The study on leprosy epidemiological phenomenon on the decreasing leprosy prevalence rate has been done by many leprologists before.
A pilot study has been done in the field in Okinawa Island to observe some epidemiological phenomena on the tendency of the decreasing leprosy incidence rate. Okinawa was the most severe leprosy endemic area in Japan and in 1967 leprosy incidence rate was 0.181‰, but recently the situation has been very much improved and the incidence rate became 0.017 ‰ in 1980. Moreover, in Okinawa, leprosy out-patient treatment has been put into operation since 1961 and it has been easy to collect the data on newly-detected patients.
The following results were obtained in the stage when leprosy incidence rate showed decreasing tendency.
1. Leprosy incidence rate, lepromatous incidence rate, child incidence rate and child ratio (under 14 years old) showed decreasing tendency altogether, especially child incidence rate and child ratio were decreasing at a faster pace than leprosy incidence rate.
2. Lepromatous ratio and aged ratio (over 65 years old) were increasing in this stage on the contrary.
3. At this stage the difference between male ratio and female ratio were reducing, especially the difference between both ratio of child patients were reductng at a faster rate than in case of adults.
4. The cross phenomenon in the curve of child ratio and aged ratio was a remarkable sign and the phenomenon previously suggests the decrease of leprosy incidence rate.
5. In this stage the peak of the onset age curve moved from the younger group to the older group.

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One of the growth factor of M. lepraemurium in 1% Ogawa yolk medium might be a reducing action of the yolk protein fraction. Fgg white medium contained the water insoluble fraction of egg yolk did not permit the good growth of M. lepraemurium, however, growth of M. lepraemurium corresponding to egg yolk medium was seen on the above medium added with a high molecular fraction of water boiled extract of egg yolk which did not accererate the growth of M. lepraemurium on the minimal medium. As the water insoluble fraction of egg yolk was a high molecular lipoprotein particle, purification of growth factor could not be achieved. Thioglycolate, reduced glutation or cysteine accerarated the growth of M. lepraemurium on the Kirchner agar medium contained 10% bovine serum.

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Avian tubercle bacilli, strain Kirchberg, were inoculated subcutaneously on the thorax of 5 inbred strains of mice, C3H, C57BL/6, DDD, BALB/c and KK, in order to examine the development of local and visceral lesions at varying time intervals.
In almost all the mice of each strain, a small, hard and sharply defined nodular infiltrate developed at the inoculation site within 2 to 3 weeks. At about 10 to 15 weeks, the nodule stopped growing and tended to regress spontaneously. And in all the mice examined the visceral lesions were slight at that time.
At the late stage of infection, however, there were remarkable differences in their visceral lesions among the tested strains of mice, while subcutaneous lesions were still slight. At 35 to 50 weeks, extensive involvement was found in the lung, liver and spleen of KK and BALB/c mice. And many bacilli-loaded cells were found in the lung of C57BL/6 mice. On the contrary, visceral lesions of C3H and DDD mice were very slight even in the late stage of infection.
Mouse strain differences in visceral lesions of the subcutaneously infected mice showed similar tendencies to those of the intraperitoneally infected mice.

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C3H/He strain mice, approximately 6 weeks of age, were inoculated intraperitoneally with 0.5mg of M. avium, strain Kirchberg. The inoculum was prepared by the grinding method from a 13-day culuture of M. avium grown on 1% Ogawa's medium. The susceptibility of these mice to the bacilli was evaluated mainly by the average survival
The susceptibility of mice of 5 inbred strains, C3H, C57BL/6, DDD, BALB/c and KK, was also examined by the same manner above mentioned, as controls.
Clear differences in the host resistance to
M. avium
could be demonstrated in these strains, markedly between C3H and the other strains, including C3H/He. Average survival time and grade of visceral lesions of C3H/He mice were intermediate to those observed in C3H and the other 4 strains, but far distant to those of C3H strain.

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Effects of vitamin K₃ and B₁₂ on the growth of M. lepraeimurium in NC-5 medium containing soluble starch were investigated. The results obtained were as follows. It was significantly indicated that vitamin K³ as well as B¹² remarkably stimulated the primary growth of M. lepraerurium, and the optimal concentrations of each vitamin were 0.005μg and 0.4μg/ml, respectively.

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Pantoyl lactone was known showing an cytokinetic effect on the agent long multinucleate filaments of certain mutant strains of E. coli induced after exposure to radiation. Effect of pantoyl lactone on the growth of M. lepraemurium in cell-free medium was tested in order to see whether pantoyl lactone had the same effect on M. lepraemurium as on filamentous E. coli. The results obtained were as follows:
1. The numbers of bacilli in NDO-5 medium were significantly increased by addition of 0.005M pantoyl lactone.
2. Remarkable growth stimulation of M. lepraemurium in the slide culture system was observed when added with 0.005M pantoyl lactone.
3. However, filament formation of M. lepraemurium was still not changed by additions of any dosis of pantoyl lactone.

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One of the most susceptible strains, C3H mice and the most resistant strain, C57BL/6 mice were inoculated with 5×10⁹ bacilli of ^{Mycobacterium epraemurium} subcutaneously and lymphocyte transformation test of spleen cells to the specific antigen and polyclonal mitogens was performed and DTH to PPD and unrelated intigen, SRBC and HGG was also tested at various stages during the course of infection.
In LTT to the specific antigen, MLM, C3H mice showed almost low response until 24 weeks after infection, though C57BL/6 mice showed high response at 7th, 8th and 13 week significantly. In contrast, in LTT to T-cell mitogens, PHA and ConA, C3H mice developed the blastogenic response as high as seen in C57BL/6 mice even at 24th week after infection, though C3H mice were heavily loaded with bacilli at this stage.
Response to B-cell mitogen, LPS was rather high in C3H mice than in C57BL/6 mice.
Delayed type hypersensitivity response to MLM and PPD measured on footpad was high in C57BL/6 mice at 7th week after MLM infection. C3H mice also developed response to PPD in spite of low responsiveness to the specific antigen, MLM.
DTH response to unrelated antigen, SRBC and HGG were tested in MLM infected C3H mice. DTH to SRBC at 13th and 17th week was not different between MLM infected and uninfected control mice. DTH to HGG at 17th week was also the same as that of control mice.
These results showed that MLM infected C3H mice were not able to respond to the specific antigen, MLM, but general depression of cell mediated immunity was not observed in these mice until 24th week after infection.
As the facts above mentioned are similar to our early observations on human leprosy, the suppression of specific immune response is discussed in relation to antileprous immunity in human and experimental murine leprosy.

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