Feeding trials of Listeria monocytogenes with a nonhuman primate model.

One of the major unanswered questions regarding the presence of Listeria monocytogenes in foods is how many cells must be ingested in order to cause illness. To answer this question, studies were undertaken by using Macaca fascicularis (cynomolgus monkey) as an animal model. Healthy nonhuman primates were dosed with various concentrations of L. monocytogenes suspended in sterile whole milk. Final concentrations of 10(5), 10(7), and 10(9) total cells of the organism were used; a control was also included. Blood samples, as well as fecal and nasal specimens, were taken at various time intervals. Only animals that received 10(9) cells of L. monocytogenes became noticeably ill, with symptoms of septicemia, irritability, loss of appetite, and occasional diarrhea. Monkeys that received 10(7) and 10(9) cells shed L. monocytogenes in the feces for approximately 21 days. In monkeys that received the dose of 10(9) cells, severe lymphopenia and neutrophilia occurred within 48 h. In a separate trial, monkeys received Maalox to reduce the gastric acidity of the stomach. However, no substantial differences were observed between Maalox-treated and control monkeys.