Fleas may also get the disease when they bite the infected human being with bubonic plague [I.sub.HB] or septicemic plague [I.sub.HS] at the rates [[GAMMA].sub.hbf] and [[GAMMA].sub.hsf], respectively.
The human population may get the disease in one of the following ways: when the infected flea [I.sub.F] bites and infects the susceptible human being [S.sub.H] at a rate [[GAMMA].sub.fh]; when they interact with one another; this can be with either a person with pneumonic plague [I.sub.HP] through airborne transmission or septicemic plague [I.sub.HS] through physical or sexual contact at the rates [[GAMMA].sub.hph] and [[GAMMA].sub.hsh], respectively.
The proportion of [E.sub.H] progresses and becomes infected by bubonic plague [I.sub.HB], septicemic plague [I.sub.HS], or pneumonic plague [I.sub.HP] at the rate [[alpha].sub.2] and proportion to [v.sub.1], [v.sub.2], or [v.sub.3], respectively.
The rodent population may get a disease in one of the following ways: when the infected flea [I.sub.F] bites and infects the susceptible rodent [S.sub.R] at a rate [[GAMMA].sub.fr], through interaction between rodents themselves, which may be with rodent infected by pneumonic plague [I.sub.RP] or septicemic plague [I.sub.RS] at the rates [[GAMMA].sub.rpr] and [[GAMMA].sub.rsr], respectively.
After 2 to 7 days the subgroup [E.sub.R] become infected and capable of transmitting the disease; the fraction of it progresses and becomes infected by bubonic plague [I.sub.RB], septicemic plague [I.sub.RS], or pneumonic plague [I.sub.RP] at the rate [[gamma].sub.2] and proportional to [[tau].sub.1], [[tau].sub.2], or [[tau].sub.3] respectively.
Occupations of white (A) and native (B) patients with bubonic, mixed, or septicemic plague
, Johannesburg, South Africa, 1904.
pestis only in blood vessels of numerous tissues, consistent with septicemic plague
Of these, bubonic plague accounted for 327 (83.9%) cases and 44 (13.5%) deaths; primary septicemic plague, for 49 (12.6%) cases and 11 (22.4%) deaths; and primary pneumonic plague, for seven (1.8%) cases and four (57.1%) deaths.
Septicemic plague without obvious lymphadenopathy is more difficult to diagnose because the manifestations are nonspecific (e.g., elevated temperature, chills, abdominal pain, nausea, vomiting, diarrhea, tachycardia, tachypnea, and hypotension) (4).
Bubonic and septicemic plague
are not transmissible from person to person, but if left untreated, plague bacteria can spread hematogenously to the lungs, resulting in secondary pneumonic plague.
Cases of pneumonic plague in the United States have occurred secondary to septicemic plague
or as a result of direct exposure (i.e., primary) to respiratory droplets from infected cats (5,6).