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Dr. Rieux resolved to compile this chronicle…to state quite simply what we learn in a time of pestilence: that there are more things to admire in men than to despise.

—Albert Camus: The Plague


Zoonoses are infections in humans acquired by direct or indirect contact with animals. There are many zoonoses and more are being recognized (Table 36–1), but those covered herein are of great importance historically and still occur. The three principle species/variants of Brucella and their associated animals are abortus (cattle), melitensis (sheep and goats), and suis (pigs) in whom they cause genitourinary tract disease. 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. 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. Because the infection is localized in reticuloendothelial organs, there are few physical findings unless the liver or spleen becomes enlarged. When patients develop a cycling pattern of nocturnal fevers, the disease has been called undulant fever. The diagnosis is made by culturing blood or retrospectively by serology.

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. Most cases begin with a painful swollen lymph node (bubo) from which the bacteria rapidly spread to the bloodstream. Pneumonic plague (Black Death) is produced by pulmonary seeding from the bloodstream or is acquired directly from another patient with hemorrhagic pneumonia. All forms cause a toxic picture with shock and death within a few days. No other disease regularly kills previously healthy persons so rapidly. Y pestis is readily recovered on media used for other Enterobacteriaceae from aspirates of lymph nodes, blood cultures, and sputum in patients with pneumonia.

Tularemia is a disease of wild mammals caused by Francisella tularensis. Humans usually become infected by direct contact with infected animals or through the bite of a vector (tick or deer fly). Contact with water contaminated by ill animals has also been well documented. The illness is characterized by a local ulcer with high fever and severe constitutional symptoms. Typhoid-like illness with systemic symptoms only has been described. The epidemiology of tularemia and many features of the clinical infection are similar to those of plague.

Pasteurella multocida is found normally in the respiratory tract of many companion and other domestic and wild animals. When humans sustain a penetrating bite or scratch, most often by a cat, a rapidly destructive local soft tissue infection results.

TABLE 36–1Some Important Bacterial Zoonotic Infections

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