We continued to clinically monitor 3 groups of vaccinated [BCG or BCG + 2 doses of heat-killed M. leprae (HKML)] and one group of unvaccinated (control) rhesus monkeys (RM) and sooty mangabey monkeys (SMM) to determine long term clinical consequences of vaccination/live M. leprae challenge. There were 10 RM and 7 SMM per group. The clinical results were as follows 1) vaccination with BCG alone protected RM by 70%; 2) BCG + low dose (LD) or high dose (HD) HKML protected RM by 85%; 3) BCG vaccination significantly slowed the rate and the degree of leprosy progress in SMM, but failed to significantly diminish the total number of animals developing clinical symptoms long term (long-term protection by BCG alone would have probably been significant if there had been a larger number of SMM/group); and 4) BCG + LD or HD HKML vaccination exacerbated leprosy susceptibility in SMM. Since we have observed that RM tend (approx. 80%) to develop paucibacillary (PB) forms of leprosy and SMM develop (approx. 80%)multibacillary (MB) forms, the data suggest that BCG + HKML protect animals that have a predisposition towards susceptibility to PB forms of leprosy, but exacerbate susceptibility of animals with an innate predisposition towards MB forms of leprosy. BCG appears to protect very effectively at the PB end and less so at the MB end of the spectrum. A report from Vietnam agrees with our observations in that BCG + HKML only offered protection in geographic areas which had high percentages of PB-prone humans. Since it cannot be readily predicted which individuals are susceptible to PB vs. MB forms of leprosy, it is recommended that BCG + HKML not be utilized for vaccination of humans. BCG alone, on the other hand appears to be safe and effective over the entire range of leprosy types, especially so at the PB end of the spectrum. These observations help to explain the extreme variability (from 20-80%) in the protective rates of BCG vaccination against leprosy among various human populations (which vary in the proportions of PB:MB forms of leprosy). Our BCG results are in agreement with these reports of field trials among human populations.
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