  Humans, domestic animals, orchard crops, and ornamental plants are commonly
treated with antibiotics in response to bacterial infection. By curing
infectious individuals, antibiotic therapy might limit the spread of contagious
disease among hosts. But an antibiotic`s suppression of within-host pathogen
density might also reduce the probability that the host is otherwise removed
from infectious status prior to therapeutic recovery. When rates of both
recovery via treatment and other removal events (e.g., isolation or mortality)
depend directly on within-host pathogen density, antibiotic treatment can relax
the overall removal rate sufficiently to increase between-host disease
transmission. To explore this dependence, a deterministic within-host dynamics
drives the infectious host's time-dependent probability of disease
transmission, as well as the probabilistic duration of the infectious period.
At the within-host scale, the model varies (1) inoculum size, (2) bacterial
self-regulation, (3) the time between infection and initiation of therapy, and
(4) antibiotic efficacy. At the between-host scale the model varies (5) the
size/susceptibility of groups randomly encountered by an infectious host.
Results identify conditions where antibiotic treatment can increase duration of
a host`s infectiousness, and consequently increase the expected number of new
infections. At lower antibiotic efficacy, treatment might convert a rare,
serious bacterial disease into a common, but treatable infection.
