Salmonella in the American cockroach: evaluation of vector potential through dosed feeding experiments

Restrained American cockroaches, Periplaneta americana L., were fed graded doses of Salmonella typhimurium ranging from 1-6 X 10(3) to 2-0 X 10(6), and their faeces assayed daily for the pathogen. Only 4 specimens out of 117 demonstrated multiplication of salmonellas, which was unrelated to size of input. When data of persistence regardless of actual numbers were expressed as percentage Salmonella-positive faecal-days, and these transformed to probits, a graph of percentage-positive faecal-days versus log dose allowed a calculation of the CD 50, or contaminative dose required for 50% of the faecal-days to be infective. The CD 50 for this cockroach species was 1-4 X 10(6) Salmonella.