"Although on the surface, seem to have been similar, we are not convinced that the epidemic in the 14th century and the present day bubonic plague are the same," says Dr. James Wood, professor of anthropology and demography at Penn State. "Old descriptions of disease symptoms are usually too non-specific to be a reliable basis for diagnosis."
The researchers note that it was the symptom of lymphatic swelling that led 19th century bacteriologists to identify the 14th century epidemic as bubonic plague.
"The symptoms of the Black Death included high fevers, fetid breath, coughing, vomiting of blood and foul body odor," says Rebecca Ferrell, graduate student in anthropology. "Other symptoms were red bruising or hemorrhaging of skin and swollen lymph nodes. Many of these symptoms do appear in bubonic plague, but they can appear in many other diseases as well."
The researchers, who also include Sharon DeWitt-Avina, Penn State graduate student in anthropology, Stephen Matthews and Mark Shriver, both professors in the Population Research Institute at Penn State, and Darryl Holman, assistant professor of anthropology, University of Washington, Seattle, are investigating church records and other documents from England to reconstruct the virulence, spacial diffusion and temporal dynamics of the Black Death.
They are looking especially closely at bishops' records of the replacement of priests in several English dioceses. Although these records are often incomplete and difficult to interpret, they clearly show that many priests died during the epidemic period of 1349 to 1350.
"These records indicate that the spread of the Black Death was more rapid than we formerly believed," Wood told attendees today (April 12) at the annual meeting of the American Association of Physical Anthropologists in Buffalo, N.Y. "This disease appears to spread too rapidly among humans to be something that must first be established in wild rodent populations, like bubonic plague. An analysis of the priests' monthly mortality rates during the epidemic shows a 45-fold greater risk of death than during normal times, a level of mortality far higher than usually associated with bubonic plague."
Modern bubonic plague typically needs to reach a high frequency in the rat population before it spills over into the human community via the flea vector. Historically, epidemics of bubonic plague have been associated with enormous die-offs of rats.
"There are no reports of dead rats in the streets in the 1300s of the sort common in more recent epidemics when we know bubonic plague was the causative agent," says Wood.
Instead of being spread by animals and insect vectors, the researchers believe that the Black Death was transmitted through person-to-person contact, as are measles and smallpox. The geographic pattern of the disease seems to bear this out, since the disease spread rapidly along roadways and navigable rivers and was not slowed down by the kinds of geographical barrier that would restrict the movement of rodents.
"It is possible that the Black Death was caused by any of a number of infectious organisms, but we are not ready to pinpoint the causative agent," says Wood. "The Black Death was too quickly identified with bubonic plague in the past. Indeed, historians took what was known about the bubonic plague and used it erroneously to fill in the many gaps in our picture of the Black Death. We do not want to make the same mistake by identifying some other possible cause prematurely."
The researchers do not rule out the possibility that the Black Death might have been caused by an ancestor of the modern plague bacillus, which might later have mutated into the insect-borne disease of rodents that we now call bubonic plague. The fact is that we can only trace modern bubonic plague reliably back to the late 18th century or early 19th century, according to Wood. Who knows when it first emerged?
"We too often make the assumption that while a lot of things change in the interaction of infectious diseases and human hosts, the microbe itself stays more or less the same," says Wood. "This is wrong. If anything is likely to change, it is a microbe that goes through millions of generations and an equal number of chances to mutate over a few centuries. We see no reason to think that the Black Death pathogen still exists in anything like its original form."