Abstract
Capnocytophaga canimorsus 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, Capnocytophaga canis and Capnocytophaga cynodegmi, cause rare, mild local infections. To date, nine capsular serovars have been described in C. canimorsus. Here, we serotyped 112 strains of Capnocytophaga spp. isolated from human infections. The C. canimorsus 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 C. canis and ten strains of C. cynodegmi and, surprisingly, found one C. canis and three C. cynodegmi strains to be of capsular serovar B. This observation prompted us to test 34 dog-isolated C. canis and 16 dog-isolated C. cynodegmi strains. We found four C. canis strains belonging to serovar A and one belonging to serovar F. In contrast, no dog-isolated C. cynodegmi 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 C. canimorsus species.
Highlights
Our study confirms the high prevalence of capsular serovars A, B, and C among the strains of C. canimorsus isolated from human infections (86/96, 89.6%), and we describe two new serovars (L and M)
Capsular typing of human-isolated C. canimorsus strains Since the C. canis species has been only recently described[3–5], we tested whether any of the 102 strains from the Collection University of Gothenburg (CCUG) identified as C. canimorsus might be reclassified as C. canis
We found that the 96 C. canimorsus strains from the CCUG belonged to only six serovars, with A, B, and C being heavily dominant (89.6%), followed by the D serovar (7.3%) (Fig. 2a)
Summary
Introduction Capnocytophaga is aGram-negative bacterial genus in the phylum Bacteroidetes that represents part of the normal oral flora of domestic animals and humans[1]. Four Capnocytophaga species are present in dog and cat oral cavities: C. canimorsus; C. cynodegmi; C. canis; and C. stomatis[2–4]. Two studies have reported an incidence of 0.5–0.7 cases per million inhabitants per year[10,11], in 2016, a retrospective study performed in the Helsinki area in Finland reported an incidence of 4.1 cases per million inhabitants per year[12]. The discrepancy between these estimates might result from the difficulty in diagnosing C. canimorsus infections, presumably because of the slow and fastidious growth of these bacteria in culture[2]
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