Intestinal immunity to poliovirus develops only after repeated infections in monkeys.

To establish gut immunity in monkeys to polio-virus type 1, we fed four juvenile bonnet monkeys (Macaca radiata) with 100 median infectious doses of the Mahoney strain of virus. The duration of viral shedding in the throat and faeces and the titre of virus in faeces were measured. Eighteen days after the last monkey stopped shedding virus, they were inoculated once again with the same amount of virus. All monkeys got re-infected; the duration and titres of viral shedding were very similar to those after the first inoculation. This was interpreted to mean a lack of effective gut immunity. After a third inoculation the re-infection was of very short duration, and viral titres were one log10 less, indicating a high degree of gut immunity. These results suggest that one infection by poliovirus does not induce gut immunity; at least two infections are necessary to induce it. Even after two consecutive infections, immunity was not sufficient to prevent re-infection.