![]() ![]() Most cases begin with a painful swollen lymph node (bubo) from which the bacteria rapidly spread to the bloodstream. Yersinia pestis causes plague, an infection of rodents that is transmitted to humans by the bite of infected fleas and is the most explosively virulent disease known owing to its complex array of mechanisms to avoid host defenses. The diagnosis is made by culturing blood or retrospectively by serology. ![]() When patients develop a cycling pattern of nocturnal fevers, the disease has been called undulant fever. Because the infection is localized in reticuloendothelial organs, there are few physical findings unless the liver or spleen becomes enlarged. In humans Brucella evade toll-like receptors (TLRs) and innate immunity, survive in macrophages by inhibiting myeloperoxidase and lysosome fusion, and produce a chronic illness characterized by fever, night sweats, and weight loss lasting weeks to months. Humans such as farmers, slaughterhouse workers, and veterinarians become infected directly by occupational contact or indirectly by consumption of contaminated animal products such as milk. The three principle species of Brucella and their associated animals are abortus (cattle), melitensis (sheep and goats), and suis (pigs) in whom they cause genitourinary tract disease. There are many zoonoses and more are being recognized ( Table 36–1), but those covered herein are of great importance historically and still occur. Zoonoses are infections in humans acquired by direct or indirect contact with animals. ![]()
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