, L. (2011). Plague Historians in Lab Coats Past & Present, 213 (1), 267-290 DOI: 10.1093/pastj/gtr014
 Mitchell, P. (2011). Retrospective diagnosis and the use of historical texts for investigating disease in the past International Journal of Paleopathology, 1 (2), 81-88 DOI: 10.1016/j.ijpp.2011.04.002
 Raoult D (2011). Molecular, epidemiological, and clinical complexities of predicting patterns of infectious diseases. Frontiers in microbiology, 2 PMID: 21687417
 Scasciamacchia S, Serrecchia L, Giangrossi L, Garofolo G, Balestrucci A, Sammartino G, et al. Plague epidemic in the Kingdom of Naples, 1656–1658. Emerging Infectious Disease. 2012 Jan . http://dx.doi.org/10.3201/eid1801.110597