Body Lice, Yersinia pestis Orientalis, and Black Death

To the Editor: Wild rodent fl eas are the most common vectors of Yersinia pestis, the plague agent (1). The human body louse (Pediculus humanus) has been proposed as a probable additional vector during historical epidemics (2) because human cases of louse-borne plague have been suspected (3) and body louse–borne plague has been demonstrated experimentally with rabbits (4). Using rabbits, we tested the ability of the 3 Y. pestis main biovars to produce a successful rabbit-louse-rabbit-louse cycle of transmission (4). Two New Zealand White (Oryctolagus cuniculi) rabbits were inoculated intravenously with phosphate-buffered saline alone (negative controls) or phosphate-buffered saline containing 109 CFU of Y. pestis biotype Nairobi-Rattus Antiqua, biotype 14–47 Medievalis, or biotype 6/69M Orientalis. PCR ensured detection of the virulence factor–encoding plasmids. The rabbits inoculated with biotypes Antiqua, Medievalis, or Orientalis had septicemia of ≈2 × 103 CFU/mL of blood 14 hours postinoculation and died at 20–22 hours, 18–20 hours, or 16–18 hours postinoculation, respectively. In contrast, the negative control rabbits remained healthy for 3 weeks. Five minutes postinoculation, 150 uninfected lice fed for 1 hour on rabbits and took an equivalent blood meal as measured by weight, regardless of the rabbit used. Y. pestis was isolated from all 120 randomly tested lice and their feces. Five days postinfection, the death rate of Orientalis-fed lice (95.3%) was signifi cantly higher than that of the control (4%), Antiqua-fed, (78.6%), and Medievalis-fed (74%) (p<0.0001) lice. One third of Orientalis-infected lice remained alive 3 days after the contaminating blood meal. Lice fed on septicemic rabbits further fed on 2 uninfected rabbits for 1 hour daily for up to 6 days. The rabbits bitten by Orientalis-infected lice had 2.7 × 102 CFU/mL of blood 4 days postinfection and died 1 day later. In contrast, the rabbits bitten by Antiqua-infected or Medievalis-infected lice looked healthy and lacked septicemia 3 weeks after challenge. New groups of 150 uninfected lice fed for 1 hour daily on Orientalis-infected rabbits started to die earlier than did lice fed on Antiqua, Medievalis (1 vs. 2–3 days after blood meal), and uninfected rabbits. Furthermore, 21 days after their fi rst blood meal, lice fed on Orientalis-infected rabbits had a signifi cantly higher death rate (90%) than did control (3%) (p<0.0001), Antiqua-infected (16%), and Medievalisinfected (10%) lice; the latter values were signifi cantly higher than that of the negative controls (p = 0.046). Y. pestis could be cultured only from lice and their feces if the lice were fed on rabbits previously bitten by Orientalisinfected lice (online Appendix Figure, www.cdc.gov/EID/content/16/5/892appF.htm). Our observation that body lice effectively transmitted Y. pestis through a complete cycle of transmission confi rms previous experimental (4) and fi eld observations of experimental transmission that used body lice collected from plague patients from the same family in the absence of any other ectoparasite (3). Transmission of Orientalis but not Antiqua or Medievalis organisms did not result merely from experimental bias because negative controls remained negative, data were duplicated, rabbits exhibited equivalent bacteremia, and lice took equivalent blood meals regardless of biotype. Our observations shed new light on the Black Death, a medieval epidemic of plague (5). Historical records indicate that persons with the Black Death had bubonic plague, indicating an ectoparasite-borne transmission (1). Pulex irritans fl eas were documented in a medieval setting in Viking Greenland (6). However, their poor competence (7) and the Black Death that swept Russia and Scandinavia are not fully compatible with fl ea-borne transmission alone. Ten infected lice are suffi cient for plague transmission (4), and our observation that one third of infected lice remained alive 3 days after infection indicates that an index plague patient carrying as few as 30 body lice could be a source for plague up to 3 days after dying. This fi gure was highly plausible during the Black Death because body lice currently infest almost 85% of homeless persons, with a mean of 57 lice per person (8). Although the role of fl eas as vectors of Y. pestis from rodents to humans is undisputed, this tabulation sustains the potential role of body lice as an additional vector of plague from human to human during the Black Death (2). Paleomicrobiology suggested that most historical cases in Europe resulted from Orientalis (5). This observation challenged the scenario that Antiqua, Medievalis, and Orientalis were responsible for ancient, medieval, and modern plague pandemics, respectively (9). The latter scenario had been hypothesized after the biotypes were observed to have a geographic repartition matching that of the hypothetical sources of the 3 historical pandemics (10) and was further propagated as dogma without further confi rmation. Our data support an alternative scenario of the historical plague epidemics transmitted by body lice, with Orientalis being the only such louseborne transmissible biotype. This point justifi es studies during ongoing epidemics in cold countries, keeping in mind the need to understand and control re-emerging plague in modern populations exposed to body lice.

present in some skeletons from port cities in France, or that body lice might, under certain circumstances, transmit the Orientalis biotype of Y. pestis; their work appears careful and considered. However, given the differences mentioned above and improved knowledge on the rapidity of virus mutation and worldwide transmission potential, we merely argue that the simplest explanation for medieval plagues has yet to be ruled out: that they may have resulted from a human-to-human transmitted virus. Adding complexity to an already complicated etiologic theory, and stating such as historical fact based on limited geography and sample size, does not seem congruent with Occam's razor.  (3,4). Rattus rattus is commonly recognized as the vertebrate host of fl ea-borne plague that swept through Europe in the 1300s, killing >50% of the population. Davis believed this explanation did not fi t what he knew of the ecologic requirements of fl eas and black rats. He studied reports of archeologic excavations and reviewed poems, medieval bestiaries, and paintings and concluded that these rats were scarce during the Black Death era.

Mark Welford and Brian Bossak
His theory, based on historical information and investigative trips to Europe, was that invasive rats, if present, mostly occurred in low densities in port areas, not in rural inland areas. He noted that the expected rodent die-offs with bubonic plague were not associated with human epidemics and that rodent fl eas would not have been active during winter to transmit plague. Flea-borne transmission from rodents usually causes a few deaths per household, but deaths of entire households commonly occurred in the medieval epidemics. Human-tohuman transmission of pneumonic plague must have occurred, but as described by Ayyadurai et al., there was evidence of human bubonic plague, suggesting vector involvement. Davis did not present a viable reservoir/vector hypothesis for plague transmission; this and the later, well-known association of R. rattus and other rodents with plague throughout the world, may partially explain why his ideas received little attention. The fi nding that human body lice can be bubonic plague vectors suggests a mechanism for humanto-human transmission continuing during winter in inland areas and, as suggested by the authors, could also explain total deaths in households.
In Response: Commenting on our recent demonstration that the human body louse was a likely vector of Black Death (the medieval European plague epidemics) (1), Welford and Bossak (2) point out that quantitative and qualitative inconsistencies in data for Black Death and modern plague argue against concluding that Yersinia pestis is the etiologic agent of Black Death (3). These authors acknowledge the paleomicrobiologic demonstration of Y. pestis in human remains collected at ports, yet they argue that such demonstration remains to be performed for human remains collected from inland burial sites (2).
Careful review of the literature indicates that 3 unrelated scientifi c teams have now demonstrated the presence of Y. pestis-specifi c biomolecules in 14th-18th-century human remains in 11 sites in Europe. These locations include 7 nonport, inland sites <650 km from the coasts (4-6) in addition to 3 Justinian (nonport) locations (4,7). Therefore, the fact that Y. pestis was the etiologic agent of Black Death can no longer be disputed; the inconsistencies correctly noted by Welford and Bossak actually question the reservoir and the vector of Y. pestis during the Black Death and the following epidemics rather than its cause. McLean and Fall remind us that the cumulative work of their mentor, David E. Davis, suggested that black rat ectoparasites could not have been likely vectors of medieval plague in Europe, based on the facts that expected die-offs of rats were not reported and that the rodents' fl eas would not have been active during winter in medieval Europe (8).
McLean and Fall acknowledge that our experimental data pave the way toward an alternative scenario of body louse-borne transmission of the Black Death. Such transmission of Y. pestis was observed by Blanc and Baltazard during a cluster of bubonic plague cases in households in Morocco during World War II (9). These authors demonstrated that the body louse could be infected when living on a septicemic patient, could stay alive for 7 days with infectious feces, and could transmit plague (9). Demonstration of Y. pestis in human lice collected from Black Death burials would be a step toward understanding the epidemiology of Black Death; this technically demanding approach has been successfully used to assess the transmission of typhus in soldiers of Napoleon's Grand Army buried in Vilnius, Lithuania (10).
In agreement with these observations and those reported by Welford and Bossak (3), our work clearly indicates that Y. pestis could be effi ciently transmitted by the human louse (1), a potential vector of Black Death because of its high prevalence in medieval Europe (11). Far from "adding complexity to an already complicated etiology theory," as Welford and Bossak stated, it seems to us that the cumulative evidence provided by paleomicrobiologic demonstration and by our recent work (1) clarifi es the epidemiology of the Black Death and the subsequent epidemics. Louse transmission of Y. pestis also explains inconsistencies rightly noted by Welford and Bossak and provides a reason for the current plague cases in poor areas of the world where poor hygiene is common. A search for alternative hypotheses, including the previous viral hypothesis for Black Death, may not be necessary (2).
The analogic reasoning based on observations of current infectious diseases cannot be applied to the medieval Black Death. Paleomicrobiologic evidence and historical data force us to change the paradigm and to question the established dogma about the epidemiology of plague. McLean and Fall remind us that, even in science, alternative hypotheses have trouble challenging dogma (8). Black Death is one of many areas at the intersection of microbiology and history for which many hypotheses have been proposed and none has received confi rmation; these hypotheses have been repeated for so long that they became accepted as demonstrated truths.