Although earlier studies on the possibility of basing oral typhoid vaccines on heat killed Salmonella typhi Ty 2 have shown that some protection may be afforded, few workers have given serious consideration to the oral route of inoculation as a practical method for the prophylaxis-of typhoid fever.
An attempt to use oral killed vaccines was also stimulated by the local immunity of the intestine after oral vaccination and many advantages as compared with subcutaneous vaccination.
Hence, four types of oral vaccine, acetone, heat, alcohol, formalin killed typhoid vaccine, were= prepared experimentally with S. typhi Ty 2 and in order to examine the protective effect and immunogenic value of these different vaccines mice were vaccinated intragastrically-with one or several doses of 0:2-ml (1010 cells in saline) of vaccines respectively through a polyethylene tube at intervals of 3 hours.
The mouse sera were tested for agglutinins 8 days later. Differences were found in the patterns-of S. typhi antibodies. In acetone, heat and alcohol killed vaccines 0 antibodies were found most frequently, and in order of Vi and H antibodies the frequency was low, while in formalin killed vaccine almost same, patterns of antibodies were shown and frequency of mice showing all three-antibodies was lower than in the other .vaccines in mice given IX 1010 to 4X 1010 cells.
In general a large dose of vaccine was required to obtain the high frequency. In mouse, protection test 2 types of vaccine, acetone and heat killed vaccine, were employed.
Mice were challenged peritoneallywith 100 LD50 (10´ cells) viable S. typil Ty 2 in 5% muck_ 10 days after oral immunization with one or several doses of 0.2 ml (1030 cells) of vaccines. Acetone killed vaccine protected 50 percent of the mice immunized with 6X10" cells and.. challenged. (p <0.05) Heat killed vaccine did not protect mice given as many as 9 X.101° cells.
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