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 significantly 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 first blood meal, lice fed on Orientalis-infected rabbits had a significantly higher death rate (90%) than did control (3%)(p< 0.0001), Antiqua-infected (16%), and Medievalisinfected (10%) lice; the latter values were significantly 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/16/5/892-appF. htm).