Salmonella Typhimurium definitive type 104 with chromosomally encoded resistance to five or
more antimicrobial drugs (R-type ACSSuT+) has been reported increasingly frequently as the
cause of human and animal salmonellosis since 1990. Among animal isolates from the
northwestern United States (NWUS), R-type ACSSuT+ Typhimurium isolates increased
through the early 1990s to comprise 73% of Typhimurium isolates by 1995, but subsequently
decreased to comprise only 30% of isolates during 1998. NWUS S. Typhimurium R-type
ACSSuT+ were consistently (99%) phage typed as DT104 or the closely related DTu302. S.
Typhimurium isolates from cattle with primary salmonellosis, randomly selected from a
national repository, from NWUS were more likely to exhibit R-type ACSSuT+ (19/24, 79%)
compared to isolates from other quadrants (17/71, 24%; P < 0.01). Human patients infected
with R-type ACSSuT+ resided in postal zip code polygons of above average cattle farm
density (P < 0.05), while patients infected with other R-types showed no similar tendency.
Furthermore, humans infected with R-type ACSSuT+ Typhimurium were more likely to
report direct contact with livestock (P < 0.01) than humans infected with other R-types.