zoonoses and communicable diseases common to ... - PAHO/WHO

288 BACTERIOSESPearl disease, a tuberculous peritonitis or pleurisy, is another form sometimesobserved in infected herds in countries with no tuberculosis control program.It is estimated that about 5% of tuberculous cows, especially in advanced cases,have tuberculous uterine lesions or tuberculous metritus, and that 1% to 2% havetuberculous mastitis. This clinical form not only has public health repercussions, butalso serves as a source of infection for calves nursed naturally or artificially. One ofthe main signs of tuberculosis acquired by the oral route is swelling of the retropharyngeallymph nodes. In calves, the primary lesion is usually located in the mesentericlymph nodes and the intestinal mucosa is not affected.The disease appears more frequently in older animals because the disease ischronic and because older animals have had more time to be exposed to the infection.The infection is more prevalent among dairy cattle than among beef cattle,not only because their useful economic life is longer, but because dairy cattle arein closer contact with one another when gathered for milking or when housed indairy sheds.Cattle are resistant to the M. avium complex (MAC) and rarely suffer progressivetuberculosis due to these agents. Nevertheless, they are very important in controlprograms because cattle can become paraspecifically sensitized to mammaliantuberculin, leading to difficulties in diagnosis. M. avium infects cattle through thedigestive tract. When lesions are present, they are generally limited to the intestineand mesenteric lymph nodes. However, lesions can occasionally be found in thelungs and regional lymph nodes but not in other tissues, indicating that the entryroute may sometimes be the respiratory tract. Lesions tend to heal spontaneously.Bovine-to-bovine transmission of M. avium infection does not occur (see the chapter,“Diseases Caused by Nontuberculous Mycobacteria”).Cattle are very resistant to M. tuberculosis, and rarely develop anatomicopathologiclesions. In several countries, M. tuberculosis has been isolated from the lymphnodes of some positive reactors to tuberculin that showed no lesions in postmortemexamination. Again in this instance, the infection’s importance lies in sensitizingthese animals to tuberculin.An experiment comparing the pathogenicity of M. africanum, M. bovis, and M.tuberculosis for calves inoculated intravenously showed that M. africanum (at leastthe strain used in this experiment) was as pathogenic for calves as M. bovis (deKantor et al., 1979).SWINE: This species is susceptible to the following agents: M. bovis, M. aviumcomplex, and M. tuberculosis. M. bovis is the most pathogenic and invasive forswine and is the cause of most cases of generalized tuberculosis.The principal route of infection is the digestive tract through consumption of contaminatedmilk or milk products, kitchen and abattoir scraps, and excreta fromtuberculous fowl and cattle. The primary infection complex is found in the oropharynxand the submaxillary lymph nodes, or in the intestines and the mesenteric lymphnodes. The lesions are usually confined in the primary complex. Chronic lesions arenot found in single organs, as they often are in cattle. Prevalence is lower in younganimals than in adults, but the former show a greater tendency toward generalizationof the infection. Eradication programs for bovine tuberculosis directly help toreduce the infection rate among swine. In the US in 1924, tuberculous lesions werefound in 15.2% of hogs butchered, while in 1989, they were found in only 0.67%.