Capnocytophaga canimorsus is a fastidious, slow-growing, Gram-negative rod of the genus Capnocytophaga. It is a commensal bacterium in the normal gingival flora of canine and feline species, but can cause illness in humans. Transmission may occur through bites, licks, or even close proximity with animals. C. canimorsus generally has low virulence in healthy individuals, but has been observed to cause severe, even grave, illness in persons with pre-existing conditions. The pathogenesis of C. canimorsus is still largely unknown, but increased clinical diagnoses have fostered an interest in the bacillus. Treatment with antibiotics is effective in most cases, but the most important yet basic diagnostic tool available to clinicians remains the knowledge of recent exposure to canines or felines. History Capnocytophaga canimorsus was first observed in 1976 by Bobo and Newton. The pair isolated a previously unknown Gram-negative bacterium from a patient presenting with meningitis in addition to sepsis. The patient had been previously exposed to two canine bites on two consecutive days from two different dogs. Noting the coincidence between the timing of the bites with the onset of symptoms, Butler et al. analyzed 17 similar cases of patients presenting with either sepsis or meningitis from 1961 to 1975. The cases had been sent to the CDC for examination due to the presence of an unknown Gram-negative bacillus isolated from infected individuals. Butler notified the CDC of the high incidence of dog bites in connection with the infections. The CDC could not identify the organism, so they applied the name CDC group DF-2. DF-2 stands for dysgonic fermenter, meaning that the bacterium is a slow-growing, fermentative bacillus. In 1989, while analyzing the properties of the unknown bacterium, Weaver et al. noted many similarities to bacteria of the genus Capnocytophaga. Later that same year, Brenner et al. proposed the name Capnocytophaga canimorsus after examining the morphology, G+C% content, and motility of the species. Etymology The name Capnocytophaga is derived from the Greek word kapnos, meaning "smoke", and given here because of its dependence on carbon dioxide for growth. It was added to distinguish this genus from the Cytophaga genus, originating from the Greek words of kytos (meaning "cell"), and phagein (meaning "eat"). The species name of canimorsus comes from the Latin words canis and morsus, meaning "dog" and "bite" respectively. Epidemiology In the United States, 50% of Americans will be bitten by dogs during the course of their lifetimes; 1 million