Effect of para-aminosalicylic acid (PAS) alone and in association with streptomycin on experimental ocular tuberculosis.

THE therapeutic effects of streptomycin and of para-aminosalicylic acid (PAS) in the control of many forms of experimentally induced tuberculosis and in' the treatment of clinical tuberculosis are now well known. Although streptomycin is the most powerful antituberculosis drug which has so far been fully evaluated, evidence is accumulating that a combination of streptomycin and PAS is even more effective. (Karlson and Feldman, 1949; Medical Research Council, 1950). So far it has not proved possible to assess how much of this increased effect is due to the prevention of the emergence of streptomycin-resistant strains. When this investigation was begun, we were unable to find any reference to the use of PAS alone or in combination with streptomycin in the treatment of experimentally induced or clinical tuberculosis of the eye. Several authors have since published their experiences in this field of investigation. Rees and Robson (1949) have shown that PAS had a definite beneficial effect on tuberculous infection of the cornea in rabbits, though it was less effective than streptomycin. In their experience a combination of streptomycin and PAS was more effective than either drug used alone. Rees and Robson (1950) recorded similar observations in tuberculous infection of the cornea in mice. Gardner and others (1949) found that PAS given by the intravitreous method was effective against tuberculous infection of the cornea of rabbits but much less so than streptomycin. Bietti (1950), using PAS and streptomycin separately and in combination in the treatment of experimental tuberculosis of the eye, noted that streptomycin was superior to PAS but that the best results were obtained by a combination of both drugs. The present experiments were carried out with a view to determining whether PAS was an effective therapeutic agent in experimental ocular tuberculosis; and also whether when used in combination with streptomycin it enhanced the known beneficial effect of the latter. Leopold and Nichols (1946) have shown that bacteriostatic levels of streptomycin can be obtained in the intra-ocular fluid of rabbits