The three common species, B. melitensis, B. suis , and B. abortus , can be differentiated on the basis of cultural, metabolic, and serologic features. Once thought not to be pathogenic for humans, B. canis has been found in 16 cases of human infection, several of which were attributed to pet dogs. A few cases acquired from caribou and moose have been seen in Alaska. The organism penetrates the mucous membranes of the oropharynx or enters through minor breaks in the skin, reaches the lymphatics, traverses the lymph node barriers, and then invades the bloodstream. Thereafter, any tissue can be involved.
Lesions are most common in the spleen, liver, lymph nodes, and bone marrow.
The inflammatory reaction produces nonspecific granulomas. Because of the insidious nature of many Brucella infections, the incubation period is difficult to define, varying from a few days to several months. Symptoms include intermittent fever, chills, weakness, malaise, myalgia, headache, weight loss, and anorexia. Occasionally, there are localizing complaints or physical findings such as pain or tenderness over the vertebrae or in the distribution of a nerve root, acute arthritis, splenomegaly, and cardiac murmurs. Pregnancy may end in spontaneous abortion.