Capsular serovars of virulent <i>Capnocytophaga canimorsus</i> are shared by the closely related species <i>C. canis</i> and <i>C. cynodegmi</i>

<i>Capnocytophaga canimorsus</i> is a dog oral commensal bacterium that causes rare but life-threatening generalized infections in humans who have been in contact with its animal hosts. Two other dog commensals, <i>Capnocytophaga canis</i> and <i>Capnocytophaga cynodegmi</i>, cause rare, mild local infections. To date, nine capsular serovars have been described in <i>C. canimorsus</i>. Here, we serotyped 112 strains of <i>Capnocytophaga</i> spp. isolated from human infections. The <i>C. canimorsus</i> strains (86 of 96, 89.6%) belonged to serovars A, B, or C with relative frequencies of approximately 30% for each serovar. The high prevalence of the A, B, and C serovars in strains isolated from humans, compared to the previously described low prevalence of these serovars among dog isolates (7.6%), confirms that these three serovars are more virulent to humans than other serovars and suggests that the low incidence of disease may be linked to the low prevalence of the A, B, and C serovars in dogs. We serotyped six strains of <i>C. canis</i> and ten strains of <i>C. cynodegmi</i> and, surprisingly, found one <i>C. canis</i> and three <i>C. cynodegmi</i> strains to be of capsular serovar B. This observation prompted us to test 34 dog-isolated <i>C. canis</i> and 16 dog-isolated <i>C. cynodegmi</i> strains. We found four <i>C. canis</i> strains belonging to serovar A and one belonging to serovar F. In contrast, no dog-isolated <i>C. cynodegmi</i> strain could be typed with the available antisera. This work demonstrates that virulence-associated capsular polysaccharides (A, B, and C) are not specific to the <i>C. canimorsus</i> species.