Table 2.

Summary of fleabite challenges

Mouse	Mice bitten by fleas infected with:
	Wild type		Δpla		Δpla (complemented)
	Infective fleabites	Outcome	Infective fleabites	Outcome	Infective fleabites	Outcome
1	1	B	6	S	3	B
2	2	B	6	S	5	B
3	2	B	3	S	7	B
4	6	B	3	—	8	B
5	9	B	4	—	8	B
6	10	B	5	—	12	B
7	3	S	5	—	6	S
8	10	S	6	—	11	S
9	6	—	8	—	8	—
10	7	—	9	—	9	—
Median	6		5.5		8

Shown is the cumulative number of bites from infective fleas, which had the characteristic bacterial blockage of the proventricular valve in the flea foregut that is prerequisite for efficient transmission (see refs. 4 and 38). The median number of infective bites received by each group was not significantly different (P > 0.6 by Mann–Whitney test). B, bubonic plague; S, primary septicemic plague; —, no disease. Disease type and outcome were significantly different for the Δpla group compared with the other two (P < 0.05 by χ² test).