Immunological problems in paratuberculosis.

Paratuberculosis, or Johne's disease, is caused that was not known to be infected with paraby an acid-fast bacillus, Mycobacterium paratuberculosis, and the imported sheep were tuberculosis, and affects several species of rumiclinically healthy. Five of these twenty sheep nants, particularly sheep and cattle. It consists transmitted the disease to healthy contacts and of an insidious and chronic inflammation of the were thus proven to be carriers of the infection intestinal mucosa and the mesenteric lymph (3, 4). Paratuberculosis had not been known to glands draining the affected part. The inflamoccur in Iceland previously. mation may lead to extreme thickening of the Paratuberculosis is of great economic consemucosa and submucosa. It is most commonly quence to sheep and cattle raising in many located in the lowest part of the ileum adjacent countries in all parts of the world, and methods to the ileocecal junction, but in advanced cases of control have therefore been actively sought. the lesions may extend into the cecum. The With few exceptions, which will be mentioned causative organisms may usually be found in the later, the only method employed to control the diseased tissue, often in enormous numbers. spread of the disease has been attempts at early Even advanced lesions of paratuberculosis are diagnosis and subsequent destruction of infected sometimes found, however, in which only few animals. It appears that this method has nowhere or no acid-fast bacilli can be demonstrated proved very effective. The reason for this is, in microscopically. the writer's opinion, that early diagnosis of From six months to several years usually clinical cases is difficult and also .that latent elapse after effective transmission of paratuberinfections are common, and healthy carriers culosis until symptoms of the disease become therefore play an important role in the epiclinically noticeable, and the course of the clinical zootiology of paratuberculosis. phase of disease is also very protracted. When the Most immunological studies of paratuberdisease reaches a stage where clinical symptoms culosis have either been aimed at developing are in evidence, it is fatal in the large majority of diagnostic procedures or they have attempted to cases. On the other hand, there is reason to lay the foundations of a satisfactory procedure believe that latent infections which never become for active immunization. This is natural, as clinically recognizable are quite common in measures taken to control the spread of the infected herds, or in other words that many of disease must necessarily be based on immunothe animas, that are infected develop immunity logical methods of diagnosis or prevention. If a early and never show clinical signs of the infecsatisfactory method of active immunization could tion. Taylor, in England (1), isolated M. parabe developed, this would probably be the method tuberculosis from the ileocecal lymph nodes of 15 of choice. per cent of healthy cattle that came to the It is natural that immunological studies of slaughterhouse. He believes that the present paratuberculosis should have been fashioned comparatively ineffectual methods for isolating after corresponding work on infections with other the organism do not detect nearly all infected species of mycobacteria, such as mammalian animals and that many more "healthy" animals tuberculosis, avian tuberculosis and leprosy. in infected areas may be expected to be infected, That is, workers have tried to develop satisperhaps 25 to 30 per cent. Rankin (2) also found factory methods for testing skin sensitivity in a considerable number of cattle latently inparatuberculosis with tuberculin-like products, to fected. develop a complement fixation reaction for diagEpizootiological evidence is also available to nostic purposes, and finally to develop a vaccine suggest that latent infections are common. For capable of provoking active resistance to paraexample, 20 sheep were introduced into Iceland tuberculosis. from Germany in 1933. They came from a herd A general description of the various aspects of