A comparison of the virulence in Guinea-pigs of south indian and British tubercle bacilli

Abstract

Summary Cultures of tubercle bacilli were obtained from 83 South Indian and 29 British patients with radiological evidence of tuberculosis, who were aged 12 years or more and had not had more than two weeks of chemotherapy All the cultures were sensitive to isoniazid and streptomycin and all were proved to be tubercle bacilli by a variety of identification tests. All the Indian cultures were catalase positive. Of the 63 Indian and 14 British cultures examined, all yielded positive niacin tests, indicating that they were of human type. The guinea-pig virulence of these cultures was investigated in five series of experiments, two in London, two in London, two in Madras and one at Porton. In these experiments three breeds of animals were used, one of which was bred both in London and in Madras. A dose of 1 or o·1 mg. bacilli was injected intramuscularly, the guinea-pigs were killed six and twelve weeks later and the amount of disease assessed by a score. As assessed by scores, guinea-pig mortality and culture of guinea-pig spleens, the virulence of Indian cultures was on the average lower than that of British cultures. The Indian cultures had a wide range of virulence, about 30 per cent being as virulent as British cultures, while the remainder were less virulent. The least virulent culture produced little more than local lesions, and healing occurred between six and twelve weeks, demonstrated both by scores, spleen culture and histological examination. Histological evidence of healing occurred to a greater extent in the spleen and liver than in the lungs. The British cultures were of homogenous high virulence. The best measurement of virulence was found to be the ‘mean index’, defined as the score divided by the survival time of the animal. The mean index combined the results of scores and mortality in both the 6-week and 12-week animals, so that additional information was obtained without loss of precision in measuring virulence. As judged by the mean index, Indian cultures were less virulent than British cultures in all the five series of experiments. The scoring procedure was found to be reliable and repeatable in serial experiments. Different observers obtained closely similar scores on the same animals and the results of experiments could be accurately duplicated when carried out in different countries. The Mantoux reactions of guinea-pigs tested with 100 TU of Old Tuberculin four weeks after infection with Indian cultures were found to be on average the same as those infected with British cultures, but the reactions were more variable in size.